Soil and crop responses to basalt, tephrite and lime amendments in a 16-month maize field trial

Enhanced weathering (EW) of silicate minerals has been proposed as a carbon dioxide removal (CDR) strategy with potential co-benefits for soil health and crop productivity. Among the proposed feedstocks, basalt is considered a good candidate feedstock due to its lower heavy metal content compared to olivine-rich rocks such as dunite. However, basaltic feedstocks differ substantially in mineralogical composition, and the implications of this variability for EW co-benefits remain poorly understood, particularly in relatively fertile agricultural soils. In this field experiment, we applied two basalt (Bolsdorfer and Hühnerberg) and a tephrite (Eifelgold) amendment (all at 50 ton ha-1), which differed in mineralogical composition, as well as lime (applied at 2 ton ha-1), to a Belgian maize field (Gleyic Podzol with loamy-sand texture; topsoil pH = 6.66 ± 0.03; cation exchange capacity (CEC) = 7.6 ± 0.13 cmol+ kg-1). We assessed the feedstocks’ potential to affect topsoil (0-10 cm) volumetric water content (VWC), chemical fertility and crop performance over 16 months. Hühnerberg, Bolsdorfer and lime increased pore water alkalinity (+87.9%, +65.1%, +51.6% respectively) and DIC (+111.0%, +97.2%, 74.4% respectively), indicating active weathering. Other treatment effects were modest and mostly short-lived. Bolsdorfer, Hühnerberg and lime significantly increased soil pH (+0.52 ± 0.06, + 0.53 ± 0.09, +0.37 ± 0.08 respectively; mean ± SE) and base saturation (+21.1% ± 4.0, +18.8% ± 6.0, +20.8% ± 6.8 respectively; mean ± SE) after the first growing season, but these early responses dissipated over time. None of the treatments significantly affected CEC, while effects on VWC were growing season- and treatment-specific. Crop biomass and quality, including plant Ni and Cr content, were unaffected, aside from minor shifts in nutrient concentrations. Overall, EW co-benefits for soil water content, soil chemical fertility and crop biomass were limited and dependent on amendment mineralogy. The observed short-term improvements in soil chemical fertility did not translate into greater crop biomass, suggesting that the agronomic benefits of basalt amendments may be limited in relatively fertile, near-neutral soils. Importantly, the absence of adverse effects on crop biomass and quality suggests that silicate rock application did not compromise crop performance under the conditions investigated.