COVER CROPS AND RESIDUAL PHOSPHORUS: PATHWAYS TO SOIL ORGANIC MATTER

Soil organic matter is a key component of soil fertility and nutrient cycling in agricultural systems, playing a central role in long-term production sustainability. Continuous management practices can enhance or impair these processes, particularly in systems dependent on non-renewable inputs such as phosphate fertilizers. In this context, this study evaluated 15 years of the interaction between winter cover crops and residual soil phosphate addition on soil organic carbon (SOC), total nitrogen (N) stocks, and chemical properties in a long-term field experiment in a clayey soil from southern Brazil. The experiment comprised seven cropping seasons with P application using sources of contrasting solubility (rock phosphate and soluble phosphate), followed by eight seasons relying exclusively on residual soil P. Winter cover crops included common vetch (Vicia sativa), white lupin (Lupinus albus), fodder radish (Raphanus sativus), ryegrass (Lolium multiflorum), and black oat (Avena strigosa), as well as a fallow treatment. The results of SOC and N stocks were significantly influenced by cover crops, regardless of phosphate source. Fodder radish and black oat promoted higher SOC stocks in the 0–30 cm soil layer, exceeding 90 Mg ha⁻¹, while fallow showed lower values, 83 Mg ha⁻¹. Black oat also resulted in the highest soil N stocks, ~8 Mg ha⁻¹, whereas other cover crops and fallow treatments remained <7 Mg ha⁻¹. These results indicate that the integration of residual soil phosphorus with cover crop management sustains soil organic matter and nutrient cycling over the long term, reinforcing the importance of integrated strategies for agricultural sustainability.