Substitution of Inorganic Nitrogen Fertilizer with Green Manure (GM) Increased Yield Stability by Improving C Input and Nitrogen Recovery Efficiency in Rice Based Cropping System

A long-term field experiment was carried out (since 2008) for evaluating the effects of different substitution rates of inorganic nitrogen (N) fertilizer by green manure (GM) on yield stability and N balance under double rice cropping system. Treatments included, (1) N-0 (no N fertilizer and no green manure); (2) N-100 (recommended rate of N fertilizer and no green manure); (3) N-100-M (recommended rate of N fertilizer and green manure); (4) N-80-M (80% of recommended N fertilizer and green manure); (5) N-60-M (60% of recommended N fertilizer and green manure); and (6) M (green manure without N fertilization). Results showed that, among all treatments, annual crop yield under N-80-M treatment was highest. Crop yield did not show significant differences between N-100-M and N-80-M treatments. Substitution of different N fertilizer rates by GM reduced the yield variability index. Compared to the N-0 treatment, yield variability index of early rice under N-100-M, N-80-M, and N-60-M treatments was decreased by 11%, 26%, and 36%, respectively. Compared to the N-0 treatment, yield variability index of late rice was decreased by 12%, 38%, 49%, 47%, and 24% under the N-100, N-100-M, N-80-M, N-60-M, and M treatments, respectively. During period of 2009-2013 and 2014-2018, nitrogen recovery efficiency (NRE) was highest under N-80-M treatment and N balance was highest under N-100 treatment. NRE of all treatments with GM was increased over the time from 2009-2013 to 2014-2018. All treatments with GM showed increasing trend of SOC over the years. Substitution of N fertilizer by GM also increased C inputs and soil C:N ratio compared to the N-100 and N-0 treatments. Boosted regression model indicated that C input, N uptake and AN were most influencing factors of crop yield. Thus, we concluded that N fertilization rates should be reduced by 20% under GM rotation to attain high yield stability of double rice cropping system through increasing NRE and C inputs.