Beyond inorganic carbon: Soil organic carbon as key pathway for carbon sequestration in Enhanced Weathering

Beyond inorganic carbon: Soil organic carbon as key pathway for carbon sequestration in Enhanced Weathering | 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 Article Beyond inorganic carbon: Soil organic carbon as key pathway for carbon sequestration in Enhanced Weathering Laura Steinwidder, Lucilla Boito, Patrick Frings, Harun Niron, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5672251/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 21 Jul, 2025 Read the published version in Global Change Biology → Version 1 posted You are reading this latest preprint version Abstract Enhanced weathering captures CO2 via two pathways: Carbonate formation and leaching of weathering products. Here, we look beyond those two pathways, identifying other CO2 sinks and sources. While processes such as clay formation or organic matter decomposition reduce the efficiency of enhanced weathering, organic matter stabilisation could contribute to C storage. In a 15 month mesocosm experiment including two different types of silicates (basalt and steel slag) inorganic CO2 sequestration indeed remained negligible (below 0.1 t CO2/ha) due to clay formation. Also organic matter decomposition increased in silicate amended treatments (basalt +0.9 and slag +1.1 t CO2/ha released), further lowering the CO2 removal efficiency of enhanced weathering. Other organic C pathways could however contribute substantially to C storage. Aggregate formation and the storage of C within them was promoted in silicate amended treatments (basalt +106 and slag +73 % organic C stored in aggregates >250μm). Next to that, the association of organic C to minerals was determined another possible organic C sink. These results underline the urge for reliable ways to quantify CO2 removal not only including inorganic but also organic carbon dynamics. Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation Biological sciences/Biochemistry/Biogeochemistry/Carbon cycle Earth and environmental sciences/Biogeochemistry/Carbon cycle Earth and environmental sciences/Solid Earth sciences/Geochemistry Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Soils store large amounts of carbon, making them a valuable C sink which could contribute to climate change mitigation efforts (IPCC, 2023a ; Lehmann et al., 2020 ; Soussana et al., 2019 ). While there have been many studies investigating soil organic carbon (SOC) sequestration over the past few decades (Basile-Doelsch et al., 2020 ; Cotrufo et al., 2013 ; Lehmann & Kleber, 2015 ), inorganic carbon (IC) sequestration for climate change mitigation is only gaining attention in recent years (Hartmann et al., 2013 ; Huang et al., 2024 ; Vicca et al., 2022 ). IC sequestration typically occurs during silicate weathering, when CO 2 is converted into bicarbonate (HCO 3 − ) (Eq. 1, simplified example of Dunite weathering). The bicarbonate then either precipitates in the soil as pedogenic carbonates, or leaches out, eventually ending up in the ocean. Either way, the captured CO 2 is trapped for thousands of years, forming a stable carbon sink (Hartmann et al., 2013 ). Mg 2 SiO 4 + 4 CO 2 + 4 H 2 O -> 2 Mg 2+ + 4 HCO 3 − + H 4 SiO 4 (1) Enhanced weathering (EW) as a CO 2 removal technology aims to speed up the natural weathering process by finely grinding silicate rocks before applying them to soils. Common silicates for EW include basalt, due to its high abundance and low heavy metal contents (Lewis et al., 2021 ). By-products of industrial processes such as steel slags are also suitable for EW: they are not only available in large amounts but also weather comparably rapid while not requiring any mining activity (Renforth, 2019 ). Estimations of the inorganic carbon removal potential of EW range from 0.5-4 Gt CO 2 /year (Beerling et al., 2020 ; Fuss et al., 2018 ; IPCC, 2023b) to up to 95 Gt CO 2 /year (Strefler et al., 2018 ; Taylor et al., 2016 ), highlighting the large uncertainty still present in measurement procedures and global extrapolations. While much effort is directed at assessing IC removal more precisely, the impact of EW on SOC cycling has been largely overlooked. With over 90% of the global soil CO 2 fluxes originating from organic processes (3.7 and 0.3 Pg CO 2 /year from organic and inorganic fluxes (IPCC, 2023a )), disregarding its role could be detrimental to climate change mitigation efforts. Emissions from SOC could completely offset IC sequestration by EW. Enhanced SOC stabilisation, on the contrary, could further increase C sequestration. Nevertheless, carbon capture by EW is typically calculated from mineral weathering rates (e.g. Amann et al., 2020 ; Kelland et al., 2020 ), therefore considering only inorganic carbon fluxes and neglecting the potentially much larger fluxes associated with organic carbon. Soil mineral amendments, whether carbonates for liming or silicates for EW, alter key soil properties (e.g. pH and nutrient availability) which are known to control SOC dynamics. Increased dissolved organic carbon concentrations in pore water and in leachates have been reported post-amendment in both, liming (Ahmad et al., 2013 ) and EW (Vienne et al., 2024 ) experiments, confirming the impact of mineral amendments on SOC cycling. Yet, few studies have explicitly investigated the effects of EW on SOC dynamics (Buss et al., 2023 ; Klemme et al., 2022 ; Niron et al., 2024 ; Vienne et al., 2024 ; J. Xu et al., 2024 ; T. Xu et al., 2024 ; Yan et al., 2023 ). Buss et al. ( 2023 ) showed that SOC content was higher in silicate amended treatments than control treatments in a six-month pot experiment. In agreement with this, Vienne et al. ( 2024 ) found indications for reduced decomposition with basalt addition, albeit only in the absence of earthworms. Conversely, the modelling study of Klemme et al. ( 2022 ), as well as the experimental studies of Yan et al. ( 2023 )d Xu et al. ( 2024 ) suggest that silicate amendment could trigger CO 2 release by stimulating SOC decomposition. These inconsistent results of models and experiments investigating SOC responses to EW are not entirely surprising. First, the SOC cycle contains a high degree of complexity. It involves a variety of processes with potentially opposing effects. For example, a silicate induced pH increase could increase SOM decomposition (Wang & Kuzyakov, 2024 ), while the active weathering process could simultaneously promote physico-chemical stabilisation of SOC (Kleber et al., 2021 ). Second, varying experimental conditions further contribute to the uncertainty. Especially the presence or absence of plants could alter the outcome of experiments, as they are major drivers of both, the soil weathering process (Ibarra et al., 2019 ; Schwartzman & Volk, 1989 ) and SOC dynamics (Basile-Doelsch et al., 2020 ; Liang et al., 2017 ). Considering plant-soil interactions is therefore key to better understand EW-SOC interactions. Here, we conducted a 15-month mesocosm experiment with basalt and steel slag to test the effects of EW on IC and SOC dynamics in the presence and absence of plants. We hypothesised that SOC decomposition intensifies shortly after silicate application and that once a new steady-state in the SOC cycle is established, SOC stabilisation will be the dominant process. Results Evidence of weathering but limited inorganic CO 2 removal Silicate weathering produces alkalinity, thereby affecting pH, dissolved inorganic carbon (DIC) and cation concentrations in soil porewater (Eq. 1) . In a 15-month mesocosm experiment silicate amendment increased porewater pH ( Fig. 1 ) , DIC ( Fig. 2 ) and alkalinity (Figure S1) through weathering. The presence of plants amplified these effects. Also base cation concentrations in various soil pools (exchangeable, carbonate, (hydr)oxide and SOM) were affected by silicate application (Table S1 , Fig. 3 ) . Basalt amendment increased extractable Ca, Mg and Na concentrations, while BOF application increased extractable Ca concentrations (BOF slag does not contain any Na and only traces of K and Mg). No effect of maize was found. Interestingly, amounts of extractable base cations were highest at the beginning of the experiment (days 9 and 101) after which they decreased (day 435, Fig. 3 ). While there were some treatment effects in leachate base cation concentrations (Table S2) , this pool showed minimal potential for CO 2 removal. Even though an average volume of 66.4 L (± a standard error of 5.8 L) and 86.4 L (± 7.5 L) leached out of planted and unplanted treatments respectively, less than 0.5% of cations contained in the silicates were leached ( Fig. 3 ) . Decreases in soil extractable base cations can therefore not be explained by leaching, suggesting that the cations remained in the soil, where they must have transitioned to a more stable mineral pool as they could not be extracted anymore. These results indicate that primary mineral dissolution dominated during the first 101 days of the experiment, afterwards mineral formation became the prevailing process. Weathering rates (Table S3) and inorganic CO 2 removal ( Table 1 ) were estimated based on base cation release from applied silicates ( Fig. 3 ) . Inorganic CO 2 removal occurred during the first months of the experiment (until day 101) when primary mineral weathering was the dominant process. The subsequent decrease in extractable cations resulted in negative inorganic CO 2 removal (i.e. CO 2 release) over the remaining course of the experiment when secondary mineral formation was the dominating process. In this period, base cations became part of mineral lattices, not requiring bicarbonates for charge balance anymore. Table 1 Realised and potential future inorganic CO 2 removal in tons/ha ± 95% confidence intervals (calculations require the aggregation of replicates, confidence intervals are used to give an indication of statistical significances, statistical analysis of raw unaggregated data can be found in Table S1 ). ‘Total’ gives the CO 2 removal of the entire experimental period. The theoretical maximum CO 2 removal of basalt and BOF slag equals 19.45 and 4.60 tons CO 2 /ha respectively (with application rates of 50 tons basalt/ha and 5 tons BOF/ha). Basalt Day 0–9 t CO 2 /ha Day 9-101 t CO 2 /ha Day 101–435 t CO 2 /ha Total t CO 2 /ha unplanted Realised inorganic CO 2 removal Potential future inorganic CO 2 removal 0.28 1.52 0.38 0.40 -0.58 -0.13 0.08 ± 0.09 1.79 ± 1.04 planted Realised inorganic CO 2 removal Potential future inorganic CO 2 removal 0.28 1.52 0.06 0.51 -0.30 -1.14 0.04 ± 0.17 0.89 ± 0.88 BOF slag Day 0–9 t CO 2 /ha Day 9-101 t CO 2 /ha Day 101–435 t CO 2 /ha Total t CO 2 /ha unplanted Realised inorganic CO 2 removal Potential future inorganic CO 2 removal 0.02 0.03 0.22 1.64 -0.15 -1.01 0.07 ± 0.10 0.66 ± 1.01 planted Realised inorganic CO 2 removal Potential future inorganic CO 2 removal 0.02 0.03 0.16 2.82 -0.13 -1.74 0.04 ± 0.07 2.84 ± 0.80 Increased SOM decomposition further reduced CO 2 removal efficiency Depending on which silicate was applied, different responses in SOM decomposition were found. Dissolved organic carbon (DOC) concentrations in soil porewater increased with BOF slag application and remained high throughout the first growing season and shortly after. In contrast, basalt amendment did not significantly affect porewater DOC concentrations ( Fig. 4 ) . Consistent with the above results, increases in SOM decomposition in silicate amended treatments were especially clear for BOF treatments during the first growing season ( Fig. 5 , Fig. 6 ) . Deeper insight into SOM decomposition is provided by the δ 13 C signatures of soil CO 2 effluxes in unplanted treatments, which increased by 1.7 ± 0.5‰ (mean ± standard error) in BOF treatments, and by 1.1 ± 0.4‰ (not significantly) in Basalt treatments relative to the control ( Fig. 7 ) . Considering no plants are present, CO 2 exclusively originated from SOM decomposition. Changes in isotopic signatures thus suggest that silicate application triggered the release of SOC from pools which were not decomposed (to such a degree) in the control treatment. In agreement with these results, signatures of soil C at the end of the experiment were lower for the BOF treatment than for the control (Figure S2) . These results confirm that silicate application increased SOM decomposition shortly after application for BOF treatments. Effects in basalt treatments were smaller and remained unsignificant. Similar trends were found in SOC measurements, SOC stocks declined over time in BOF treatments, while basalt treatments maintained stable SOC levels (p = 0.077, Fig. 6 , bottom). Note that silicate application also increased root biomass (particularly during the first growing season, Figure S3 ), which is reflected in increased rhizosphere respiration ( Fig. 5 , Fig. 6 ) . Stabilised OC potentially dominating C sink during EW In soils, two major mechanisms are known to stabilise OC, both of which were evaluated here: The association of OC with minerals and the formation of stable aggregates and the storage of OC within them. OC bound to amorphous minerals was investigated via soil extractions. Here, no treatment effect was found in soil samples (Figure S4) , but expected changes are small and consequently difficult to detect. The same extractions were therefore also carried out on pure silicate samples recovered from mesh bags which were buried in the soil. These samples are independent of soil heterogeneity or application rates, thus yielding less ambiguous results. Here, a clear stabilising effect of BOF slag appeared: SOC bound to amorphous minerals increased by 129% (p < 0.001, 10.6 ± 0.4 mg C/g BOF slag at the start of the experiment vs 24.3 ± 0.4 mg C/g BOF slag at the end, no maize effect was found, Figure S5 ). No change was found in basalt samples. These findings suggest that the formation of mineral associated OC could be an alternative C sink to inorganic CO 2 removal (depending on the applied silicate). A size fractionation was conducted to investigate SOC accumulation in different aggregate classes. There was no silicate effect on SOC in the finest fraction, where the formation of mineral associated OC is expected (possibly due to soil heterogeneity). Silicate amendment, however, clearly stimulated the formation of aggregates and the storage of SOC within them (Fig. 8 , Table S5 ). Relative to control treatments, C stored in aggregates > 250 µm increased by 106% ( = + 1.6 ± 0.4 g C/kg soil) in basalt and 73% ( = + 1.1 ± 0.3 g C/kg soil) in BOF treatments, without significant effects of maize. Next to the formation of organo-mineral complexes, SOC sequestration in macroaggregates could therefore be another alternative C sink during EW. Discussion Attempts to quantify CO 2 removal by EW often implicitly assume that weathering products either end up in carbonates or leach out of the soil, eventually reaching the ocean. Here, we looked beyond those two pathways with the aim of identifying and quantifying other CO 2 sinks and sources relevant for EW. Our findings indeed suggest that current assessments overlook critical aspects of C removal. Carbonate formation and leaching were minor contributors to overall CO 2 removal. Less than 0.5% of added cations had leached out after 15 months, most of the cations were thus retained in the soil. Still, cations did not end up in carbonates. Instead, the majority was found on the exchangeable complex or in (hydr)oxides ( Fig. 3 ) , therefore not requiring the dissolution of CO 2 to ensure charge balance. The reduction in cation concentrations over time further suggests that cations slowly crystallised and became part of mineral lattices, rendering a de-sorption and therefore CO 2 sequestration within the next decades to centuries unlikely. Yet ‘potential’ inorganic CO 2 removal (i.e., estimates including changes in the exchangeable pool) is commonly estimated, relying on the assumption that cations will desorb at a later time and therefore remove CO 2 (e.g. in Kelland et al. ( 2020 ), Reershemius et al. ( 2023 ), Reynaert et al. ( 2023 ), Vienne et al. ( 2024 ), Niron et al. ( 2024 )). The possibility that these cations instead become part of crystalline phases – as suggested by our results – is usually not considered. Dietzen & Rosing ( 2023 ) also detected a decrease in exchangeable cations in their field study. They argued, however, that those cations had leached out and therefore removed CO 2 , but leachate concentrations to verify this were not available. Clay formation could provide an alternative explanation for such reductions. This underlines that the unclear fate of cations could lead to overestimations in inorganic CO 2 removals. For natural systems it is well known that increasing soil water solute concentrations lead to declining primary mineral dissolution rates as reaction affinity decreases and the formation of secondary minerals such as clays is stimulated (Blume et al., 2016 ; Maher et al., 2009 ). Given that weathering rates are several orders of magnitude larger with EW as compared to natural weathering, the formation of secondary mineral phases could be substantial. A close natural analogue to EW is subglacial grinding of bedrock to produce fine and highly reactive glacial flour (Blackburn et al., 2019 ; Hatton et al., 2021 ). Fresh subglacial material comprises to a large extent of amorphous Si, Al and Fe phases (Rampe et al., 2022 ), which are known to be precursors for more crystalline phases such as phyllosilicates (Baronas et al., 2021 ; Behrens et al., 2015 ), though the mechanisms remain understudied. There is also evidence for an increase in clay mineral content in glacial forefield chronosequences (Mavris et al., 2010 ), and evidence for an illuviation process immobilising cations (Zhou et al., 2016 ). This highlights the possibility that crystalline phases, likely aluminosilicate clay minerals, could act as a thus-far unconsidered cation sink. Such a maturation of amorphous phases, as suggested by our results, can seriously reduce the efficiency of inorganic CO 2 removal by EW. These low efficiencies in inorganic CO 2 removal underline the need for a broader understanding of C removal during EW. Our findings demonstrate that OC fluxes dominate in soils, even with application of EW. For example, changes in OC pools were ~ 20 times larger than changes in IC pools ( Fig. 6 ) . Clearly, effects of EW on SOC dynamics - whether positive or negative - need to be taken into account as they can act as additional C sinks or sources. Our hypothesis of increased SOM decomposition shortly after silicate application was indeed confirmed, leading to a further reduction in CO 2 removal efficiency. In fact, for BOF treatments, increases in decomposition released more CO 2 than weathering removed (+ 1.8 t CO 2 /ha released vs 0.08 t CO 2 /ha removed in unplanted treatments, + 1.1 t CO 2 /ha released vs 0.04 t CO 2 /ha removed in planted treatments, Fig. 6 ). While also basalt treatments showed some indication of increased decomposition ( Fig. 6 , Fig. 7 ) trends were less clear. Thus, depending on which silicate is applied different responses in SOM decomposition can be expected. Soil pH is known to be a major driver of SOM decomposition as it controls microbial community composition and activity, and drives SOM (de)stabilisation mechanisms (Wang & Kuzyakov, 2024 ). The stronger pH increase in BOF as compared to basalt treatments ( Fig. 1 ) might explain the differing responses. Indeed, experimental addition of wollastonite to different soil types showed that the greater the pH increase, the larger the SOM decomposition (Yan et al., 2023 ). SOC losses thus seem especially likely with larger pH increases. While these pH-driven increases in SOM decomposition lower the efficiency of EW, it should be taken into account that in an agricultural setting silicate amendments would likely replace liming, a common practice used to raise soil pH. Schroeder et al. ( 2024 ) suggested that also with liming, the microbial response depends on the initial soil pH and the magnitude of the pH shift. Thus, lime application could likely lead to similar responses in SOM decomposition as silicate application, while in many cases no CO 2 would be removed via weathering. In contrast to SOM decomposition, SOM stabilisation via mineral association and aggregate formation provided alternative C sinks for EW. In fact, contributions of these processes are potentially substantial, especially in the longer term. Weathering of primary minerals facilitates the formation of highly reactive secondary clay-sized minerals such as phyllosilicates, metal (hydr)oxides and aluminosilicates, which influence SOC stabilising processes (Basile-Doelsch et al., 2015 ; Totsche et al., 2018 ). Soil cation extractions indeed indicated the formation of (hydr)oxides and we even suspect the formation of other reactive clay-sized secondary minerals, suggesting our hypothesis regarding increased SOM stabilisation in the long-term could be correct. Similar to SOM decomposition, also SOM stabilisation appeared to depend on the type of silicate that was applied. While OC stabilisation in BOF samples more than doubled, OC levels in basalt samples remained unchanged. Decision making prosses regarding the applied silicate type should therefore also take into account effects on SOC dynamics and not only the materials potential for inorganic C removal. Aggregate formation and the storage of C within them was clearly increased regardless of silicate type. While in tilled agricultural soils the durability of this C sink is questionable (Lavallee et al., 2020 ), sustainable farming practices applying no or reduced tillage could greatly benefit from improved soil aggregation and increased SOC levels (Angst et al., 2023 ). While these results suggest that OC could actually be the central C sink during EW it should also be noted that the evolution of those so far understudied processes are still relatively unclear in terms of long-term development and durability. In summary, inorganic processes played a minor role in CO 2 removal. SOM decomposition further lowered removal efficiencies. However, increased root inputs, MAOM formation and the formation of large aggregates are alternative pathways indirectly contribute to CO 2 sequestration. In our experiment, the contribution of these pathways was substantial. Although the impact of EW on SOC is clearly relevant, its long-term impact is yet to be verified. These results underline the need for a better understanding of long-term SOC dynamics in response to EW and for reliable ways to quantify CO 2 removal considering the soil-plant system holistically. The estimation of inorganic CO 2 removals – as commonly done - could lead to unrealistic predictions since it neither takes into account the possibility of secondary mineral formation nor the effects of EW on SOC. Methods Experimental set-up Mesocosms with a diameter of 50 cm and a height of 60 cm were equipped with a leaching system and established outside at experimental sites of the University of Antwerp (51°09'41.8"N, 4°24'30.8"E), where they were exposed to local weather conditions. The experiment started on 1st of June 2022 and lasted for 435 days (~ 15 months). The regional climate is temperate maritime with mild winters and warm summers (Kottek et al., 2006 ). In 2022, the annual mean temperature was 12.2°C with an annual precipitation of 701.4 mm, in 2023 the annual mean temperature was 12.1°C and annual precipitation was 1011.4 mm. While 2022 was exceptionally warm and dry, 2023 was exceptionally warm and wet (Royal Meteorological Institute Belgium, 2024 ). Mesocosms were filled with a sandy loam soil (69.5% sand, 28.1% silt, 1.8% clay) originating from a pasture in Zandhoven, Belgium. According to the Belgian soil map the soil is categorised as a plaggic anthrosol (Dondeyne et al., 2014 ). The soil was slightly acidic (pH = 5.5 ± 0.3) and low in SOC (0.8 ± 0.05% with a δ 13 C of -25.1 ± 0.36‰), characteristics typical for agricultural soils. At the start of the experiment, the soil had a cation exchange capacity of 3.92 ± 0.64 meq/100g, base saturation 66.62 ± 0.14%, electrical conductivity 84.78 ± 17.32 µS/cm, total N 0.07 ± 0.02% and bulk density 1.4 ± 0.02 g/cm 3 . Silicate amendments were mixed into the top 20 cm of the soil using a cement mixer to ensure an even distribution. Deeper soil layers (20–60 cm depth) therefore did not initially contain any amendment. The application of two different types of silicates was tested: basalt (type: durubas, obtained from RPBL, Germany), and basic oxygen furnace (BOF, obtained from ArcelorMittal, Belgium) slag, an artificial silicate produced as a by-product of steel production (supplementary section 4.1) . Application rates of basalt and BOF slag were 50 and 5 t/ha respectively. BOF slag was applied at a much lower application rate due to its unstable structure and therefore extremely high weathering rates. In total, six treatments were established. These included three different types of amendments (BOF, basalt, no amendment), each present as unplanted and planted treatment. During growing seasons (June-September 2022 and June-August 2023) 2 maize plants ( Zea mays ) were sown in planted mesocosms. Due to seed unavailability a different variety (with very similar traits) was used in the second growing season (first season golden midget , second season tom thumb ). The growing seasons lasted for 3–4 months, typical for these maize varieties. Before sowing, limited amounts of N, P and K fertilisers (NH 4 NO 3 : 270 kg/ha, P 2 O 5 : 20 kg/ha and K 2 SO 4 : 90 kg/ha) were added to each mesocosm. During dry spells in summer, mesocosms were irrigated with rainwater collected in belowground tanks to avoid severe drought stress. Outside of the growing seasons, all pots were left bare. The experiment started with 42 pots (six replicates for bare soil treatments, eight replicates for planted treatments). At the end of the first growing season half of the replicates of each treatment were sampled destructively and therefore removed from the experiment. The experiment thus continued with 21 pots (three replicates for bare soil treatments, four replicates for planted treatments) after the first growing season ended. Analyses of plants After each growing season, aboveground biomass was cut and dried at 70°C for 48 h. Tassle, stem, leaves and corn were weighed separately. Ground maize leaves and corn were analysed for Ca, Mg, K, Al and Fe concentrations via digestion with 70% HNO 3 (60°C for 30 minutes) and 30% H 2 O 2 after which samples were heated to 120°C for 90 minutes. After cooling and filtering concentrations were measured via ICP-OES (Thermo Scientific, iCAP 6300 duo). To estimate belowground biomass, soil cores (one directly below each plant and one in between) were taken at three different depths (0–20 cm, 20–40 cm and 40–60 cm). Soil was then washed off with tap water on a sieve with 1 mm mesh-size and remaining roots were picked out and dried at 70°C for 48 h and weighed. Root biomass was estimated assuming that samples taken below plants reflect root growth in half of the mesocosm, while samples taken between plants reflect the other half. Soil sequential extractions To unravel organic and inorganic soil C dynamics, two different types of soil sequential extractions were carried out, each at three points in time (day 9 = before planting, day 101 = end of first growing season, day 435 = end of second growing season/end of experiment). Sequential extractions adapted from Tessier et al. ( 1979 ) and Uhlig and Von Blanckenburg ( 2019 ) aim at quantifying the cation content of different soil pools, which can be used for weathering rate calculations and inorganic CO 2 removal estimations (Niron et al., 2024 ; Vienne et al., 2024 ). Cation extractions were carried out in four extraction steps ( Table 2 , supplementary section 4.2) on 0.5 g of ground and air-dried soil. After each step cations (Mg, Ca, Na, K, Fe, Al) plus Si concentrations in extractant solution were quantified on a Varian 720-ES or SpectroGreen ICP-OES following the analytical protocol of (Schuessler et al., 2016 ). Table 2 Four different extraction steps of cation extractions including definition of targeted pools Extract Chemical Heat bath Rotator Targeted Pool 1 1 M Ammonium acetate (CH 3 COONH 4 ) 1 h (15 rpm) Exchangeable cations 2 1 M Acetic acid (CH 3 COOH) 2 h (15 rpm) Cations in carbonates 3 0.05 M Hydroxylamine (NH 2 OH) in 0.5 M HCl 5 h (80°C) Invert manually (every 30 min) Cations in (hydr)oxides 4 9.8 M Hydrogen peroxide (H 2 O 2 ) in 0.01 M HNO 3 5 h (70°C) Invert manually (every 30 min) Cations in SOM Sequential extractions adapted from Heckman et al. ( 2018 ) focus on mineral associated organic matter and were caried out on 1 g of ground and air-dried soil. The sum of both extraction steps is assumed to represent SOC bound to amorphous minerals ( Table 3 , supplementary section 4.2) . Dissolved organic C concentrations in extractant solutions were determined using a continuous flow analyser (Skalar, SAN++). Extractions were also carried out on pure silicate samples recovered from mesh bags (0.325 mm mesh size, thus roots can grow inside) which were buried in silicate amended mesocosms at the beginning of the experiment. Table 3 Two different steps of MAOM extractions including definition of targeted pools Step Chemical Rotator Targeted Pool 1 0.1 M Sodium pyrophosphate (Na 4 P 2 O 7 ) 16 h SOC bound to amorphous minerals 2 0.1 M Hydroxylamine (NH 2 OH) in 0.25 M HCl 16 h Wet sieving To quantify the amount of SOC in different aggregate size classes (similar to Six et al. ( 1998 )) undisturbed soil core samples were taken at 10 cm depth. Air dried samples were submerged in water and set aside for 5 minutes for slaking. Samples (about 100 g each) were then passed through various sieves (2mm, 250 µm and 63 µm) stacked on top of each other using a shaker and 500 mL of deionised water to wash the samples through. After the fractionation, samples were dried and SOC contents determined via loss on ignition (550°C for 4 hours). The fraction > 2 mm comprised mostly small stones and a few larger aggregates. This fraction contributed only 1.05 ± 0.63% to the total sample mass and was therefore considered negligible. Collection and analysis of soil pore water and leachates Porewater samples were collected 20 times throughout the experimental period via Rhizon flex samplers (Rhizosphere Research Products) installed at 5–10 cm depth. Leachates were collected 16 times via glass bottles connected to the bottom of the mesocosms. After collection a SpectroGreen ICP-OES was used to quantify Ca, Mg, K, Na, Al, Fe and Si concentrations. Alkalinity was measured on a continuous flow analyser (Skalar, SAN++), dissolved inorganic and organic carbon with a TOC analyser (Skalar, Formacs) and pH with a pH/Conductometer at 25°C (914 Metrohm). Measurement and analysis of soil CO 2 efflux The soil CO 2 efflux (in ppm/s) and its isotopic signature δ 13 C (in ‰ relative to Vienna Pee Dee Belemnite) was measured 22 times throughout the experiment with a cavity ring-down spectroscopy analyser (Picarro, G2131-i). To do so, a custom built chamber connected to the analyser was positioned on stainless steel collars which were inserted into the surface of each mesocosm at the beginning of the experiment. For each measurement, the CO 2 concentration was monitored for 7–15 minutes, aiming for an increase of at least 100 ppm (to minimise the error on C source partitioning). Soil CO 2 efflux was calculated from the rate of CO 2 increase over time. To partition the flux into rhizosphere respiration and SOM decomposition two-pool mixing models ( Eq. 2 , supplementary section 4.3) were used. First, the δ 13 C of the CO 2 efflux was calculated as the intercept of a linear regression between measured δ 13 C and inverse measured CO 2 (Keeling, 1958 ). δ 13 C of maize roots was measured by an elemental analyser coupled to an isotope ratio mass spectrometer (EA-IRMS), δ 13 C of SOM obtained from flux measurements of unplanted treatments. $$\:{f}_{\text{r}\text{h}\text{i}\text{z}\text{o}}\:=\:\frac{({{\delta\:}}^{13}\text{C}\:{\text{C}\text{O}}_{2}\:\text{e}\text{f}\text{f}\text{l}\text{u}\text{x}\:-\:{{\delta\:}}^{13}\text{C}\:\text{S}\text{O}\text{M})}{({{\delta\:}}^{13}\text{C}\:\text{r}\text{o}\text{o}\text{t}\text{s}\:-\:{{\delta\:}}^{13}\text{C}\:\text{S}\text{O}\text{M})}$$ 2 After determining the fraction f rhizo accounting for rhizosphere processes, the total CO 2 efflux can be partitioned into CO 2 originating from plants versus SOM (Eqs. 3 and 4). Rhizosphere respiration = \(\:\:{f}_{\text{r}\text{h}\text{i}\text{z}\text{o}}\text{*}\:\text{s}\text{o}\text{i}\text{l}\:{\text{C}\text{O}}_{2}\:\text{e}\text{f}\text{f}\text{l}\text{u}\text{x}\) ( 3 ) SOM decomposition = \(\:{(1-f}_{\text{r}\text{h}\text{i}\text{z}\text{o}})\:\text{*}\:\text{s}\text{o}\text{i}\text{l}\:{\text{C}\text{O}}_{2}\:\text{e}\text{f}\text{f}\text{l}\text{u}\text{x}\) ( 4 ) These calculations assume no contribution of inorganic processes (weathering and carbonate formation). Given that inorganic CO 2 fluxes in our system were minor compared to organic CO 2 fluxes (see Fig. 6 ), we consider their influence on the CO 2 flux partitioning negligible. Weekly/bi-weekly soil CO 2 efflux measurements were used to reconstruct daily soil CO 2 effluxes based on daily soil temperature and soil water content measurements, with the aim of calculating cumulative fluxes. To this end, the model of Eq. 5 (following Vicca et al. (2014)) was fit separately for each treatment (Figure S8, Figure S9) . Daily CO 2 effluxes were then predicted based on the daily soil temperature and soil water content of each mesocosm and cumulated over time. log 10 (soil CO 2 efflux) = a + b * soil temp + c * SWC + d * SWC 2 ( 5 ) Calculation of inorganic CO 2 removal Realised inorganic CO 2 removal was calculated considering leached out cations and cations bound in carbonates ( Table 4 ) . To this end, the change in cation concentration in those two pools since the start of the experiment was expressed relative to the respective control treatment and converted to ton CO 2 /ha (Niron et al., 2024 ; Vienne et al., 2024 ). Note, that we only consider the change in cation concentrations over time instead of using absolute values. Silicate amended treatments had higher cation concentrations even before the weathering process started, as extractions also remove cations from the silicates themselves. By focussing on the change over time, we avoid taking into account these cations. Cation concentration in the silicate amended layer (0–20 cm) and below (20–40 and 40–60 cm) were included in the calculations. Cations bound in carbonates were assumed to only sequester half of the originally formed bicarbonates, since 50% of the CO 2 is released during carbonate formation. A correction for downstream losses (during fluvial transport, due to ocean chemistry, etc.) was not considered necessary since the exported alkalinity flux in leachates was negligible compared to within-soil storage. Cations in the soil exchangeable pool and in the SOM pool are considered as potential inorganic CO 2 removal, happening in the near future and therefore relevant for climate change mitigation. Once cations are released back into the soil solution their charge will be counterbalanced by HCO 3 − , therefore capturing CO 2 . Note that, the SOM cation pool is thus considered inorganic CO 2 removal. The amount of cations in this pool does not directly correspond to a certain amount of stabilised SOC. Therefore, this pool does not quantify organic C stabilisation. The processes of cation uptake by plants and (hydr)oxide formation is considered CO 2 neutral, and is therefore not included in CO 2 removal calculations. Maize harvest would lead to the removal of cations which would thus not be returned to the soil solution where they would be charge-balanced by HCO 3 − . (Hydr)oxide weathering could lead to CO 2 sequestration, if cations are released again, but this process is expected to happen at very slow pace (Tessier et al., 1979 ; Uhlig & Von Blanckenburg, 2019 ) and is therefore considered negligible in a climate change mitigation context. Weathering rate calculations, on the contrary, consider all cation pools (leachate, soil and plant pools, supplementary section 4.4 ) Table 4 Definition of cation pools and their effect on inorganic CO 2 removal Cation pool Inorganic CO 2 removal Leachate Realised removal Carbonate Exchangeable Potential future removal SOM Hydr(oxide) No removal Plant Statistical analyses As most measurements were repeated over time, linear mixed models with mesocosm as a random effect and time as a fixed effect were used to test for the effects of silicate addition (no amendment, basalt, BOF slag), maize (unplanted, planted) and their interactions. Interaction effects between silicate-maize, silicate-time and maize-time were tested and included in the final model if statistically significant. For each analysis, normality and homoscedasticity were assessed visually as well as by the Shapiro-Wilk and Levene’s test. In the few cases where the assumptions were invalid, a log or square root transformation was used. All statistical tests were conducted in R, using lme4 (Bates et al., 2015 ), pbkrtest (Halekoh & Højsgaard, 2014 ), lmerTest (Kuznetsova et al., 2017 ) and emmeans (Lenth, 2024 ). Weathering and inorganic CO 2 removal rates, as well as cumulated CO 2 effluxes required the aggregation of replicates, therefore yielding one value per treatment. To evaluate differences between treatments 95% confidence intervals were calculated from standard errors: non-overlapping confidence intervals were considered indicative of a significant difference between treatments. Additionally, statistical analyses using linear mixed models (as specified above) were carried out on raw unaggregated datasets. 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Geoderma , 267 , 78–91. https://doi.org/10.1016/j.geoderma.2015.12.024 Additional Declarations There is NO Competing Interest. Supplementary Files Supplementary.pdf Supplementary GA.png Graphical abstract Cite Share Download PDF Status: Published Journal Publication published 21 Jul, 2025 Read the published version in Global Change Biology → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-5672251","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":394647906,"identity":"c804e102-6fc3-4825-8aeb-28aa1a7cda0d","order_by":0,"name":"Laura Steinwidder","email":"data:image/png;base64,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","orcid":"","institution":"Biobased Sustainability Engineering (SUSTAIN), Department of Bioscience Engineering, University of Antwerp","correspondingAuthor":true,"prefix":"","firstName":"Laura","middleName":"","lastName":"Steinwidder","suffix":""},{"id":394647907,"identity":"0ed60afb-85ce-46e1-85e0-f160c3d052f9","order_by":1,"name":"Lucilla Boito","email":"","orcid":"","institution":"University of Antwerp","correspondingAuthor":false,"prefix":"","firstName":"Lucilla","middleName":"","lastName":"Boito","suffix":""},{"id":394647908,"identity":"e0da670a-dd3f-4207-b6d0-2b2da476bf15","order_by":2,"name":"Patrick Frings","email":"","orcid":"","institution":"German Research Centre for Geosciences (GFZ)","correspondingAuthor":false,"prefix":"","firstName":"Patrick","middleName":"","lastName":"Frings","suffix":""},{"id":394647909,"identity":"0863710d-bb74-4944-af6f-00659c5caa0c","order_by":3,"name":"Harun Niron","email":"","orcid":"","institution":"University of Antwerp","correspondingAuthor":false,"prefix":"","firstName":"Harun","middleName":"","lastName":"Niron","suffix":""},{"id":394647910,"identity":"cde55a9b-f777-46e3-bbbf-3afac92e85e1","order_by":4,"name":"Jet Rijnders","email":"","orcid":"","institution":"University of Antwerp","correspondingAuthor":false,"prefix":"","firstName":"Jet","middleName":"","lastName":"Rijnders","suffix":""},{"id":394647911,"identity":"5852165f-5093-43a7-a322-748964218a9e","order_by":5,"name":"Anthony de Schutter","email":"","orcid":"","institution":"KU Leuven","correspondingAuthor":false,"prefix":"","firstName":"Anthony","middleName":"","lastName":"de Schutter","suffix":""},{"id":394647912,"identity":"495ca75c-49eb-491d-a56d-548753844123","order_by":6,"name":"Arthur Vienne","email":"","orcid":"","institution":"University of Antwerp","correspondingAuthor":false,"prefix":"","firstName":"Arthur","middleName":"","lastName":"Vienne","suffix":""},{"id":394647913,"identity":"3fcabfd4-5445-4265-953d-d3ab1380cc74","order_by":7,"name":"Sara Vicca","email":"","orcid":"https://orcid.org/0000-0001-9812-5837","institution":"University of Antwerp","correspondingAuthor":false,"prefix":"","firstName":"Sara","middleName":"","lastName":"Vicca","suffix":""}],"badges":[],"createdAt":"2024-12-18 22:00:20","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5672251/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5672251/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1111/gcb.70340","type":"published","date":"2025-07-22T00:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":75015978,"identity":"6363099e-bef3-40d4-b068-4eb022e2249a","added_by":"auto","created_at":"2025-01-29 12:36:16","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":30772,"visible":true,"origin":"","legend":"\u003cp\u003eEffect of silicate addition on soil porewater pH (collected from 10 cm depth). Values represent the average of all pH measurements taken throughout the entire experimental period. Error bars represent standard errors.\u003cstrong\u003e \u003c/strong\u003eP-values \u0026lt;0.05, \u0026lt;0.01 and \u0026lt;0.001 are represented by *, ** and ***. Values below p-values represent the change relative to control (for silicate effect) and unplanted (for maize effect) treatments.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/54bc91d8204f401633cf254f.png"},{"id":75015609,"identity":"29889175-995b-498d-92c2-3ab9cd329ea1","added_by":"auto","created_at":"2025-01-29 12:28:15","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":62136,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal dynamics of dissolved inorganic carbon (DIC) concentrations in porewater of topsoil. Grey boxes indicate 1\u003csup\u003est\u003c/sup\u003e and 2\u003csup\u003end\u003c/sup\u003e growing season; error bars display standard errors; p-values \u0026lt;0.05, \u0026lt;0.01 and \u0026lt;0.001 are represented by *, ** and ***; values below p-values represent the change relative to control (for silicate effect) and unplanted (for maize effect) treatments.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/758a10119d6c830e6dae366c.png"},{"id":75015979,"identity":"0cfcd6d7-b3eb-43ef-8c6c-162ee9f79b59","added_by":"auto","created_at":"2025-01-29 12:36:16","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":75189,"visible":true,"origin":"","legend":"\u003cp\u003eChange in extractable base cation levels (Ca, Mg, K, Na) at three points in time relative to the respective control treatment. Beginning of the experiment = day 9, end of first growing season = day 101, end of second growing season = day 435. The total amount of cations added with silicate application is set to equal 100% (corresponding to 8.87 mol total alkalinity/mesocosm for basalt and 2.05 mol total alkalinity/mesocosm for BOF treatments). Leachate and plant pools are not visible due to low concentrations. Error bars show 95% confidence intervals of cations realising inorganic CO\u003csub\u003e2\u003c/sub\u003e removal (CDR, i.e., leachates + carbonate pool) and cations realising CDR + cations contributing to potential future CDR (i.e., leachates + carbonate + exchangeable + SOM pools). Confidence intervals of each pool are provided in \u003cstrong\u003eTable S4 \u003c/strong\u003e(calculations require the aggregation of replicates, lack of overlap of 95% confidence intervals is assumed indicative for a significant difference; statistical analysis of raw unaggregated data can be found in \u003cstrong\u003eTable S1\u003c/strong\u003e).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/2a302d2f93b962517acae0a3.png"},{"id":75015623,"identity":"a2cfca09-ffc8-49e5-a4a0-405ba4aba048","added_by":"auto","created_at":"2025-01-29 12:28:16","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":57336,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal dynamics of dissolved organic carbon (DOC) concentrations in porewater of topsoil. Grey boxes indicate 1\u003csup\u003est\u003c/sup\u003e and 2\u003csup\u003end\u003c/sup\u003e growing season; error bars display standard errors; p-values \u0026lt;0.05, \u0026lt;0.01 and \u0026lt;0.001 are represented by *, ** and ***; values below p-values represent the change relative to control (for silicate effect) and unplanted (for maize effect) treatments.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/46a2d4acb5c6d8ccc8e15c62.png"},{"id":75015614,"identity":"33a3b2c9-fd27-481b-ad53-13b81697d19c","added_by":"auto","created_at":"2025-01-29 12:28:16","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":102950,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal dynamics of soil CO\u003csub\u003e2\u003c/sub\u003e effluxes (SCE) partitioned into rhizosphere respiration (darker colour) and SOM decomposition (lighter colour). For this figure measured values were interpolated, each measured value is shown including standard error; p-values \u0026lt;0.05, \u0026lt;0.01 and \u0026lt;0.001 are represented by *, ** and ***; values below p-values represent the change relative to the control treatment.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/99007e9a70e40387076723db.png"},{"id":75015631,"identity":"6fab9ded-f4c7-42ad-8f46-9cd16c6537e1","added_by":"auto","created_at":"2025-01-29 12:28:17","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":65183,"visible":true,"origin":"","legend":"\u003cp\u003eTop panel: cumulative soil CO\u003csub\u003e2\u003c/sub\u003e fluxes, partitioned into rhizosphere respiration, SOM decomposition and realised inorganic CO\u003csub\u003e2\u003c/sub\u003e removal due to EW. Error bars display 95% confidence intervals (calculations require the aggregation of replicates, 95% confidence intervals are used to give an indication of statistical significances, unaggregated raw data can be found in \u003cstrong\u003eFigure 5\u003c/strong\u003e). Bottom panel: changes in soil C. Inorganic C changes are not visible due to low concentrations ranging from 0.1 to -0.06 t C/ha. Error bars display standard errors; Con=control, Bas=basalt, BOF=BOF slag.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/afd9d710445dbbee65dc4c41.png"},{"id":75015984,"identity":"0aad4305-554b-427b-ad53-b870acc261a1","added_by":"auto","created_at":"2025-01-29 12:36:17","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":31251,"visible":true,"origin":"","legend":"\u003cp\u003eTemporal dynamics of the\u003cstrong\u003e \u003c/strong\u003eδ\u003csup\u003e13\u003c/sup\u003eC signatures of soil CO\u003csub\u003e2\u003c/sub\u003e effluxes of unplanted treatments during the first growing season. Error bars display standard errors; p-values \u0026lt;0.05, \u0026lt;0.01 and \u0026lt;0.001 are represented by *, ** and ***; values below p-values represent the change relative to the control treatment.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/2b04c237d8c6c9419fb2bf60.png"},{"id":75015981,"identity":"2876a237-6816-43b7-833f-ce57d5ac3f8e","added_by":"auto","created_at":"2025-01-29 12:36:16","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":37995,"visible":true,"origin":"","legend":"\u003cp\u003eSOC content in each aggregate size class (\u0026gt;250, 250-63, \u0026lt;63 μm) at the end of the experiment. Grey lines display SOC concentrations at the beginning of the experiment. Error bars show standard errors of total SOC concentrations. Statistical results can be found in \u003cstrong\u003eTable S5.\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/23ba95a7aa46a595f5966b70.png"},{"id":87436596,"identity":"69b49fef-76bf-4b65-9dbd-4bbe86144ff0","added_by":"auto","created_at":"2025-07-23 18:48:20","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1420586,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/cd16308f-a3b3-4b51-a13b-70eedb4d5c46.pdf"},{"id":75015611,"identity":"26e00242-206c-4219-a5d4-2c9e774b8c48","added_by":"auto","created_at":"2025-01-29 12:28:16","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1030923,"visible":true,"origin":"","legend":"Supplementary","description":"","filename":"Supplementary.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/0720f78f28a45f5521893c6c.pdf"},{"id":75015977,"identity":"cf880545-8a13-4b83-8756-92ca37436b9d","added_by":"auto","created_at":"2025-01-29 12:36:15","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":116898,"visible":true,"origin":"","legend":"\u003cp\u003eGraphical abstract\u003c/p\u003e","description":"","filename":"GA.png","url":"https://assets-eu.researchsquare.com/files/rs-5672251/v1/d82c75af05bcb002a1d0c10a.png"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Beyond inorganic carbon: Soil organic carbon as key pathway for carbon sequestration in Enhanced Weathering","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSoils store large amounts of carbon, making them a valuable C sink which could contribute to climate change mitigation efforts (IPCC, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e; Lehmann et al., \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Soussana et al., \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). While there have been many studies investigating soil organic carbon (SOC) sequestration over the past few decades (Basile-Doelsch et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Cotrufo et al., \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Lehmann \u0026amp; Kleber, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), inorganic carbon (IC) sequestration for climate change mitigation is only gaining attention in recent years (Hartmann et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Huang et al., \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Vicca et al., \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). IC sequestration typically occurs during silicate weathering, when CO\u003csub\u003e2\u003c/sub\u003e is converted into bicarbonate (HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e) (Eq.\u0026nbsp;1, simplified example of Dunite weathering). The bicarbonate then either precipitates in the soil as pedogenic carbonates, or leaches out, eventually ending up in the ocean. Either way, the captured CO\u003csub\u003e2\u003c/sub\u003e is trapped for thousands of years, forming a stable carbon sink (Hartmann et al., \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMg\u003csub\u003e2\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;4 CO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;+\u0026thinsp;4 H\u003csub\u003e2\u003c/sub\u003eO -\u0026gt; 2 Mg\u003csup\u003e2+\u003c/sup\u003e + 4 HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e + H\u003csub\u003e4\u003c/sub\u003eSiO\u003csub\u003e4\u003c/sub\u003e \u003cb\u003e(1)\u003c/b\u003e\u003c/p\u003e \u003cp\u003eEnhanced weathering (EW) as a CO\u003csub\u003e2\u003c/sub\u003e removal technology aims to speed up the natural weathering process by finely grinding silicate rocks before applying them to soils. Common silicates for EW include basalt, due to its high abundance and low heavy metal contents (Lewis et al., \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). By-products of industrial processes such as steel slags are also suitable for EW: they are not only available in large amounts but also weather comparably rapid while not requiring any mining activity (Renforth, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2019\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eEstimations of the inorganic carbon removal potential of EW range from 0.5-4 Gt CO\u003csub\u003e2\u003c/sub\u003e/year (Beerling et al., \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Fuss et al., \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; IPCC, 2023b) to up to 95 Gt CO\u003csub\u003e2\u003c/sub\u003e/year (Strefler et al., \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Taylor et al., \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), highlighting the large uncertainty still present in measurement procedures and global extrapolations. While much effort is directed at assessing IC removal more precisely, the impact of EW on SOC cycling has been largely overlooked. With over 90% of the global soil CO\u003csub\u003e2\u003c/sub\u003e fluxes originating from organic processes (3.7 and 0.3 Pg CO\u003csub\u003e2\u003c/sub\u003e/year from organic and inorganic fluxes (IPCC, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2023a\u003c/span\u003e)), disregarding its role could be detrimental to climate change mitigation efforts. Emissions from SOC could completely offset IC sequestration by EW. Enhanced SOC stabilisation, on the contrary, could further increase C sequestration. Nevertheless, carbon capture by EW is typically calculated from mineral weathering rates (e.g. Amann et al., \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Kelland et al., \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), therefore considering only inorganic carbon fluxes and neglecting the potentially much larger fluxes associated with organic carbon.\u003c/p\u003e \u003cp\u003eSoil mineral amendments, whether carbonates for liming or silicates for EW, alter key soil properties (e.g. pH and nutrient availability) which are known to control SOC dynamics. Increased dissolved organic carbon concentrations in pore water and in leachates have been reported post-amendment in both, liming (Ahmad et al., \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) and EW (Vienne et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) experiments, confirming the impact of mineral amendments on SOC cycling. Yet, few studies have explicitly investigated the effects of EW on SOC dynamics (Buss et al., \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Klemme et al., \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Niron et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Vienne et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; J. Xu et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; T. Xu et al., \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Yan et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Buss et al. (\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) showed that SOC content was higher in silicate amended treatments than control treatments in a six-month pot experiment. In agreement with this, Vienne et al. (\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) found indications for reduced decomposition with basalt addition, albeit only in the absence of earthworms. Conversely, the modelling study of Klemme et al. (\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), as well as the experimental studies of Yan et al. (\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2023\u003c/span\u003e)d Xu et al. (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) suggest that silicate amendment could trigger CO\u003csub\u003e2\u003c/sub\u003e release by stimulating SOC decomposition.\u003c/p\u003e \u003cp\u003eThese inconsistent results of models and experiments investigating SOC responses to EW are not entirely surprising. First, the SOC cycle contains a high degree of complexity. It involves a variety of processes with potentially opposing effects. For example, a silicate induced pH increase could increase SOM decomposition (Wang \u0026amp; Kuzyakov, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), while the active weathering process could simultaneously promote physico-chemical stabilisation of SOC (Kleber et al., \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Second, varying experimental conditions further contribute to the uncertainty. Especially the presence or absence of plants could alter the outcome of experiments, as they are major drivers of both, the soil weathering process (Ibarra et al., \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Schwartzman \u0026amp; Volk, \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e1989\u003c/span\u003e) and SOC dynamics (Basile-Doelsch et al., \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Liang et al., \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Considering plant-soil interactions is therefore key to better understand EW-SOC interactions.\u003c/p\u003e \u003cp\u003eHere, we conducted a 15-month mesocosm experiment with basalt and steel slag to test the effects of EW on IC and SOC dynamics in the presence and absence of plants. We hypothesised that SOC decomposition intensifies shortly after silicate application and that once a new steady-state in the SOC cycle is established, SOC stabilisation will be the dominant process.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eEvidence of weathering but limited inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003cp\u003eSilicate weathering produces alkalinity, thereby affecting pH, dissolved inorganic carbon (DIC) and cation concentrations in soil porewater \u003cb\u003e(Eq.\u0026nbsp;1)\u003c/b\u003e. In a 15-month mesocosm experiment silicate amendment increased porewater pH \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e, DIC \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e and alkalinity \u003cb\u003e(Figure S1)\u003c/b\u003e through weathering. The presence of plants amplified these effects.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAlso base cation concentrations in various soil pools (exchangeable, carbonate, (hydr)oxide and SOM) were affected by silicate application \u003cb\u003e(Table S1\u003c/b\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. Basalt amendment increased extractable Ca, Mg and Na concentrations, while BOF application increased extractable Ca concentrations (BOF slag does not contain any Na and only traces of K and Mg). No effect of maize was found.\u003c/p\u003e \u003cp\u003eInterestingly, amounts of extractable base cations were highest at the beginning of the experiment (days 9 and 101) after which they decreased (day 435, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). While there were some treatment effects in leachate base cation concentrations \u003cb\u003e(Table S2)\u003c/b\u003e, this pool showed minimal potential for CO\u003csub\u003e2\u003c/sub\u003e removal. Even though an average volume of 66.4 L (\u0026plusmn;\u0026thinsp;a standard error of 5.8 L) and 86.4 L (\u0026plusmn;\u0026thinsp;7.5 L) leached out of planted and unplanted treatments respectively, less than 0.5% of cations contained in the silicates were leached \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. Decreases in soil extractable base cations can therefore not be explained by leaching, suggesting that the cations remained in the soil, where they must have transitioned to a more stable mineral pool as they could not be extracted anymore. These results indicate that primary mineral dissolution dominated during the first 101 days of the experiment, afterwards mineral formation became the prevailing process.\u003c/p\u003e \u003cp\u003eWeathering rates \u003cb\u003e(Table S3)\u003c/b\u003e and inorganic CO\u003csub\u003e2\u003c/sub\u003e removal \u003cb\u003e(\u003c/b\u003eTable\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e were estimated based on base cation release from applied silicates \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. Inorganic CO\u003csub\u003e2\u003c/sub\u003e removal occurred during the first months of the experiment (until day 101) when primary mineral weathering was the dominant process. The subsequent decrease in extractable cations resulted in negative inorganic CO\u003csub\u003e2\u003c/sub\u003e removal (i.e. CO\u003csub\u003e2\u003c/sub\u003e release) over the remaining course of the experiment when secondary mineral formation was the dominating process. In this period, base cations became part of mineral lattices, not requiring bicarbonates for charge balance anymore.\u003c/p\u003e \u003cp\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\u003eRealised and potential future inorganic CO\u003csub\u003e2\u003c/sub\u003e removal in tons/ha\u0026thinsp;\u0026plusmn;\u0026thinsp;95% confidence intervals (calculations require the aggregation of replicates, confidence intervals are used to give an indication of statistical significances, statistical analysis of raw unaggregated data can be found in \u003cb\u003eTable S1\u003c/b\u003e). \u0026lsquo;Total\u0026rsquo; gives the CO\u003csub\u003e2\u003c/sub\u003e removal of the entire experimental period. The theoretical maximum CO\u003csub\u003e2\u003c/sub\u003e removal of basalt and BOF slag equals 19.45 and 4.60 tons CO\u003csub\u003e2\u003c/sub\u003e/ha respectively (with application rates of 50 tons basalt/ha and 5 tons BOF/ha).\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colspan=\"4\" nameend=\"c6\" namest=\"c3\"\u003e \u003cp\u003eBasalt\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eDay 0\u0026ndash;9\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003eDay 9-101\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003eDay 101\u0026ndash;435\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003eTotal\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eunplanted\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRealised inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003cp\u003ePotential future inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.38\u003c/p\u003e \u003cp\u003e0.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.58\u003c/p\u003e \u003cp\u003e-0.13\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.08\u0026thinsp;\u0026plusmn;\u0026thinsp;0.09\u003c/p\u003e \u003cp\u003e1.79\u0026thinsp;\u0026plusmn;\u0026thinsp;1.04\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eplanted\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRealised inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003cp\u003ePotential future inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.28\u003c/p\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.06\u003c/p\u003e \u003cp\u003e0.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.30\u003c/p\u003e \u003cp\u003e-1.14\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.17\u003c/p\u003e \u003cp\u003e0.89\u0026thinsp;\u0026plusmn;\u0026thinsp;0.88\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colspan=\"4\" nameend=\"c6\" namest=\"c3\"\u003e \u003cp\u003e\u003cb\u003eBOF slag\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e\u003cb\u003eDay 0\u0026ndash;9\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e\u003cb\u003eDay 9-101\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e\u003cb\u003eDay 101\u0026ndash;435\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e\u003cb\u003eTotal\u003c/b\u003e\u003c/p\u003e \u003cp\u003et CO\u003csub\u003e2\u003c/sub\u003e/ha\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eunplanted\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRealised inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003cp\u003ePotential future inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.22\u003c/p\u003e \u003cp\u003e1.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.15\u003c/p\u003e \u003cp\u003e-1.01\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.07\u0026thinsp;\u0026plusmn;\u0026thinsp;0.10\u003c/p\u003e \u003cp\u003e0.66\u0026thinsp;\u0026plusmn;\u0026thinsp;1.01\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eplanted\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eRealised inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003cp\u003ePotential future inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.02\u003c/p\u003e \u003cp\u003e0.03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.16\u003c/p\u003e \u003cp\u003e2.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e-0.13\u003c/p\u003e \u003cp\u003e-1.74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c6\"\u003e \u003cp\u003e0.04\u0026thinsp;\u0026plusmn;\u0026thinsp;0.07\u003c/p\u003e \u003cp\u003e2.84\u0026thinsp;\u0026plusmn;\u0026thinsp;0.80\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\u003eIncreased SOM decomposition further reduced CO\u003csub\u003e2\u003c/sub\u003e removal efficiency\u003c/p\u003e \u003cp\u003eDepending on which silicate was applied, different responses in SOM decomposition were found. Dissolved organic carbon (DOC) concentrations in soil porewater increased with BOF slag application and remained high throughout the first growing season and shortly after. In contrast, basalt amendment did not significantly affect porewater DOC concentrations \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eConsistent with the above results, increases in SOM decomposition in silicate amended treatments were especially clear for BOF treatments during the first growing season \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. Deeper insight into SOM decomposition is provided by the δ\u003csup\u003e13\u003c/sup\u003eC signatures of soil CO\u003csub\u003e2\u003c/sub\u003e effluxes in unplanted treatments, which increased by 1.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5\u0026permil; (mean\u0026thinsp;\u0026plusmn;\u0026thinsp;standard error) in BOF treatments, and by 1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u0026permil; (not significantly) in Basalt treatments relative to the control \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. Considering no plants are present, CO\u003csub\u003e2\u003c/sub\u003e exclusively originated from SOM decomposition. Changes in isotopic signatures thus suggest that silicate application triggered the release of SOC from pools which were not decomposed (to such a degree) in the control treatment. In agreement with these results, signatures of soil C at the end of the experiment were lower for the BOF treatment than for the control \u003cb\u003e(Figure S2)\u003c/b\u003e. These results confirm that silicate application increased SOM decomposition shortly after application for BOF treatments. Effects in basalt treatments were smaller and remained unsignificant. Similar trends were found in SOC measurements, SOC stocks declined over time in BOF treatments, while basalt treatments maintained stable SOC levels (p\u0026thinsp;=\u0026thinsp;0.077, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, bottom). Note that silicate application also increased root biomass (particularly during the first growing season, \u003cb\u003eFigure S3\u003c/b\u003e), which is reflected in increased rhizosphere respiration \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eStabilised OC potentially dominating C sink during EW\u003c/p\u003e \u003cp\u003eIn soils, two major mechanisms are known to stabilise OC, both of which were evaluated here: The association of OC with minerals and the formation of stable aggregates and the storage of OC within them. OC bound to amorphous minerals was investigated via soil extractions. Here, no treatment effect was found in soil samples \u003cb\u003e(Figure S4)\u003c/b\u003e, but expected changes are small and consequently difficult to detect. The same extractions were therefore also carried out on pure silicate samples recovered from mesh bags which were buried in the soil. These samples are independent of soil heterogeneity or application rates, thus yielding less ambiguous results. Here, a clear stabilising effect of BOF slag appeared: SOC bound to amorphous minerals increased by 129% (p\u0026thinsp;\u0026lt;\u0026thinsp;0.001, 10.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mg C/g BOF slag at the start of the experiment vs 24.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 mg C/g BOF slag at the end, no maize effect was found, \u003cb\u003eFigure S5\u003c/b\u003e). No change was found in basalt samples. These findings suggest that the formation of mineral associated OC could be an alternative C sink to inorganic CO\u003csub\u003e2\u003c/sub\u003e removal (depending on the applied silicate).\u003c/p\u003e \u003cp\u003eA size fractionation was conducted to investigate SOC accumulation in different aggregate classes. There was no silicate effect on SOC in the finest fraction, where the formation of mineral associated OC is expected (possibly due to soil heterogeneity). Silicate amendment, however, clearly stimulated the formation of aggregates and the storage of SOC within them (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e8\u003c/span\u003e, \u003cb\u003eTable S5\u003c/b\u003e). Relative to control treatments, C stored in aggregates\u0026thinsp;\u0026gt;\u0026thinsp;250 \u0026micro;m increased by 106% (\u0026thinsp;=\u0026thinsp;+\u0026thinsp;1.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 g C/kg soil) in basalt and 73% (\u0026thinsp;=\u0026thinsp;+\u0026thinsp;1.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3 g C/kg soil) in BOF treatments, without significant effects of maize. Next to the formation of organo-mineral complexes, SOC sequestration in macroaggregates could therefore be another alternative C sink during EW.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eAttempts to quantify CO\u003csub\u003e2\u003c/sub\u003e removal by EW often implicitly assume that weathering products either end up in carbonates or leach out of the soil, eventually reaching the ocean. Here, we looked beyond those two pathways with the aim of identifying and quantifying other CO\u003csub\u003e2\u003c/sub\u003e sinks and sources relevant for EW. Our findings indeed suggest that current assessments overlook critical aspects of C removal. Carbonate formation and leaching were minor contributors to overall CO\u003csub\u003e2\u003c/sub\u003e removal. Less than 0.5% of added cations had leached out after 15 months, most of the cations were thus retained in the soil. Still, cations did not end up in carbonates. Instead, the majority was found on the exchangeable complex or in (hydr)oxides \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e, therefore not requiring the dissolution of CO\u003csub\u003e2\u003c/sub\u003e to ensure charge balance.\u003c/p\u003e \u003cp\u003eThe reduction in cation concentrations over time further suggests that cations slowly crystallised and became part of mineral lattices, rendering a de-sorption and therefore CO\u003csub\u003e2\u003c/sub\u003e sequestration within the next decades to centuries unlikely. Yet \u0026lsquo;potential\u0026rsquo; inorganic CO\u003csub\u003e2\u003c/sub\u003e removal (i.e., estimates including changes in the exchangeable pool) is commonly estimated, relying on the assumption that cations will desorb at a later time and therefore remove CO\u003csub\u003e2\u003c/sub\u003e (e.g. in Kelland et al. (\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), Reershemius et al. (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), Reynaert et al. (\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), Vienne et al. (\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2024\u003c/span\u003e), Niron et al. (\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e)). The possibility that these cations instead become part of crystalline phases \u0026ndash; as suggested by our results \u0026ndash; is usually not considered. Dietzen \u0026amp; Rosing (\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2023\u003c/span\u003e) also detected a decrease in exchangeable cations in their field study. They argued, however, that those cations had leached out and therefore removed CO\u003csub\u003e2\u003c/sub\u003e, but leachate concentrations to verify this were not available. Clay formation could provide an alternative explanation for such reductions. This underlines that the unclear fate of cations could lead to overestimations in inorganic CO\u003csub\u003e2\u003c/sub\u003e removals.\u003c/p\u003e \u003cp\u003eFor natural systems it is well known that increasing soil water solute concentrations lead to declining primary mineral dissolution rates as reaction affinity decreases and the formation of secondary minerals such as clays is stimulated (Blume et al., \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Maher et al., \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). Given that weathering rates are several orders of magnitude larger with EW as compared to natural weathering, the formation of secondary mineral phases could be substantial. A close natural analogue to EW is subglacial grinding of bedrock to produce fine and highly reactive glacial flour (Blackburn et al., \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2019\u003c/span\u003e; Hatton et al., \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Fresh subglacial material comprises to a large extent of amorphous Si, Al and Fe phases (Rampe et al., \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2022\u003c/span\u003e), which are known to be precursors for more crystalline phases such as phyllosilicates (Baronas et al., \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Behrens et al., \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), though the mechanisms remain understudied. There is also evidence for an increase in clay mineral content in glacial forefield chronosequences (Mavris et al., \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), and evidence for an illuviation process immobilising cations (Zhou et al., \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). This highlights the possibility that crystalline phases, likely aluminosilicate clay minerals, could act as a thus-far unconsidered cation sink. Such a maturation of amorphous phases, as suggested by our results, can seriously reduce the efficiency of inorganic CO\u003csub\u003e2\u003c/sub\u003e removal by EW.\u003c/p\u003e \u003cp\u003eThese low efficiencies in inorganic CO\u003csub\u003e2\u003c/sub\u003e removal underline the need for a broader understanding of C removal during EW. Our findings demonstrate that OC fluxes dominate in soils, even with application of EW. For example, changes in OC pools were ~\u0026thinsp;20 times larger than changes in IC pools \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. Clearly, effects of EW on SOC dynamics - whether positive or negative - need to be taken into account as they can act as additional C sinks or sources.\u003c/p\u003e \u003cp\u003eOur hypothesis of increased SOM decomposition shortly after silicate application was indeed confirmed, leading to a further reduction in CO\u003csub\u003e2\u003c/sub\u003e removal efficiency. In fact, for BOF treatments, increases in decomposition released more CO\u003csub\u003e2\u003c/sub\u003e than weathering removed (+\u0026thinsp;1.8 t CO\u003csub\u003e2\u003c/sub\u003e/ha released vs 0.08 t CO\u003csub\u003e2\u003c/sub\u003e/ha removed in unplanted treatments, +\u0026thinsp;1.1 t CO\u003csub\u003e2\u003c/sub\u003e/ha released vs 0.04 t CO\u003csub\u003e2\u003c/sub\u003e/ha removed in planted treatments, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). While also basalt treatments showed some indication of increased decomposition \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e, Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e trends were less clear. Thus, depending on which silicate is applied different responses in SOM decomposition can be expected. Soil pH is known to be a major driver of SOM decomposition as it controls microbial community composition and activity, and drives SOM (de)stabilisation mechanisms (Wang \u0026amp; Kuzyakov, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). The stronger pH increase in BOF as compared to basalt treatments \u003cb\u003e(\u003c/b\u003eFig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e might explain the differing responses. Indeed, experimental addition of wollastonite to different soil types showed that the greater the pH increase, the larger the SOM decomposition (Yan et al., \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). SOC losses thus seem especially likely with larger pH increases. While these pH-driven increases in SOM decomposition lower the efficiency of EW, it should be taken into account that in an agricultural setting silicate amendments would likely replace liming, a common practice used to raise soil pH. Schroeder et al. (\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e2024\u003c/span\u003e) suggested that also with liming, the microbial response depends on the initial soil pH and the magnitude of the pH shift. Thus, lime application could likely lead to similar responses in SOM decomposition as silicate application, while in many cases no CO\u003csub\u003e2\u003c/sub\u003e would be removed via weathering.\u003c/p\u003e \u003cp\u003eIn contrast to SOM decomposition, SOM stabilisation via mineral association and aggregate formation provided alternative C sinks for EW. In fact, contributions of these processes are potentially substantial, especially in the longer term. Weathering of primary minerals facilitates the formation of highly reactive secondary clay-sized minerals such as phyllosilicates, metal (hydr)oxides and aluminosilicates, which influence SOC stabilising processes (Basile-Doelsch et al., \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Totsche et al., \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Soil cation extractions indeed indicated the formation of (hydr)oxides and we even suspect the formation of other reactive clay-sized secondary minerals, suggesting our hypothesis regarding increased SOM stabilisation in the long-term could be correct. Similar to SOM decomposition, also SOM stabilisation appeared to depend on the type of silicate that was applied. While OC stabilisation in BOF samples more than doubled, OC levels in basalt samples remained unchanged. Decision making prosses regarding the applied silicate type should therefore also take into account effects on SOC dynamics and not only the materials potential for inorganic C removal. Aggregate formation and the storage of C within them was clearly increased regardless of silicate type. While in tilled agricultural soils the durability of this C sink is questionable (Lavallee et al., \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), sustainable farming practices applying no or reduced tillage could greatly benefit from improved soil aggregation and increased SOC levels (Angst et al., \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). While these results suggest that OC could actually be the central C sink during EW it should also be noted that the evolution of those so far understudied processes are still relatively unclear in terms of long-term development and durability.\u003c/p\u003e \u003cp\u003eIn summary, inorganic processes played a minor role in CO\u003csub\u003e2\u003c/sub\u003e removal. SOM decomposition further lowered removal efficiencies. However, increased root inputs, MAOM formation and the formation of large aggregates are alternative pathways indirectly contribute to CO\u003csub\u003e2\u003c/sub\u003e sequestration. In our experiment, the contribution of these pathways was substantial. Although the impact of EW on SOC is clearly relevant, its long-term impact is yet to be verified. These results underline the need for a better understanding of long-term SOC dynamics in response to EW and for reliable ways to quantify CO\u003csub\u003e2\u003c/sub\u003e removal considering the soil-plant system holistically. The estimation of inorganic CO\u003csub\u003e2\u003c/sub\u003e removals \u0026ndash; as commonly done - could lead to unrealistic predictions since it neither takes into account the possibility of secondary mineral formation nor the effects of EW on SOC.\u003c/p\u003e "},{"header":"Methods","content":"\u003cp\u003eExperimental set-up\u003c/p\u003e \u003cp\u003eMesocosms with a diameter of 50 cm and a height of 60 cm were equipped with a leaching system and established outside at experimental sites of the University of Antwerp (51\u0026deg;09'41.8\"N, 4\u0026deg;24'30.8\"E), where they were exposed to local weather conditions. The experiment started on 1st of June 2022 and lasted for 435 days (~\u0026thinsp;15 months). The regional climate is temperate maritime with mild winters and warm summers (Kottek et al., \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). In 2022, the annual mean temperature was 12.2\u0026deg;C with an annual precipitation of 701.4 mm, in 2023 the annual mean temperature was 12.1\u0026deg;C and annual precipitation was 1011.4 mm. While 2022 was exceptionally warm and dry, 2023 was exceptionally warm and wet (Royal Meteorological Institute Belgium, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Mesocosms were filled with a sandy loam soil (69.5% sand, 28.1% silt, 1.8% clay) originating from a pasture in Zandhoven, Belgium. According to the Belgian soil map the soil is categorised as a plaggic anthrosol (Dondeyne et al., \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). The soil was slightly acidic (pH\u0026thinsp;=\u0026thinsp;5.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3) and low in SOC (0.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.05% with a δ\u003csup\u003e13\u003c/sup\u003eC of -25.1\u0026thinsp;\u0026plusmn;\u0026thinsp;0.36\u0026permil;), characteristics typical for agricultural soils. At the start of the experiment, the soil had a cation exchange capacity of 3.92\u0026thinsp;\u0026plusmn;\u0026thinsp;0.64 meq/100g, base saturation 66.62\u0026thinsp;\u0026plusmn;\u0026thinsp;0.14%, electrical conductivity 84.78\u0026thinsp;\u0026plusmn;\u0026thinsp;17.32 \u0026micro;S/cm, total N 0.07\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02% and bulk density 1.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.02 g/cm\u003csup\u003e3\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSilicate amendments were mixed into the top 20 cm of the soil using a cement mixer to ensure an even distribution. Deeper soil layers (20\u0026ndash;60 cm depth) therefore did not initially contain any amendment. The application of two different types of silicates was tested: basalt (type: durubas, obtained from RPBL, Germany), and basic oxygen furnace (BOF, obtained from ArcelorMittal, Belgium) slag, an artificial silicate produced as a by-product of steel production \u003cb\u003e(supplementary section 4.1)\u003c/b\u003e. Application rates of basalt and BOF slag were 50 and 5 t/ha respectively. BOF slag was applied at a much lower application rate due to its unstable structure and therefore extremely high weathering rates.\u003c/p\u003e \u003cp\u003eIn total, six treatments were established. These included three different types of amendments (BOF, basalt, no amendment), each present as unplanted and planted treatment. During growing seasons (June-September 2022 and June-August 2023) 2 maize plants (\u003cem\u003eZea mays\u003c/em\u003e) were sown in planted mesocosms. Due to seed unavailability a different variety (with very similar traits) was used in the second growing season (first season \u003cem\u003egolden midget\u003c/em\u003e, second season \u003cem\u003etom thumb\u003c/em\u003e). The growing seasons lasted for 3\u0026ndash;4 months, typical for these maize varieties. Before sowing, limited amounts of N, P and K fertilisers (NH\u003csub\u003e4\u003c/sub\u003eNO\u003csub\u003e3\u003c/sub\u003e: 270 kg/ha, P\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e: 20 kg/ha and K\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e: 90 kg/ha) were added to each mesocosm. During dry spells in summer, mesocosms were irrigated with rainwater collected in belowground tanks to avoid severe drought stress. Outside of the growing seasons, all pots were left bare. The experiment started with 42 pots (six replicates for bare soil treatments, eight replicates for planted treatments). At the end of the first growing season half of the replicates of each treatment were sampled destructively and therefore removed from the experiment. The experiment thus continued with 21 pots (three replicates for bare soil treatments, four replicates for planted treatments) after the first growing season ended.\u003c/p\u003e \u003cp\u003eAnalyses of plants\u003c/p\u003e \u003cp\u003eAfter each growing season, aboveground biomass was cut and dried at 70\u0026deg;C for 48 h. Tassle, stem, leaves and corn were weighed separately. Ground maize leaves and corn were analysed for Ca, Mg, K, Al and Fe concentrations via digestion with 70% HNO\u003csub\u003e3\u003c/sub\u003e (60\u0026deg;C for 30 minutes) and 30% H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e after which samples were heated to 120\u0026deg;C for 90 minutes. After cooling and filtering concentrations were measured via ICP-OES (Thermo Scientific, iCAP 6300 duo).\u003c/p\u003e \u003cp\u003eTo estimate belowground biomass, soil cores (one directly below each plant and one in between) were taken at three different depths (0\u0026ndash;20 cm, 20\u0026ndash;40 cm and 40\u0026ndash;60 cm). Soil was then washed off with tap water on a sieve with 1 mm mesh-size and remaining roots were picked out and dried at 70\u0026deg;C for 48 h and weighed. Root biomass was estimated assuming that samples taken below plants reflect root growth in half of the mesocosm, while samples taken between plants reflect the other half.\u003c/p\u003e \u003cp\u003eSoil sequential extractions\u003c/p\u003e \u003cp\u003eTo unravel organic and inorganic soil C dynamics, two different types of soil sequential extractions were carried out, each at three points in time (day 9\u0026thinsp;=\u0026thinsp;before planting, day 101\u0026thinsp;=\u0026thinsp;end of first growing season, day 435\u0026thinsp;=\u0026thinsp;end of second growing season/end of experiment).\u003c/p\u003e \u003cp\u003eSequential extractions adapted from Tessier et al. (\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1979\u003c/span\u003e) and Uhlig and Von Blanckenburg (\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) aim at quantifying the cation content of different soil pools, which can be used for weathering rate calculations and inorganic CO\u003csub\u003e2\u003c/sub\u003e removal estimations (Niron et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Vienne et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Cation extractions were carried out in four extraction steps \u003cb\u003e(\u003c/b\u003eTable\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, \u003cb\u003esupplementary section 4.2)\u003c/b\u003e on 0.5 g of ground and air-dried soil. After each step cations (Mg, Ca, Na, K, Fe, Al) plus Si concentrations in extractant solution were quantified on a Varian 720-ES or SpectroGreen ICP-OES following the analytical protocol of (Schuessler et al., \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eFour different extraction steps of cation extractions including definition of targeted pools\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"5\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExtract\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eChemical\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eHeat bath\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eRotator\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eTargeted Pool\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 M Ammonium acetate (CH\u003csub\u003e3\u003c/sub\u003eCOONH\u003csub\u003e4\u003c/sub\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e1 h (15 rpm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eExchangeable cations\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e1 M Acetic acid (CH\u003csub\u003e3\u003c/sub\u003eCOOH)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e2 h (15 rpm)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCations in carbonates\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.05 M Hydroxylamine (NH\u003csub\u003e2\u003c/sub\u003eOH) in 0.5 M HCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5 h (80\u0026deg;C)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eInvert manually (every 30 min)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCations in (hydr)oxides\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e4\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e9.8 M Hydrogen peroxide (H\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e2\u003c/sub\u003e) in 0.01 M HNO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e5 h (70\u0026deg;C)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eInvert manually (every 30 min)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003eCations in SOM\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\u003eSequential extractions adapted from Heckman et al. (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) focus on mineral associated organic matter and were caried out on 1 g of ground and air-dried soil. The sum of both extraction steps is assumed to represent SOC bound to amorphous minerals \u003cb\u003e(\u003c/b\u003eTable\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, \u003cb\u003esupplementary section 4.2)\u003c/b\u003e. Dissolved organic C concentrations in extractant solutions were determined using a continuous flow analyser (Skalar, SAN++). Extractions were also carried out on pure silicate samples recovered from mesh bags (0.325 mm mesh size, thus roots can grow inside) which were buried in silicate amended mesocosms at the beginning of the experiment.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eTwo different steps of MAOM extractions including definition of targeted pools\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStep\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eChemical\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eRotator\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eTargeted Pool\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.1 M Sodium pyrophosphate (Na\u003csub\u003e4\u003c/sub\u003eP\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e7\u003c/sub\u003e)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16 h\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eSOC bound to amorphous minerals\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003e2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e0.1 M Hydroxylamine (NH\u003csub\u003e2\u003c/sub\u003eOH) in 0.25 M HCl\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e16 h\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\u003eWet sieving\u003c/p\u003e \u003cp\u003eTo quantify the amount of SOC in different aggregate size classes (similar to Six et al. (\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e1998\u003c/span\u003e)) undisturbed soil core samples were taken at 10 cm depth. Air dried samples were submerged in water and set aside for 5 minutes for slaking. Samples (about 100 g each) were then passed through various sieves (2mm, 250 \u0026micro;m and 63 \u0026micro;m) stacked on top of each other using a shaker and 500 mL of deionised water to wash the samples through. After the fractionation, samples were dried and SOC contents determined via loss on ignition (550\u0026deg;C for 4 hours). The fraction\u0026thinsp;\u0026gt;\u0026thinsp;2 mm comprised mostly small stones and a few larger aggregates. This fraction contributed only 1.05\u0026thinsp;\u0026plusmn;\u0026thinsp;0.63% to the total sample mass and was therefore considered negligible.\u003c/p\u003e \u003cp\u003eCollection and analysis of soil pore water and leachates\u003c/p\u003e \u003cp\u003ePorewater samples were collected 20 times throughout the experimental period via Rhizon flex samplers (Rhizosphere Research Products) installed at 5\u0026ndash;10 cm depth. Leachates were collected 16 times via glass bottles connected to the bottom of the mesocosms. After collection a SpectroGreen ICP-OES was used to quantify Ca, Mg, K, Na, Al, Fe and Si concentrations. Alkalinity was measured on a continuous flow analyser (Skalar, SAN++), dissolved inorganic and organic carbon with a TOC analyser (Skalar, Formacs) and pH with a pH/Conductometer at 25\u0026deg;C (914 Metrohm).\u003c/p\u003e \u003cp\u003eMeasurement and analysis of soil CO\u003csub\u003e2\u003c/sub\u003e efflux\u003c/p\u003e \u003cp\u003eThe soil CO\u003csub\u003e2\u003c/sub\u003e efflux (in ppm/s) and its isotopic signature δ\u003csup\u003e13\u003c/sup\u003eC (in \u0026permil; relative to Vienna Pee Dee Belemnite) was measured 22 times throughout the experiment with a cavity ring-down spectroscopy analyser (Picarro, G2131-i). To do so, a custom built chamber connected to the analyser was positioned on stainless steel collars which were inserted into the surface of each mesocosm at the beginning of the experiment. For each measurement, the CO\u003csub\u003e2\u003c/sub\u003e concentration was monitored for 7\u0026ndash;15 minutes, aiming for an increase of at least 100 ppm (to minimise the error on C source partitioning).\u003c/p\u003e \u003cp\u003eSoil CO\u003csub\u003e2\u003c/sub\u003e efflux was calculated from the rate of CO\u003csub\u003e2\u003c/sub\u003e increase over time. To partition the flux into rhizosphere respiration and SOM decomposition two-pool mixing models \u003cb\u003e(\u003c/b\u003eEq.\u0026nbsp;\u003cspan refid=\"Equ1\" class=\"InternalRef\"\u003e2\u003c/span\u003e, \u003cb\u003esupplementary section 4.3)\u003c/b\u003e were used. First, the δ\u003csup\u003e13\u003c/sup\u003eC of the CO\u003csub\u003e2\u003c/sub\u003e efflux was calculated as the intercept of a linear regression between measured δ\u003csup\u003e13\u003c/sup\u003eC and inverse measured CO\u003csub\u003e2\u003c/sub\u003e (Keeling, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e1958\u003c/span\u003e). δ\u003csup\u003e13\u003c/sup\u003eC of maize roots was measured by an elemental analyser coupled to an isotope ratio mass spectrometer (EA-IRMS), δ\u003csup\u003e13\u003c/sup\u003eC of SOM obtained from flux measurements of unplanted treatments.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$$\\:{f}_{\\text{r}\\text{h}\\text{i}\\text{z}\\text{o}}\\:=\\:\\frac{({{\\delta\\:}}^{13}\\text{C}\\:{\\text{C}\\text{O}}_{2}\\:\\text{e}\\text{f}\\text{f}\\text{l}\\text{u}\\text{x}\\:-\\:{{\\delta\\:}}^{13}\\text{C}\\:\\text{S}\\text{O}\\text{M})}{({{\\delta\\:}}^{13}\\text{C}\\:\\text{r}\\text{o}\\text{o}\\text{t}\\text{s}\\:-\\:{{\\delta\\:}}^{13}\\text{C}\\:\\text{S}\\text{O}\\text{M})}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eAfter determining the fraction f\u003csub\u003erhizo\u003c/sub\u003e accounting for rhizosphere processes, the total CO\u003csub\u003e2\u003c/sub\u003e efflux can be partitioned into CO\u003csub\u003e2\u003c/sub\u003e originating from plants versus SOM (Eqs.\u0026nbsp;3 and 4).\u003c/p\u003e \u003cp\u003eRhizosphere respiration =\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:{f}_{\\text{r}\\text{h}\\text{i}\\text{z}\\text{o}}\\text{*}\\:\\text{s}\\text{o}\\text{i}\\text{l}\\:{\\text{C}\\text{O}}_{2}\\:\\text{e}\\text{f}\\text{f}\\text{l}\\text{u}\\text{x}\\)\u003c/span\u003e\u003c/span\u003e (\u003cb\u003e3\u003c/b\u003e)\u003c/p\u003e \u003cp\u003eSOM decomposition = \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{(1-f}_{\\text{r}\\text{h}\\text{i}\\text{z}\\text{o}})\\:\\text{*}\\:\\text{s}\\text{o}\\text{i}\\text{l}\\:{\\text{C}\\text{O}}_{2}\\:\\text{e}\\text{f}\\text{f}\\text{l}\\text{u}\\text{x}\\)\u003c/span\u003e\u003c/span\u003e (\u003cb\u003e4\u003c/b\u003e)\u003c/p\u003e \u003cp\u003eThese calculations assume no contribution of inorganic processes (weathering and carbonate formation). Given that inorganic CO\u003csub\u003e2\u003c/sub\u003e fluxes in our system were minor compared to organic CO\u003csub\u003e2\u003c/sub\u003e fluxes (see Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e), we consider their influence on the CO\u003csub\u003e2\u003c/sub\u003e flux partitioning negligible.\u003c/p\u003e \u003cp\u003eWeekly/bi-weekly soil CO\u003csub\u003e2\u003c/sub\u003e efflux measurements were used to reconstruct daily soil CO\u003csub\u003e2\u003c/sub\u003e effluxes based on daily soil temperature and soil water content measurements, with the aim of calculating cumulative fluxes. To this end, the model of \u003cb\u003eEq.\u0026nbsp;5\u003c/b\u003e (following Vicca et al. (2014)) was fit separately for each treatment \u003cb\u003e(Figure S8, Figure S9)\u003c/b\u003e. Daily CO\u003csub\u003e2\u003c/sub\u003e effluxes were then predicted based on the daily soil temperature and soil water content of each mesocosm and cumulated over time.\u003c/p\u003e \u003cp\u003elog\u003csub\u003e10\u003c/sub\u003e (soil CO\u003csub\u003e2\u003c/sub\u003e efflux)\u0026thinsp;=\u0026thinsp;a\u0026thinsp;+\u0026thinsp;b * soil temp\u0026thinsp;+\u0026thinsp;c * SWC\u0026thinsp;+\u0026thinsp;d * SWC\u003csup\u003e2\u003c/sup\u003e (\u003cb\u003e5\u003c/b\u003e)\u003c/p\u003e \u003cp\u003eCalculation of inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003cp\u003e \u003cem\u003eRealised\u003c/em\u003e inorganic CO\u003csub\u003e2\u003c/sub\u003e removal was calculated considering leached out cations and cations bound in carbonates \u003cb\u003e(\u003c/b\u003eTable\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e\u003cb\u003e)\u003c/b\u003e. To this end, the change in cation concentration in those two pools since the start of the experiment was expressed relative to the respective control treatment and converted to ton CO\u003csub\u003e2\u003c/sub\u003e/ha (Niron et al., \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Vienne et al., \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2024\u003c/span\u003e). Note, that we only consider the change in cation concentrations over time instead of using absolute values. Silicate amended treatments had higher cation concentrations even before the weathering process started, as extractions also remove cations from the silicates themselves. By focussing on the change over time, we avoid taking into account these cations. Cation concentration in the silicate amended layer (0\u0026ndash;20 cm) and below (20\u0026ndash;40 and 40\u0026ndash;60 cm) were included in the calculations. Cations bound in carbonates were assumed to only sequester half of the originally formed bicarbonates, since 50% of the CO\u003csub\u003e2\u003c/sub\u003e is released during carbonate formation. A correction for downstream losses (during fluvial transport, due to ocean chemistry, etc.) was not considered necessary since the exported alkalinity flux in leachates was negligible compared to within-soil storage.\u003c/p\u003e \u003cp\u003eCations in the soil exchangeable pool and in the SOM pool are considered as \u003cem\u003epotential\u003c/em\u003e inorganic CO\u003csub\u003e2\u003c/sub\u003e removal, happening in the near future and therefore relevant for climate change mitigation. Once cations are released back into the soil solution their charge will be counterbalanced by HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e, therefore capturing CO\u003csub\u003e2\u003c/sub\u003e. Note that, the SOM cation pool is thus considered inorganic CO\u003csub\u003e2\u003c/sub\u003e removal. The amount of cations in this pool does not directly correspond to a certain amount of stabilised SOC. Therefore, this pool does not quantify organic C stabilisation.\u003c/p\u003e \u003cp\u003eThe processes of cation uptake by plants and (hydr)oxide formation is considered CO\u003csub\u003e2\u003c/sub\u003e neutral, and is therefore not included in CO\u003csub\u003e2\u003c/sub\u003e removal calculations. Maize harvest would lead to the removal of cations which would thus not be returned to the soil solution where they would be charge-balanced by HCO\u003csub\u003e3\u003c/sub\u003e\u003csup\u003e\u0026minus;\u003c/sup\u003e. (Hydr)oxide weathering could lead to CO\u003csub\u003e2\u003c/sub\u003e sequestration, if cations are released again, but this process is expected to happen at very slow pace (Tessier et al., \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e1979\u003c/span\u003e; Uhlig \u0026amp; Von Blanckenburg, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) and is therefore considered negligible in a climate change mitigation context. Weathering rate calculations, on the contrary, consider all cation pools (leachate, soil and plant pools, \u003cb\u003esupplementary section 4.4\u003c/b\u003e)\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eDefinition of cation pools and their effect on inorganic CO\u003csub\u003e2\u003c/sub\u003e removal\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=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCation pool\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eInorganic CO\u003csub\u003e2\u003c/sub\u003e removal\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eLeachate\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eRealised removal\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCarbonate\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eExchangeable\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003ePotential future removal\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eSOM\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eHydr(oxide)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003eNo removal\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ePlant\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\u003eStatistical analyses\u003c/p\u003e \u003cp\u003eAs most measurements were repeated over time, linear mixed models with mesocosm as a random effect and time as a fixed effect were used to test for the effects of silicate addition (no amendment, basalt, BOF slag), maize (unplanted, planted) and their interactions. Interaction effects between silicate-maize, silicate-time and maize-time were tested and included in the final model if statistically significant. For each analysis, normality and homoscedasticity were assessed visually as well as by the Shapiro-Wilk and Levene\u0026rsquo;s test. In the few cases where the assumptions were invalid, a log or square root transformation was used. All statistical tests were conducted in R, using lme4 (Bates et al., \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2015\u003c/span\u003e), pbkrtest (Halekoh \u0026amp; H\u0026oslash;jsgaard, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2014\u003c/span\u003e), lmerTest (Kuznetsova et al., \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) and emmeans (Lenth, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eWeathering and inorganic CO\u003csub\u003e2\u003c/sub\u003e removal rates, as well as cumulated CO\u003csub\u003e2\u003c/sub\u003e effluxes required the aggregation of replicates, therefore yielding one value per treatment. To evaluate differences between treatments 95% confidence intervals were calculated from standard errors: non-overlapping confidence intervals were considered indicative of a significant difference between treatments. Additionally, statistical analyses using linear mixed models (as specified above) were carried out on raw unaggregated datasets.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eWe thank Guy van den Broeck and Jan Segers for their support in setting up the experiment and Jasper Roussard, Sarah Janse, Emma Pellegrini, Mart\u0026iacute;n Carrera Larrea and Mariana Rodriguez for their assistance with experimental measurements. This research was supported by the Research Foundation Flanders (FWO) through the following grands: 1174925N, 1S06325N, G0A4821N, G000821N.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAhmad, W., Singh, B., Dijkstra, F. A., \u0026amp; Dalal, R. C. (2013). 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Gongga, SW China. \u003cem\u003eGeoderma\u003c/em\u003e, \u003cem\u003e267\u003c/em\u003e, 78\u0026ndash;91. https://doi.org/10.1016/j.geoderma.2015.12.024\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5672251/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5672251/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"Enhanced weathering captures CO2 via two pathways: Carbonate formation and leaching of weathering products. Here, we look beyond those two pathways, identifying other CO2 sinks and sources. While processes such as clay formation or organic matter decomposition reduce the efficiency of enhanced weathering, organic matter stabilisation could contribute to C storage. In a 15 month mesocosm experiment including two different types of silicates (basalt and steel slag) inorganic CO2 sequestration indeed remained negligible (below 0.1 t CO2/ha) due to clay formation. Also organic matter decomposition increased in silicate amended treatments (basalt +0.9 and slag +1.1 t CO2/ha released), further lowering the CO2 removal efficiency of enhanced weathering. Other organic C pathways could however contribute substantially to C storage. Aggregate formation and the storage of C within them was promoted in silicate amended treatments (basalt +106 and slag +73 % organic C stored in aggregates \u003e250μm). Next to that, the association of organic C to minerals was determined another possible organic C sink. These results underline the urge for reliable ways to quantify CO2 removal not only including inorganic but also organic carbon dynamics.","manuscriptTitle":"Beyond inorganic carbon: Soil organic carbon as key pathway for carbon sequestration in Enhanced Weathering","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-01-29 12:28:11","doi":"10.21203/rs.3.rs-5672251/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"b368a145-d5c3-4ee4-a226-e4d84aefbcd8","owner":[],"postedDate":"January 29th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":42058645,"name":"Earth and environmental sciences/Climate sciences/Climate change/Climate-change mitigation"},{"id":42058646,"name":"Biological sciences/Biochemistry/Biogeochemistry/Carbon cycle"},{"id":42058647,"name":"Earth and environmental sciences/Biogeochemistry/Carbon cycle"},{"id":42058648,"name":"Earth and environmental sciences/Solid Earth sciences/Geochemistry"}],"tags":[],"updatedAt":"2025-07-23T18:48:14+00:00","versionOfRecord":{"articleIdentity":"rs-5672251","link":"https://doi.org/10.1111/gcb.70340","journal":{"identity":"global-change-biology","isVorOnly":true,"title":"Global Change Biology"},"publishedOn":"2025-07-22 00:00:00","publishedOnDateReadable":"July 22nd, 2025"},"versionCreatedAt":"2025-01-29 12:28:11","video":"","vorDoi":"10.1111/gcb.70340","vorDoiUrl":"https://doi.org/10.1111/gcb.70340","workflowStages":[]},"version":"v1","identity":"rs-5672251","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5672251","identity":"rs-5672251","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}