Green manure incorporation with reductions in chemical fertilizer inputs improves rice yield and soil organic matter accumulation

A 6-year (2011–2016) field experiment was performed to explore the effects of partial substitution of chemical fertilizer by green manure on rice yield, sustainability yield index, and the building-up of different fractions of soil organic carbon, soil nitrogen, and phosphorus. The experiment included no fertilization (NF), chemical fertilizer only (CF100), and Chinese milk vetch (Astragalus sinicus L.) incorporation with 80%, 60%, 40%, 20%, and 0% of total N, P, and K supplied from chemical fertilizer (MVCF80, MVCF60, MVCF40, MVCF20, and MVCF0, respectively) treatments. The soil organic carbon fractions, soil nitrogen, and phosphorus fraction contents were measured over 6 years. In comparison with CF100 treatment, the MVCF80, MVCF60, and MVCF40 treatments significantly increased rice yield between 2013 and 2016, thus improving sustainability yield index. The soil organic carbon fractions increased 15–58%, 16–61%, 14–50%, and 12–33% in the MVCF80, MVCF60, MVCF40, and MVCF20 treatments, respectively, compared with the CF100 treatment (p < 0.05). The easily oxidizable nitrogen, acid hydrolysable pool II nitrogen, total nitrogen, NaOH extractable phosphorus, HCl extractable phosphorus, and total phosphorus contents in the MVCF40 treatment were 17%, 28%, 9%, 12%, 15%, and 8% higher than those in the CF100 treatment (p < 0.05). The MVCF60 and MVCF80 treatments further increased the contents of these nitrogen and phosphorus fractions compared with the CF100 treatment. Stepwise multiple linear regression analysis showed that the average yield was positively influenced by the contents of total phosphorus, easily oxidizable nitrogen, and dissolved organic nitrogen, and that the sustainability yield index was positively influenced by the contents of easily oxidizable carbon and total organic carbon. Chinese milk vetch incorporation with a 20–40% reduction in chemical fertilizer inputs may be a potential fertilization practice for improving rice productivity and sustainability.