Effects of long-term green manure and reducing nitrogen applications on rice yield and soil nutrient content

Green manure is a natural clean organic source of fertilizer. Chinese milk vetch is the most popular winter green manure crop in rice-growing regions of China. In this study, a long-term field experiment was conducted at an experimental station managed by National Observation and Research Station of Farmland Ecosystem in Qiyang, China to explore suitable application amount of nitrogen fertilizer when returning green manure(Chinese milk vetch, Astragalus sinicus L.) to the field in the double-season rice area of southern Hunan. The double cropping rice experiment began from the winter of 2008. It lasted 9 years. A total of six treatments with different fertilization practices were included: 1) CK, without fertilization; 2) MV, Chinese milk vetch only; 3) FFP100%, conventional application rate of chemical nitrogen fertilizer; 4) MV+FFP100%, Chinese milk vetch combined with 100% conventional application rate of chemical nitrogen fertilizer; 5) MV+FFP80%, Chinese milk vetch combined with 80% conventional application rate of chemical nitrogen fertilizer; 6) MV+FFP60%, Chinese milk vetch combined with 60% conventional application rate of chemical nitrogen fertilizer. The yield of rice and soil nutrient content were measured. The nitrogen agronomic efficiency and nitrogen partial factor productivity, coefficient of variation and sustainable index of yield were calculated. The results showed that the treatments of green manure combined with different chemical nitrogen fertilizer application rate kept and even increased annual rice yield. The treatments of MV combined with reducing nitrogen application also increased the nitrogen agronomic efficiency and nitrogen partial factor productivity. Compared to FFP100%, the treatments MV+FFP100%, MV+FFP80% and MV+FFP60% resulted in the increase of long-term average of nitrogen agronomic efficiency by 34.63%, 64.91% and 115.26% in early rice, respectively, and by 29.69%, 56.23% and 72.09% in late rice, respectively(P<0.05). Compared to that of FFP100%, the long-term averages of nitrogen partial factor productivity of MV+FFP100%, MV+FFP80%, MV+FFP60% increased by 8.73%, 35.06% and 78.92% in early rice, respectively, and 7.51%, 32.90% and 68.06% in late rice, respectively. Among all the treatments, regardless of early and late rice, the treatment of MV+FFP60% had the smallest coefficient of variation of yield and the highest sustainable index. Soil organic matter, total nitrogen, total phosphorus and total potassium were not significantly different among the treatments of FFP100%, MV, MV+FFP100%, MV+FFP80%, MV+FFP60% (P>0.05). But, they were higher than the initial values in 2018. The treatment of Chinese milk vetch combined with 60% conventional application rate of chemical nitrogen fertilizer could achieve high and stable yield, and improve the nitrogen use efficiency and slowly improve soil fertility. Therefore, Chinese milk vetch combined with 60% conventional application rate of chemical nitrogen fertilizer was suggested as the best fertilization mode for double-cropping rice planting area in southern Hunan. In this case, the chemical fertilizer application rate could be decreased by 40% compared to the conventional fertilizer practice.