Exploring soil genes as proxies for short-term SOC and N dynamics: Insights from a four-year on-farm experimentation

The sustainability of different management practices to enhance soil organic carbon (SOC) sequestration and reduce nitrogen (N) losses remains debated due to site-specific variability. Investigating soil physicochemical and biological factors is crucial, and soil functional genes can serve as key indicators of nutrients cycling and soil health. The present 4-year on-farm experiment in northeastern Italy, developed in collaboration with local farmers and a land reclamation authority, investigated the effects of two organic fertilizers (compost or digestate), a triticale (X triticosecale) cover crop treatment (CC) under rainfed vs. irrigated conditions on SOC, total N (nitrates - NO3- and Total Kjeldahl Nitrogen -TKN), and soil N and C functional genes in a biennial maize-soybean rotation. Compost application led to higher short-term SOC accumulation and greater GH48 cellulolytic enzyme activity than digestate supposedly due to its more recalcitrant molecules. In compost treatments, higher SOC levels enhanced microbial denitrification (nosZ). While triticale CC did not impact SOC, it reduced soil NO3content. Digestate exhibited greater competition with microbial N fixation (lower nifH gene copies) than compost. Elevated NO3- levels in the soil stimulated nitrification activity (AOA) across all management practices. A general increase in N, SOC, and microbial-mediated activities throughout the four experimental years suggested an overall enhancement of soil biological fertility. Agronomic management strategies influenced N and SOC dynamics, and functional genes linked to C and N cycling showing their potential as proxies for tracking these changes. However, while gene abundance was sensitive to management practices, environmental conditions also influenced the results, making it challenging to isolate single key factors.