Sustainable Rice Farming: The Benefits of Substituting Fertilizer-N with Milk Vetch for Improved Soil Structure and Quality

Abstract Purpose Chinese milk vetch (MV) is widely used in rice yield enhancement because of the huge nitrogen (N) substitution potential. However, the proper substitution rate of MV for N fertilizer and its effect on carbon sequestration and nutrient retention in soil aggregates remains unknown. Method A 10-year field experiment was conducted to investigate the effects of different MV substitution rates on soil aggregate stability, nutrient retention, and soil quality in a double rice cropping system. The treatments included no fertilizer (CK), 100% NPK fertilizer (N₁₀₀), recommended N supply by different proportions of MV (N₈₀G₂₀, N₆₀G₄₀, N₄₀G₆₀, N₂₀G₈₀) Result Compared with the N₁₀₀ treatment, the N₈₀G₂₀ and the N₆₀G₄₀ treatment increased the mean weight diameter (MWD) by 4.2% and 5.3%, and the geometric mean diameter (GMD) by 7.7% and 12.1%, respectively. The N₆₀G₄₀ treatment significantly increased the labile organic carbon content and carbon pool management index by 24.7% and 45.0%, respectively. N₈₀G₂₀ and N₆₀G₄₀ treatments directly increased total nitrogen (TN) and total phosphorus (TP) in macro-aggregates (> 0.25mm), and improved the contribution of total nutrients in > 2mm aggregate. Compared with the N₁₀₀ treatment, the N₆₀G₄₀ treatment improved TN, TP and TK by 6.0%, 9.3% and 5.6%. Incorporating MV improved the soil quality index (SQI), with N₆₀G₄₀ treatment improved the most by 34.1%. And the grain yield increased significantly with the increasing SQI. Substituting 20–60% of N by MV can sustain grain yield. However, a higher substitution rate significantly reduced grain yield, particularly in the early rice. Conclusions Consequently, Incorporating MV to substitute 20–40% N fertilizer can enhance soil structure by improving the proportion of macro-aggregates, thereby improving nutrient retention and soil quality. This study provides a sustainable and eco-friendly approach in the double rice cropping systems.