Early-stage effects of carbon-rich soil amendments stimulate retention-related nitrogen genes while maintaining nitrogen and yield levels

RCW, particularly at high doses (RCW-HD), increased soil organic carbon and microbial biomass at an early stage. Despite a 50% reduction in organic fertiliser use, RCW-HD did not reduce N availability and crop productivity, suggesting improved N use efficiency. Several N-cycling gene abundances were elevated under CTL compared to RCW-HD, including the nitrification-related pmoA-amoA (+42%) and pmoC-amoC (+72%), and the denitrification-related nosZ (+14%). The RCW-HD no-till system increased nitrate reduction assimilation (+13% nrtABC) and favoured N-fixing bacterial genera such as Terrihabitans, Ferriphaselus, Azospira and Rhodopseudomonas. Soil depth significantly influenced 72% of the N-cycling genes, with key genes being more abundant at the surface. These results highlight the potential of RCW to improve N retention and soil fertility, while reducing fertiliser dependence and greenhouse gas emissions. They also support sustainable practices in regenerative agriculture by highlighting how microbiomes contribute to the efficiency of nitrogen cycling.

Keywords: Shotgun metagenomics; Biogeochemical process; Microbial activity; Organic agriculture; Soil organic matter; Soil regeneration.