Synergistic effects of basalt and earthworms: enhanced nutrient availability and altered heavy metal dynamics

Abstract. Enhanced weathering (EW), the application of crushed silicate rocks such as basalt to soils, has been proposed as a carbon dioxide removal (CDR) strategy. Besides CO₂ sequestration, EW can influence soil chemistry and crop nutrition, providing agronomic co-benefits, but it may also release heavy metals with potential risks for food safety. Soil organisms such as earthworms, which strongly influence nutrient cycling, and mineral dissolution, may further interact with EW processes, yet their role remains poorly understood. Here, we conducted a full-factorial experiment to test the combined effects of basalt and earthworms on soil chemistry, and growth, nutrients and heavy metal concentrations of Zea mays. Basalt significantly increased porewater pH, alkalinity, and nutrient concentration (Ca, Mg, Na, and Si). Earthworms further amplified most of these responses, suggesting increased weathering rates. Plant nutrient concentrations were in general positively affected by basalt and earthworms, while plant biomass remained unaffected. Furthermore, basalt decreased heavy metal concentrations in the aboveground plant parts, but V, Ni, and Al accumulated in their roots with basalt application. Basalt and earthworms thus show promise in improving agricultural soils, but trade-offs related to heavy metal accumulation must be addressed to optimize their use in sustainable agriculture.