Organic co-amendments reshape soil microbial community responses to enhanced rock weathering in croplands

Soil microbial responses to enhanced rock weathering (ERW) in agricultural systems remain poorly characterized, despite the known role of microbes in biogeochemical processes and mineral weathering. We evaluated how a crushed andesitic metavolcanic rock amendment, applied alone or with organic co-amendments (compost and/or biochar), affected bacterial and fungal communities over a three-year maize field trial. Adding rock alone altered bacterial alpha diversity in the second year, with only isolated differences in the third. Fungal alpha diversity shifted only under organic co-application in year two and was otherwise unaffected by rock. Differential abundance analysis showed that rock and organic additions selectively enriched microbial taxa with reported roles in mineral weathering, organic matter decomposition, and nutrient cycling. This compositional shift tracked shifts in soil chemistry: canonical correspondence analysis showed organic carbon and nitrogen as the strongest correlates early on, and pH and ions later. Together, these results show how ERW reshapes microbial community composition over time as environmental conditions change. Individual taxa were replaced by others, but this turnover did not cause a lasting drop in overall diversity. The changes depended on whether organic co-amendments were present, showing that ERW's effects on soil microbes are altered by organic inputs.