Substitution of fertilizer-N by green manure improves the sustainability of yield in double-rice cropping system in south China

Rice–rice rotation is the most important intensive cropping system for food security in China. So far, few studies have examined sustainability of double-rice cropping system using partial substitution of fertilizer N (FN) by green manure (GM). The effects of 100% FN (N100) and different substitution rates of FN by GM (80%, 60%, 40% and 20% FN plus 20%, 40%, 60% and 80% N through GM, and represented respectively by N80M20, N60M40, N40M60 and N20M80) on the rice productivity and N-supplying capacity of paddy soil were evaluated in double-rice system from 2008 to 2013. Soil organic matter and total N content in the 0–15 cm layer and rice grain yield of early and late rice annually increased in N80M20 and N60M40 plots, but decreased in N100, N40M60 and N20M80 plots. Compared with N100 plots, the NH4+-N content and agronomic efficiency of applied N significantly increased in N80M20 and N60M40 plots. The grain yield and sustainable yield index of rice crops were improved in N80M20 and N60M40 plots, while declined in N40M60 and N20M80. Soil NO3−-N content decreased significantly under partial substitutions of GM for FN. It can be concluded that the appropriate substitution of GM for FN (e.g., 20–40%) is beneficial for improving the productivity and sustainability of paddy field under double-rice cropping system.