Effects of basalt amendment and mycorrhizal inoculation on soil chemical properties and maize growth

of heating the soil at 80 °C for four hours. Pasteurization therefore resulted in an increase in DOC (Fig. S18), which has been shown to increase silicate weathering rates (Perez-Fodich and Derry, 2019). In contrast with our hypothesis, AMF did not significantly increase basalt weathering rates . This outcome may be partly attributed to the experimental conditions : pasteurization likely increased nutrient availability (Hu et al., 2019) and, combined 335 with fertilizer application (NPK; 96 – 10 – 39.5 kg ha-1), it ensured sufficient nutrient levels. Consequently, AMF might have played a limited role in basalt bio -weathering, as the experimental conditions may have reduced the need for AMF-driven nutrient mobilization from the mineral substrate. 4.1.1 Reconciling contrasting weathering proxies 340 Pore water pH, alkalinity, Ca and Mg increased with basalt application, with no AMF effect, whereas pore water DIC showed a significant basalt x AMF x time interaction (Fig. 3 -4). The observed increase in pore water DIC with basalt application was reinforced by AMF towards the end of the experiment, as the fungal hyphae developed . The observed increase in pore water DIC with AMF might suggest that AMF increased basalt weathering rates. However, other weathering proxies such as pore water pH, alkalinity, Ca and Mg did not show similar patterns. This discrepancy between DIC and the 345 other weathering indicators may be due to different reasons. .CC-BY-NC 4.0 International licenseperpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for thisthis version posted November 4, 2025. ; https://doi.org/10.1101/2025.11.03.686277doi: bioRxiv preprint 17 While silicate weathering is expected to increase DIC through (bi)carbonate production, the observed increase in DIC may also reflect elevated soil partial CO₂ pressure ( pCO₂). Pore water DIC is governed by carbonate equilibria, which are sensitive to transient environmental conditions such as pore water pH and pCO₂ (Hartmann et al., 2013; Zeebe and Wolf -350 Gladrow, 2001) . Indeed, higher pCO 2 can increase dissolved H 2CO3 and therefore DIC. As the pore water pH in our experiment was > 6.5 in the second half of the experiment (Fig. 3), the dissolved H 2CO3 likely speciated into (bi)carbonate ions within the soil solution, leading to elevated DIC. A second possible explanation for the discrepancy between pore water DIC and alkalinity lies in the role of the anions in 355 alkalinity. Total alkalinity is the sum of base cation charges, minus the sum of conservative anion charges (e.g. Cl -, SO42-, PO43-, NO3-) (Barker, 2013; Wolf-Gladrow et al., 2007). While DIC increased with AMF, other anions may have decreased, leading to a null effect of AMF on alkalinity. We did not find a decrease in pore water NO3-, SO42-- and PO 43- with AMF (Fig. 4; Fig. S 3), but organic anions may still have contributed to the observed effect. During the middle of the growing season, basalt treatments without AMF tended to have lower DOC compared to basalt treatments with AMF (Fig. S18), 360 partially supporting this hypothesis. Finally, cation uptake may have also played a role. Plants and AMF act as a sink for cations, and a positive effect of AMF on this plant and AMF sink may have contributed to the observed decoupling between DIC and base cations. In this case, we would observe an increase in Ca and Mg stocks in the plant and/or AMF tissues. However, basalt only increased Mg stocks 365 in plants, without an effect on Ca stocks (Fig . S19). Therefore, the plant sink hypothesis can only partially explain the observed mismatch between DIC and other weathering proxies such as pore water cations. Although we cannot fully disentangle the divergence of different weathering proxies, the inconsistencies in their responses highlight the limitations of relying on a single proxy to assess weathering dynamics, as it may not adequately capture the 370 complexity of the system (see also Iff et al. (2024) ). According to the explicit conservative expression for total alkalinity (Wolf-Gladrow et al., 2007) , DIC and alkalinity consist of distinct chemical species, thus complicating the interpretation of these weathering proxies. Therefore, our results suggest that cation accounting, rather than carbon accounting, may provide a more reliable estimate of weathering dynamics, as proposed by Bijma et al. (2025). 4.2 Basalt, but not AMF, improved soil fertility measures 375 Basalt, but not AMF, increased soil pH, CEC and base saturation (Fig. 2). The observed increases in soil pH, CEC, and base saturation upon basalt application align with our hypothesis of basalt improving soil fertility. The observed increase in bas e saturation was attributable to an increase in soil exchangeable Ca, Mg and Na due to basalt application (Fig. S20). Indeed, during weathering of basalt, protons are consumed and cations are released. Additionally, weathering of primary minerals promotes the form ation of metal (hydr)oxides and secondary silicate phases, ultimately increasing the content of clay -sized 380 .CC-BY-NC 4.0 International licenseperpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for thisthis version posted November 4, 2025. ; https://doi.org/10.1101/2025.11.03.686277doi: bioRxiv preprint 18 particles in the soil and therefore CEC (Righi and Meunier, 1995) . Our results corroborate previous work that observed increased soil pH and CEC upon basalt application to tropical soils (Anda et al., 2015; Conceição et al., 2022; Gillman, 1980). An increase in pH was also found in experiments conducted in using relatively younger, less cation -depleted soils upon basalt or dunite application (Rijnders et al., 2023; Rijnders et al., 2025; Skov et al., 2024; Ten Berge et al., 2012) . These consistent results show that EW can contribute to soil restoration by counteracting soil acidification and improving 385 CEC and availability of exchangeable bases, which are a common problem associated with intensive agriculture (Ashitha et al., 2021; Goulding, 2016). 4.2.1 Limited AMF and basalt effects on soil nutrients While basalt amendment increased soil CEC and pH, it generally did not affect soil nutrient concentrations, with a few exceptions. Despite an increase in exchangeable Ca and Mg, only Mg increased in the pore water as well. This might be at 390 least partially explained by weathering kinetics: forsterite, a Mg -rich olivine, weathers relatively quickly compared to other Ca-bearing minerals within the applied basalt, such as labradorite, a Ca -enriched plagioclase feldspar (Gudbrandsson et al., 2011). Nevertheless, basalt increased exchangeable Ca (Fig. 6; Fig. S20), indicating that Ca was released during weathering, as observed by Li and Dong (2013) , Niron et al. (2024) and Vienne et al. (2022) . Our pore water and exchangeable Ca

also indicate that, once released, Ca was rapidly adsorbed onto soil exchange sites rather than remaining in solution. 395 The observed Mg and Ca increases in the exchangeable fraction are in agreement with existing literature, as application of basalt supplies new cations that can bind to soil particles, increasing base saturation (Gillman, 1980; Gillman et al., 2001, 2002; Te Pas et al., 2023) . Increases in exchangeable Ca and Mg were also reported by Buss et al. (2024), whereas in a mesocosm experiment by Niron et al. (2024) , basalt application did not affect any of the measured cations (Ca, Mg, K, Na, 400 Fe and Al) in the exchangeable fraction, thus contrasting our findings. On the other hand, in our experiment exchangeable Zn, an important micronutrient, decreased with basalt application in the two top layers, a result consistent with previous research (Buss et al., 2024; Desmalles et al., 2025) . Generally, above a pH of 6 exchangeable Zn contents tend to be very low (Blume et al., 2010) . Therefore, the observed increase in soil pH from 5.65 ±0.11 to 6.36 ±0.17 following basalt application may have contributed to the reduction in exchangeable Zn. 405 Our finding of increased exchangeable Mg, Ca and Na (albeit not in all soil layers) is cautiously encouraging, given the global imperative to restore and maintain soil nutrient levels to support sustainable agricultural productivity (Un General Assembly, 2015) . Indeed, the exchangeable pool consists of cations adsorbed onto soil particles that can be readily exchanged with the soil solution (Tessier et al., 1979) , representing a key reservoir of nutrients, from which plant roots 410 primarily acquire elements essential for growth (Roy et al., 2006) . At the same time, even though basalt contains Fe, and in smaller amounts also K, which are important plant nutrients, their concentration in the exchangeable and pore water pools did not increase with basalt application, stressing the need to be cautious when expecting soil nutrient replenishment co - .CC-BY-NC 4.0 International licenseperpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for thisthis version posted November 4, 2025. ; https://doi.org/10.1101/2025.11.03.686277doi: bioRxiv preprint 19 benefits from basalt application. Moreover, the observed decrease in exchangeable Zn underscores the need for careful evaluation of both unintended side-effects as well as potential co-benefits associated with enhanced weathering. 415 Finally, AMF did not significantly affect (micro)nutrient concentrations in the soil pore water nor in the soil exchangeable complex. Our results are thus in contrast with our hypotheses, as we expected increases in (micro)nutrient availability and/o r uptake with AMF. This outcome may be at least partially attributed to the experimental conditions , namely the pasteurization process and the fertilizer addition, as explored at the end of section 4 .1. In addition, t he soil used in this experiment was 420 relatively young and not particularly catio n-depleted, therefore likely resulting in adequate nutrient availability, possibly masking any basalt- and/or AMF-driven effects on soil nutrients (Hu et al., 2019). 4.2.2 Potential for future soil organic C stabilization and clay-sized particles formation Most of the agriculturally relevant (micro)nutrients present in the basalt (Ca, Mg, Fe and Na) increased in the reducible poo l 425 in the 20 -40 cm layer, and Ca and Si also increased in the oxidizable pool in the same layer (Table S7). The reducible and oxidizable pools comprise cations bound to metal (hydr)oxides and organic matter respectively (Tessier et al., 1979). Cations in these fractions are expected to be retained in the soil for a relatively long time, with plants able to access these nutri ents only upon weathering (Niron et al., 2024; Palandri and Kharaka, 2004) . Nevertheless, metal (hydr)oxide formation can ultimately increase CEC and govern SOC stabilization processes (Basile-Doelsch et al., 2015; Beerling et al., 2020; Manning 430 et al., 2024; Totsche et al., 2017) , with downstream benefits for plant growth and soil fertility, particularly in agricultural soils which are often depleted in organic matter (Rusco et al., 2001) . These processes can improve soil structure, water retention, nutrient availability, and biological activity, thereby supporting overall soil health (Bot and Benites, 2005) . Although the total stock of cations in the oxidizable fraction was relatively small compared to other soil pools, the observe d increases in Ca and Si following basalt application suggest potential for organic matter stabilization with enhanced 435 weathering. 4.3 Limited basalt and AMF effects on crop yield and quality In line with our soil nutrient data and contrary to our hypotheses, plant nutrient concentrations were generally not affected by basalt application or AMF presence, with the exception of Mg, which generally increased with basalt. Increased plant Mg can be directly linked with the observed increase in pore water Mg concentrations and Mg exchange capacity, leading to 440 elevated Mg uptake. Our results are supported by research carried out on corn after basalt application (Boniao et al., 2006; Rijnders et al., 2025) , and on ryegrass (Ten Berge et al., 2012) and soybean (Moretti et al., 2019) after dunite application. These consistent findings indicate that EW can help alleviate Mg deficiency, which often occurs in agricultural systems that are fertilized with N, P, and K as uptake competition between these elements and Mg arise (Guo et al., 2016). 445 .CC-BY-NC 4.0 International licenseperpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for thisthis version posted November 4, 2025. ; https://doi.org/10.1101/2025.11.03.686277doi: bioRxiv preprint 20 Nevertheless, our plant data shows only limited evidence for a positive effect of basalt and AMF on crop yield and quality, challenging the assumption that EW and AMF consistently deliver agronomic co -benefits. This outcome may be at least partially attributed to the experimental conditions : mycorrhizal symbiosis is established by the allocation of photosynthetic C from the plant to the fungus, and an important driver of this symbiosis is low bioavailability of soil P (Leake and Read, 2017). The sufficient P availability in our experiment (due to pasteurization and fertilization) might have led to lower energy 450 investment in AMF production, hence possibly explaining the lack of basalt effect on AMF colonization and hyphal length. In addition, the short experimental duration of our trial (113 days) might have limited the AMF effect on crop yield and quality. Indeed, a meta -analysis by Qin et al. (2022) identified experimental duration as the most important factor limiting the influence of AMF on crop growth, followed by soil texture, with a smaller AMF effect on sandy soils (>50% sand). 455 Given the short experimental duration of the present experiment and texture of the used soil (sandy loam; 69.5% sand, 28.1% silt, 1.8% clay), it is likely that both factors played a role in the observed lack of AMF effect on plant biomass in our stu dy. The observed lack of a plant biomass response is similar to results by Vienne et al. (2022) , where the application of 50 ton ha-1 of basalt did not significantly affect potato growth. In other experiments, on the other hand, basalt application increased 460 biomass of spring oat (Skov et al., 2024) , sorghum (Kelland et al., 2020) , and maize (Rijnders et al., 2025) . Additional counter evidence is provided by the review of Swoboda et al. (2022) , which focused primarily on silicate rock powder applications in highly weathered, nutrient -poor soils, and which reported a positive silicate effect on crop biomass for almost all studies using mafic and ultramafic rocks as feedstock. These contrasting effects across soils and climatic regions emphasize the context-dependency of the effect of EW on crops. 465 4.4 Need for monitoring of toxic trace elements Our findings highlight the need for careful monitoring of soil and plant heavy metal concentrations following basalt amendment. Given the observed increases in pore water Zn and Ni, our results do not support the hypothesis that basalt application does not elevate the availability of heavy metals in the soil. This is noteworthy, as Ni release is among the primary environmental safety concerns associated with EW in agricultural contexts (Suhrhoff, 2022; Te Pas et al., 2023) . 470 During the 113 -day experimental period, average pore water Ni concentrations exceeded the freshwater Environmental Quality Standards (EQS) for both the EU (4 µg l⁻¹) and Australia (8 µg l⁻¹) in both control and basalt treatments (11.67 ±1.2 and 26.8 ±4.9 µg l⁻¹ respectively). This result might be partially due to background Ni present in the soil used for this experiment. Nevertheless, the observed concentrations remained below regulatory limits established by other jurisdictions, such as the United States (52 µg l⁻¹) and Canada (200 µg l⁻¹). 475 Plant heavy metal concentrations were generally unaffected by basalt amendment, with the notable exception of an increase in Ni in corn tissue (0.65 ±0.0004 and 0.42 ±0.0001 mg kg -1 for basalt +AMF and control -AMF respectively), therefore only .CC-BY-NC 4.0 International licenseperpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for thisthis version posted November 4, 2025. ; https://doi.org/10.1101/2025.11.03.686277doi: bioRxiv preprint 21 partially supporting the hypothesis that basalt amendment does not increase plant heavy metal concentrations. Despite the increase, corn Ni concentrations remained below the EU regulatory threshold of 0.8 mg kg -1 which will come into effect in 480 2026 (European Commission, 2024). Our results therefore partially contrast with previous research , where Ni concentrations did not increase upon basalt application (Kelland et al., 2020; Rijnders et al., 2025; Vienne et al., 2022). Heavy metal concentrations in the pore water and soil exchangeable complex were mostly not affected by AMF. On the other hand, AMF increased Cr concentration in the corn tissue. Therefore, our hypothesis that AMF presence would decrease 485 heavy metal availability is not supported by our data. This finding underscores that AMF inoculation does not universally confer benefits and may, under certain conditions such as in co -deployment with silicate amendment, pose risks to food safety. It is important to note that Cr concentrations in corn tissue were, regardless of treatment, higher than the average Cr concentrations found in common vegetables as identified by WHO (World Health Organization, 1988) , which is in the range of 0.005–0.03 mg Cr kg-1 fresh weight (Fig. S21). 490 5 Conclusion Despite a synergistic effect of basalt and arbuscular mycorrhizal fungi (AMF) on pore water dissolved inorganic carbon (DIC), our 113 -day mesocosm experiment did not reveal a significant AMF -induced enhancement of weathering rates, as assessed through cation mass balance, pore water alkalinity and pore water pH. These findings suggest that pore water DIC alone is not a reliable proxy for weathering dynamics in soil systems, given its sensitivity to a range of dynamic soil 495 parameters such as partial CO 2 pressure and pH. Notably, AMF had limited influence on nutrient availability, crop yield and quality under the fertilized conditions of this study. Longer -term experiments or those conducted under nutrient -limited conditions are needed to verify whether this lack of a short-term effect persists over time. In contrast to AMF, basalt amendment improved several indicators of soil fertility, including soil pH, cation exchange 500 capacity (CEC) and base saturation (particularly exchangeable Ca and Mg). These findings support the role of enhanced weathering in mitigating soil acidification and restoring base cation availability. However, these improvements did not translate into enhanced crop yield or nutrient status, with the exception of increased Mg concentrations in plant tissues. Therefore, our results indicate that the anticipated agronomic co -benefits of enhanced weathering are likely context - dependent and cannot be generalized to all soils and ecosystems. Moreover, although the increase of toxic trace elements in 505 soil and plants was limited, our results stress the importance of carefully evaluating the environmental safety of enhanced weathering for the soil under consideration. .CC-BY-NC 4.0 International licenseperpetuity. It is made available under a preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in The copyright holder for thisthis version posted November 4, 2025. ; https://doi.org/10.1101/2025.11.03.686277doi: bioRxiv preprint 22 Code and data availability The data and model code in support of our findings are openly available in Zenodo at 510 https://doi.org/10.5281/zenodo.16813184 (Boito et al., 2025). Author contributions LB and JR contributed equally to this manuscript. LB and JR: conceptualization, data curation, formal analysis, investigation, project administration, software, supervision, visualization, writing – original draft, writing – review and editing. LS: conceptualization, data curation, formal analysis, funding acquisition, investigation, software, methodology, 515 project administration, supervision, writing – review and editing. PF: investigation, methodology , writing – review and editing. MM: investigation, writing – review and editing. AV: conceptualization, formal analysis , funding acquisition, software, writing – review and editing. EV: methodology, writing – review and editing. SV: conceptualization, formal analysis, funding acquisition, methodology, project administration, resources, supervision, writing – review and editing. Competing interests 520 SV is a member of the editorial board of journal Biogeosciences.

We thank Emma Pellegrini , Martín “Gato” Carrera Larrea , Sarah Janse and Jasper Roussard for their invaluable contributions in data collection. We thank the Helmholtz Laboratory for the Geochemistry of the Earth Surface (HELGES) for carrying out elemental analyses of the porewater and the sequential extraction protocol of soil samples. 525 Financial support This project has received funding from Fonds Research Foundation -Flanders (FWO), project Grants No G000821N, G0A4821N; and from the University of Antwerp, Grant No FN 5423001. A.V. and L.S. were financially supported by the Fonds Research Foundation Flanders (FWO) Ph.D. Fellowship (Grant Nos 1S06325N and 1174925N respectively).

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