Dynamics of sulfate reduction regulate arsenic mobilization and speciation in paddy soils in response to root exudates

Dynamics of sulfate reduction regulate arsenic mobilization and speciation in paddy soils in response to root exudates | 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 Dynamics of sulfate reduction regulate arsenic mobilization and speciation in paddy soils in response to root exudates Guo-hao Zhang, Hong-yan Wang, Jun-jie Lin, Guo-xin Sun, Zheng Chen, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6692895/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 02 Jan, 2026 Read the published version in Plant and Soil → Version 1 posted 6 You are reading this latest preprint version Abstract Background and Aims Root exudates have been proposed to influence arsenic (As) mobility and speciation in paddy soils. However, how sulfur-rich conditions interacting with root exudates affect iron (Fe)-sulfur-driven As mobilization and speciation remains unclear. Methods As speciation and Fe mineralogy were characterized in an 28-day anoxic microcosm experiment using paddy soils, in which typical root exudates including citric acid, oxalic acid and glucose were introduced under under varying sulfate concentrations of 0 mM , 5 mM and 25 mM . Results All root exudates accelerated the reduction of Fe (oxy)hydroxides, and a corresponding increase in As concentrations of 11.6–46.3% in pore water of paddy soils. Oxalic acid had the strongest promoting effect, followed by citric acid and glucose. Although sulfate reduction further enhanced Fe (oxy)hydroxides dissolution, the concurrent formation of Fe sulfides sequestrated a portion of released As, resulting in 4.6%-22.5% lower pore water As compared to exudate alone. Elevated sulfide fluxes promoted greater As immobilization via Fe sulfides formation. Compared to the sulfate-free treatments, sulfate reduction facilitated the formation of dimethylarsenic and dimethylmonothioarsenate by 52.6-127.5% and 14.3–99.9%, respectively. Conclusions Root exudates can enhance As bioavailability in the rhizosphere by promoting its mobilization. In contrast, sulfate reductions may partially counteract As release through incorporating into sulfide minerals. The extent of As immobilization via sulfate reduction appears to depend on sulfate fluxes. This study highlights the critical role of sulfate reduction in regulating As mobilization and speciation in rhizosphere soils. Paddy soil Root exudates Sulfate reduction Fe (oxy)hydroxides transformation Methylated As Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Arsenic (As) contamination is widespread in both terrestrial and aquatic environments, particularly in South and Southeast Asia, where geogenic As-enriched groundwater is prevalent (Aftabtalab et al. 2022 ; Mukherjee et al. 2024 ). The irrigation of As-contaminated groundwater has elevated As accumulation in paddy soils, a problem further exacerbated by anthropogenic activities such as mining and pesticide application (Hong et al. 2023 ; Qiao et al. 2023 ). Under periodic flooding conditions, As is mobilized into pore water, and efficiently translocated into rice plants primarily via silicic acid transport pathways (Ma et al. 2008 ; Zhu et al. 2008 ). As a result, rice consumption has become a major exposure pathway to As for population dependent on a rice-based diet (Hussain et al. 2021 ). Thus, a comprehensive understanding of the biogeochemical cycling of As in paddy soils, particularly within rhizosphere, is critical for predicting and mitigating As uptake by rice. The As mobilization, speciation and redistributions in paddy soils are closely governed by the redox transformation of iron (Fe) oxy(hydro)xides (Burton et al. 2011 ; Nghiem et al. 2023 ). Under oxic conditions, Fe oxy(hydro)xides act as primary minerals adsorbing or sequestrating arsenate (As(V)) (Chen et al. 2023 ; Hong et al. 2023 ). In contrast, during flooded conditions, As(V) can be released through the reductive dissolution of Fe oxy(hydro)xides, and subsequently reduced to arsenite (As(III)) (Guo et al. 2013 ; Huang et al. 2012 ). As(III) exhibits lower sorption affinity to Fe oxy(hydro)xides and is more readily mobilized into pore waters (Xiang et al. 2025 ). Additionally, Fe(Ⅱ)-catalyzed phase transformation of Fe oxy(hydro)xides influences the distribution of As species between soil and pore water phases (Wang et al. 2021 ). The formation of secondary Fe-bearing minerals such as green rust, magnetite, siderite, pyrite can further sequester As either by structural incorporation or surface adsorption, thereby reducing its concentration in pore water (Perez et al. 2020 ; Wang et al. 2018 ). Sulfur redox transformations play a critical role in influencing the fate of Fe oxy(hydr)oxides, thereby affecting As repartitioning between pore water and soils, as well as the speciation of As in pore water (Guo et al. 2016 ). Although the reductions of poorly-crystalline Fe oxy(hydr)oxides and As(V) are thermodynamically more favorable than sulfate (SO 4 2- ) reduction, increasing evidence indicates that reduction of Fe oxy(hydr)oxides, As(V), and SO 4 2- can occur concurrently in anoxic environments (Nghiem et al. 2023 ; Wu et al. 2024 ). Sulfide produced from sulfate reduction can rapidly react with Fe(II) to form Fe monosulfide minerals such as FeS (e.g., mackinawite), which can transform into more crystalline pyrite (FeS 2 ), although the diagenetic pathways remain incompletely understood (Burton et al. 2013 ). While FeS has limited capacity to adsorb or incorporate As, pyrite exhibits a strong affinity for As, facilitating its immobilization (Qiu et al. 2017 ; Wang et al. 2020a ). Concurrently, sulfide can abiotically reduce As-bearing Fe oxy(hydr)oxides, enhancing As mobilization into pore water (Guo et al. 2016 ; Kocar et al. 2010 ; Zheng et al. 2020 ). Thus, As mobility in paddy soils is governed by was complex interaction between pore water and solid-phase geochemistry, particularly sulfide fluxes and the abundance of reactive Fe oxy(hydr)oxides (Burton et al. 2014 ; Planer-Friedrich 2023 ). The impact of sulfate reduction on pore water As concentrations remain inconsistent across studies (Burton et al. 2014 ; Wang et al. 2017 ; Xu et al. 2019 ; Zheng et al. 2020 ). Furthermore, under sulfidic conditions, inorganic oxyarsenic species can undergo microbial methylation by sulfate-reducing bacteria, producing methylated As species, such as dimethylarsenate (DMA) and monomethylarsenate (MMA) (Chen et al. 2019 ; Wang et al. 2020c ). Increasing evidence also indicates the co-occurrence of thiolated As species both in aquatic systems. These include both inorganic forms such as monothioarsenate [MTA, AsS(OH) 3 ], dithioarsenate [DTA, AsS 2 (OH) 2 2- ] and organic forms such as monomethylmonothioarsenate [MMMTA, (CH 3 )AsS(OH) 2 ], dimethylmonothioarsenate [DMMTA, (CH 3 ) 2 AsS(OH)]. These thiolated species commonly co-exist with As(III) and As(V), adding complexity to arsenic speciation under reducing conditions (Besold et al. 2018 ; Wang et al. 2020c ). Root exudates, particularly organic acids and simple sugars, constitute a major source of bioavailable carbon in soils (Haichar et al. 2008 ; Liu et al. 2022b ). These compounds can stimulate microbial processes such as sulfate reduction and the reductive dissolution of Fe oxy(hydr)oxides (Jia et al. 2014 ; Liu et al. 2022a ), thereby influencing coupled Fe-S redox dynamics and the associated and speciation of As. While previous studies have explored the role of root exudates in promoting As mobilization and the underlying microbial mechanisms (Jiang et al. 2023 ; Liu et al. 2022a ; Zou et al. 2024 ), their specific effects on Fe-S mineral transformation pathways remain insufficiently understood. In this study, we conducted controlled laboratory experiments to examine the concentration, speciation, and redistribution of pore water As in response to Fe oxy(hydr)oxide transformation under the sulfidic conditions and root exudate amendments. We hypothesize that: 1) sulfate reduction critically amplifies As mobilization in paddy soil porewater, with this effect being intensified in the presence of root exudates; 2) the coupled processes of sulfate reduction and oxidation of root exudate-derived carbon promote the reduction of As(III) and the formation of methylated As species, thereby driving As partitioning from the soil solid phase into the aqueous phase. Materials and methods Paddy soil collections Surface paddy soils (0–20 cm) were collected from an As-contaminated paddy field in Qingyuan city of Guangdong province, China (23°42′ N, 113°4′ E). Due to mining activities, the paddy soils are heavily contaminated with As at a concentration of ~ 51 mg kg -1 soil, which has far exceeded the regulatory for Chinese paddy soils (Regulation 2018 ). Following transport to the laboratory, soil samples were air-dried and sieved to < 2 mm. The main physio-chemical characteristics of the soils are summarized in Table 1 . Table 1 Soil physio-chemical properties. Arsenic, As; Iron, Fe; TC, total carbon; TN, total nitrogen; SOC, soil organic carbon. Items Mean ± SD pH 6.51 ± 0.02 As (mg kg − 1 ) 50.71 ± 3.01 Fe (g kg − 1 ) 12.10 ± 0.05 TN (%) 0.25 ± 0.00 TC (%) 1.91 ± 0.00 C/N 7.68 ± 0.23 SOC (%) 1.61 ± 0.01 S (%) 0.08 ± 0.00 Sulfate and root exudate additions Citric acid (CA), oxalic acid (OA), and glucose (Gl) were selected as representative root exudates, due to their identification as key constituents of rice root exudation (Yang et al. 2023 ). The total exudate-carbon (C) input was standardized to 2% of the soil organic carbon (SOC) (De Graaff et al. 2010 ). In alignment with the variable SO 4 2− concentrations observed in paddy soils (Fulda et al. 2013 ), SO 4 2− was applied in pulsed treatments at concentrations of 0 mM , 5 mM , and 25 mM . Incubation setups 10 g of soil was placed in a 100 mL serum vial. 30 mL of deionized water resulted in a 3 cm of standing water above the soil surface. The vials were sealed tightly with 2 cm-thick butyl stoppers (Glasgerätebau Ochs, Bovenden, Germany) and aluminum caps. The soils were pre-incubated at 25°C for 7 days to re-activate microbial activity. Root exudate fractions and sodium sulfate (Na 2 SO 4 ) were added into the via after pre-incubation. The pH of the soil suspension was adjusted to 6.5 ± 0.1 using 3-(N-Morpholino)propanesulfonic acid (MOPS buffer), which has been proven to have little impact on As adsorption on soils (Jeong et al. 2010 ). Experimental treatments included 4 root exudate inputs (control vs. CA vs. OA vs. Gl) × 3 sulfate concentrations (0 mM vs. 5 mM vs. 25 mM ) × 3 replicates, a total of 36 samples. Reagents were deoxygenated by purging with nitrogen (N 2 ), to create an oxygen-free environment. Experimental samplings were processed in a glovebox (MIKROUNA, Shanghai) under a N 2 atmosphere (O 2 < 1 ppm). Pore water chemical analysis Pore water was sampled at specific intervals (0.5, 1, 3, 7, 14, 21, and 28d). Following by filtering through 0.45 µm membrane filters, the pH and Eh were immediately measured in the glovebox, using pH and SenTix ORP electrodes (Multi 3510, WTW, Germany), respectively. The concentration of SO 4 2- in the filtered aliquots was analyzed by ion chromatography (HIC-SP, Shimadzu, Japan). The concentration of sulfides was determined spectrophotometrically using the methylene blue method (Cline 1969 ). Total dissolved Fe and Fe(Ⅱ) concentrations were analyzed by ferrozine assay using the ultraviolet-visible spectrophotometer (DR6000, Hach, USA), where Fe(III) was reduced by hydroxylamine hydrochloride (Tamura et al. 1974 ). Dissolved organic carbon (DOC) concentration was measured with a total organic carbon (TOC) analyzer (TOC-L, Shimadzu, Japan). As concentration and speciation analysis 1.0 mL of pore water samples were added to 10 mM DTPA (neutralized to pH 7.5) and preserved at -20°C within seven days before pore water As concentration and speciation analysis. Total As concentration in pore water was measured using a hydride-generation flame atomic absorption spectrophotometer (TAS-990AFG, Purkinje, Beijing). Pore water As speciation was analyzed by anion exchange chromatography (Dionex ICS-1100, Thermo Scientific, USA) coupled to an inductively coupled pharma-mass spectrometry (ICP-MS, NexION 350X, PerkinElmer, Inc., Shelton, CT USA). Briefly, As species were separated using an anion-exchange column (IonPac AS23, 250 mm × 4 mm, Dionex) under alkaline mobile phases (20 mM NH 4 CO 3 , pH 10.7) to protect acid-sensitive compounds during rapid elution (Yuan et al. 2021 ). Separated species were then directly introduced into the ICP-MS, equipped with a Type C0.5 Glass Nebulizer, for quantification. The As(III), As(V), DMA, MMA and DMMTA were separated during speciation analysis. The average recovery of total As across all measured samples was 109 ± 19%, indicating that predominant As species were effectively captured during the analytical procedure. Sequential extraction for solid-phase Fe and As After incubations, fresh soils were directly used for sequential extraction of Fe oxy(hydr)oxides as well as related As partition. A six-step extraction was applied referring to Wang et al., (2020), specifically as (1) “ligand-displaceable” (1 M NaH 2 PO 4 , pH = 5.0, C1); (2) “organic matter-bound” (0.1 M Na 4 P 2 O 7 , pH = 7, C2); (3) “acid volatile sulfides (AVS) and carbonate precipitates” (1 M CH 3 COONa, pH = 4.5, C3); (4) “amorphous Fe(oxy)hydroxides and magnetite precipitates” (0.2 M (NH 4 ) 2 C 2 O 4 , pH = 3, C4); (5) “Fe(II) sulfides-precipitated nominally from dissolution of pyrite” (12 M HNO 3 , C5); (6) “crystalline Fe(oxy)hydroxides-precipitated” (dithionite-citrate-bicarbonate (DCB), C6). Specifically, 0.5 g of fresh soil was weighed into a 50 mL centrifuge tube and shaken at 280 rpm. The extraction solutions were centrifuged at 4000 rpm for 10 minutes, followed by filtration through 0.45 µm cellulose acetate filters. Then samples were washed once using 50 mL ultrapure water and combined with the extraction solutions. Total As and Fe concentrations were measured according to previous descriptions. Statistical analyses A linear mixed-effects model was employed to evaluate the interacting effects of root exudates, sulfate concentration, and their interactions on As concentrations using R lme4 package (v1.1-36). Partial least-squares path modeling (PLS-PM) was performed to elucidate the pathways by which root exudates and sulfate reduction influence pore water As concentrations, utilizing the plspm package (v0.5.1) (Sanchez 2013 ; Zhang et al. 2020 ). The PLS-PM analysis included the following latent variables: root exudates, sulfate, Fe(oxy)hydroxides, Fe sulfides, pore water redox conditions, and total arsenic in pore water. Fe(oxy)hydroxides included amorphous Fe(oxy)hydroxides and crystalline Fe(oxy)hydroxides. Fe sulfides included AVS and pyrite-derived Fe species. Pore water redox condition included soil pore water Eh and Fe 2+ concentration. Path coefficients quantify the direction and strength of the linear relationships among variables, and explained variability (R²) were estimated in models. Models were evaluated using the Goodness of Fit (GoF) statistic. Results Dynamics of As concentration In control, the As concentration increased rapidly in first three days, and followed by a steady increase to 116.6 μg L -1 over the 28-day incubation period (Fig. 1a). Presence of root exudates elevated pore water As concentrations ( P < 0.05) (Fig. 1e), with As concentrations reaching 149.5 μg L -1 for citric acid, 134.7 μg L -1 for oxalic acid, and 130.2 μg L -1 for glucose, respectively. In contrast to root exudates, sulfate addition suppressed As release ( P < 0.05, Fig. 1). In the absence of root exudates, 5 mM and 25 mM sulfate addition reduced As concentration from 116.6 μg L -1 to 102.2 μg L -1 and 90.3 μg L -1 , respectively (Fig. 1a and e). In the presence of root exudates, 25 mM sulfate reduced the pore water As concentration to 110.8-130.2 μg L -1 , averaging 11.5-19.7 μg L -1 lower than treatments containing root exudates alone. Overall, sulfate reduction suppressed As mobilization to a similar extent, regardless of the presence or absence of root exudates. Dynamics of Fe and S in pore waters In control, the pore water Eh decreased and stabilized at ~ -200 mV (vs Ag/AgCl) after 10 days of incubation (Fig. S1). Root exudates treatment exhibited a more rapid and pronounced decline in Eh, reaching lower values of -250 mV. Concurrently, the sulfate reduction occurred rapidly, with the highest rate observed within 24 h, coinciding with a sharp decrease in pore water DOC (Figs. 2a-h, S2). The combined addition of root exudates and sulfate further accelerated SO 4 2- reduction. Notably, the maximum sulfate reduction rate was observed in setup of 25 mM sulfate, with peak values ranging from approximately 7 to 12 mM d -1 . Furthermore, under identical sulfate levels, the SO 4 2- concentrations were decreased to similar levels across different root exudate treatments. In control, Fe(II) concentration increased gradually and stabilized at 418 μM (Fig. 2j). In the presence of root exudates alone, the reduction of Fe(oxy)hydroxides was significantly accelerated, reaching peaks of 695 μM in the presence of citric acids and 987 μM in presence of oxalic acids, respectively. This was followed by a rapid decline to 342 and 388 μM , respectively (Figs. 2k and l). Sulfate supplementation further stimulated Fe(II) release with a rapid increase in Fe(II) concentrations, coinciding with the initial phase of accelerated sulfate reduction, which subsequently declined in later stages. Pore water As speciation The detectable As species in pore water mainly included As(III), As(V), DMA, and DMMTA. At the end of the incubation, the concentrations of As(III) and As(V) were 69 μg L -1 and 52 μg L -1 , respectively, together accounting for 98% of the total As in control. Root exudates enhanced As(III) concentration to 70-85 μg L -1 . Regardless of root exudate additions, the sulfate reduction marginally reduced As(III) concentrations by 3.0-37 μg L -1 . The organoarsenical species DMA and DMMTA were detected, with concentrations ranging from 1.4 to 41 μg L⁻¹ (Fig. 3i-p). Both DMA and DMTA concentrations were initially increased and then rapidly declined over time. Sulfate addition significantly enhanced their concentrations. In control, the DMA and DMMTA had peak concentrations of 4.9 and 7.7 μg L - ¹, respectively. In the treatment of 25 mM sulfates, DMMTA and DMA were increased by 93% and 111% without root exudate, by 27% and 87% with citric acid, by 100% and 126% with oxalic acid, and by 32% and 128% with glucose, respectively. Fe mineralogy and As partitions In pristine paddy soils, the solid-phase As was mainly incorporated into crystalline Fe(oxy)hydroxides, which accounted ~92% of total As (Table S1 ). At the end of the incubation, the Fe(oxy) hydroxides-bound As decreased by 1.1-2.7 mg kg -1 soil ( P < 0.05, Fig. 4a, c, e and g), while the Fe sulfides-bound As was increased by 0.9-1.7 mg kg -1 across all the treatments ( P < 0.05, Fig. 4a, c, e and g). Notably, root exudate additions decreased ligand-displaceable As content by 11.1-21.6%. Sulfate reductions reduced Fe(oxy)hydroxides-incorporated As by 0.38-1.22 mg kg -1 , while enhancing the iron sulfides-incorporated As by 0.10-0.85 mg kg -1 . Overall, As showed significant correlations with both Fe(oxy)hydroxides and iron sulfides ( P < 0.05, Fig. 5). Influencing pathways of root exudates and sulfate inputs for As mobilization The PLS-PM analysis revealed pathways of root exudates and sulfate inputs in controlling pore water As (Fig. 6). The results indicate that both root exudates and sulfate have non-significant direct effects on pore water As concentration, while indirect pathways playing a more critical role. Root exudates stimulated As release by enhancing Fe(oxy)hydroxides dissolutions, while reduction of added sulfates enhanced formation of iron sulfides and incorporated As into their structures, therefore partly counteracting the influences from root exudates. Discussion Influences of root exudates for As mobilization and speciation Root exudates enhanced As release into pore water (Fig. 1 ), which is consistent with previous studies (Jiang et al. 2023 ; Liu et al. 2022a ; Yang et al. 2023 ; Zou et al. 2024 ). Root exudate fractions, including citric acids, oxalic acids and glucose, can act as the organic carbon sources to promote microbial Fe(oxy)hydroxides reduction, which would initially cause As release into pore water (Jia et al. 2014 ; Liu et al. 2022a ; Zecchin et al. 2023 ). Furthermore, under the equal carbon input, citric acids and oxalic acids had greater influences for As release, compared to the glucose. The low-molecular weight acids increases negative charge of mineral surfaces, thereby reducing As adsorption sites on minerals (Geng et al. 2020 ; Onireti et al. 2017 ). The carboxyl groups of organic acids could coordinate with surface Fe, thereby decreasing As re-adsorbed into mineral surfaces (Grafe et al. 2002 ; Mikutta et al. 2010 ). Critically, even under pH-buffer (MOPS, pH = 6.5) conditions, citric and oxalic acids persistently promoted As release (Fig. 1 ), suggesting pH-independent ligand competition dominates over proton-induced dissolution. Therefore, although the re-incorporation of As into secondary Fe minerals may partly counteract the As release, root exudates may reduce the availability of As adsorption sites on mineral surfaces. As mainly existed as As(III) speciation in control and root exudate treatments (Fig. 3 ). Root exudates slightly enhanced As methylation, which is also referred in previous studies that carbon sources could enhance As methylations (Wang et al. 2023 ; Yan et al. 2020 ; Zhao et al. 2013 ). Furthermore, addition of root exudates not only increased the peak concentration of DMA but also shortened the time to reach the peak (Fig. 3 i-l). This phenomenon could be due to following reasons: 1) exogenous carbon serving as a high-quality electron donor enhanced the activity of heterotrophic arsenic-methylating microorganisms (Yang et al. 2018 ); 2) carbon sources may facilitate the establishment of anaerobic conditions and enhance As release, thereafter providing more As substrates for As methylations (Liu et al. 2022a ). Influences of sulfate reduction on As mobilization and speciation Contrary to our hypothesis, the incorporation of As into Fe sulfides could partly counteract the increase of sulfate reduction-induced As release (Fig. 1 ). The rapid SO 4 2− reduction was observed and coincided with rapid oxidation of organic carbon substrates at the beginning of anerobic incubation experiments (Fig. 2 e-h and S2). The sulfides formed from sulfate reduction induced rapid reductive dissolution of iron (oxyhydr)oxides, leading to an increase in Fe(II) concentration in pore water (Fig. 2 j-m). This process was significantly enhanced in presence of root exudates. It suggested that root exudates promoted dissimilatory sulfate reduction, therefore enhanced sulfide production by 1–6 times (Fig. 2 e-h). However, there is no difference in incorporated As of iron sulfides under 0 mM SO 4 2− between root exudates-free and exudates treatments (Fig. 4 ). This may be attributed to the fact that iron sulfides formation is constrained by sulfide fluxes instead of organic substrates. When sulfur flux is insufficient (e.g., under low sulfate concentrations), sulfide generation remains constrained, thereby limiting the formation of Fe sulfides and incorporate more As into Fe sulfides structures (Holmkvist et al. 2011 ; Wilkin and Ford 2006 ). Previous studies suggesting that SO 4 2− reductions enhance As concentration in groundwater (Guo et al. 2016 ; Nghiem et al. 2023 ). However, our result indicated that although sulfate reduction transiently enhanced the reductive dissolution of Fe(oxy)hydroxides in paddy soils, it ultimately leads to a significant decrease in pore water As concentrations (Chen et al. 2021 ; Xu et al. 2019 ; Yan et al. 2022 ). Formation of pyrite could be controlled by supply rate of sulfide (Fig. 4 ). Either low availabilities of organic carbon or low sulfate fluxes would hinder formation of pyrite in groundwater systems (Pi et al. 2017 ; Wang et al. 2021 ). Abundant organic carbon, especially under supply of root exudates in paddy soils could provide prerequisite for pyrite formation upon sulfate pulse in paddy soils (Fig. 4 ). Although sulfate reduction decreased As release from paddy soils, sulfate reductions enhance As methylation and methylthiolation processes. Sulfate reducing bacteria e.g., Clostridium sp BXM, could actively participate in As methylations through their specific arsenic methyltransferases (ArsM) (Chen et al. 2019 ; Wang et al. 2015 ). Sulfate reduction processes rapidly decreased the Eh to be level favorable for activities of sulfate reducing bacteria, and generated substantial As(Ⅲ) for subsequent methylation processes (Chen et al. 2021 ). In this study, DMA emerged as the predominant arsenic methylation product (Fig. 3 i-j), whereas the conventionally recognized precursor MMA was undetectable (Chen et al. 2019 ; Jia et al. 2013 ). It could be due to that MMA generated by sulfate-reducing bacteria was assimilated and further methylated to DMA by synergistic microbial communities (Hemmat-Jou et al. 2024 ; Jia et al. 2013 ; Marapakala et al. 2012 ). The DMMTA and DMA suggest similar behaviors under sulfate reduction, as significant correlations between the concentration of DMMTA and DMA occurred (Fig. S5). It indicate that DMMTA is the primary product DMA thiolation through chemical exchange of hydroxy with sulfide produced by sulfate reducing bacteria (Wang et al. 2020c ). Overall, our results suggest the possibility to apply sulfate fertilizer to remediate As contaminations in paddy soils. The formation of methylated both organic and inorganic As species might be mitigated through water management that regulate soil redox conditions (Wang et al. 2020b ). Conclusion This study indicated that in flooded paddy soil, although co-presences of root exudates and sulfate enhance the reductive dissolution of Fe(oxy)hydroxides, formation of iron sulfides could counteract part of the As release into pore water (Fig. 7 ). Root exudates predominantly enhance As mobilization through serving as carbon source to facilitate microbial reduction of Fe(oxy)hydroxides and displacing ligand-displaceable As, particularly in the presence of LMOWAs. The sulfate reduction could partly counteract this mobilization effect via incorporating As into Fe sulfides structures. The pristine organic carbon and root exudates provided sufficient carbon substrates to simulate high sulfide flux and thereafter pyrite formation. Thus, As can be efficiently incorporated into pyrite structures. This discovery significantly refines the conceptual framework of As transformation mechanisms, particularly under sulfate-reducing conditions in the rhizosphere. Given that root exudation in natural paddy systems is a continuous and dynamic process shaped by plant growth stages and environmental stressors, the static exudate additions used in this study may underestimate the transient coupling between exudate flux and sulfate reduction rates. Future experiments should focus on mimic field-realistic carbon release to resolve how temporally variable organic inputs influence the Fe-S-As in paddy rhizosphere soil. Abbreviations As arsenic As(V) arsenate As(III) arsenite S sulfur SO 4 2- sulfate S 2- sulfide SOC soil organic carbon DOC dissolved organic carbon TN total nitrogen CA citric acid OA oxalic acid Gl glucose AVS acid volatile sulfides DMA dimethyl arsenate MMA monomethylated arsenate MMMTA monomethylmonothioarsenate DMMTA dimethylmonothioarsenate PLS-PM partial least-squares path modeling LMOWAs low-molecular weight acids Declarations Completing interests The authors have no competing interests to declare. Funding This work was supported by National Natural Science Foundation of China (No. 22494682, 42207268, 32471702), and Jiangsu (BK20220454). Author Contributions Hong-Yan Wang designed the incubation experiment; Guo-Hao Zhang conducted incubations, measurements and results analysis, Guo-Hao Zhang conducted the writing with conceptual input of Hong-Yan Wang; Jun-Jie Lin, Guo-Xin Sun, Zheng Chen and Zhi-Guo Yu edited and reviewed this paper. Acknowledgements Authors thanks for the help from Yaqin Wang for As speciation analysis. This work was supported by National Natural Science Foundation of China (No. 22494682, 42207268, 32471702), and Jiangsu (BK20220454). Data Availability The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. 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Environ Res 249:118421. https://doi.org/10.1016/j.envres.2024.118421 Supplementary Files Rawdata.xls Supplementarymaterials.doc Cite Share Download PDF Status: Published Journal Publication published 02 Jan, 2026 Read the published version in Plant and Soil → Version 1 posted Editorial decision: Major revisions 17 Aug, 2025 Reviewers agreed at journal 17 Jun, 2025 Reviewers invited by journal 17 Jun, 2025 Editor invited by journal 23 May, 2025 Editor assigned by journal 23 May, 2025 First submitted to journal 22 May, 2025 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. 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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-6692895","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":472824368,"identity":"b4e4fb69-92c5-472f-8f34-0a171b723d54","order_by":0,"name":"Guo-hao Zhang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABFUlEQVRIie3QMUsDMRTA8RcCuSVya0LF+wrv6CCi0q/yjoIudhU3Dw7Spe4VFL9CR90igZsO547XXaHgYhHUXFfvro4O+Y/h/XhJAEKhf1jMuavp6vtAcDb9WAPI7bHsIXpqzrCu+DCOCqvnzbDYQbCqUK8Mz25nJQ22k7sILAmRhGCL5QUOTx/dfvKQs/rNQHLYIdicqCYpOXoynlROYgk8vTeQPuXthCuySEqJhriJ8USAGOwZILTtRKgsV/5u0pO0OPIkMRB99hEpHSgiUnpWjjnzBEoQvI+oyAgka9F/csluzLl/S1bouxeVLjrIyMXvq82XvTZ+BWzM8Sgp3PP69fIk6drSFsub9X+fD4VCodCvfgAZ+lrp3eLfIQAAAABJRU5ErkJggg==","orcid":"","institution":"Nanjing University of Information Science and Technology School of Hydrology and Water Resources Engineering","correspondingAuthor":true,"prefix":"","firstName":"Guo-hao","middleName":"","lastName":"Zhang","suffix":""},{"id":472824369,"identity":"8b0d0836-cb17-4e5c-864a-5a35521d8e94","order_by":1,"name":"Hong-yan Wang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Hong-yan","middleName":"","lastName":"Wang","suffix":""},{"id":472824370,"identity":"8206d18e-87c0-4056-8d0e-d33c329bfb2c","order_by":2,"name":"Jun-jie Lin","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jun-jie","middleName":"","lastName":"Lin","suffix":""},{"id":472824371,"identity":"817a6543-99a5-47d4-9a5c-c96790906d2a","order_by":3,"name":"Guo-xin Sun","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Guo-xin","middleName":"","lastName":"Sun","suffix":""},{"id":472824372,"identity":"b661fdc2-776f-4e9a-836f-747c23048520","order_by":4,"name":"Zheng Chen","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Zheng","middleName":"","lastName":"Chen","suffix":""},{"id":472824373,"identity":"6b548930-090a-4ea7-a53c-29fb8e689ea2","order_by":5,"name":"Zhi-guo Yu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Zhi-guo","middleName":"","lastName":"Yu","suffix":""}],"badges":[],"createdAt":"2025-05-18 16:38:24","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6692895/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6692895/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s11104-025-08220-w","type":"published","date":"2026-01-02T15:57:01+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":85078840,"identity":"c03241f2-23fb-48e5-bf2d-f5f95c3516f7","added_by":"auto","created_at":"2025-06-20 17:10:10","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":1682803,"visible":true,"origin":"","legend":"\u003cp\u003eDynamics of arsenic (As) concentrations in pore water under different treatments over the incubation period (a-d), and As concentrations at the end of incubation (e). Lowercase letters in \u003cstrong\u003eFig. 1e\u003c/strong\u003e denote statistically significant differences among sulfate treatments, while uppercase letters denote significant differences among root exudate treatments. *** indicate statistically significance at \u003cem\u003eP \u003c/em\u003e\u0026lt; 0.001.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/5adb25ba957a36cee80f8ff4.jpeg"},{"id":85077987,"identity":"ab210e0f-9f2c-495c-9643-6f06d56cd6b6","added_by":"auto","created_at":"2025-06-20 17:02:10","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":1812687,"visible":true,"origin":"","legend":"\u003cp\u003eInfluences of root exudates fractions and sulfate addition on concentrations of sulfate (a-d), sulfate reduction rates (e-h) and Fe(II) concentration\u003csup\u003e \u003c/sup\u003e(j-m) over the incubation period.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/db5b9d58554c9467f806e06e.jpeg"},{"id":85078842,"identity":"0f6bf2b4-e73f-458f-8d30-9f471ef62d2c","added_by":"auto","created_at":"2025-06-20 17:10:10","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2634740,"visible":true,"origin":"","legend":"\u003cp\u003eInfluences\u003cstrong\u003e \u003c/strong\u003eof root exudates and sulfate addition on concentrations of As(Ⅲ) (a-d), As (Ⅴ)(e-h), dimethylarsenate (DMA) (i-l) and dimethylmonothioarsenate (DMMTA) (m-p) over incubations.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/e476c53e3e3d07b14124efe4.jpeg"},{"id":85077988,"identity":"a3c77c8e-de18-4554-8a9a-e2256772bc3a","added_by":"auto","created_at":"2025-06-20 17:02:10","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":1762617,"visible":true,"origin":"","legend":"\u003cp\u003eDifferences in As and Fe contents in the extracts from each sequential extraction step between incubated and raw soil. (C1: ligand-displaceable, C2: organic matter-bound, C3: AVS (acid volatile sulfides), carbonates-precipitates, C4: amorphous Fe(oxy)hydroxides and magnetite-precipitated, C5: Fe(II) sulfides-precipitated nominally from dissolution of pyrite, C6: crystalline Fe(oxy)hydroxides-precipitated). The uppercase letters denote significant differences among root exudates treatments (P \u0026lt; 0.05).\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/278b01df2e47396f194e3357.jpeg"},{"id":85078844,"identity":"8d5e3afc-9d7d-4b76-8dc4-49d012b8fbb9","added_by":"auto","created_at":"2025-06-20 17:10:10","extension":"jpeg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1872267,"visible":true,"origin":"","legend":"\u003cp\u003eCorrelations between iron (Fe) content and associated total arsenic (As) in extracted iron sulfides and Fe (oxy)hydroxides\u003c/p\u003e","description":"","filename":"floatimage5.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/7dd5e9dcd173684509f2dd71.jpeg"},{"id":85079840,"identity":"72186baa-e17a-4523-b42d-dc06f3ec7bb7","added_by":"auto","created_at":"2025-06-20 17:26:10","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":124020,"visible":true,"origin":"","legend":"\u003cp\u003ePatrial least squares path modelling (PLS-PM) illustrating the pathways through which root exudates and sulfate inputs influence total arsenic (As) concentrations in pore water of paddy soils. Red and blue arrows indicate significant positive and negative pathways, respectively, while grey arrows represent non-significant paths. R\u003csup\u003e2\u003c/sup\u003e values denote the proportion of variance explained for each dependent variable. GOF indicates the models’ goodness of fit (GOF).\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/b7980fd58b35b8478fb2afe2.jpeg"},{"id":85079055,"identity":"7674f271-b1ce-4e98-b8e2-211f4e38921c","added_by":"auto","created_at":"2025-06-20 17:18:10","extension":"jpeg","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":318602,"visible":true,"origin":"","legend":"\u003cp\u003eConceptual model of paddy soil As mobilization under root exudates and sulfate input\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/c9bcbec769560a5af36bd4b8.jpeg"},{"id":99545188,"identity":"064aaa86-ddaf-4eeb-a679-5b6adfd25a2e","added_by":"auto","created_at":"2026-01-05 16:01:22","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":10824242,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/6228993f-ba12-402a-ade7-6e194f00fecb.pdf"},{"id":85079054,"identity":"fab90b2b-bcfe-404a-9079-95ad34e5c777","added_by":"auto","created_at":"2025-06-20 17:18:10","extension":"xls","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":80896,"visible":true,"origin":"","legend":"","description":"","filename":"Rawdata.xls","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/97bc870ab7e625b3932123bb.xls"},{"id":85079056,"identity":"ecfafc9b-721d-424d-8ce5-884ea79c9f33","added_by":"auto","created_at":"2025-06-20 17:18:10","extension":"doc","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":6434816,"visible":true,"origin":"","legend":"","description":"","filename":"Supplementarymaterials.doc","url":"https://assets-eu.researchsquare.com/files/rs-6692895/v1/8b6a85ad169b68dd8428e1ef.doc"}],"financialInterests":"","formattedTitle":"Dynamics of sulfate reduction regulate arsenic mobilization and speciation in paddy soils in response to root exudates","fulltext":[{"header":"Introduction","content":"\u003cp\u003eArsenic (As) contamination is widespread in both terrestrial and aquatic environments, particularly in South and Southeast Asia, where geogenic As-enriched groundwater is prevalent (Aftabtalab et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Mukherjee et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The irrigation of As-contaminated groundwater has elevated As accumulation in paddy soils, a problem further exacerbated by anthropogenic activities such as mining and pesticide application (Hong et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Qiao et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Under periodic flooding conditions, As is mobilized into pore water, and efficiently translocated into rice plants primarily via silicic acid transport pathways (Ma et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Zhu et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2008\u003c/span\u003e). As a result, rice consumption has become a major exposure pathway to As for population dependent on a rice-based diet (Hussain et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Thus, a comprehensive understanding of the biogeochemical cycling of As in paddy soils, particularly within rhizosphere, is critical for predicting and mitigating As uptake by rice.\u003c/p\u003e \u003cp\u003eThe As mobilization, speciation and redistributions in paddy soils are closely governed by the redox transformation of iron (Fe) oxy(hydro)xides (Burton et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Nghiem et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Under oxic conditions, Fe oxy(hydro)xides act as primary minerals adsorbing or sequestrating arsenate (As(V)) (Chen et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Hong et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). In contrast, during flooded conditions, As(V) can be released through the reductive dissolution of Fe oxy(hydro)xides, and subsequently reduced to arsenite (As(III)) (Guo et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Huang et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). As(III) exhibits lower sorption affinity to Fe oxy(hydro)xides and is more readily mobilized into pore waters (Xiang et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Additionally, Fe(Ⅱ)-catalyzed phase transformation of Fe oxy(hydro)xides influences the distribution of As species between soil and pore water phases (Wang et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). The formation of secondary Fe-bearing minerals such as green rust, magnetite, siderite, pyrite can further sequester As either by structural incorporation or surface adsorption, thereby reducing its concentration in pore water (Perez et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eSulfur redox transformations play a critical role in influencing the fate of Fe oxy(hydr)oxides, thereby affecting As repartitioning between pore water and soils, as well as the speciation of As in pore water (Guo et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Although the reductions of poorly-crystalline Fe oxy(hydr)oxides and As(V) are thermodynamically more favorable than sulfate (SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e) reduction, increasing evidence indicates that reduction of Fe oxy(hydr)oxides, As(V), and SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e can occur concurrently in anoxic environments (Nghiem et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Wu et al. \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Sulfide produced from sulfate reduction can rapidly react with Fe(II) to form Fe monosulfide minerals such as FeS (e.g., mackinawite), which can transform into more crystalline pyrite (FeS\u003csub\u003e2\u003c/sub\u003e), although the diagenetic pathways remain incompletely understood (Burton et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). While FeS has limited capacity to adsorb or incorporate As, pyrite exhibits a strong affinity for As, facilitating its immobilization (Qiu et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2020a\u003c/span\u003e). Concurrently, sulfide can abiotically reduce As-bearing Fe oxy(hydr)oxides, enhancing As mobilization into pore water (Guo et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Kocar et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Zheng et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Thus, As mobility in paddy soils is governed by was complex interaction between pore water and solid-phase geochemistry, particularly sulfide fluxes and the abundance of reactive Fe oxy(hydr)oxides (Burton et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Planer-Friedrich \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The impact of sulfate reduction on pore water As concentrations remain inconsistent across studies (Burton et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Zheng et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eFurthermore, under sulfidic conditions, inorganic oxyarsenic species can undergo microbial methylation by sulfate-reducing bacteria, producing methylated As species, such as dimethylarsenate (DMA) and monomethylarsenate (MMA) (Chen et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2020c\u003c/span\u003e). Increasing evidence also indicates the co-occurrence of thiolated As species both in aquatic systems. These include both inorganic forms such as monothioarsenate [MTA, AsS(OH)\u003csub\u003e3\u003c/sub\u003e], dithioarsenate [DTA, AsS\u003csub\u003e2\u003c/sub\u003e(OH)\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e] and organic forms such as monomethylmonothioarsenate [MMMTA, (CH\u003csub\u003e3\u003c/sub\u003e)AsS(OH)\u003csub\u003e2\u003c/sub\u003e], dimethylmonothioarsenate [DMMTA, (CH\u003csub\u003e3\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eAsS(OH)]. These thiolated species commonly co-exist with As(III) and As(V), adding complexity to arsenic speciation under reducing conditions (Besold et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2020c\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eRoot exudates, particularly organic acids and simple sugars, constitute a major source of bioavailable carbon in soils (Haichar et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022b\u003c/span\u003e). These compounds can stimulate microbial processes such as sulfate reduction and the reductive dissolution of Fe oxy(hydr)oxides (Jia et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e), thereby influencing coupled Fe-S redox dynamics and the associated and speciation of As. While previous studies have explored the role of root exudates in promoting As mobilization and the underlying microbial mechanisms (Jiang et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e; Zou et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), their specific effects on Fe-S mineral transformation pathways remain insufficiently understood.\u003c/p\u003e \u003cp\u003eIn this study, we conducted controlled laboratory experiments to examine the concentration, speciation, and redistribution of pore water As in response to Fe oxy(hydr)oxide transformation under the sulfidic conditions and root exudate amendments. We hypothesize that: 1) sulfate reduction critically amplifies As mobilization in paddy soil porewater, with this effect being intensified in the presence of root exudates; 2) the coupled processes of sulfate reduction and oxidation of root exudate-derived carbon promote the reduction of As(III) and the formation of methylated As species, thereby driving As partitioning from the soil solid phase into the aqueous phase.\u003c/p\u003e"},{"header":"Materials and methods","content":"\u003cp\u003ePaddy soil collections\u003c/p\u003e \u003cp\u003eSurface paddy soils (0\u0026ndash;20 cm) were collected from an As-contaminated paddy field in Qingyuan city of Guangdong province, China (23\u0026deg;42\u0026prime; N, 113\u0026deg;4\u0026prime; E). Due to mining activities, the paddy soils are heavily contaminated with As at a concentration of ~\u0026thinsp;51 mg kg\u003csup\u003e-1\u003c/sup\u003e soil, which has far exceeded the regulatory for Chinese paddy soils (Regulation \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Following transport to the laboratory, soil samples were air-dried and sieved to \u0026lt;\u0026thinsp;2 mm. The main physio-chemical characteristics of the soils are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\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\u003eSoil physio-chemical properties. Arsenic, As; Iron, Fe; TC, total carbon; TN, total nitrogen; SOC, soil organic carbon.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"2\"\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 \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eItems\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMean\u0026thinsp;\u0026plusmn;\u0026thinsp;SD\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\u003e6.51\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAs (mg kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e50.71\u0026thinsp;\u0026plusmn;\u0026thinsp;3.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eFe (g kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e12.10\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTN (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.25\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTC (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.91\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eC/N\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e7.68\u0026thinsp;\u0026plusmn;\u0026thinsp;0.23\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSOC (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e1.61\u0026thinsp;\u0026plusmn;\u0026thinsp;0.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eS (%)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e \u003cp\u003e0.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.00\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\u003eSulfate and root exudate additions\u003c/p\u003e \u003cp\u003eCitric acid (CA), oxalic acid (OA), and glucose (Gl) were selected as representative root exudates, due to their identification as key constituents of rice root exudation (Yang et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The total exudate-carbon (C) input was standardized to 2% of the soil organic carbon (SOC) (De Graaff et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). In alignment with the variable SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e concentrations observed in paddy soils (Fulda et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e was applied in pulsed treatments at concentrations of 0 \u003cem\u003emM\u003c/em\u003e, 5 \u003cem\u003emM\u003c/em\u003e, and 25 \u003cem\u003emM\u003c/em\u003e.\u003c/p\u003e \u003cp\u003eIncubation setups\u003c/p\u003e \u003cp\u003e10 g of soil was placed in a 100 mL serum vial. 30 mL of deionized water resulted in a 3 cm of standing water above the soil surface. The vials were sealed tightly with 2 cm-thick butyl stoppers (Glasger\u0026auml;tebau Ochs, Bovenden, Germany) and aluminum caps. The soils were pre-incubated at 25\u0026deg;C for 7 days to re-activate microbial activity. Root exudate fractions and sodium sulfate (Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e) were added into the via after pre-incubation. The pH of the soil suspension was adjusted to 6.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.1 using 3-(N-Morpholino)propanesulfonic acid (MOPS buffer), which has been proven to have little impact on As adsorption on soils (Jeong et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Experimental treatments included 4 root exudate inputs (control vs. CA vs. OA vs. Gl) \u0026times; 3 sulfate concentrations (0 \u003cem\u003emM\u003c/em\u003e vs. 5 \u003cem\u003emM\u003c/em\u003e vs. 25 \u003cem\u003emM\u003c/em\u003e) \u0026times; 3 replicates, a total of 36 samples. Reagents were deoxygenated by purging with nitrogen (N\u003csub\u003e2\u003c/sub\u003e), to create an oxygen-free environment. Experimental samplings were processed in a glovebox (MIKROUNA, Shanghai) under a N\u003csub\u003e2\u003c/sub\u003e atmosphere (O\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;\u0026lt;\u0026thinsp;1 ppm).\u003c/p\u003e \u003cp\u003ePore water chemical analysis\u003c/p\u003e \u003cp\u003ePore water was sampled at specific intervals (0.5, 1, 3, 7, 14, 21, and 28d). Following by filtering through 0.45 \u0026micro;m membrane filters, the pH and Eh were immediately measured in the glovebox, using pH and SenTix ORP electrodes (Multi 3510, WTW, Germany), respectively. The concentration of SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e in the filtered aliquots was analyzed by ion chromatography (HIC-SP, Shimadzu, Japan). The concentration of sulfides was determined spectrophotometrically using the methylene blue method (Cline \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e1969\u003c/span\u003e). Total dissolved Fe and Fe(Ⅱ) concentrations were analyzed by ferrozine assay using the ultraviolet-visible spectrophotometer (DR6000, Hach, USA), where Fe(III) was reduced by hydroxylamine hydrochloride (Tamura et al. \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1974\u003c/span\u003e). Dissolved organic carbon (DOC) concentration was measured with a total organic carbon (TOC) analyzer (TOC-L, Shimadzu, Japan).\u003c/p\u003e \u003cp\u003eAs concentration and speciation analysis\u003c/p\u003e \u003cp\u003e1.0 mL of pore water samples were added to 10 \u003cem\u003emM\u003c/em\u003e DTPA (neutralized to pH 7.5) and preserved at -20\u0026deg;C within seven days before pore water As concentration and speciation analysis. Total As concentration in pore water was measured using a hydride-generation flame atomic absorption spectrophotometer (TAS-990AFG, Purkinje, Beijing). Pore water As speciation was analyzed by anion exchange chromatography (Dionex ICS-1100, Thermo Scientific, USA) coupled to an inductively coupled pharma-mass spectrometry (ICP-MS, NexION 350X, PerkinElmer, Inc., Shelton, CT USA). Briefly, As species were separated using an anion-exchange column (IonPac AS23, 250 mm \u0026times; 4 mm, Dionex) under alkaline mobile phases (20 \u003cem\u003emM\u003c/em\u003e NH\u003csub\u003e4\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e, pH 10.7) to protect acid-sensitive compounds during rapid elution (Yuan et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Separated species were then directly introduced into the ICP-MS, equipped with a Type C0.5 Glass Nebulizer, for quantification. The As(III), As(V), DMA, MMA and DMMTA were separated during speciation analysis. The average recovery of total As across all measured samples was 109\u0026thinsp;\u0026plusmn;\u0026thinsp;19%, indicating that predominant As species were effectively captured during the analytical procedure.\u003c/p\u003e \u003cp\u003eSequential extraction for solid-phase Fe and As\u003c/p\u003e \u003cp\u003eAfter incubations, fresh soils were directly used for sequential extraction of Fe oxy(hydr)oxides as well as related As partition. A six-step extraction was applied referring to Wang et al., (2020), specifically as (1) \u0026ldquo;ligand-displaceable\u0026rdquo; (1 \u003cem\u003eM\u003c/em\u003e NaH\u003csub\u003e2\u003c/sub\u003ePO\u003csub\u003e4\u003c/sub\u003e, pH\u0026thinsp;=\u0026thinsp;5.0, C1); (2) \u0026ldquo;organic matter-bound\u0026rdquo; (0.1 \u003cem\u003eM\u003c/em\u003e Na\u003csub\u003e4\u003c/sub\u003eP\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e, pH\u0026thinsp;=\u0026thinsp;7, C2); (3) \u0026ldquo;acid volatile sulfides (AVS) and carbonate precipitates\u0026rdquo; (1 \u003cem\u003eM\u003c/em\u003e CH\u003csub\u003e3\u003c/sub\u003eCOONa, pH\u0026thinsp;=\u0026thinsp;4.5, C3); (4) \u0026ldquo;amorphous Fe(oxy)hydroxides and magnetite precipitates\u0026rdquo; (0.2 \u003cem\u003eM\u003c/em\u003e (NH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eC\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, pH\u0026thinsp;=\u0026thinsp;3, C4); (5) \u0026ldquo;Fe(II) sulfides-precipitated nominally from dissolution of pyrite\u0026rdquo; (12 \u003cem\u003eM\u003c/em\u003e HNO\u003csub\u003e3\u003c/sub\u003e, C5); (6) \u0026ldquo;crystalline Fe(oxy)hydroxides-precipitated\u0026rdquo; (dithionite-citrate-bicarbonate (DCB), C6). Specifically, 0.5 g of fresh soil was weighed into a 50 mL centrifuge tube and shaken at 280 rpm. The extraction solutions were centrifuged at 4000 rpm for 10 minutes, followed by filtration through 0.45 \u0026micro;m cellulose acetate filters. Then samples were washed once using 50 mL ultrapure water and combined with the extraction solutions. Total As and Fe concentrations were measured according to previous descriptions.\u003c/p\u003e \u003cp\u003eStatistical analyses\u003c/p\u003e \u003cp\u003eA linear mixed-effects model was employed to evaluate the interacting effects of root exudates, sulfate concentration, and their interactions on As concentrations using R \u003cem\u003elme4\u003c/em\u003e package (v1.1-36). Partial least-squares path modeling (PLS-PM) was performed to elucidate the pathways by which root exudates and sulfate reduction influence pore water As concentrations, utilizing the \u003cem\u003eplspm\u003c/em\u003e package (v0.5.1) (Sanchez \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). The PLS-PM analysis included the following latent variables: root exudates, sulfate, Fe(oxy)hydroxides, Fe sulfides, pore water redox conditions, and total arsenic in pore water. Fe(oxy)hydroxides included amorphous Fe(oxy)hydroxides and crystalline Fe(oxy)hydroxides. Fe sulfides included AVS and pyrite-derived Fe species. Pore water redox condition included soil pore water Eh and Fe\u003csup\u003e2+\u003c/sup\u003e concentration. Path coefficients quantify the direction and strength of the linear relationships among variables, and explained variability (R\u0026sup2;) were estimated in models. Models were evaluated using the Goodness of Fit (GoF) statistic.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eDynamics of As concentration\u003c/p\u003e\n\u003cp\u003eIn control, the As concentration increased rapidly in first three days, and followed by a steady increase to 116.6 \u0026mu;g L\u003csup\u003e-1 \u003c/sup\u003eover the 28-day incubation period (Fig. 1a). Presence of root exudates elevated pore water As concentrations (\u003cem\u003eP\u003c/em\u003e \u0026lt; 0.05) (Fig. 1e), with As concentrations reaching 149.5 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e for citric acid, 134.7 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e for oxalic acid, and 130.2 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e for glucose, respectively.\u003c/p\u003e\n\u003cp\u003eIn contrast to root exudates, sulfate addition suppressed As release (\u003cem\u003eP \u003c/em\u003e\u0026lt; 0.05, Fig. 1). In the absence of root exudates, 5 \u003cem\u003emM\u003c/em\u003e and 25 \u003cem\u003emM\u003c/em\u003e sulfate addition reduced As concentration from 116.6 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e to 102.2 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e and 90.3 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e, respectively (Fig. 1a and e). In the presence of root exudates, 25 \u003cem\u003emM\u003c/em\u003e sulfate reduced the pore water As concentration to 110.8-130.2 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e, averaging 11.5-19.7 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e lower than treatments containing root exudates alone. Overall, sulfate reduction suppressed As mobilization to a similar extent, regardless of the presence or absence of root exudates.\u003c/p\u003e\n\u003cp\u003eDynamics of Fe and S in pore waters\u003c/p\u003e\n\u003cp\u003eIn control, the pore water Eh decreased and stabilized at ~ -200 mV (vs Ag/AgCl) after 10 days of incubation (Fig. S1). Root exudates treatment exhibited a more rapid and pronounced decline in Eh, reaching lower values of -250 mV. Concurrently, the sulfate reduction occurred rapidly, with the highest rate observed within 24 h, coinciding with a sharp decrease in pore water DOC (Figs. 2a-h, S2). The combined addition of root exudates and sulfate further accelerated SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e reduction. Notably, the maximum sulfate reduction rate was observed in setup of 25 \u003cem\u003emM\u003c/em\u003e sulfate, with peak values ranging from approximately 7 to 12 \u003cem\u003emM\u003c/em\u003e d\u003csup\u003e-1\u003c/sup\u003e. Furthermore, under identical sulfate levels, the SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e concentrations were decreased to similar levels across different root exudate treatments.\u003c/p\u003e\n\u003cp\u003eIn control, Fe(II) concentration increased gradually and stabilized at 418 \u003cem\u003e\u0026mu;M\u003c/em\u003e (Fig. 2j). In the presence of root exudates alone, the reduction of Fe(oxy)hydroxides was significantly accelerated, reaching peaks of 695 \u003cem\u003e\u0026mu;M\u003c/em\u003e in the presence of citric acids and 987 \u003cem\u003e\u0026mu;M\u003c/em\u003e in presence of oxalic acids, respectively. This was followed by a rapid decline to 342 and 388 \u003cem\u003e\u0026mu;M\u003c/em\u003e, respectively (Figs. 2k and l). Sulfate supplementation further stimulated Fe(II) release with a rapid increase in Fe(II) concentrations, coinciding with the initial phase of accelerated sulfate reduction, which subsequently declined in later stages.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePore water As speciation\u003c/p\u003e\n\u003cp\u003eThe detectable As species in pore water mainly included As(III), As(V), DMA, and DMMTA. At the end of the incubation, the concentrations of As(III) and As(V) were 69 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e and 52 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e, respectively, together accounting for 98% of the total As in control. Root exudates enhanced As(III) concentration to 70-85 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e. Regardless of root exudate additions, the sulfate reduction marginally reduced As(III) concentrations by 3.0-37 \u0026mu;g L\u003csup\u003e-1\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eThe organoarsenical species DMA and DMMTA were detected, with concentrations ranging from 1.4 to 41 \u0026mu;g L⁻\u0026sup1; (Fig. 3i-p). Both DMA and DMTA concentrations were initially increased and then rapidly declined over time. Sulfate addition significantly enhanced their concentrations. In control, the DMA and DMMTA had peak concentrations of 4.9 and 7.7 \u0026mu;g L\u003csup\u003e-\u003c/sup\u003e\u0026sup1;, respectively. In the treatment of 25 \u003cem\u003emM\u003c/em\u003e sulfates, DMMTA and DMA were increased by 93% and 111% without root exudate, by 27% and 87% with citric acid, by 100% and 126% with oxalic acid, and by 32% and 128% with glucose, respectively.\u003c/p\u003e\n\u003cp\u003eFe mineralogy and As partitions\u003c/p\u003e\n\u003cp\u003eIn pristine paddy soils, the solid-phase As was mainly incorporated into crystalline Fe(oxy)hydroxides, which accounted ~92% of total As (Table \u003cstrong\u003eS1\u003c/strong\u003e). At the end of the incubation, the Fe(oxy) hydroxides-bound As decreased by 1.1-2.7 mg kg\u003csup\u003e-1\u003c/sup\u003e soil (\u003cem\u003eP\u003c/em\u003e \u0026lt; 0.05, Fig. 4a, c, e and g), while the Fe sulfides-bound As was increased by 0.9-1.7 mg kg\u003csup\u003e-1\u003c/sup\u003e across all the treatments (\u003cem\u003eP\u003c/em\u003e \u0026lt; 0.05, Fig. 4a, c, e and g). Notably, root exudate additions decreased ligand-displaceable As content by 11.1-21.6%. Sulfate reductions reduced Fe(oxy)hydroxides-incorporated As by 0.38-1.22 mg kg\u003csup\u003e-1\u003c/sup\u003e, while enhancing the iron sulfides-incorporated As by 0.10-0.85 mg kg\u003csup\u003e-1\u003c/sup\u003e. Overall, As showed significant correlations with both Fe(oxy)hydroxides and iron sulfides (\u003cem\u003eP \u003c/em\u003e\u0026lt; 0.05, Fig. 5).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eInfluencing pathways of root exudates and sulfate inputs for As mobilization\u003c/p\u003e\n\u003cp\u003eThe PLS-PM analysis revealed pathways of root exudates and sulfate inputs in controlling pore water As (Fig. 6). The results indicate that both root exudates and sulfate have non-significant direct effects on pore water As concentration, while indirect pathways playing a more critical role. Root exudates stimulated As release by enhancing Fe(oxy)hydroxides dissolutions, while reduction of added sulfates enhanced formation of iron sulfides and incorporated As into their structures, therefore partly counteracting the influences from root exudates.\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eInfluences of root exudates for As mobilization and speciation\u003c/p\u003e \u003cp\u003eRoot exudates enhanced As release into pore water (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), which is consistent with previous studies (Jiang et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e; Yang et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Zou et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Root exudate fractions, including citric acids, oxalic acids and glucose, can act as the organic carbon sources to promote microbial Fe(oxy)hydroxides reduction, which would initially cause As release into pore water (Jia et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Liu et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e; Zecchin et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Furthermore, under the equal carbon input, citric acids and oxalic acids had greater influences for As release, compared to the glucose. The low-molecular weight acids increases negative charge of mineral surfaces, thereby reducing As adsorption sites on minerals (Geng et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Onireti et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The carboxyl groups of organic acids could coordinate with surface Fe, thereby decreasing As re-adsorbed into mineral surfaces (Grafe et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2002\u003c/span\u003e; Mikutta et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2010\u003c/span\u003e). Critically, even under pH-buffer (MOPS, pH\u0026thinsp;=\u0026thinsp;6.5) conditions, citric and oxalic acids persistently promoted As release (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), suggesting pH-independent ligand competition dominates over proton-induced dissolution. Therefore, although the re-incorporation of As into secondary Fe minerals may partly counteract the As release, root exudates may reduce the availability of As adsorption sites on mineral surfaces.\u003c/p\u003e \u003cp\u003eAs mainly existed as As(III) speciation in control and root exudate treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). Root exudates slightly enhanced As methylation, which is also referred in previous studies that carbon sources could enhance As methylations (Wang et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Zhao et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Furthermore, addition of root exudates not only increased the peak concentration of DMA but also shortened the time to reach the peak (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei-l). This phenomenon could be due to following reasons: 1) exogenous carbon serving as a high-quality electron donor enhanced the activity of heterotrophic arsenic-methylating microorganisms (Yang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e); 2) carbon sources may facilitate the establishment of anaerobic conditions and enhance As release, thereafter providing more As substrates for As methylations (Liu et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022a\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eInfluences of sulfate reduction on As mobilization and speciation\u003c/p\u003e \u003cp\u003eContrary to our hypothesis, the incorporation of As into Fe sulfides could partly counteract the increase of sulfate reduction-induced As release (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The rapid SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e reduction was observed and coincided with rapid oxidation of organic carbon substrates at the beginning of anerobic incubation experiments (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee-h and S2). The sulfides formed from sulfate reduction induced rapid reductive dissolution of iron (oxyhydr)oxides, leading to an increase in Fe(II) concentration in pore water (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ej-m). This process was significantly enhanced in presence of root exudates. It suggested that root exudates promoted dissimilatory sulfate reduction, therefore enhanced sulfide production by 1\u0026ndash;6 times (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee-h). However, there is no difference in incorporated As of iron sulfides under 0 \u003cem\u003emM\u003c/em\u003e SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e between root exudates-free and exudates treatments (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). This may be attributed to the fact that iron sulfides formation is constrained by sulfide fluxes instead of organic substrates. When sulfur flux is insufficient (e.g., under low sulfate concentrations), sulfide generation remains constrained, thereby limiting the formation of Fe sulfides and incorporate more As into Fe sulfides structures (Holmkvist et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Wilkin and Ford \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2006\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePrevious studies suggesting that SO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2\u0026minus;\u003c/sup\u003e reductions enhance As concentration in groundwater (Guo et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Nghiem et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). However, our result indicated that although sulfate reduction transiently enhanced the reductive dissolution of Fe(oxy)hydroxides in paddy soils, it ultimately leads to a significant decrease in pore water As concentrations (Chen et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Xu et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Yan et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Formation of pyrite could be controlled by supply rate of sulfide (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). Either low availabilities of organic carbon or low sulfate fluxes would hinder formation of pyrite in groundwater systems (Pi et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Abundant organic carbon, especially under supply of root exudates in paddy soils could provide prerequisite for pyrite formation upon sulfate pulse in paddy soils (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eAlthough sulfate reduction decreased As release from paddy soils, sulfate reductions enhance As methylation and methylthiolation processes. Sulfate reducing bacteria e.g., \u003cem\u003eClostridium\u003c/em\u003e sp BXM, could actively participate in As methylations through their specific arsenic methyltransferases (ArsM) (Chen et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Sulfate reduction processes rapidly decreased the Eh to be level favorable for activities of sulfate reducing bacteria, and generated substantial As(Ⅲ) for subsequent methylation processes (Chen et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). In this study, DMA emerged as the predominant arsenic methylation product (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ei-j), whereas the conventionally recognized precursor MMA was undetectable (Chen et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Jia et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). It could be due to that MMA generated by sulfate-reducing bacteria was assimilated and further methylated to DMA by synergistic microbial communities (Hemmat-Jou et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Jia et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Marapakala et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The DMMTA and DMA suggest similar behaviors under sulfate reduction, as significant correlations between the concentration of DMMTA and DMA occurred (Fig. S5). It indicate that DMMTA is the primary product DMA thiolation through chemical exchange of hydroxy with sulfide produced by sulfate reducing bacteria (Wang et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2020c\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eOverall, our results suggest the possibility to apply sulfate fertilizer to remediate As contaminations in paddy soils. The formation of methylated both organic and inorganic As species might be mitigated through water management that regulate soil redox conditions (Wang et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2020b\u003c/span\u003e).\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study indicated that in flooded paddy soil, although co-presences of root exudates and sulfate enhance the reductive dissolution of Fe(oxy)hydroxides, formation of iron sulfides could counteract part of the As release into pore water (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eRoot exudates predominantly enhance As mobilization through serving as carbon source to facilitate microbial reduction of Fe(oxy)hydroxides and displacing ligand-displaceable As, particularly in the presence of LMOWAs. The sulfate reduction could partly counteract this mobilization effect via incorporating As into Fe sulfides structures. The pristine organic carbon and root exudates provided sufficient carbon substrates to simulate high sulfide flux and thereafter pyrite formation. Thus, As can be efficiently incorporated into pyrite structures. This discovery significantly refines the conceptual framework of As transformation mechanisms, particularly under sulfate-reducing conditions in the rhizosphere. Given that root exudation in natural paddy systems is a continuous and dynamic process shaped by plant growth stages and environmental stressors, the static exudate additions used in this study may underestimate the transient coupling between exudate flux and sulfate reduction rates. Future experiments should focus on mimic field-realistic carbon release to resolve how temporally variable organic inputs influence the Fe-S-As in paddy rhizosphere soil.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Abbreviations","content":"\u003ctable width=\"634\"\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eAs\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003earsenic\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eAs(V)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003earsenate\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eAs(III)\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003earsenite\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003esulfur\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eSO\u003csub\u003e4\u003c/sub\u003e\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003esulfate\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eS\u003csup\u003e2-\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003esulfide\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eSOC\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003esoil organic carbon\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eDOC\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003edissolved organic carbon\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eTN\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003etotal nitrogen\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eCA\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003ecitric acid\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eOA\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003eoxalic acid\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eGl\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003eglucose\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eAVS\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003eacid volatile sulfides\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eDMA\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003edimethyl arsenate\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eMMA\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003emonomethylated arsenate\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eMMMTA\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003emonomethylmonothioarsenate\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eDMMTA\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003edimethylmonothioarsenate\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003ePLS-PM\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003epartial least-squares path modeling\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd width=\"151\"\u003e\n\u003cp\u003eLMOWAs\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd width=\"483\"\u003e\n\u003cp\u003elow-molecular weight acids\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompleting interests\u003c/h2\u003e \u003cp\u003eThe authors have no competing interests to declare.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThis work was supported by National Natural Science Foundation of China (No. 22494682, 42207268, 32471702), and Jiangsu (BK20220454).\u003c/p\u003e\u003ch2\u003eAuthor Contributions\u003c/h2\u003e \u003cp\u003eHong-Yan Wang designed the incubation experiment; Guo-Hao Zhang conducted incubations, measurements and results analysis, Guo-Hao Zhang conducted the writing with conceptual input of Hong-Yan Wang; Jun-Jie Lin, Guo-Xin Sun, Zheng Chen and Zhi-Guo Yu edited and reviewed this paper.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eAuthors thanks for the help from Yaqin Wang for As speciation analysis. This work was supported by National Natural Science Foundation of China (No. 22494682, 42207268, 32471702), and Jiangsu (BK20220454).\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e \u003cp\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eAftabtalab A, Rinklebe J, Shaheen SM, Niazi NK, Moreno-Jim\u0026eacute;nez E, Schaller J, Knorr K-H (2022) Review on the interactions of arsenic, iron (oxy)(hydr)oxides, and dissolved organic matter in soils, sediments, and groundwater in a ternary system. 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Environ Res 249:118421. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.envres.2024.118421\u003c/span\u003e\u003cspan address=\"10.1016/j.envres.2024.118421\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":true,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"plant-and-soil","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plso","sideBox":"Learn more about [Plant and Soil](https://www.springer.com/journal/11104)","snPcode":"11104","submissionUrl":"https://submission.nature.com/new-submission/11104/3","title":"Plant and Soil","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Paddy soil, Root exudates, Sulfate reduction, Fe (oxy)hydroxides transformation, Methylated As","lastPublishedDoi":"10.21203/rs.3.rs-6692895/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6692895/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003eBackground and Aims\u003c/h2\u003e \u003cp\u003eRoot exudates have been proposed to influence arsenic (As) mobility and speciation in paddy soils. However, how sulfur-rich conditions interacting with root exudates affect iron (Fe)-sulfur-driven As mobilization and speciation remains unclear.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eAs speciation and Fe mineralogy were characterized in an 28-day anoxic microcosm experiment using paddy soils, in which typical root exudates including citric acid, oxalic acid and glucose were introduced under under varying sulfate concentrations of 0 \u003cem\u003emM\u003c/em\u003e, 5 \u003cem\u003emM\u003c/em\u003e and 25 \u003cem\u003emM\u003c/em\u003e.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eAll root exudates accelerated the reduction of Fe (oxy)hydroxides, and a corresponding increase in As concentrations of 11.6\u0026ndash;46.3% in pore water of paddy soils. Oxalic acid had the strongest promoting effect, followed by citric acid and glucose. Although sulfate reduction further enhanced Fe (oxy)hydroxides dissolution, the concurrent formation of Fe sulfides sequestrated a portion of released As, resulting in 4.6%-22.5% lower pore water As compared to exudate alone. Elevated sulfide fluxes promoted greater As immobilization via Fe sulfides formation. Compared to the sulfate-free treatments, sulfate reduction facilitated the formation of dimethylarsenic and dimethylmonothioarsenate by 52.6-127.5% and 14.3\u0026ndash;99.9%, respectively.\u003c/p\u003e\u003ch2\u003eConclusions\u003c/h2\u003e \u003cp\u003eRoot exudates can enhance As bioavailability in the rhizosphere by promoting its mobilization. In contrast, sulfate reductions may partially counteract As release through incorporating into sulfide minerals. The extent of As immobilization via sulfate reduction appears to depend on sulfate fluxes. This study highlights the critical role of sulfate reduction in regulating As mobilization and speciation in rhizosphere soils.\u003c/p\u003e","manuscriptTitle":"Dynamics of sulfate reduction regulate arsenic mobilization and speciation in paddy soils in response to root exudates","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-06-20 17:02:05","doi":"10.21203/rs.3.rs-6692895/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Major revisions","date":"2025-08-17T04:54:55+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"","date":"2025-06-18T03:25:07+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-06-18T03:06:55+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Plant and Soil","date":"2025-05-23T06:16:20+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-05-23T06:05:42+00:00","index":"","fulltext":""},{"type":"submitted","content":"Plant and Soil","date":"2025-05-22T08:20:59+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"plant-and-soil","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"plso","sideBox":"Learn more about [Plant and Soil](https://www.springer.com/journal/11104)","snPcode":"11104","submissionUrl":"https://submission.nature.com/new-submission/11104/3","title":"Plant and Soil","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"5d5b9c4c-a647-4fc6-b854-f46dbed9b1de","owner":[],"postedDate":"June 20th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-01-05T15:58:54+00:00","versionOfRecord":{"articleIdentity":"rs-6692895","link":"https://doi.org/10.1007/s11104-025-08220-w","journal":{"identity":"plant-and-soil","isVorOnly":false,"title":"Plant and Soil"},"publishedOn":"2026-01-02 15:57:01","publishedOnDateReadable":"January 2nd, 2026"},"versionCreatedAt":"2025-06-20 17:02:05","video":"","vorDoi":"10.1007/s11104-025-08220-w","vorDoiUrl":"https://doi.org/10.1007/s11104-025-08220-w","workflowStages":[]},"version":"v1","identity":"rs-6692895","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6692895","identity":"rs-6692895","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}