Long-term green manuring to substitute partial chemical fertilizer simultaneously improving crop productivity and soil quality in a double-rice cropping system

Rotation with Chinese milk vetch (MV) as winter green manure in double-rice cropping system is an efficient approach to substitute chemical fertilizer (CF). However, the optimal substitution rate of MV for CF and its long-term effects on crop productivity and soil quality remain unknown. We conducted a 13-year field experiment with different substitution rates of MV for CF (22.5 t ha−1 fresh MV incorporation with 100%, 80%, 60%, and 40% of the recommended nitrogen (N) and potassium (K) supply, represented by MV+F100, MV+F80, MV+F60 and MV+F40, respectively) to investigate its long-term effects on yield sustainability and soil quality in double rice cropping system. Compared with the sole application of CF, the MV+F100 and MV+F80 treatments significantly increased the early rice yields by 15.6% and 11.3% and the late rice yields by 9.3% and 6.3%, respectively. Utilization of MV to substitute 0–40% of the N and K in early rice season and 0–20% of the N and K in late rice season significantly increased the soil quality index (SQI). However, further increasing the substitution rate decreased the sustainability yield index (SYI) and SQI. The soil available phosphorus had a stronger effect on the early and late rice grain yield than the other soil physicochemical properties. Meanwhile, rice grain yields significantly increased with increasing SQI, and the apparent N, P and K balances positively affects SQI. The SYIs of early and late rice crops linearly increased with increasing SQIs, while the coefficient of variance in grain yield (CVy) decreased with increasing SQI. Consequently, long-term utilization of MV to substitute 0–40% of the N and K in early rice season and 0–20% of the N and K in late rice season can simultaneously improves yield sustainability and soil quality.