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

Abstract. 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 constrained. 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-day mesocosm experiment with Zea mays using a Belgian, sandy loam soil, we investigated the effect of basalt 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. Basalt application, but not AMF, improved soil pH, cation exchange capacity, base saturation, and generally increased exchangeable Ca and Mg, whereas most other nutrients in the pore water remained unaffected. 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. These results suggest that risk for heavy metal contamination is not generic but may arise under specific environmental conditions. Finally, despite a synergistic effect of basalt and AMF on pore water DIC, we found no indication that AMF enhanced basalt weathering rates. Overall, AMF had limited influence on soil fertility indicators and crop performance. 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, these chemical enhancements did not increase maize growth here, indicating that the agronomic benefits of basalt are context-dependent.