Limited effect of basalt amendment and mycorrhizal inoculation on soil chemical properties and maize growth on a nutrient-rich soil

Enhanced weathering (EW) of silicate minerals has emerged as a promising carbon dioxide removal (CDR) strategy, with potential benefits for soil fertility and crop performance. However, the soil processes that determine these co-benefits remain poorly explained. In particular, interactions between basalt amendments and soil biota such as arbuscular mycorrhizal fungi (AMF) may influence nutrient mobilization and plant uptake, but these effects have rarely been quantified. In a 113 d mesocosm experiment with Zea mays using a Belgian, sandy loam Plaggic anthrosoil (pH = 5.6), we investigated the effect of basalt (applied at 50 t ha ⁻¹ ) and AMF inoculation on soil properties, nutrient and heavy metal availability, and crop yield and quality. We also assessed potential AMF-driven bio-weathering via cation mass balance and pore water dissolved inorganic carbon (DIC), pH, and alkalinity measurements. In our experiment, basalt application, but not AMF, improved soil pH (+0.71 ± 0.14), cation exchange capacity (+0.43 ± 0.18 meq 100 g ⁻¹ ), base saturation (+13.6 %± 3.5 %), and generally increased exchangeable Ca and Mg in the 0–40 cm soil profile (+80.2 ± 24.6 and +23.9 ± 4.4 mg per kg soil respectively), whereas most other nutrients in the pore water remained unaffected (all values are mean ± SE). Crop yield and quality were largely unaltered by basalt or AMF, except for an increase in plant Mg with basalt application. Moreover, heavy metal availability and plant uptake were also generally unaffected, with the notable exception of soil pore water and corn Ni, which increased with basalt (+0.02 ± 0.005 mg L ⁻¹ ; +0.0002 ± 0.0001 mg per g dry biomass; mean ± SE). These results suggest that risks associated with heavy metal contamination are not generic, but may depend on local soil conditions. In our experiment, elevated background Ni concentrations likely contributed to pore water Ni concentrations exceeding the environmental quality standards of the European Union and Australia even in the control treatments. Finally, we found no clear evidence that AMF enhanced basalt weathering rates. Overall, AMF had limited influence on soil chemical fertility and crop performance. On the other hand, basalt application improved key soil chemical indicators and increased the exchangeable fractions of Ca and Mg, demonstrating its role as a soil improver. Unlike several studies conducted in more acidic soils however, these chemical enhancements did not increase maize growth here, suggesting limited short-term agronomic benefits of basalt under the fertile soil studied here.