Effects of olive mill wastewater on soil leachates composition under Tunisian climatic conditions: a lysimeter pilot study

Effects of olive mill wastewater on soil leachates composition under Tunisian climatic conditions: a lysimeter pilot study | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Effects of olive mill wastewater on soil leachates composition under Tunisian climatic conditions: a lysimeter pilot study Emna Kammoun, Christian Buchmann This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7185457/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Tunisia, as a major olive oil producer, generates substantial quantities of olive mill wastewater (OMW), frequently applied to agricultural soils due to its fertilizing potential. However, OMW contains high levels of (poly)phenolic compounds, which can persist in soils and thereby affect basic soil properties, soil wettability, and pose risks of groundwater contamination. The semi-arid to arid climate of Tunisia, characterized by pronounced seasonal variations, may strongly influence the degradation, leaching, and environmental fate of OMW-derived compounds. This study aimed to investigate the dynamics of OMW application in soil columns under controlled conditions simulating Tunisian seasonal climates. Soil lysimeters were used to monitor soil leachate quality over 18 weeks, encompassing two winter periods, a spring, and a summer season. Parameters analyzed in leachates included soluble phenolic compounds (SPC), pH, electrical conductivity (EC), water drop penetration time (WDPT), and dissolved organic carbon (DOC) quality via SUVA 254 . Results showed that wet winter conditions promoted OMW percolation, leading to elevated SPC concentrations in leachates, while moderate spring conditions favored degradation processes, reducing SPC and soil water repellency. Hot and dry summer conditions induced polymerization and (re)accumulation of OMW-derived compounds at the soil surface, whereas the second winter period exhibited lower SPC levels than the first. The findings highlight the significant role of seasonal climatic conditions on OMW behavior in soils, underlining the need for season-specific management strategies to minimize environmental risks associated with its utilization as soil amendment. Olive mill wastewater (OMW) (Poly)phenol dynamics Soil water repellency Lysimeter study Seasonal dynamics Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Tunisia, one of the world’s major olive oil producers, generates large volumes of olive mill wastewater (OMW) annually, posing significant environmental and economic challenges. Despite its potential as a soil amendment, since it provides high amounts of water, organic matter (OM), nitrogen, phosphorous, potassium and magnesium (Chaari et al. 2014 ; Chatzistathis and Koutsos 2017 ; Magdich et al. 2012 ; Tamimi et al. 2017 ). Although several studies have already demonstrated that OMW promotes soil organic matter (SOM) formation (Ayoub et al. 2014 ; Peikert et al. 2017 ; Tamimi et al. 2016 ), OMW-derived OM is very rich in (poly)phenols with loads estimated to be about 1000 times higher than in domestic wastewater (Niaounakis and Halvadakis 2006 ). Within the last years, it has already been shown that OMW-related (poly)phenols can negatively impact the environment, e.g., by accumulating in soil or leaching to the groundwater, thereby increasing soil water repellency (SWR) and inhibiting plant growth and soil microbial activity (Mekki et al. 2007 , 2008 ; Peikert et al. 2017 ). Polymerization reactions of (poly)phenols into larger molecules further promote abiotic acidification and repellency effects (C. Buchmann et al. 2015 ; M. Kurtz et al. 2015 ; Peikert et al. 2015 ; Steinmetz et al. 2015 ; Tamimi et al. 2016 ) as overall function of temperature, soil moisture, SOM content and pH (Diehl and Schaumann 2007 ; Lebron et al. 2012 ; Täumer et al. 2005 ). The semi-arid to arid climate of Tunisia, with its pronounced seasonal variations, can strongly influence the fate and impacts of these compounds. Sustainable management of OMW requires a thorough understanding of the dynamics and spatiotemporal fate of OMW-derived (poly)phenols in soils, as well as the identification of optimal conditions that enable effective and environmentally safe utilization while minimizing potential adverse effects. Many field studies have shown contradictory results for the strength and persistence of (poly)phenol-related effects in soil, whereby the environmental conditions in terms of seasonal variations were one of the main influencing factors. For instance, a rapid decrease in phenolic compounds to almost 50% of their initial concentration was recorded within the first three weeks following OMW application to soil (Saadi et al. 2007 ; Sierra et al. 2001 ; Tsiknia et al. 2014 ). In the same context, Buchmann et al. ( 2015 ) reported a 40% reduction of the total phenolic content (TPC) within two days after OMW application under moist conditions. Also, Tamimi et al. ( 2016 ) reported disappearing SWR with a significantly reduced soluble TPC during the first rain season, mainly caused by the (re)mobilization, degradation and leaching of OMW-derived compounds. However, other studies have shown a high persistence of soluble OMW-OM even after rainy seasons, which promoted their immobilization in the topsoil layers (Steinmetz et al. 2015 ). Diamantis et al. ( 2013 ) found that moderate moisture and temperature in spring reduced OMW-related toxicity and SWR. In contrast, hot and dry conditions, especially in summer, ought to promote condensation of OMW-derived amphiphilic compounds, also leading to SWR. Sunlight and drought during spring and summer can induce polymerization processes, causing soluble OMW-OM to migrate back to the soil surface by capillary action, accumulate there, and persist upon drying (Steinmetz et al. 2015 ; Tamimi et al. 2016 ). These partly contradictory findings highlight the need for more targeted and realistic research on the behavior and fate of specific OMW-derived compounds under varying environmental conditions. Although several field studies have investigated the impact of OMW on groundwater contamination (Boukhoubza et al. 2008 ; Kapellakis et al. 2015 ; Zenjari and Nejmeddine 2001 ), most rely on edge-of-field monitoring, which is often non-reproducible and subject to high variability (Duncan et al. 2016 ). Studies specifically examining the composition and concentrations of OMW-derived (poly)phenols in soil leachates—and their relevance for groundwater contamination—remain scarce, underscoring the importance of monitoring leaching dynamics during and after OMW application (Tamimi et al. 2016 ). To our knowledge, realistic laboratory experiments assessing (poly)phenol leaching under different seasonal conditions have not yet been conducted in detail. Lysimeter studies offer a promising approach due to minimal sample preparation, automated extraction, and high reproducibility, bridging the gap between complex field studies and simplified laboratory tests (Gilbert et al. 2013 ). While lysimeters have been used to evaluate OMW leaching potential (Caputo et al. 2013 ), study soil properties and plant performance after OMW spreading (Chartzoulakis et al. 2010 ), and measure water fluxes using tracers (Mohawesh et al. 2014 ), they have not yet been employed to investigate the dynamics of OMW-derived (poly)phenols under controlled, climate-relevant conditions. The aim of this study was to gain first insights into the dynamics and effects of OMW application in a soil column under different climatic conditions of Tunisia to understand the relationship between the degradation and accumulation of OMW-OM, and the changes in soil leachates quality. In this context, the composition and content of OMW-OM in the soil leachates can be used to assess degradation and transport dynamics of OMW in soil. Additionally, the knowledge on the time- and season-dependent availability of OMW-derived (poly)phenols in soil leachates might be useful to estimate the potential and degree of OMW-related groundwater contamination. For this, the time-dependent soil leachate quality during and after OMW application was monitored over 18 weeks in soil lysimeters and included four scenarios in terms of a first winter season (WS1), followed by a spring season (SP), a summer season (SS), and a final second winter season (WS2), all reflecting typical Tunisian seasons. Over the total incubation time, various parameters were assessed for the soil leachates, including the time-dependent variations of soluble phenolic compounds (SPC), pH, electrical conductivity (EC), water drop penetration time (WDPT), and the quality of dissolved organic carbon (DOC) by specific ultraviolet absorbance (SUVA 254 ). We hypothesized that 1) OMW-derived (poly)phenols are the main reason for hydrophobic effects in soil, which should be displayed in directly related SPC concentration and WDPT in OMW-treated soil leachates during all simulated seasons. However, especially the high moisture content under the wet winter conditions should promote OMW percolation through the soil matrix and therefore result in higher SPC in the soil leachates. Under spring conditions, moderate soil moisture and temperature are expected to allow for considerable OMW degradation, consequently reducing OMW-derived SWR. Moist conditions during a second wet winter simulation and after the hot and dry summer conditions are expected to further reduce the SPC in the soil leachates compared to the first winter simulation, which should be reflected in an additionally reduced SWR. Materials and methods Soil, OMW, and leachates characterization The soil used in this study was sampled from a field located in the Mediterranean country of Sfax/Tunisia (North Africa, Lambert coordinates X = 38G 70 ’50’’and 38G 73’ 80’’ Y = 8G 97 ’60’ ’and 9G 05’ 90’’ Z = 130). The sampling area was characterized by extensive arboriculture based on olive trees. The climatological data used were those of the Sfax weather station, provided by the Tunisian National Institute of Meteorology ( 2025 ). The sampling area was characterized by insufficient and irregular rainfall, the influence of the sea on temperature and humidity in summer. The average annual rainfall is about 213 mm with generally moderate temperatures, mild winters and short to long hot summers. The average maximum temperature is about 35.1°C in August and the respective minimum temperature about 6.2°C in January. The average annual temperature is 18.7°C with August as the hottest (average of 26.5°C) and January as the coldest month (average of 11.3°C), respectively. For basic physicochemical characterization, OMW and soil were processed as follows: OMW was first diluted 1:1000 (v/v) in demineralized water and filtered through 0.45 µm membranes (Whatman, United Kingdom). Soil extracts were prepared at a 1:5 (w/v) soil-to-water ratio, shaken for 24 h, centrifuged at 3,720 g for 15 min, and similarly filtered through 0.45 µm membranes. Various physicochemical parameters were measured for OMW, soil extracts, and soil leachates collected during the incubation experiment: pH and electrical conductivity (EC) were determined according to DIN ISO 11265 (1997) and DIN ISO 38404-5 (2009). Gravimetric water content was calculated on a drymass basis after ovendrying samples at 105°C for 48 h. Total carbon (TC) and total inorganic carbon (TIC) were quantified using a Multi N/C Analyzer 2100/2100S (Analytik Jena, Germany), with organic carbon (OC) obtained by subtracting TIC from TC. Total nitrogen (TN) analysis was conducted using a Vario Micro Cube (Elementar GmbH, Germany) following DIN ISO 10694 (1996). Specific UV absorbance at 254 nm (SUVA 254 ) was measured according to Weishaar et al. ( 2003 ) as a proxy for the aromaticity, and hence lability, of dissolved organic carbon (DOC) using a Specord 50 UV/VIS spectrophotometer (Analytik Jena, Germany). Total phenolic content (TPC) was assessed via Folin–Ciocalteu reagent, with results expressed as g gallic acid equivalents (GAE) L -1 based on a calibration with gallic acid (0–500 mg L⁻¹). In this regard, soluble phenolic compounds (SPC) were quantified by adding 300 µL of soil leachate to 1.5 mL of Folin–Ciocalteu reagent (diluted 1:10 in deionized water), incubating for 4 min, then adding 1.2 mL saturated Na₂CO₃ solution (200 g L⁻¹). Major cations (K⁺, Na⁺, Mg²⁺, Ca²⁺) and iron (Fe) were measured by ICPOES (Agilent 720, Germany) following microwaveassisted reverse aqua regia extraction at pH < 2, while chloride (Cl⁻) concentrations were determined by ion chromatography (881 Compact IC pro, Metrohm, Switzerland). Further soil-specific measurements included bulk dry density (DIN EN ISO 11272 2014 ), maximum waterholding capacity (WHCₘₐₓ) according to DIN ISO 11274 ( 1998 ), and soil poresize distribution according to Meyer et al. ( 2018 ) using a MiniSpec mq7.5 Relaxometer (Bruker Biospin, Germany). Lysimeter setup and soil incubation For the incubation experiment, four lysimeters (ecoTech, Germany) of 40 cm height and 30 cm diameter were filled layer-wise with the respective Tunisian soil according to Lewis & Sjöstrom ( 2010 ) to reach a final bulk density of 1.4 g cm -3 (see Fig. 1 for schematic experimental setup). During the overall 18-week incubation period, different seasons typical for Tunisia were simulated: a relatively cold and wet winter simulation (WS1), a moderate spring simulation (SPS), a hot and dry summer simulation (SS), and a second winter simulation (WS2) (Fig. 2 ). Thus, initial moisture conditions for each lysimeter were set to Tunisian winter conditions (WS1; 15% of WHC max ) by adding the respective amount of demineralized water on the top of the soil surface. One movable sprinkler head allowing uniform distribution of the irrigation and rainwater was used for mimicking season-specific rainfall events typical for Sfax (winter: 110 mm in 21 events; spring: 60 mm in 11 events and extreme summer: no rainfall). Each lysimeter was equipped with a polyamide membrane at the bottom to adjust matric potential using a dosing pump (ecoTech, Germany). The lower boundary condition was set to -83 hPa for WS1 and WS2, and to -600 hPa for SS and SPS, respectively. To monitor depth-dependent soil moisture in-situ and over time, two HydraProbes sensors (ecoTech, Germany) were placed at 10 cm and 30 cm soil depth in each lysimeter. To establish predefined soil temperature, heating cables were wrapped around the lysimeters at regular intervals and covered with insulating tape. For each seasonal scenario, the heat output generated from the heating cables was adjusted to reach the target temperature regime, consequently validated by measuring soil surface temperature for each lysimeter. Full spectrum daylight tubes (intensity: 100.000 lux for 12 h per day) were mounted above the lysimeter to allow natural light exposure and associated light-induced photoreactions on the soil surface. After pre-equilibration of 7 days, three lysimeters (OMW1-3) were irrigated with OMW, one lysimeter was set as control (only irrigated with demineralized water). The amount of OMW applied was 1.1 L (equivalent to 50 m 3 ha -1 or 14 L m -2 used for field applications) and therefore based on the recommendation of the Ministry of Environmental Protection in Tunisia for single soil application. OMW application was done manually using water gardening cans to avoid soil disturbance and to allow equal distribution. Throughout the whole incubation time, weekly leachates sampling campaigns took place, divided into 13 samplings for the first winter simulation (WS1), 7 samplings for spring simulation (SPS) and 6 samplings for the second winter simulation (WS2). Due to the overall low soil water content and missing irrigation during the summer simulation (SS), no leachate was collected. Besides the determination of basic physicochemical leachate properties already described in the previous section, soil water repellency (SWR) was assessed on the soil surface of all lysimeters via water drop penetration time (WDPT). For this, 20 water drops of each 100 µl were placed directly but randomly distributed on the topsoil in each lysimeter and the time until complete water penetration was determined, presented as the arithmetic mean of the three OMW-treated replicates and the control. The soil was considered water repellent when the WDPT exceeded 5s (Bisdom et al. 1993 ). Statistical analysis Statistical analyses were performed using R Statistics V4.3.0 (R Core Team 2020 ). Prior to conducting parametric tests, data was assessed for normality using the Shapiro-Wilk test. Homogeneity of variances was evaluated with Levene’s test. Relationships between different parameters were examined using Pearson’s product-moment correlation, differences in means across groups were analyzed with one-way ANOVA. When significant effects were detected (significance level of p < 0.05 was considered for all statistical tests), Tukey’s multiple comparison test was applied to identify significant differences between soil leachates from untreated and OMW-treated soils. Results Physicochemical properties of soil and OMW The physicochemical properties of the soil and OMW used in this study are summarized in Table 1 . Soil was alkaline (pH 8.8 ± 0.1), with very low water content (0.8 ± 0.1% dry weight) and a WHC max of 250.0 ± 6.3 mL kg⁻¹. TC was about 8.7 ± 0.1 mg L⁻¹ (4.4 ± 0.1 mg L⁻¹ C org ) and TN was 0.17 ± 0.10 mg L⁻¹. Its EC was about 64.9 ± 0.1 µS cm⁻¹, with Na + and K + concentrations of 0.001 ± 0.0003 g L⁻¹ and 0.50 ± 0.02 g L⁻¹, respectively. Ca 2+ , Fe, and Mg 2+ concentrations were 18.4 ± 0.5 g L⁻¹, 0.74 ± 0.09 g L⁻¹, and 1.73 ± 0.40 g L⁻¹, respectively. PSD was predominantly coarse (85.1 ± 0.8%) pores, followed by 9.6 ± 0.5% medium and 5.3 ± 0.3% fine pores. Table 1 Selected physicochemical properties of the investigated soil and olive mill wastewater (OMW) Parameter Soil OMW pH 8.8 ± 0.1 5.35 ± 0.1 Electrical conductivity (EC) (µS cm -1 ) 64.9 ± 0.1 530.0 ± 0.1 Water content (%) 0.8 ± 0.1 91.7 ± 0.1 Maximum water-holding capacity (WHC max ) (mL Kg -1 ) 250.0 ± 6.3 - Total Carbon (TC) (mg L -1 ) 8.7 ± 0.1 376.7 ± 0.1 Organic carbon (C org ) (mg L -1 ) 4.4 ± 0.1 298.6 ± 0.1 Total nitrogen (TN) (mg L -1 ) 0.17 ± 0.10 8.6 ± 0.1 SUVA 254 (L mg C -1 m -1 ) 0.37 ± 0.10 2.8 ± 0.1 Total phenolic content (TPC) (g GAE L -1 ) 0.036 ± 0.001 6.47 ± 0.10 Na (g L -1 ) 0.001 ± 0.0003 1.4 ± 0.4 K (g L -1 ) 0.50 ± 0.02 10.3 ± 0.3 Ca (g L -1 ) 18.4 ± 0.5 749.0 ± 0.2 Fe (g L -1 ) 0.74 ± 0.09 27.0 ± 0.5 Mg (g L -1 ) 1.73 ± 0.40 397.0 ± 0.4 Coarse pores − 10–50 µm (%) 85.1 ± 0.8 - Medium pores − 0.2–10 µm (%) 9.6 ± 0.5 - Fine pores - < 0.2 µm (%) 5.3 ± 0.3 - In contrast, OMW was strongly acidic (pH 5.35 ± 0.1) and highly conductive (530.0 ± 0.1 µS cm⁻¹), with a water content of 91.7 ± 0.1%. Its organic load was very high: TC was 376.7 ± 0.1 mg L⁻¹ (298.6 ± 0.1 mg L⁻¹ C org ), TN of 8.6 ± 0.1 mg L⁻¹, and specific UV absorbance at 254 nm (SUVA 254 ) of 2.8 L mg C⁻¹ m⁻¹. TPC reached 6.47 ± 0.10 g GAE L⁻¹. Cation concentrations were highest for Ca 2+ (749.0 ± 0.2 g L⁻¹), followed by Mg 2+ (397.0 ± 0.4 g L⁻¹), Fe (27.0 ± 0.5 g L⁻¹), K + (10.3 ± 0.3 g L⁻¹), and Na + (1.4 ± 0.4 g L⁻¹). Time- and scenario-dependent moisture dynamics The water content (WC) measured at both 10 cm (Fig. 3 a) and 30 cm (Fig. 3 b) soil depth varied not only between OMW treatment and control, but also among the three OMW replicates, revealing distinct spatial and temporal signatures. The control lysimeter exhibited a modest and predictable moisture regime: during WS1, WC increased from 9–20% and 12% at 10 cm and 30 cm soil depth, respectively. During SPS, WC decreased sharply and stabilized around 12% at 10 cm and 10% at 30 cm. During SS, WC decreased to ~ 3% and finally re-increased to 14% (10 cm) and 9% (30 cm) during WS2 without any intermittent spikes, respectively. OMW1 responded with the fastest initial infiltration: during WS1, WC at 10 cm increased from 9–26% within 24 h of the single OMW pulse, then declined steeply to 15% by the end of SPS. At 30 cm it peaked at 15% and returned to ~ 10%) more rapidly than the other OMW-treated replicates. During SS, OMW1 showed a single brief recovery spike to ~ 6% at 10 cm after an early irrigation and fell to ~ 3% until the end of the scenario. During WS2, it re-increased to ~ 15% at 10 cm and to 9% at 30 cm, but its recovery rate lagged slightly behind OMW3. In contrast, OMW2 exhibited the slowest WC decrease in the initial incubation course, its WC at 10 cm increased to 25% and declined more gradually, stabilizing around 14% during SPS. At 30 cm it peaked at 14% and maintained ~ 11% until WS2. During SS, OMW2 never rose above the 3% WC, indicating very rapid drying. During WS2, WC at 10 cm recovered only to ~ 14%, at 30 cm even only to 8%, marking the lowest re-increase among the OMW-treated replicates. OMW3 showed a high peak retention with intermediate WC decrease: during WS1, the WC at 10 cm reached 27% and decreased to finally 17% at the end of SPS. At 30 cm, WC peaked at 16% and decreased steadily to ~ 12%. Most striking, OMW3 showed two distinct WC peaks at 10 (5–7%) during SS, following mid-period irrigations. During WS2, it recovered fastest to ~ 16% at 10 cm and 9% at 30 cm. All together, these patterns show that, even under identical OMW dosing, clear differences in (1) WC peak magnitude, (2) rate of WC decline, and (3) residual WC retention under drought for OMW1-3 and the control. OMW1 drained fastest, OMW2 held soil moisture longest through spring, and OMW3 combined high peaks with notable resilience during summer drying, already underlining the importance of micro-scale soil structure heterogeneity in mediating OMW–soil water interactions. pH and EC dynamics Generally, OMW application increased EC and reduced pH in all soil leachates with respect to the untreated control soil (Fig. 4 ). Over the 18-week incubation, all three OMW-treated lysimeters exhibited substantially higher EC than the untreated control (Fig. 4 a). During the WS1, EC in OMW2 increased to 1252 ± 1 µS cm -1 , followed by OMW 3 (~ 987 ± 1 µS cm⁻¹), and OMW1 (~ 985 ± 54 µS cm⁻¹), while the control remained steady at 283 ± 19 µS cm⁻¹. During SPS, EC in OMW2 further increased to 1358 ± 118 µS cm⁻¹ and in OMW3 to 1314 ± 1 µS cm⁻¹. In contrast, OMW1 increased only to 658 ± 1 µS cm⁻¹, whereas the control fluctuated around 60 µS cm⁻¹. No leachate was collected during the SS, however, in WS2, all OMW-treated soil leachates reached the highest EC, with 1690 ± 1 µS cm⁻¹ for OMW 1, 1733 ± 1 µS cm -1 for OMW2, and 1992 ± 1 cm⁻¹ for OMW 3. EC of the control slightly decreased to finally 416 ± 1 µS cm -1 at the end of WS2. At the beginning of WS1, the pH of the control started at 7.42 ± 0.00, slightly decreased to 7.12 ± 0.00 in week 2, and recovered to 7.32 ± 0.01 by week 4 (Fig. 4 b). OMW1 started at 8.09 ± 0.06, decreased to 7.05 ± 0.01 in week 2, and re-increased to 7.32 ± 0.03 by week 4. Also, OMW2 showed this pattern, starting at a pH of 8.11 ± 0.00, decreasing to 7.10 ± 0.01, and re-increasing to 7.43 ± 0.07 at the end of WS1. OMW3 exhibited the largest fluctuations by decreasing during the first two weeks from 7.70 ± 0.00 to the lowest pH recorded (6.47 ± 0.03) before re-increasing again to finally 7.75 ± 0.15. At the beginning of SPS, the pH of the control was 7.57 ± 0.02, OMW1 7.97 ± 0.01, OMW2 7.55 ± 0.01, and OMW3 7.61 ± 0.08. However, at the end of SPS, the control had increased to 7.78 ± 0.04, OMW2 to 7.78 ± 0.00, and OMW3 to 7.88 ± 0.02. Only the pH of OMW1 slightly decreased to finally 7.79 ± 0.02. Regarding WS2, pH values peaked across all treatments towards more alkaline values, with 8.06 ± 0.00 for the control, 8.04 ± 0.00 for OMW1, 8.06 ± 0.01 for OMW2, and 8.21 ± 0.01 for OMW3. At the end of WS2, the control reached its highest pH of 8.32 ± 0.00, OMW1-2 showed with 8.15 ± 0.05, 7.99 ± 0.01, and 7.97 ± 0.02, were also still alkaline, respectively. Soluble OMW-OM dynamics Over the 18-week incubation, all three OMW-treated soil lysimeter showed markedly different leachate composition regarding OMW-OM compared to the untreated control, with each phase (WS1, SPS, SS, WS2) revealing distinct trends (Fig. 5 ). During WS1, TOC in OMW1 and OMW2 leachates significantly increased above both OMW3 and the control (Fig. 5 a). Already within the first incubation week, OMW1 peaked at 180 ± 0.5 mg L⁻¹ and OMW2 at 50 ± 0.5 mg L⁻¹, both significantly higher (p < 0.05) than OMW3 and the control with 19.2 ± 0.5 mg L⁻¹ and 14.6 ± 0.9 mg L⁻¹, respectively. From week 2 on, the TOC of OMW1 steeply decreased to ~ 25 mg L⁻¹ and then stabilized around 20–30 mg L⁻¹ through the end of WS1, while OMW2 increased steadily to ~ 220 mg L⁻¹ by week 4. OMW3 and the control showed only minor fluctuations around 10 mg L⁻¹. At the beginning of SPS, OMW2 increased again to 376 ± 0.5 mg L⁻¹, eight times higher than OMW1 (70 ± 0.5 mg L⁻¹), OMW3 (50 ± 0.5 mg L⁻¹), and the control (< 30 mg L⁻¹; p < 0.05). After the dry and hot SS, where no leachate was collected, WS2 caused a decrease of TOC for all lysimeters: OMW1 and the control were at ~ 20–30 mg L⁻¹, OMW3 at 80 ± 0.5 mg L⁻¹, and OMW2 at 50 ± 0.5 mg L⁻¹, respectively. At the end of week 18, no significant differences were observed anymore. Soluble phenolic compounds (SPC) followed a similar but less pronounced pattern: during WS1, SPC were highest in OMW1 (0.04 ± 0.01 g L⁻¹; p < 0.05), intermediate in OMW2 (0.025 ± 0.01 g L⁻¹) and lowest in OMW3 (0.005 ± 0.01 g L⁻¹), while the control remained below 0.002 g L⁻¹ (Fig. 5 b). During SPS, SPC in OMW1 decreased toward control levels, whereas OMW2 remained at ~ 0.02 g L⁻¹, and both OMW3 and the control below 0.005 g L⁻¹. At the end of WS2, SPC concentrations across all treatments decreased below 0.005 g L⁻¹, which was far lower than the incoming OMW concentration of 6.47 ± 0.10 g L⁻¹. SUVA 254 also peaked in the WS1 with OMW1 showing 0.40 L mg C⁻¹ m⁻¹ and OMW2 0.20 L mg C⁻¹ m⁻¹, both significantly higher than the control (0.07 L mg C⁻¹ m⁻¹, p < 0.05) (Fig. 5 c). OMW3 was in the range of the control (0.30 L mg C⁻¹ m⁻¹) and did therefor not differ significantly. During SPS, SUVA 254 decreased to ~ 0.05 L mg C⁻¹ m⁻¹ for both OMW1 and OMW3, and to ~ 0.15 L mg C⁻¹ m⁻¹ for OMW2, all comparable to the control. During WS2, only OMW2 exhibited a slight re-increase (~ 0.12 L mg C⁻¹ m⁻¹), while OMW1 and OMW3 remained at control levels (< 0.10 L mg C⁻¹ m⁻¹), with no significant differences among treatments. Soil surface and water repellency dynamics WDPT measurements on the soil surface demonstrated that OMW application significantly (p < 0.05) increased topsoil water repellency in OMW1–3 compared to the MQ-water control in every seasonal scenario (Fig. 6 b). All values for WS1, SPS, SS and WS2 are the mean of the three OMW treatments. For WS1, immediately after OMW application under simulated winter conditions, the soil surface exhibited a continuous, glossy crust with extensive vesiculation and protruding micro-aggregates, and repellency was at its peak: over 80% of measurement spots exhibited WDPT > 600 s (severe water repellency), with a small number of spots failing to infiltrate even after 3600 s. On average only 5 ± 2% of drops soaked in within 0–5 s, 5 ± 1% in 5–60 s, 20 ± 3% in 60–600 s, 60 ± 4% in 600–3600 s, and 10 ± 2% remained unpenetrated at 3600 s. During the spring simulation (SPS), the surface developed a dense, semi-consolidated dark crust with fewer nodular peaks and the onset of fine fractures, and repellency decreased but remained significant (p < 0.05): approximately 0% of drops infiltrated within 0–5 s, 5 ± 2% within 5–60 s, 50 ± 5% within 60–600 s, 30 ± 3% within 600–3600 s, and 15 ± 2% beyond 3600 s. Across SPS, 70% of spots still exhibited WDPT > 60 s. After the dry and hot summer phase (SS), the crust had transitioned to a matte-brown, uniform granular matrix with minimal relief, and repellency persisted: when leachate collection resumed in week 14, 90 ± 3% of drops infiltrated in 0–5 s, 8 ± 2% in 5–60 s, and 2 ± 1% in 60–600 s, with no drops exceeding 600 s. Despite this apparent recovery in the 0–5 s class, statistical comparison among WS1, SPS, SS and WS2 remained significant (p < 0.05), indicating that SS repellency differed from other phases. In the second winter simulation (WS2), the soil surface presented a light-brown, finely granulated layer devoid of vesicle formation, and hydrophobicity re-emerged at intermediate intensity: 85 ± 4% of drops soaked in within 0–5 s, 10 ± 2% in 5–60 s, and 5 ± 1% in 60–600 s, with no spots requiring more than 600 s. Overall, 70% of all spots across SS and WS2 showed WDPT > 60 s, confirming that significant repellency persisted beyond WS1. By contrast, the MQ-water control remained fully wettable (100% of drops in the 0–5 s class) at every time point. Discussion This study provides insights into the dynamics and fate of olive mill wastewater (OMW)-derived compounds and their related effects in soils under Tunisian season conditions. Using controlled lysimeter experiments, we demonstrated clear season-dependent differences in the leaching, accumulation, and potential transformation of OMW. The findings confirm and complement previous field and laboratory investigations across various Mediterranean regions but also highlight specific mechanisms observable only under controlled conditions. OMW-derived compounds underwent a dynamic interplay of dissolution, microbial transformation, sorption, polymerization, and physical immobilization. The dominance of each process shifted according to climatic conditions, profoundly influencing their overall and spatio-temporal fate. In this context, the underlying processes and mechanisms can be conceptualized as follows: during the first winter simulation (WS1), the combination of high soil moisture and relatively low temperature created conditions that enhanced the dissolution and downward movement of soluble OMW-derived compounds, as evidenced by elevated SPC, TOC, and SUVA 254 values in the collected leachates, peaking at significantly higher concentrations than measured in the control lysimeter. As demonstrated by Kurtz et al. ( 2021 ) during a five-year field study on a semi-arid olive orchard, annual OMW applications (50–150 m³ ha⁻¹ y⁻¹) significantly increased soil water content and TPC in the top 0–10 cm, showing overall dose-dependent accumulation patterns between OMW application rate, TPC and DOC. Moreover, the soil surface in WS1 developed a continuous, glossy crust with extensive vesiculation and protruding microaggregates, which corresponded to the maximum SWR recorded in this phase (over 80% of WDPT measurements > 600 s). Various field experiments and lysimeter studies have consistently demonstrated an increased SWR after OMW application, particularly in the upper soil layers. For example, Tamimi et al. ( 2016 ) found that WDPT increased for OMW-treated soil, especially after summer and winter applications, indicating moderate to high water repellency in the topsoil. Similarly, other studies showed persistent SWR and reduced saturated hydraulic conductivity in OMW-treated soils, with most hydrophobic compounds immobilized in the upper 5 cm (Bombino et al. 2021 ; Chaâri et al. 2022 ). On the one hand, the high matric potential together with the low soil temperatures during WS1 likely promoted the adsorption of OMW-OM onto fine silt and clay surfaces (Yaakoubi et al. 2024 ). On the other hand, when especially OMW-derived amphiphilic molecules diffuse into soil micro- and mesopore domains, they bind preferentially to hydrophobic domains within SOM, forming microscale coatings that further impeded water infiltration (Cajot et al. 2025 ; Chai et al. 2022 ; Hammecker et al. 2022 ). However, temperature-induced shrinkage and swelling of the clay fraction may have further generated transient fissures, acting as preferential flow paths that enhanced localized leaching while leaving adjacent zones highly enriched in polymerizing (poly)phenols (Beven and Germann 2013 ; Magdich et al. 2022 ; Mekki et al. 2007 ). However, soil microbial activity was likely constrained by low temperatures, as indicated by the persistence of high SPC levels throughout the winter period, and by elevated values for SUVA 254 in leachates, suggesting the presence of aromatic, less-degraded OM (Beven and Germann 2013 ; Pietikåinen et al. 2005 ). Moreover, the winter period also came along with noticeable acidification of soil leachates in terms of lower pH values, aligning with field observations that OMW application can reduce soil pH due to the organic acid content and subsequent biogeochemical reactions (Tamimi et al. 2016 ). Elevated EC values in winter leachates further indicated the mobilization of salts and ions from OMW, increasing the salinity risk for deeper soil horizons and groundwater (Bouhia et al. 2023 ; Khalil et al. 2024 ). Concerning the seasonal effects, the results are consistent with field studies conducted in Bait Reema and Gilat, where OMW application in winter resulted in significant leaching of phenolic compounds due to increased infiltration and preferential flow pathways (Peikert et al. 2015 ; Tamimi et al. 2016 ). During the simulated spring period (SPS), moderate temperatures and soil moisture created an environment favorable for soil microbial degradation of OMW-OM, as indicated by a significant decrease in SPC in leachates compared to WS1. SPC decrease was accompanied by a reduction in SUVA 254 values, indicating a shift toward less aromatic, more degraded DOM fractions (Kellerman et al. 2020 ; Wang et al. 2023 ). Such observations align with earlier reports from other incubation studies showing that microbial communities rapidly degrade soluble phenolic compounds under moderate conditions, leading to a significant decrease in toxicity and mobility of OMW-OM (C. Buchmann et al. 2015 ; Tamimi et al. 2016 ). In general, warm temperatures and intermittent moisture are well-known to stimulate extracellular enzyme production by saprotrophic fungi and bacteria, accelerating oxidative breakdown of low-molecular-weight organic compounds such as phenols (Ferguson and Lindo 2025 ; Min et al. 2015 ; Pallandt et al. 2025 ). Thus, soil microbial hotspots likely formed microsites of intensive biodegradation that accounted for the sharp decline in SPC and SUVA 254 (Kellerman et al. 2020 ; Saarela et al. 2024 ; Schimel and Schaeffer 2012 ). For instance, Saarela et al. ( 2024 ) observed that microbial hotspots or zones of elevated microbial activity can drive rapid DOM transformation, accompanied by reduced SUVA 254 values, indicating a reduction in the aromaticity of the remaining DOM pool. Additionally, SWR decreased moderately during spring, reflected in lower WDPT values. This reduction was accompanied by the development of a dense, semi-consolidated dark crust that likely created preferential flow channels, still allowing limited water infiltration despite sustained high repellency. Although microbial activity gradually degraded the OMW-induced hydrophobic organic coatings on soil particles, evidenced by a measurable decline in soil water repellency (SWR), a residual level of repellency persisted, indicating the enduring presence of recalcitrant hydrophobic compounds resistant to biodegradation (Albalasmeh and Mohawesh 2023 ; Doerr et al. 2000 ; Mekki et al. 2006 ). Despite the improvement in leachate quality, the elevated EC values persisted, indicating that the ions from the OMW are still contributing to the salinity of the soil. This matches observations that negative effects of OMW can persist even after the degradation of toxic organic compounds, potentially impacting soil structure and crop health in the long term (Kavvadias et al. 2015 ; M. P. Kurtz et al. 2021 ; Mahmoud et al. 2022 ; Regni et al. 2021 ). The simulated summer period (SS) was characterized by high temperatures and severe soil drought, conditions that significantly altered the fate of OMW-derived compounds. Although no leachate was collected during this season due to the absence of rain events and thus percolating water, several already known mechanisms point to significant physicochemical transformation dynamics in soil: dry and hot conditions, and especially at elevated soil salinity, strongly inhibit soil microbial activity (Oustani et al. 2025 ; Vázquez et al. 2025 ). Instead, abiotic transformation dynamics become more relevant, such as polymerization and condensation reactions of OMW-derived phenols and amphiphilic compounds (Bombino et al. 2021 ; F. El Hassani et al. 2023 ; F. Z. El Hassani et al. 2020 ). Although such reactions typically result in the formation of larger, often more hydrophobic macromolecules that may persist and even enhance SWR (Peikert et al. 2015 ; Steinmetz et al. 2015 , 2019 ), the observed reduction of hydrophobicity-indicating WDPT classes during SS indicates rater a decreased relevance or activity of OMW-derived hydrophobic compounds. However, (poly)phenols can undergo abiotic binding and condensation reactions with other OM fractions such as humic acids, resulting in their immobilization and transformation into bound, less hydrophobic forms (Vinken et al. 2005 ). Furthermore, the evaporation taking place upon soil drying promotes the upward transport of salts and other soluble OM to the soil surface, thereby modulating SWR through competing mechanisms: elevated ionic strength compresses the electrical double layer and increases solution surface tension, raising the solid–water contact angle and reinforcing hydrophobic coatings, while at moderate salinity, cation-induced flocculation aggregates particles and redistributes organic films, transiently mitigating water repellency (Jajarmi et al. 2023 ; Tang et al. 2024 ). Furthermore, higher salinity has been shown to induce the formation of DOM with higher molecular weight, degree of oxidation, and lability, as well as lower C:N ratio, aromaticity, and increased vulnerability to degradation (Zhu et al. 2023 ). Consequently, the dynamic interplay between salt‐driven reinforcement of hydrophobic barriers and salinity‐mediated aggregation highlights the complex nature of SWR and underscores the need to account for both processes in OMW utilization strategies. During the second winter simulation (WS2), soil rewetting following the summer drought enabled the (re)mobilization and (re)availability of the previously immobilized OMW-derived compounds (C. Buchmann et al. 2015 ; Tamimi et al. 2016 , 2017 ). However, the lysimeter results indicated only a modest leaching of SPC during WS2 compared to WS1, suggesting that while some soluble compounds were released, a significant fraction of OMW-OM remained physically immobilized or polymerized and thus resistant to further dissolution. Aso field data from Kurtz et al. ( 2021 ) showed that biological indicators, including bait-lamina consumption, Collembola, and Acari abundance, recovered between successive winter applications, supporting the disappearance of toxic phenol-related effects over time. Despite lower SPC levels in the collected soil leachates, WDPT remained elevated, indicating that hydrophobic compounds partly persisted in soil, consistent with observations that polymerized OMW-OM can remain in soil over multiple seasons, maintaining hydrophobicity and potentially impacting soil-water relations over extended periods (Doerr et al. 2000 ). Furthermore, EC values in the leachates during the second winter simulation also remained elevated, indicating an ongoing remobilization of ionic constituents, possibly due to the dissolution of salts concentrated at the soil surface during summer evaporation. This aligns with the results of Tamimi et al. ( 2016 ) and Kurtz et al. ( 2021 ), who demonstrated that surface evaporation drives upward water fluxes in the soil profile, resulting in capillary-mediated, time- and dose-dependent accumulation of Na⁺, K⁺, Ca²⁺, and Cl⁻ in the upper layers. Taken together, seasonal dynamics highlight that the environmental fate of OMW-OM is governed by a complex balance between dissolution, microbial transformation, physical immobilization, and abiotic polymerization, which collectively shape the risks and benefits associated with the utilization of OMW on soil. The results further underline the importance of aligning OMW management practices with seasonal climatic patterns to mitigate environmental risks (Dich et al. 2025 ; Vaz et al. 2024 ). It seems that OMW application during spring emerges as the most favorable option, as it allows for effective soil microbial degradation of labile organic compounds, while minimizing potential leaching risks. In contrast, OMW application in winter should be approached with caution due to the high potential for rapid leaching of (poly)phenols and salts into deeper soil layers and potentially to the groundwater. Summer applications, while avoiding immediate leaching, pose the risk of salt and OMW-OM accumulation in the topsoil layer, potentially fostering hydrophobic dynamics through abiotic transformations that impair soil water dynamics, soil microbial activity, and plant growth. Therefore, sustainable management of OMW as a soil amendment should include careful consideration of application timing, coupled with monitoring of soil hydrophobicity and salinity, particularly following dry summer periods. Implementing controlled irrigation during spring applications may further support microbial degradation processes and dilute salt concentrations, thus reducing long-term impacts on soil functioning and groundwater quality. Moreover, systematic monitoring of leachate composition in regions receiving OMW applications seems essential to detect and mitigate unwanted environmental implications (Geng et al. 2025 ). Integrating the gained knowledge of seasonally governed processes into practical OMW management strategies will increase the agricultural benefits of OMW while maintaining soil health and protecting water resources as has been widely suggested for various soil amendments (Christian Buchmann et al. 2025 ; Diacono and Montemurro 2010 ). While the lysimeter approach offered precise control over climatic variables and high reproducibility, it inherently simplifies field heterogeneity. The absence of plant–soil–microbe interactions, particularly rhizosphere dynamics, may underestimate the extent of microbial degradation under natural vegetation. Additionally, the single soil type and OMW batch limit the generalizability of our findings across the diverse textures and compositions found in Tunisian orchards. Future studies should incorporate dynamic water table fluctuations, additional soil matrices, and co-application of amendments (e.g., biochar or compost) to evaluate how these factors modulate (poly)phenol fate and soil hydrophobicity at landscape scales. Conclusion This study demonstrated that the environmental fate of olive mill wastewater (OMW)-derived compounds, especially OMW-OM, in Tunisian soils was tightly controlled by seasonal climatic conditions. During the simulated Tunisian winter periods, elevated soil moisture promoted SPC percolation, yet leachate concentrations remained substantially below the original OMW input levels, indicating a limited risk of deeper soil contamination under the controlled application rate. In spring, moderate temperatures and soil moisture enhanced soil microbial activity, which significantly reduced SPC, TOC, and SUVA 254 values in the collected soil leachates and partially alleviated soil water repellency. Conversely, the relatively hot and dry summer conditions suppressed biodegradation and favored abiotic polymerization of OMW-derived OM, resulting in the accumulation of hydrophobic compounds and residues at the soil surface that promoted persistent soil water repellency. For the sustainable utilization of OMW as a soil amendment, application timing and soil management need to align with seasonal dynamics. Based on this study, spring emerged as the optimal period for OMW application, when soil microbial degradation effectively reduces labile (poly)phenols while minimizing leaching risks. Application rates should be kept moderate and paired with controlled irrigation to slow percolation, dilute soluble salts, and promote sorption and soil microbial stabilization of OMW-OMW. Systematic monitoring of soil leachate composition, soil hydrophobicity, and salinity is recommended to maintain groundwater quality and soil health. In this regard, further investigations into the spatial-temporal persistence of OMW-OM and soil hydrophobicity within the soil profile should be evaluated to gain more detailed knowledge on the effects of OMW. Declarations Competing Interests: The authors declare no competing interests Financial support Laboratory measurements and data collection were supported by the facilities of RPTU Kaiserslautern-Landau, Campus Landau and conducted within the framework of the project “TRILAT-OLIVEOIL” funded by the Deutsche Forschungsgemeinschaft (grant number SCHA 849/13) Author Contribution Conceptualization: C.B.,E.K.; Methodology and experimental setup: C.B., E.K.; Material preparation and data collection: E.K.; Data evaluation and interpretation: E.K.; Writ-ing—original draft preparation: C.B.,E.K.; Writing—review and editing: C.B.,E.K.; Supervision: C.B. Acknowledgement We kindly thank Gabriele E. Schaumann for her valuable feedback on the manuscript. Data Availability The data that supports the findings of this study are available from the corresponding author upon reasonable request. References Albalasmeh, A. A., & Mohawesh, O. E. (2023). 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E., Keren, Y., Bukhanovsky, N., Borisover, M., Abo Garfha, M., et al. (2015). Characterization of topsoils subjected to poorly controlled olive oil mill wastewater pollution in West Bank and Israel. Agriculture, Ecosystems and Environment , 199 (1), 176–189. https://doi.org/10.1016/j.agee.2014.08.025 Pietikåinen, J., Pettersson, M., & Bååth, E. (2005). Comparison of temperature effects on soil respiration and bacterial and fungal growth rates. FEMS Microbiology Ecology , 52 (1), 49–58. https://doi.org/10.1016/j.femsec.2004.10.002 R Core Team. (2020). R: A language and environment for statistical computing (R Version 4.3. 0). R Foundation for Statistical Computing. Regni, L., Pezzolla, D., Ciancaleoni, S., Marozzi, G., Albertini, E., Gigliotti, G., & Proietti, P. (2021). Long-Term Effects of Amendment with Olive Mill Wastewater on Soil Chemical Properties, Microbial Community, and Olive Tree Vegetative and Productive Activities. Agronomy , 11 (12), 2562. https://doi.org/10.3390/agronomy11122562 Saadi, I., Laor, Y., Raviv, M., & Medina, S. (2007). Land spreading of olive mill wastewater: Effects on soil microbial activity and potential phytotoxicity. Chemosphere , 66 (1), 75–83. https://doi.org/10.1016/j.chemosphere.2006.05.019 Saarela, T., Zhu, X., Jäntti, H., Ohashi, M., Ide, J., Siljanen, H., et al. (2024). The influence of dissolved organic matter composition on microbial degradation and carbon dioxide production in pristine subarctic rivers. Boreal Environment Research , 29 . Schimel, J. P., & Schaeffer, S. M. (2012). Microbial control over carbon cycling in soil. Frontiers in Microbiology , 3 , 348. https://doi.org/10.3389/fmicb.2012.00348 Sierra, J., Martı́, E., Montserrat, G., Cruañas, R., & Aguilar, M. A. (2001). Characterisation and evolution of a soil affected by olive oil mill wastewater disposal. Science of The Total Environment , 279 (1), 207–214. https://doi.org/10.1016/S0048-9697(01)00783-5 Steinmetz, Z., Kurtz, M. P., Dag, A., Zipori, I., & Schaumann, G. E. (2015). The seasonal influence of olive mill wastewater applications on an orchard soil under semi-arid conditions. Journal of Plant Nutrition and Soil Science , 178 (4), 641–648. https://doi.org/10.1002/jpln.201400658 Steinmetz, Z., Kurtz, M. P., Zubrod, J. P., Meyer, A. H., Elsner, M., & Schaumann, G. E. (2019). Biodegradation and photooxidation of phenolic compounds in soil—A compound-specific stable isotope approach. Chemosphere , 230 , 210–218. https://doi.org/10.1016/j.chemosphere.2019.05.030 Tamimi, N., Diehl, D., Njoum, M., Marei, A., & Schaumann, G. E. (2016). Effects of olive mill wastewater disposal on soil: Interaction mechanisms during different seasons. Journal of Hydrology and Hydromechanics , 64 (2), 176–195. https://doi.org/10.1515/johh-2016-0017 Tamimi, N., Schaumann, G. E., & Diehl, D. (2017). The fate of organic matter brought into soil by olive mill wastewater application at different seasons. Journal of Soils and Sediments , 17 (4), 901–916. https://doi.org/10.1007/s11368-016-1584-1 Tang, L., Chen, Y., Jian, Q., Cheng, Z., & Ding, W. (2024). Effects of chemical solution components on the contact angle of typical minerals in soil: quartz, orthoclase and plagioclase. Scientific Reports , 14 (1). https://doi.org/10.1038/s41598-024-71117-8 Täumer, K., Stoffregen, H., & Wessolek, G. (2005). Determination of repellency distribution using soil organic matter and water content. Geoderma , 125 (1–2), 107–115. https://doi.org/10.1016/j.geoderma.2004.07.004 Tsiknia, M., Tzanakakis, V. A. A. V. A., Oikonomidis, D., Paranychianakis, N. V. N. V., & Nikolaidis, N. P. N. P. (2014). Effects of olive mill wastewater on soil carbon and nitrogen cycling. Applied Microbiology and Biotechnology , 98 (6), 2739–2749. https://doi.org/10.1007/s00253-013-5272-4 Vaz, T., Quina, M. M. J., Martins, R. C., & Gomes, J. (2024). Olive mill wastewater treatment strategies to obtain quality water for irrigation: A review. Science of The Total Environment , 931 , 172676. https://doi.org/10.1016/j.scitotenv.2024.172676 Vázquez, E., Teutscherová, N., Almorox, J., Cámara, J., Kasschau, K. D., & Benito, M. (2025). The accumulation of mineral nitrogen in soil during drying events is affected by soil management. Soil and Tillage Research , 252 , 106623. https://doi.org/10.1016/j.still.2025.106623 Vinken, R., Schäffer, A., & Ji, R. (2005). Abiotic association of soil-borne monomeric phenols with humic acids. Organic Geochemistry , 36 (4), 583–593. https://doi.org/10.1016/j.orggeochem.2004.10.016 Wang, Y.-H., Zhang, P., He, C., Yu, J.-C., Shi, Q., Dahlgren, R. A., et al. (2023). Molecular signatures of soil-derived dissolved organic matter constrained by mineral weathering. Fundamental Research , 3 (3), 377–383. https://doi.org/10.1016/j.fmre.2022.01.032 Weishaar, J. L., Aiken, G. R., Bergamaschi, B. A., Fram, M. S., Fujii, R., & Mopper, K. (2003). Evaluation of Specific Ultraviolet Absorbance as an Indicator of the Chemical Composition and Reactivity of Dissolved Organic Carbon. Environmental Science & Technology , 37 (20), 4702–4708. https://doi.org/10.1021/es030360x Yaakoubi, A., Aganchich, B., Meddich, A., & Wahbi, S. (2024). Land spreading of olive mill wastewater (OMW): Biodegradation of organic matter and polyphenols in soil and effect on the activity of the total soil microflora. Water Practice & Technology , 19 (2), 297–310. https://doi.org/10.2166/wpt.2024.013 Zenjari, B., & Nejmeddine, A. (2001). Impact of spreading olive mill wastewater on soil characteristics: laboratory experiments. Agronomie , 21 , 749–755. Zhu, X., Xie, L., Ma, Y., Wang, L., Pang, Q., Peng, F., et al. (2023). Effects of drought-rewetting processes and salinity variations on dissolved organic matter (DOM) transformation and bacterial communities in lacustrine sediments. Journal of Soils and Sediments , 23 (11), 4055–4068. https://doi.org/10.1007/s11368-023-03611-x Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7185457","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":490216465,"identity":"a3a69f75-1182-4510-ac7d-2be3a751f08e","order_by":0,"name":"Emna Kammoun","email":"","orcid":"","institution":"IES Landau, RPTU University Kaiserslautern-Landau","correspondingAuthor":false,"prefix":"","firstName":"Emna","middleName":"","lastName":"Kammoun","suffix":""},{"id":490216466,"identity":"080f8466-cc7b-4061-87d6-6584521c8180","order_by":1,"name":"Christian Buchmann","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAx0lEQVRIiWNgGAWjYFACHgYGxgYQg/kAROAA8VrYEhgYEkjTwmNAnBb+Bt5jEj93bJM3l+759rnwx2EGvuMN+LVIHOBLk+w9c9tw55yzm2fPSDjMIHmGgDUGDDxm0oxttxk33MjdzMwD1GJwI4E4LfYbbuQ8hmi5/4A4LYlALcxQW/DrYJA4zJds2dt2O3nDjTRj5hlp6TySZwg4jL+99+CNn223bTfcSH7MXGBjLcd3/AABa5jR2DwE1OPRPgpGwSgYBaMADgAOmURLrXPKTQAAAABJRU5ErkJggg==","orcid":"","institution":"IES Landau, RPTU University Kaiserslautern-Landau","correspondingAuthor":true,"prefix":"","firstName":"Christian","middleName":"","lastName":"Buchmann","suffix":""}],"badges":[],"createdAt":"2025-07-22 09:53:07","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7185457/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7185457/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":87602694,"identity":"78993de4-cadb-4ec7-bd5f-f59ee3785fc1","added_by":"auto","created_at":"2025-07-25 17:10:33","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":72178,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic layout of the laboratory lysimeter used for the 18-week incubation study. Each lysimeter (30 cm diameter × 40 cm height) was filled with soil and monitored for moisture at 10 and 30 cm depth (green arrows). Olive mill wastewater (OMW) was applied at the surface (purple arrows) and followed by simulated rainfall events (blue arrows) under artificial sunlight (red arrows). Temperature was adjusted using heating cables wrapped and thermically sealed around the lysimeter. The upper boundary condition was defined by the time‐variable OMW/rainfall inflow, while the lower boundary condition was controlled by the matric potential at 40 cm depth using a permeable polyamide membrane, beneath which leachate was collected (blue outlet).\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7185457/v1/48c3b98653dc50b6ff4075bd.jpg"},{"id":87602699,"identity":"88a00915-2ba4-4048-933b-7382a8236909","added_by":"auto","created_at":"2025-07-25 17:10:33","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":38397,"visible":true,"origin":"","legend":"\u003cp\u003eSeasonal simulation after OMW application for a total of 18 weeks (WS1: 1\u003csup\u003est\u003c/sup\u003e winter simulation, SPS: spring simulation, SS: summer simulation, WS2: 2\u003csup\u003end\u003c/sup\u003e winter simulation) with respective temperatures and rainfall events (precipitation in mm and total number of events for each scenario)\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7185457/v1/b107768786048920b7dd96d5.jpg"},{"id":87601917,"identity":"498e5975-7336-4ed2-8619-a354c6f98a3e","added_by":"auto","created_at":"2025-07-25 17:02:33","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":97662,"visible":true,"origin":"","legend":"\u003cp\u003eSoil water content at (a) 10 cm and (b) 30 cm soil depth for control (grey) and olive mill wastewater (OMW)–treated soil lysimeters (OMW1 (red), OMW2 (black) and OMW3 (blue)) over the 18-week incubation period, subdivided into four seasonal scenarios indicated by the solid vertical lines: WS1 (first winter, weeks 1–4), SPS (spring, weeks 4–8), SS (summer, weeks 8–14) and WS2 (second winter, weeks 14–18). Downward arrows indicate the irrigation events with demineralized water during the respective seasonal scenario.\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7185457/v1/1508106f08829429f7a1ce20.jpg"},{"id":87601922,"identity":"4955ef23-9c60-45f4-9bc0-346578e87d5a","added_by":"auto","created_at":"2025-07-25 17:02:33","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":98115,"visible":true,"origin":"","legend":"\u003cp\u003ea) Electrical conductivity (EC) and b) pH for control and olive mill wastewater (OMW)–treated soil lysimeters OMW1-3 over the 18-week incubation period, subdivided into four seasonal scenarios indicated by the solid vertical lines: WS1 (first winter, weeks 1–4), SPS (spring, weeks 4–8), and WS2 (second winter, weeks 14–18). *No leachates were collected during the dry and hot SS (weeks 8-14).\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7185457/v1/ff3ab4a11ad712aba0a6e0ab.jpg"},{"id":87602697,"identity":"12df194a-aa06-4a5b-8434-223559256f77","added_by":"auto","created_at":"2025-07-25 17:10:33","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":64382,"visible":true,"origin":"","legend":"\u003cp\u003ea) Total organic carbon (TOC), b) soluble phenolic compounds (SPC) and c) specific ultraviolet absorbance (SUVA\u003csub\u003e254\u003c/sub\u003e) for control and olive mill wastewater (OMW)–treated soil lysimeters OMW1-3 over the 18-week incubation period, subdivided into four seasonal scenarios indicated by the solid vertical lines: WS1 (first winter, weeks 1–4), SPS (spring, weeks 4–8), SS (summer, weeks 8–14) and WS2 (second winter, weeks 14–18). No leachates were collected during the dry and hot SS.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7185457/v1/33fd76a9f03e169dd8f6733c.jpg"},{"id":87603003,"identity":"154abaa8-a9b1-48cf-b7ea-e700c9669b05","added_by":"auto","created_at":"2025-07-25 17:18:33","extension":"jpg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":80736,"visible":true,"origin":"","legend":"\u003cp\u003ea)\u003cstrong\u003e \u003c/strong\u003eexemplary\u003cstrong\u003e \u003c/strong\u003etemporal development of the soil surface after OMW application and during the different seasonal scenarios;\u003cstrong\u003e \u003c/strong\u003eb)\u003cstrong\u003e \u003c/strong\u003erelative frequencies of soil water drop penetration times (WDPT) according to Bisdom et al. (1993), presented as the mean of OMW1-3 and the control. WS1 (first winter, weeks 1–4), SPS (spring, weeks 4–8), and WS2 (second winter, weeks 14–18). Error bars represent standard deviations.\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7185457/v1/8ae8867561ac73ac9b28fa0a.jpg"},{"id":93755600,"identity":"7587d6e4-81c7-42cf-9859-eebfa4eae1cf","added_by":"auto","created_at":"2025-10-17 08:40:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1214729,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7185457/v1/c0bacd62-1020-4733-b085-17f916178aff.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effects of olive mill wastewater on soil leachates composition under Tunisian climatic conditions: a lysimeter pilot study","fulltext":[{"header":"Introduction","content":"\u003cp\u003eTunisia, one of the world\u0026rsquo;s major olive oil producers, generates large volumes of olive mill wastewater (OMW) annually, posing significant environmental and economic challenges. Despite its potential as a soil amendment, since it provides high amounts of water, organic matter (OM), nitrogen, phosphorous, potassium and magnesium (Chaari et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Chatzistathis and Koutsos \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Magdich et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Tamimi et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Although several studies have already demonstrated that OMW promotes soil organic matter (SOM) formation (Ayoub et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Peikert et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), OMW-derived OM is very rich in (poly)phenols with loads estimated to be about 1000 times higher than in domestic wastewater (Niaounakis and Halvadakis \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Within the last years, it has already been shown that OMW-related (poly)phenols can negatively impact the environment, e.g., by accumulating in soil or leaching to the groundwater, thereby increasing soil water repellency (SWR) and inhibiting plant growth and soil microbial activity (Mekki et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2007\u003c/span\u003e, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Peikert et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Polymerization reactions of (poly)phenols into larger molecules further promote abiotic acidification and repellency effects (C. Buchmann et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; M. Kurtz et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Peikert et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Steinmetz et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) as overall function of temperature, soil moisture, SOM content and pH (Diehl and Schaumann \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Lebron et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; T\u0026auml;umer et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). The semi-arid to arid climate of Tunisia, with its pronounced seasonal variations, can strongly influence the fate and impacts of these compounds. Sustainable management of OMW requires a thorough understanding of the dynamics and spatiotemporal fate of OMW-derived (poly)phenols in soils, as well as the identification of optimal conditions that enable effective and environmentally safe utilization while minimizing potential adverse effects.\u003c/p\u003e\u003cp\u003eMany field studies have shown contradictory results for the strength and persistence of (poly)phenol-related effects in soil, whereby the environmental conditions in terms of seasonal variations were one of the main influencing factors. For instance, a rapid decrease in phenolic compounds to almost 50% of their initial concentration was recorded within the first three weeks following OMW application to soil (Saadi et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2007\u003c/span\u003e; Sierra et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Tsiknia et al. \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). In the same context, Buchmann et al. (\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) reported a 40% reduction of the total phenolic content (TPC) within two days after OMW application under moist conditions. Also, Tamimi et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) reported disappearing SWR with a significantly reduced soluble TPC during the first rain season, mainly caused by the (re)mobilization, degradation and leaching of OMW-derived compounds. However, other studies have shown a high persistence of soluble OMW-OM even after rainy seasons, which promoted their immobilization in the topsoil layers (Steinmetz et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Diamantis et al. (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) found that moderate moisture and temperature in spring reduced OMW-related toxicity and SWR. In contrast, hot and dry conditions, especially in summer, ought to promote condensation of OMW-derived amphiphilic compounds, also leading to SWR. Sunlight and drought during spring and summer can induce polymerization processes, causing soluble OMW-OM to migrate back to the soil surface by capillary action, accumulate there, and persist upon drying (Steinmetz et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). These partly contradictory findings highlight the need for more targeted and realistic research on the behavior and fate of specific OMW-derived compounds under varying environmental conditions.\u003c/p\u003e\u003cp\u003eAlthough several field studies have investigated the impact of OMW on groundwater contamination (Boukhoubza et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Kapellakis et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Zenjari and Nejmeddine \u003cspan citationid=\"CR78\" class=\"CitationRef\"\u003e2001\u003c/span\u003e), most rely on edge-of-field monitoring, which is often non-reproducible and subject to high variability (Duncan et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Studies specifically examining the composition and concentrations of OMW-derived (poly)phenols in soil leachates\u0026mdash;and their relevance for groundwater contamination\u0026mdash;remain scarce, underscoring the importance of monitoring leaching dynamics during and after OMW application (Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). To our knowledge, realistic laboratory experiments assessing (poly)phenol leaching under different seasonal conditions have not yet been conducted in detail. Lysimeter studies offer a promising approach due to minimal sample preparation, automated extraction, and high reproducibility, bridging the gap between complex field studies and simplified laboratory tests (Gilbert et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). While lysimeters have been used to evaluate OMW leaching potential (Caputo et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), study soil properties and plant performance after OMW spreading (Chartzoulakis et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), and measure water fluxes using tracers (Mohawesh et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), they have not yet been employed to investigate the dynamics of OMW-derived (poly)phenols under controlled, climate-relevant conditions.\u003c/p\u003e\u003cp\u003eThe aim of this study was to gain first insights into the dynamics and effects of OMW application in a soil column under different climatic conditions of Tunisia to understand the relationship between the degradation and accumulation of OMW-OM, and the changes in soil leachates quality. In this context, the composition and content of OMW-OM in the soil leachates can be used to assess degradation and transport dynamics of OMW in soil. Additionally, the knowledge on the time- and season-dependent availability of OMW-derived (poly)phenols in soil leachates might be useful to estimate the potential and degree of OMW-related groundwater contamination. For this, the time-dependent soil leachate quality during and after OMW application was monitored over 18 weeks in soil lysimeters and included four scenarios in terms of a first winter season (WS1), followed by a spring season (SP), a summer season (SS), and a final second winter season (WS2), all reflecting typical Tunisian seasons. Over the total incubation time, various parameters were assessed for the soil leachates, including the time-dependent variations of soluble phenolic compounds (SPC), pH, electrical conductivity (EC), water drop penetration time (WDPT), and the quality of dissolved organic carbon (DOC) by specific ultraviolet absorbance (SUVA\u003csub\u003e254\u003c/sub\u003e).\u003c/p\u003e\u003cp\u003eWe hypothesized that 1) OMW-derived (poly)phenols are the main reason for hydrophobic effects in soil, which should be displayed in directly related SPC concentration and WDPT in OMW-treated soil leachates during all simulated seasons. However, especially the high moisture content under the wet winter conditions should promote OMW percolation through the soil matrix and therefore result in higher SPC in the soil leachates. Under spring conditions, moderate soil moisture and temperature are expected to allow for considerable OMW degradation, consequently reducing OMW-derived SWR. Moist conditions during a second wet winter simulation and after the hot and dry summer conditions are expected to further reduce the SPC in the soil leachates compared to the first winter simulation, which should be reflected in an additionally reduced SWR.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cp\u003e\u003cb\u003eSoil, OMW, and leachates characterization\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe soil used in this study was sampled from a field located in the Mediterranean country of Sfax/Tunisia (North Africa, Lambert coordinates X\u0026thinsp;=\u0026thinsp;38G 70 \u0026rsquo;50\u0026rsquo;\u0026rsquo;and 38G 73\u0026rsquo; 80\u0026rsquo;\u0026rsquo; Y\u0026thinsp;=\u0026thinsp;8G 97 \u0026rsquo;60\u0026rsquo; \u0026rsquo;and 9G 05\u0026rsquo; 90\u0026rsquo;\u0026rsquo; Z\u0026thinsp;=\u0026thinsp;130). The sampling area was characterized by extensive arboriculture based on olive trees. The climatological data used were those of the Sfax weather station, provided by the Tunisian National Institute of Meteorology (\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). The sampling area was characterized by insufficient and irregular rainfall, the influence of the sea on temperature and humidity in summer. The average annual rainfall is about 213 mm with generally moderate temperatures, mild winters and short to long hot summers. The average maximum temperature is about 35.1\u0026deg;C in August and the respective minimum temperature about 6.2\u0026deg;C in January. The average annual temperature is 18.7\u0026deg;C with August as the hottest (average of 26.5\u0026deg;C) and January as the coldest month (average of 11.3\u0026deg;C), respectively.\u003c/p\u003e\u003cp\u003eFor basic physicochemical characterization, OMW and soil were processed as follows: OMW was first diluted 1:1000 (v/v) in demineralized water and filtered through 0.45 \u0026micro;m membranes (Whatman, United Kingdom). Soil extracts were prepared at a 1:5 (w/v) soil-to-water ratio, shaken for 24 h, centrifuged at 3,720 g for 15 min, and similarly filtered through 0.45 \u0026micro;m membranes.\u003c/p\u003e\u003cp\u003eVarious physicochemical parameters were measured for OMW, soil extracts, and soil leachates collected during the incubation experiment: pH and electrical conductivity (EC) were determined according to DIN ISO 11265 (1997) and DIN ISO 38404-5 (2009). Gravimetric water content was calculated on a drymass basis after ovendrying samples at 105\u0026deg;C for 48 h. Total carbon (TC) and total inorganic carbon (TIC) were quantified using a Multi N/C Analyzer 2100/2100S (Analytik Jena, Germany), with organic carbon (OC) obtained by subtracting TIC from TC. Total nitrogen (TN) analysis was conducted using a Vario Micro Cube (Elementar GmbH, Germany) following DIN ISO 10694 (1996). Specific UV absorbance at 254 nm (SUVA\u003csub\u003e254\u003c/sub\u003e) was measured according to Weishaar et al. (\u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) as a proxy for the aromaticity, and hence lability, of dissolved organic carbon (DOC) using a Specord 50 UV/VIS spectrophotometer (Analytik Jena, Germany). Total phenolic content (TPC) was assessed via Folin\u0026ndash;Ciocalteu reagent, with results expressed as g gallic acid equivalents (GAE) L\u003csup\u003e-1\u003c/sup\u003e based on a calibration with gallic acid (0\u0026ndash;500 mg L⁻\u0026sup1;). In this regard, soluble phenolic compounds (SPC) were quantified by adding 300 \u0026micro;L of soil leachate to 1.5 mL of Folin\u0026ndash;Ciocalteu reagent (diluted 1:10 in deionized water), incubating for 4 min, then adding 1.2 mL saturated Na₂CO₃ solution (200 g L⁻\u0026sup1;). Major cations (K⁺, Na⁺, Mg\u0026sup2;⁺, Ca\u0026sup2;⁺) and iron (Fe) were measured by ICPOES (Agilent 720, Germany) following microwaveassisted reverse aqua regia extraction at pH\u0026thinsp;\u0026lt;\u0026thinsp;2, while chloride (Cl⁻) concentrations were determined by ion chromatography (881 Compact IC pro, Metrohm, Switzerland).\u003c/p\u003e\u003cp\u003eFurther soil-specific measurements included bulk dry density (DIN EN ISO 11272 \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), maximum waterholding capacity (WHCₘₐₓ) according to DIN ISO 11274 (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1998\u003c/span\u003e), and soil poresize distribution according to Meyer et al. (\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) using a MiniSpec mq7.5 Relaxometer (Bruker Biospin, Germany).\u003c/p\u003e\u003cp\u003e\u003cb\u003eLysimeter setup and soil incubation\u003c/b\u003e\u003c/p\u003e\u003cp\u003eFor the incubation experiment, four lysimeters (ecoTech, Germany) of 40 cm height and 30 cm diameter were filled layer-wise with the respective Tunisian soil according to Lewis \u0026amp; Sj\u0026ouml;strom (\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) to reach a final bulk density of 1.4 g cm\u003csup\u003e-3\u003c/sup\u003e (see Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e for schematic experimental setup).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDuring the overall 18-week incubation period, different seasons typical for Tunisia were simulated: a relatively cold and wet winter simulation (WS1), a moderate spring simulation (SPS), a hot and dry summer simulation (SS), and a second winter simulation (WS2) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). Thus, initial moisture conditions for each lysimeter were set to Tunisian winter conditions (WS1; 15% of WHC\u003csub\u003emax\u003c/sub\u003e) by adding the respective amount of demineralized water on the top of the soil surface. One movable sprinkler head allowing uniform distribution of the irrigation and rainwater was used for mimicking season-specific rainfall events typical for Sfax (winter: 110 mm in 21 events; spring: 60 mm in 11 events and extreme summer: no rainfall).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eEach lysimeter was equipped with a polyamide membrane at the bottom to adjust matric potential using a dosing pump (ecoTech, Germany). The lower boundary condition was set to -83 hPa for WS1 and WS2, and to -600 hPa for SS and SPS, respectively. To monitor depth-dependent soil moisture in-situ and over time, two HydraProbes sensors (ecoTech, Germany) were placed at 10 cm and 30 cm soil depth in each lysimeter. To establish predefined soil temperature, heating cables were wrapped around the lysimeters at regular intervals and covered with insulating tape. For each seasonal scenario, the heat output generated from the heating cables was adjusted to reach the target temperature regime, consequently validated by measuring soil surface temperature for each lysimeter. Full spectrum daylight tubes (intensity: 100.000 lux for 12 h per day) were mounted above the lysimeter to allow natural light exposure and associated light-induced photoreactions on the soil surface.\u003c/p\u003e\u003cp\u003eAfter pre-equilibration of 7 days, three lysimeters (OMW1-3) were irrigated with OMW, one lysimeter was set as control (only irrigated with demineralized water). The amount of OMW applied was 1.1 L (equivalent to 50 m\u003csup\u003e3\u003c/sup\u003e ha\u003csup\u003e-1\u003c/sup\u003e or 14 L m\u003csup\u003e-2\u003c/sup\u003e used for field applications) and therefore based on the recommendation of the Ministry of Environmental Protection in Tunisia for single soil application. OMW application was done manually using water gardening cans to avoid soil disturbance and to allow equal distribution. Throughout the whole incubation time, weekly leachates sampling campaigns took place, divided into 13 samplings for the first winter simulation (WS1), 7 samplings for spring simulation (SPS) and 6 samplings for the second winter simulation (WS2). Due to the overall low soil water content and missing irrigation during the summer simulation (SS), no leachate was collected. Besides the determination of basic physicochemical leachate properties already described in the previous section, soil water repellency (SWR) was assessed on the soil surface of all lysimeters via water drop penetration time (WDPT). For this, 20 water drops of each 100 \u0026micro;l were placed directly but randomly distributed on the topsoil in each lysimeter and the time until complete water penetration was determined, presented as the arithmetic mean of the three OMW-treated replicates and the control. The soil was considered water repellent when the WDPT exceeded 5s (Bisdom et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e1993\u003c/span\u003e).\u003c/p\u003e\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003eStatistical analysis\u003c/h2\u003e\u003cp\u003eStatistical analyses were performed using R Statistics V4.3.0 (R Core Team \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Prior to conducting parametric tests, data was assessed for normality using the Shapiro-Wilk test. Homogeneity of variances was evaluated with Levene\u0026rsquo;s test. Relationships between different parameters were examined using Pearson\u0026rsquo;s product-moment correlation, differences in means across groups were analyzed with one-way ANOVA. When significant effects were detected (significance level of p\u0026thinsp;\u0026lt;\u0026thinsp;0.05 was considered for all statistical tests), Tukey\u0026rsquo;s multiple comparison test was applied to identify significant differences between soil leachates from untreated and OMW-treated soils.\u003c/p\u003e\u003c/div\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cb\u003ePhysicochemical properties of soil and OMW\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe physicochemical properties of the soil and OMW used in this study are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Soil was alkaline (pH 8.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1), with very low water content (0.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1% dry weight) and a WHC\u003csub\u003emax\u003c/sub\u003e of 250.0\u0026thinsp;\u0026plusmn;\u0026thinsp;6.3 mL kg⁻\u0026sup1;. TC was about 8.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mg L⁻\u0026sup1; (4.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mg L⁻\u0026sup1; C\u003csub\u003eorg\u003c/sub\u003e) and TN was 0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10 mg L⁻\u0026sup1;. Its EC was about 64.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 \u0026micro;S cm⁻\u0026sup1;, with Na\u003csup\u003e+\u003c/sup\u003e and K\u003csup\u003e+\u003c/sup\u003e concentrations of 0.001\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0003 g L⁻\u0026sup1; and 0.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 g L⁻\u0026sup1;, respectively. Ca\u003csup\u003e2+\u003c/sup\u003e, Fe, and Mg\u003csup\u003e2+\u003c/sup\u003e concentrations were 18.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 g L⁻\u0026sup1;, 0.74\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09 g L⁻\u0026sup1;, and 1.73\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40 g L⁻\u0026sup1;, respectively. PSD was predominantly coarse (85.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8%) pores, followed by 9.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5% medium and 5.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3% fine pores.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eSelected physicochemical properties of the investigated soil and olive mill wastewater (OMW)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"3\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eParameter\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eSoil\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eOMW\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003epH\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e8.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e5.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eElectrical conductivity (EC) (\u0026micro;S cm\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e64.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e530.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eWater content (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e0.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e91.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMaximum water-holding capacity (WHC\u003csub\u003emax\u003c/sub\u003e) (mL Kg\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e250.0\u0026thinsp;\u0026plusmn;\u0026thinsp;6.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal Carbon (TC) (mg L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e8.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e376.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOrganic carbon (C\u003csub\u003eorg\u003c/sub\u003e) (mg L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e4.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e298.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal nitrogen (TN) (mg L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e0.17\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e8.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSUVA\u003csub\u003e254\u003c/sub\u003e (L mg C\u003csup\u003e-1\u003c/sup\u003e m\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e0.37\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e2.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTotal phenolic content (TPC) (g GAE L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e0.036\u0026thinsp;\u0026plusmn;\u0026thinsp;0.001\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e6.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNa (g L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e0.001\u0026thinsp;\u0026plusmn;\u0026thinsp;0.0003\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e1.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eK (g L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e0.50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e10.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCa (g L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e18.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e749.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFe (g L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e0.74\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e27.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMg (g L\u003csup\u003e-1\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e1.73\u0026thinsp;\u0026plusmn;\u0026thinsp;0.40\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e397.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCoarse pores \u0026minus;\u0026thinsp;10\u0026ndash;50 \u0026micro;m (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e85.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eMedium pores \u0026minus;\u0026thinsp;0.2\u0026ndash;10 \u0026micro;m (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e9.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eFine pores - \u0026lt; 0.2 \u0026micro;m (%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e5.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003e-\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eIn contrast, OMW was strongly acidic (pH 5.35\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1) and highly conductive (530.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 \u0026micro;S cm⁻\u0026sup1;), with a water content of 91.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1%. Its organic load was very high: TC was 376.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mg L⁻\u0026sup1; (298.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mg L⁻\u0026sup1; C\u003csub\u003eorg\u003c/sub\u003e), TN of 8.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 mg L⁻\u0026sup1;, and specific UV absorbance at 254 nm (SUVA\u003csub\u003e254\u003c/sub\u003e) of 2.8 L mg C⁻\u0026sup1; m⁻\u0026sup1;. TPC reached 6.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10 g GAE L⁻\u0026sup1;. Cation concentrations were highest for Ca\u003csup\u003e2+\u003c/sup\u003e (749.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 g L⁻\u0026sup1;), followed by Mg\u003csup\u003e2+\u003c/sup\u003e (397.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 g L⁻\u0026sup1;), Fe (27.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 g L⁻\u0026sup1;), K\u003csup\u003e+\u003c/sup\u003e (10.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 g L⁻\u0026sup1;), and Na\u003csup\u003e+\u003c/sup\u003e (1.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 g L⁻\u0026sup1;).\u003c/p\u003e\u003cp\u003e\u003cb\u003eTime- and scenario-dependent moisture dynamics\u003c/b\u003e\u003c/p\u003e\u003cp\u003eThe water content (WC) measured at both 10 cm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) and 30 cm (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) soil depth varied not only between OMW treatment and control, but also among the three OMW replicates, revealing distinct spatial and temporal signatures. The control lysimeter exhibited a modest and predictable moisture regime: during WS1, WC increased from 9\u0026ndash;20% and 12% at 10 cm and 30 cm soil depth, respectively. During SPS, WC decreased sharply and stabilized around 12% at 10 cm and 10% at 30 cm. During SS, WC decreased to ~\u0026thinsp;3% and finally re-increased to 14% (10 cm) and 9% (30 cm) during WS2 without any intermittent spikes, respectively.\u003c/p\u003e\u003cp\u003eOMW1 responded with the fastest initial infiltration: during WS1, WC at 10 cm increased from 9\u0026ndash;26% within 24 h of the single OMW pulse, then declined steeply to 15% by the end of SPS. At 30 cm it peaked at 15% and returned to ~\u0026thinsp;10%) more rapidly than the other OMW-treated replicates. During SS, OMW1 showed a single brief recovery spike to ~\u0026thinsp;6% at 10 cm after an early irrigation and fell to ~\u0026thinsp;3% until the end of the scenario. During WS2, it re-increased to ~\u0026thinsp;15% at 10 cm and to 9% at 30 cm, but its recovery rate lagged slightly behind OMW3.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIn contrast, OMW2 exhibited the slowest WC decrease in the initial incubation course, its WC at 10 cm increased to 25% and declined more gradually, stabilizing around 14% during SPS. At 30 cm it peaked at 14% and maintained\u0026thinsp;~\u0026thinsp;11% until WS2. During SS, OMW2 never rose above the 3% WC, indicating very rapid drying. During WS2, WC at 10 cm recovered only to ~\u0026thinsp;14%, at 30 cm even only to 8%, marking the lowest re-increase among the OMW-treated replicates. OMW3 showed a high peak retention with intermediate WC decrease: during WS1, the WC at 10 cm reached 27% and decreased to finally 17% at the end of SPS. At 30 cm, WC peaked at 16% and decreased steadily to ~\u0026thinsp;12%. Most striking, OMW3 showed two distinct WC peaks at 10 (5\u0026ndash;7%) during SS, following mid-period irrigations. During WS2, it recovered fastest to ~\u0026thinsp;16% at 10 cm and 9% at 30 cm.\u003c/p\u003e\u003cp\u003eAll together, these patterns show that, even under identical OMW dosing, clear differences in (1) WC peak magnitude, (2) rate of WC decline, and (3) residual WC retention under drought for OMW1-3 and the control. OMW1 drained fastest, OMW2 held soil moisture longest through spring, and OMW3 combined high peaks with notable resilience during summer drying, already underlining the importance of micro-scale soil structure heterogeneity in mediating OMW\u0026ndash;soil water interactions.\u003c/p\u003e\u003cp\u003e\u003cb\u003epH and EC dynamics\u003c/b\u003e\u003c/p\u003e\u003cp\u003eGenerally, OMW application increased EC and reduced pH in all soil leachates with respect to the untreated control soil (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Over the 18-week incubation, all three OMW-treated lysimeters exhibited substantially higher EC than the untreated control (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). During the WS1, EC in OMW2 increased to 1252\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S cm\u003csup\u003e-1\u003c/sup\u003e, followed by OMW 3 (~\u0026thinsp;987\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S cm⁻\u0026sup1;), and OMW1 (~\u0026thinsp;985\u0026thinsp;\u0026plusmn;\u0026thinsp;54 \u0026micro;S cm⁻\u0026sup1;), while the control remained steady at 283\u0026thinsp;\u0026plusmn;\u0026thinsp;19 \u0026micro;S cm⁻\u0026sup1;. During SPS, EC in OMW2 further increased to 1358\u0026thinsp;\u0026plusmn;\u0026thinsp;118 \u0026micro;S cm⁻\u0026sup1; and in OMW3 to 1314\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S cm⁻\u0026sup1;. In contrast, OMW1 increased only to 658\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S cm⁻\u0026sup1;, whereas the control fluctuated around 60 \u0026micro;S cm⁻\u0026sup1;. No leachate was collected during the SS, however, in WS2, all OMW-treated soil leachates reached the highest EC, with 1690\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S cm⁻\u0026sup1; for OMW 1, 1733\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S cm\u003csup\u003e-1\u003c/sup\u003e for OMW2, and 1992\u0026thinsp;\u0026plusmn;\u0026thinsp;1 cm⁻\u0026sup1; for OMW 3. EC of the control slightly decreased to finally 416\u0026thinsp;\u0026plusmn;\u0026thinsp;1 \u0026micro;S cm\u003csup\u003e-1\u003c/sup\u003e at the end of WS2.\u003c/p\u003e\u003cp\u003eAt the beginning of WS1, the pH of the control started at 7.42\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00, slightly decreased to 7.12\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 in week 2, and recovered to 7.32\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 by week 4 (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb). OMW1 started at 8.09\u0026thinsp;\u0026plusmn;\u0026thinsp;0.06, decreased to 7.05\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 in week 2, and re-increased to 7.32\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03 by week 4. Also, OMW2 showed this pattern, starting at a pH of 8.11\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00, decreasing to 7.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01, and re-increasing to 7.43\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07 at the end of WS1. OMW3 exhibited the largest fluctuations by decreasing during the first two weeks from 7.70\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 to the lowest pH recorded (6.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.03) before re-increasing again to finally 7.75\u0026thinsp;\u0026plusmn;\u0026thinsp;0.15.\u003c/p\u003e\u003cp\u003eAt the beginning of SPS, the pH of the control was 7.57\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02, OMW1 7.97\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01, OMW2 7.55\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01, and OMW3 7.61\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08. However, at the end of SPS, the control had increased to 7.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.04, OMW2 to 7.78\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00, and OMW3 to 7.88\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02. Only the pH of OMW1 slightly decreased to finally 7.79\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02.\u003c/p\u003e\u003cp\u003eRegarding WS2, pH values peaked across all treatments towards more alkaline values, with 8.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 for the control, 8.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00 for OMW1, 8.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 for OMW2, and 8.21\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 for OMW3. At the end of WS2, the control reached its highest pH of 8.32\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00, OMW1-2 showed with 8.15\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05, 7.99\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01, and 7.97\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02, were also still alkaline, respectively.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003cb\u003eSoluble OMW-OM dynamics\u003c/b\u003e\u003c/p\u003e\u003cp\u003eOver the 18-week incubation, all three OMW-treated soil lysimeter showed markedly different leachate composition regarding OMW-OM compared to the untreated control, with each phase (WS1, SPS, SS, WS2) revealing distinct trends (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eDuring WS1, TOC in OMW1 and OMW2 leachates significantly increased above both OMW3 and the control (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). Already within the first incubation week, OMW1 peaked at 180\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1; and OMW2 at 50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1;, both significantly higher (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) than OMW3 and the control with 19.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1; and 14.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9 mg L⁻\u0026sup1;, respectively. From week 2 on, the TOC of OMW1 steeply decreased to ~\u0026thinsp;25 mg L⁻\u0026sup1; and then stabilized around 20\u0026ndash;30 mg L⁻\u0026sup1; through the end of WS1, while OMW2 increased steadily to ~\u0026thinsp;220 mg L⁻\u0026sup1; by week 4. OMW3 and the control showed only minor fluctuations around 10 mg L⁻\u0026sup1;. At the beginning of SPS, OMW2 increased again to 376\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1;, eight times higher than OMW1 (70\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1;), OMW3 (50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1;), and the control (\u0026lt;\u0026thinsp;30 mg L⁻\u0026sup1;; p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). After the dry and hot SS, where no leachate was collected, WS2 caused a decrease of TOC for all lysimeters: OMW1 and the control were at ~\u0026thinsp;20\u0026ndash;30 mg L⁻\u0026sup1;, OMW3 at 80\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1;, and OMW2 at 50\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 mg L⁻\u0026sup1;, respectively. At the end of week 18, no significant differences were observed anymore.\u003c/p\u003e\u003cp\u003eSoluble phenolic compounds (SPC) followed a similar but less pronounced pattern: during WS1, SPC were highest in OMW1 (0.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 g L⁻\u0026sup1;; p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), intermediate in OMW2 (0.025\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 g L⁻\u0026sup1;) and lowest in OMW3 (0.005\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01 g L⁻\u0026sup1;), while the control remained below 0.002 g L⁻\u0026sup1; (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb). During SPS, SPC in OMW1 decreased toward control levels, whereas OMW2 remained at ~\u0026thinsp;0.02 g L⁻\u0026sup1;, and both OMW3 and the control below 0.005 g L⁻\u0026sup1;. At the end of WS2, SPC concentrations across all treatments decreased below 0.005 g L⁻\u0026sup1;, which was far lower than the incoming OMW concentration of 6.47\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10 g L⁻\u0026sup1;.\u003c/p\u003e\u003cp\u003eSUVA\u003csub\u003e254\u003c/sub\u003e also peaked in the WS1 with OMW1 showing 0.40 L mg C⁻\u0026sup1; m⁻\u0026sup1; and OMW2 0.20 L mg C⁻\u0026sup1; m⁻\u0026sup1;, both significantly higher than the control (0.07 L mg C⁻\u0026sup1; m⁻\u0026sup1;, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec). OMW3 was in the range of the control (0.30 L mg C⁻\u0026sup1; m⁻\u0026sup1;) and did therefor not differ significantly. During SPS, SUVA\u003csub\u003e254\u003c/sub\u003e decreased to ~\u0026thinsp;0.05 L mg C⁻\u0026sup1; m⁻\u0026sup1; for both OMW1 and OMW3, and to ~\u0026thinsp;0.15 L mg C⁻\u0026sup1; m⁻\u0026sup1; for OMW2, all comparable to the control. During WS2, only OMW2 exhibited a slight re-increase (~\u0026thinsp;0.12 L mg C⁻\u0026sup1; m⁻\u0026sup1;), while OMW1 and OMW3 remained at control levels (\u0026lt;\u0026thinsp;0.10 L mg C⁻\u0026sup1; m⁻\u0026sup1;), with no significant differences among treatments.\u003c/p\u003e\u003cp\u003e\u003cb\u003eSoil surface and water repellency dynamics\u003c/b\u003e\u003c/p\u003e\u003cp\u003eWDPT measurements on the soil surface demonstrated that OMW application significantly (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) increased topsoil water repellency in OMW1\u0026ndash;3 compared to the MQ-water control in every seasonal scenario (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). All values for WS1, SPS, SS and WS2 are the mean of the three OMW treatments.\u003c/p\u003e\u003cp\u003eFor WS1, immediately after OMW application under simulated winter conditions, the soil surface exhibited a continuous, glossy crust with extensive vesiculation and protruding micro-aggregates, and repellency was at its peak: over 80% of measurement spots exhibited WDPT\u0026thinsp;\u0026gt;\u0026thinsp;600 s (severe water repellency), with a small number of spots failing to infiltrate even after 3600 s. On average only 5\u0026thinsp;\u0026plusmn;\u0026thinsp;2% of drops soaked in within 0\u0026ndash;5 s, 5\u0026thinsp;\u0026plusmn;\u0026thinsp;1% in 5\u0026ndash;60 s, 20\u0026thinsp;\u0026plusmn;\u0026thinsp;3% in 60\u0026ndash;600 s, 60\u0026thinsp;\u0026plusmn;\u0026thinsp;4% in 600\u0026ndash;3600 s, and 10\u0026thinsp;\u0026plusmn;\u0026thinsp;2% remained unpenetrated at 3600 s.\u003c/p\u003e\u003cp\u003eDuring the spring simulation (SPS), the surface developed a dense, semi-consolidated dark crust with fewer nodular peaks and the onset of fine fractures, and repellency decreased but remained significant (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05): approximately 0% of drops infiltrated within 0\u0026ndash;5 s, 5\u0026thinsp;\u0026plusmn;\u0026thinsp;2% within 5\u0026ndash;60 s, 50\u0026thinsp;\u0026plusmn;\u0026thinsp;5% within 60\u0026ndash;600 s, 30\u0026thinsp;\u0026plusmn;\u0026thinsp;3% within 600\u0026ndash;3600 s, and 15\u0026thinsp;\u0026plusmn;\u0026thinsp;2% beyond 3600 s. Across SPS, 70% of spots still exhibited WDPT\u0026thinsp;\u0026gt;\u0026thinsp;60 s.\u003c/p\u003e\u003cp\u003eAfter the dry and hot summer phase (SS), the crust had transitioned to a matte-brown, uniform granular matrix with minimal relief, and repellency persisted: when leachate collection resumed in week 14, 90\u0026thinsp;\u0026plusmn;\u0026thinsp;3% of drops infiltrated in 0\u0026ndash;5 s, 8\u0026thinsp;\u0026plusmn;\u0026thinsp;2% in 5\u0026ndash;60 s, and 2\u0026thinsp;\u0026plusmn;\u0026thinsp;1% in 60\u0026ndash;600 s, with no drops exceeding 600 s. Despite this apparent recovery in the 0\u0026ndash;5 s class, statistical comparison among WS1, SPS, SS and WS2 remained significant (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05), indicating that SS repellency differed from other phases.\u003c/p\u003e\u003cp\u003eIn the second winter simulation (WS2), the soil surface presented a light-brown, finely granulated layer devoid of vesicle formation, and hydrophobicity re-emerged at intermediate intensity: 85\u0026thinsp;\u0026plusmn;\u0026thinsp;4% of drops soaked in within 0\u0026ndash;5 s, 10\u0026thinsp;\u0026plusmn;\u0026thinsp;2% in 5\u0026ndash;60 s, and 5\u0026thinsp;\u0026plusmn;\u0026thinsp;1% in 60\u0026ndash;600 s, with no spots requiring more than 600 s. Overall, 70% of all spots across SS and WS2 showed WDPT\u0026thinsp;\u0026gt;\u0026thinsp;60 s, confirming that significant repellency persisted beyond WS1.\u003c/p\u003e\u003cp\u003eBy contrast, the MQ-water control remained fully wettable (100% of drops in the 0\u0026ndash;5 s class) at every time point.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThis study provides insights into the dynamics and fate of olive mill wastewater (OMW)-derived compounds and their related effects in soils under Tunisian season conditions. Using controlled lysimeter experiments, we demonstrated clear season-dependent differences in the leaching, accumulation, and potential transformation of OMW. The findings confirm and complement previous field and laboratory investigations across various Mediterranean regions but also highlight specific mechanisms observable only under controlled conditions.\u003c/p\u003e\u003cp\u003eOMW-derived compounds underwent a dynamic interplay of dissolution, microbial transformation, sorption, polymerization, and physical immobilization. The dominance of each process shifted according to climatic conditions, profoundly influencing their overall and spatio-temporal fate. In this context, the underlying processes and mechanisms can be conceptualized as follows: during the first winter simulation (WS1), the combination of high soil moisture and relatively low temperature created conditions that enhanced the dissolution and downward movement of soluble OMW-derived compounds, as evidenced by elevated SPC, TOC, and SUVA\u003csub\u003e254\u003c/sub\u003e values in the collected leachates, peaking at significantly higher concentrations than measured in the control lysimeter. As demonstrated by Kurtz et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) during a five-year field study on a semi-arid olive orchard, annual OMW applications (50\u0026ndash;150 m\u0026sup3; ha⁻\u0026sup1; y⁻\u0026sup1;) significantly increased soil water content and TPC in the top 0\u0026ndash;10 cm, showing overall dose-dependent accumulation patterns between OMW application rate, TPC and DOC.\u003c/p\u003e\u003cp\u003eMoreover, the soil surface in WS1 developed a continuous, glossy crust with extensive vesiculation and protruding microaggregates, which corresponded to the maximum SWR recorded in this phase (over 80% of WDPT measurements\u0026thinsp;\u0026gt;\u0026thinsp;600 s). Various field experiments and lysimeter studies have consistently demonstrated an increased SWR after OMW application, particularly in the upper soil layers. For example, Tamimi et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) found that WDPT increased for OMW-treated soil, especially after summer and winter applications, indicating moderate to high water repellency in the topsoil. Similarly, other studies showed persistent SWR and reduced saturated hydraulic conductivity in OMW-treated soils, with most hydrophobic compounds immobilized in the upper 5 cm (Bombino et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Cha\u0026acirc;ri et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). On the one hand, the high matric potential together with the low soil temperatures during WS1 likely promoted the adsorption of OMW-OM onto fine silt and clay surfaces (Yaakoubi et al. \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). On the other hand, when especially OMW-derived amphiphilic molecules diffuse into soil micro- and mesopore domains, they bind preferentially to hydrophobic domains within SOM, forming microscale coatings that further impeded water infiltration (Cajot et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Chai et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Hammecker et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). However, temperature-induced shrinkage and swelling of the clay fraction may have further generated transient fissures, acting as preferential flow paths that enhanced localized leaching while leaving adjacent zones highly enriched in polymerizing (poly)phenols (Beven and Germann \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Magdich et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Mekki et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2007\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eHowever, soil microbial activity was likely constrained by low temperatures, as indicated by the persistence of high SPC levels throughout the winter period, and by elevated values for SUVA\u003csub\u003e254\u003c/sub\u003e in leachates, suggesting the presence of aromatic, less-degraded OM (Beven and Germann \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Pietik\u0026aring;inen et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Moreover, the winter period also came along with noticeable acidification of soil leachates in terms of lower pH values, aligning with field observations that OMW application can reduce soil pH due to the organic acid content and subsequent biogeochemical reactions (Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Elevated EC values in winter leachates further indicated the mobilization of salts and ions from OMW, increasing the salinity risk for deeper soil horizons and groundwater (Bouhia et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Khalil et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Concerning the seasonal effects, the results are consistent with field studies conducted in Bait Reema and Gilat, where OMW application in winter resulted in significant leaching of phenolic compounds due to increased infiltration and preferential flow pathways (Peikert et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eDuring the simulated spring period (SPS), moderate temperatures and soil moisture created an environment favorable for soil microbial degradation of OMW-OM, as indicated by a significant decrease in SPC in leachates compared to WS1. SPC decrease was accompanied by a reduction in SUVA\u003csub\u003e254\u003c/sub\u003e values, indicating a shift toward less aromatic, more degraded DOM fractions (Kellerman et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Such observations align with earlier reports from other incubation studies showing that microbial communities rapidly degrade soluble phenolic compounds under moderate conditions, leading to a significant decrease in toxicity and mobility of OMW-OM (C. Buchmann et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). In general, warm temperatures and intermittent moisture are well-known to stimulate extracellular enzyme production by saprotrophic fungi and bacteria, accelerating oxidative breakdown of low-molecular-weight organic compounds such as phenols (Ferguson and Lindo \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Min et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Pallandt et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Thus, soil microbial hotspots likely formed microsites of intensive biodegradation that accounted for the sharp decline in SPC and SUVA\u003csub\u003e254\u003c/sub\u003e (Kellerman et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Saarela et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Schimel and Schaeffer \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). For instance, Saarela et al. (\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) observed that microbial hotspots or zones of elevated microbial activity can drive rapid DOM transformation, accompanied by reduced SUVA\u003csub\u003e254\u003c/sub\u003e values, indicating a reduction in the aromaticity of the remaining DOM pool. Additionally, SWR decreased moderately during spring, reflected in lower WDPT values. This reduction was accompanied by the development of a dense, semi-consolidated dark crust that likely created preferential flow channels, still allowing limited water infiltration despite sustained high repellency. Although microbial activity gradually degraded the OMW-induced hydrophobic organic coatings on soil particles, evidenced by a measurable decline in soil water repellency (SWR), a residual level of repellency persisted, indicating the enduring presence of recalcitrant hydrophobic compounds resistant to biodegradation (Albalasmeh and Mohawesh \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Doerr et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Mekki et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). Despite the improvement in leachate quality, the elevated EC values persisted, indicating that the ions from the OMW are still contributing to the salinity of the soil. This matches observations that negative effects of OMW can persist even after the degradation of toxic organic compounds, potentially impacting soil structure and crop health in the long term (Kavvadias et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; M. P. Kurtz et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Mahmoud et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Regni et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2021\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eThe simulated summer period (SS) was characterized by high temperatures and severe soil drought, conditions that significantly altered the fate of OMW-derived compounds. Although no leachate was collected during this season due to the absence of rain events and thus percolating water, several already known mechanisms point to significant physicochemical transformation dynamics in soil: dry and hot conditions, and especially at elevated soil salinity, strongly inhibit soil microbial activity (Oustani et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; V\u0026aacute;zquez et al. \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Instead, abiotic transformation dynamics become more relevant, such as polymerization and condensation reactions of OMW-derived phenols and amphiphilic compounds (Bombino et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; F. El Hassani et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; F. Z. El Hassani et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Although such reactions typically result in the formation of larger, often more hydrophobic macromolecules that may persist and even enhance SWR (Peikert et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Steinmetz et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2015\u003c/span\u003e, \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), the observed reduction of hydrophobicity-indicating WDPT classes during SS indicates rater a decreased relevance or activity of OMW-derived hydrophobic compounds. However, (poly)phenols can undergo abiotic binding and condensation reactions with other OM fractions such as humic acids, resulting in their immobilization and transformation into bound, less hydrophobic forms (Vinken et al. \u003cspan citationid=\"CR74\" class=\"CitationRef\"\u003e2005\u003c/span\u003e). Furthermore, the evaporation taking place upon soil drying promotes the upward transport of salts and other soluble OM to the soil surface, thereby modulating SWR through competing mechanisms: elevated ionic strength compresses the electrical double layer and increases solution surface tension, raising the solid\u0026ndash;water contact angle and reinforcing hydrophobic coatings, while at moderate salinity, cation-induced flocculation aggregates particles and redistributes organic films, transiently mitigating water repellency (Jajarmi et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Tang et al. \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Furthermore, higher salinity has been shown to induce the formation of DOM with higher molecular weight, degree of oxidation, and lability, as well as lower C:N ratio, aromaticity, and increased vulnerability to degradation (Zhu et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Consequently, the dynamic interplay between salt‐driven reinforcement of hydrophobic barriers and salinity‐mediated aggregation highlights the complex nature of SWR and underscores the need to account for both processes in OMW utilization strategies.\u003c/p\u003e\u003cp\u003eDuring the second winter simulation (WS2), soil rewetting following the summer drought enabled the (re)mobilization and (re)availability of the previously immobilized OMW-derived compounds (C. Buchmann et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Tamimi et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). However, the lysimeter results indicated only a modest leaching of SPC during WS2 compared to WS1, suggesting that while some soluble compounds were released, a significant fraction of OMW-OM remained physically immobilized or polymerized and thus resistant to further dissolution. Aso field data from Kurtz et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e) showed that biological indicators, including bait-lamina consumption, Collembola, and Acari abundance, recovered between successive winter applications, supporting the disappearance of toxic phenol-related effects over time. Despite lower SPC levels in the collected soil leachates, WDPT remained elevated, indicating that hydrophobic compounds partly persisted in soil, consistent with observations that polymerized OMW-OM can remain in soil over multiple seasons, maintaining hydrophobicity and potentially impacting soil-water relations over extended periods (Doerr et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Furthermore, EC values in the leachates during the second winter simulation also remained elevated, indicating an ongoing remobilization of ionic constituents, possibly due to the dissolution of salts concentrated at the soil surface during summer evaporation. This aligns with the results of Tamimi et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and Kurtz et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2021\u003c/span\u003e), who demonstrated that surface evaporation drives upward water fluxes in the soil profile, resulting in capillary-mediated, time- and dose-dependent accumulation of Na⁺, K⁺, Ca\u0026sup2;⁺, and Cl⁻ in the upper layers.\u003c/p\u003e\u003cp\u003eTaken together, seasonal dynamics highlight that the environmental fate of OMW-OM is governed by a complex balance between dissolution, microbial transformation, physical immobilization, and abiotic polymerization, which collectively shape the risks and benefits associated with the utilization of OMW on soil. The results further underline the importance of aligning OMW management practices with seasonal climatic patterns to mitigate environmental risks (Dich et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Vaz et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). It seems that OMW application during spring emerges as the most favorable option, as it allows for effective soil microbial degradation of labile organic compounds, while minimizing potential leaching risks. In contrast, OMW application in winter should be approached with caution due to the high potential for rapid leaching of (poly)phenols and salts into deeper soil layers and potentially to the groundwater. Summer applications, while avoiding immediate leaching, pose the risk of salt and OMW-OM accumulation in the topsoil layer, potentially fostering hydrophobic dynamics through abiotic transformations that impair soil water dynamics, soil microbial activity, and plant growth. Therefore, sustainable management of OMW as a soil amendment should include careful consideration of application timing, coupled with monitoring of soil hydrophobicity and salinity, particularly following dry summer periods. Implementing controlled irrigation during spring applications may further support microbial degradation processes and dilute salt concentrations, thus reducing long-term impacts on soil functioning and groundwater quality. Moreover, systematic monitoring of leachate composition in regions receiving OMW applications seems essential to detect and mitigate unwanted environmental implications (Geng et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Integrating the gained knowledge of seasonally governed processes into practical OMW management strategies will increase the agricultural benefits of OMW while maintaining soil health and protecting water resources as has been widely suggested for various soil amendments (Christian Buchmann et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Diacono and Montemurro \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2010\u003c/span\u003e).\u003c/p\u003e\u003cp\u003eWhile the lysimeter approach offered precise control over climatic variables and high reproducibility, it inherently simplifies field heterogeneity. The absence of plant\u0026ndash;soil\u0026ndash;microbe interactions, particularly rhizosphere dynamics, may underestimate the extent of microbial degradation under natural vegetation. Additionally, the single soil type and OMW batch limit the generalizability of our findings across the diverse textures and compositions found in Tunisian orchards. Future studies should incorporate dynamic water table fluctuations, additional soil matrices, and co-application of amendments (e.g., biochar or compost) to evaluate how these factors modulate (poly)phenol fate and soil hydrophobicity at landscape scales.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study demonstrated that the environmental fate of olive mill wastewater (OMW)-derived compounds, especially OMW-OM, in Tunisian soils was tightly controlled by seasonal climatic conditions. During the simulated Tunisian winter periods, elevated soil moisture promoted SPC percolation, yet leachate concentrations remained substantially below the original OMW input levels, indicating a limited risk of deeper soil contamination under the controlled application rate. In spring, moderate temperatures and soil moisture enhanced soil microbial activity, which significantly reduced SPC, TOC, and SUVA\u003csub\u003e254\u003c/sub\u003e values in the collected soil leachates and partially alleviated soil water repellency. Conversely, the relatively hot and dry summer conditions suppressed biodegradation and favored abiotic polymerization of OMW-derived OM, resulting in the accumulation of hydrophobic compounds and residues at the soil surface that promoted persistent soil water repellency.\u003c/p\u003e\u003cp\u003eFor the sustainable utilization of OMW as a soil amendment, application timing and soil management need to align with seasonal dynamics. Based on this study, spring emerged as the optimal period for OMW application, when soil microbial degradation effectively reduces labile (poly)phenols while minimizing leaching risks. Application rates should be kept moderate and paired with controlled irrigation to slow percolation, dilute soluble salts, and promote sorption and soil microbial stabilization of OMW-OMW. Systematic monitoring of soil leachate composition, soil hydrophobicity, and salinity is recommended to maintain groundwater quality and soil health. In this regard, further investigations into the spatial-temporal persistence of OMW-OM and soil hydrophobicity within the soil profile should be evaluated to gain more detailed knowledge on the effects of OMW.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eCompeting Interests:\u003c/strong\u003e\u003cp\u003eThe authors declare no competing interests\u003c/p\u003e\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eFinancial support\u003c/strong\u003e\u003cp\u003eLaboratory measurements and data collection were supported by the facilities of RPTU Kaiserslautern-Landau, Campus Landau and conducted within the framework of the project \u0026ldquo;TRILAT-OLIVEOIL\u0026rdquo; funded by the Deutsche Forschungsgemeinschaft (grant number SCHA 849/13)\u003c/p\u003e\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eConceptualization: C.B.,E.K.; Methodology and experimental setup: C.B., E.K.; Material preparation and data collection: E.K.; Data evaluation and interpretation: E.K.; Writ-ing\u0026mdash;original draft preparation: C.B.,E.K.; Writing\u0026mdash;review and editing: C.B.,E.K.; Supervision: C.B.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe kindly thank Gabriele E. Schaumann for her valuable feedback on the manuscript.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe data that supports the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAlbalasmeh, A. A., \u0026amp; Mohawesh, O. E. (2023). Effects of Olive Mill Wastewater on Soil Physical and Hydraulic Properties: a Review. \u003cem\u003eWater, Air, \u0026amp; Soil Pollution\u003c/em\u003e, \u003cem\u003e234\u003c/em\u003e(1), 42. https://doi.org/10.1007/s11270-022-06055-0\u003c/li\u003e\n\u003cli\u003eAyoub, S., Al-Absi, K., Al-Shdiefat, S., Al-Majali, D., \u0026amp; Hijazean, D. (2014). 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Effects of drought-rewetting processes and salinity variations on dissolved organic matter (DOM) transformation and bacterial communities in lacustrine sediments. \u003cem\u003eJournal of Soils and Sediments\u003c/em\u003e, \u003cem\u003e23\u003c/em\u003e(11), 4055\u0026ndash;4068. https://doi.org/10.1007/s11368-023-03611-x\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Olive mill wastewater (OMW), (Poly)phenol dynamics, Soil water repellency, Lysimeter study, Seasonal dynamics","lastPublishedDoi":"10.21203/rs.3.rs-7185457/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7185457/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTunisia, as a major olive oil producer, generates substantial quantities of olive mill wastewater (OMW), frequently applied to agricultural soils due to its fertilizing potential. However, OMW contains high levels of (poly)phenolic compounds, which can persist in soils and thereby affect basic soil properties, soil wettability, and pose risks of groundwater contamination. The semi-arid to arid climate of Tunisia, characterized by pronounced seasonal variations, may strongly influence the degradation, leaching, and environmental fate of OMW-derived compounds. This study aimed to investigate the dynamics of OMW application in soil columns under controlled conditions simulating Tunisian seasonal climates. Soil lysimeters were used to monitor soil leachate quality over 18 weeks, encompassing two winter periods, a spring, and a summer season. Parameters analyzed in leachates included soluble phenolic compounds (SPC), pH, electrical conductivity (EC), water drop penetration time (WDPT), and dissolved organic carbon (DOC) quality via SUVA\u003csub\u003e254\u003c/sub\u003e. Results showed that wet winter conditions promoted OMW percolation, leading to elevated SPC concentrations in leachates, while moderate spring conditions favored degradation processes, reducing SPC and soil water repellency. Hot and dry summer conditions induced polymerization and (re)accumulation of OMW-derived compounds at the soil surface, whereas the second winter period exhibited lower SPC levels than the first. The findings highlight the significant role of seasonal climatic conditions on OMW behavior in soils, underlining the need for season-specific management strategies to minimize environmental risks associated with its utilization as soil amendment.\u003c/p\u003e","manuscriptTitle":"Effects of olive mill wastewater on soil leachates composition under Tunisian climatic conditions: a lysimeter pilot study","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-07-25 17:02:28","doi":"10.21203/rs.3.rs-7185457/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"1d034a34-3993-4f1b-b4e3-657f970d8410","owner":[],"postedDate":"July 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-10-17T08:39:39+00:00","versionOfRecord":[],"versionCreatedAt":"2025-07-25 17:02:28","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7185457","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7185457","identity":"rs-7185457","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}