Abstract:The effects of incorpotation of reduced chemical fertilizer combined with Chinese Milk Vetch (Astragalus sinicus L.) on rice yield and nitrogen use efficiency was studied in subtropical double-rice cropping system. The experiment was carried out in a double-rice cropping system, and ten different treatments (two levels of reduced chemical fertilizer combined with four different amounts of Chinese milk vetch) were set to investigate the effects of different combination on the yields and nutrient use efficiencies of early- and late-rice. The results indicated that early-rice yields of all treatments which applied with 15 000~37 500 kg/hm2 Chinese milk vetch and 80% recommend chemical fertilizer showed significant increasing, and was 5.0%~13.2% higher than that of recommended fertilizer treatment (100%F). The early-rice yields of treatments which applied with 15 000~22 500 kg/hm2 Chinese milk vetch and 60% recommend chemical fertilizer showed no significant differences with that of the 100%F treatment, while the early-rice yields of treatments applied with 30 000~37 500 kg/hm2 Chinese milk vetch and 60% recommend chemical fertilizer were 5.4%~8.5% higher than that of 100%F (P<0.05). Agronomic efficiency, partial factor productivity and recovery efficiency of applied chemical fertilizer N of all treatments with Chinese milk vetch applied were all significantly higher than that of 100%F in early-rice season, but no significant differences were found in late-rice season. When nitrogen derived from Chinese milk vetch application was counted into the total nitrogen input, agronomic efficiency of applied N of all treatments with Chinese milk vetch application showed declining trends as the Chinese milk vetch increasing in early-rice season. Compared with 100%F, agronomic efficiency of applied N of all treatments with Chinese milk vetch application showed no significant differences, but N recovery efficiency was significantly decreased when the Chinese milk vetch application was 37 500 kg/hm2 and above. Soil total nitrogen content of all treatments with Chinese milk vetch application was higher than that of 100%F, but the differences were not significant (P>0.05). However, the differences of soil available nitrogen content between 100%F and all treatments with Chinese milk vetch application were significant (P<0.05). Long-term application of Chinese milk vetch combined with chemical fertilizer showed positive effects on yields and nitrogen use efficiency in both early- and late-rice seasons in subtropical double-rice cropping system. Continuous application of Chinese milk vetch was conducive for soil organic matter and total nitrogen content, and its impact on soil available nitrogen content improvement was significant. Therefore, the combination ratio of Chinese milk vetch and chemical fertilizer not only can reduce chemical fertilizer application, but also is an effective technique to improve rice yield and soil fertility in double-cropping paddy field. Based on the experiment results and experimental area condition, 60% recommend chemical fertilizer +22 500~30 000 kg/hm2 Chinese milk vetch is appropriate.