Enhanced Rock Weathering Altered Soil Organic Carbon Fluxes in a Plant Trial.

Enhanced rock weathering (EW) is gaining attention as a promising carbon dioxide removal strategy, primarily due to its potential to sequester inorganic carbon through mineral dissolution. However, the broader biogeochemical implications of EW, particularly its effects on soil organic carbon (SOC) dynamics, as well as the role played by agriculturally relevant biota such as plants and earthworms, remain poorly understood. In a 15-month mesocosm experiment with Zea mays, we investigated how EW with basalt influences soil CO efflux (SCE; normalized for soil water content and temperature) and how this is modulated by plant and earthworm presence. Using δC-CO isotope tracing based on the C-C shift method, we partitioned normalized SCE into rhizosphere (root plus microbial respiration of rhizodeposits) and soil organic matter components. In the first growing season, basalt increased normalized SCE 2.8-fold in planted mesocosms relative to planted controls, due to elevated rhizosphere respiration. In contrast, during the second growing season, basalt significantly decreased normalized SCE 16.5-fold, but only in unplanted mesocosms. Mediation analysis revealed that basalt also indirectly influenced SCE via changes in soil water content, with the direction of this effect depending on plant presence and growing season. Basalt showed contrasting direct and indirect effects, highlighting the complexity of soil responses to silicate amendments, where multiple, sometimes opposing, processes operate simultaneously. Disentangling such effects is essential for understanding the impact of EW on soil carbon and for effective and reliable upscaling. Our findings suggest that SOM stabilization might occur after basalt application, decreasing SCE. Moreover, they demonstrate that both biotic and abiotic factors (e.g., vegetation, soil fauna, soil moisture) can modulate the impact of EW on SOC dynamics. To quantify the climate change mitigation effect of EW, research must move beyond inorganic carbon and explicitly integrate biotic and organic processes into EW assessments.