Application of milk vetch (Astragalus sinicus L.) with reduced chemical fertilizer improves rice yield and nitrogen, phosphorus, and potassium use efficiency in southern China

Milk vetch (Astragalus sinicus L.; MV) is the most common winter green manure used in rice cropping systems in southern China. The utilization of MV can achieve the dual goals of soil fertilization and increased rice yield. Two long-term field experiments were conducted in Yujiang and Gaoan in Jiangxi Province, a typical double-rice cropping area, to explore the effects of 80 % conventional fertilizer for early rice supplemented with 15.0 (G1F80), 22.5 (G2F80), 30.0 (G3F80), and 37.5 (G4F80) t ha(-1) MV on rice yield, soil fertility, nutrient uptake, and nutrient use efficiency. The results showed that, the yield data obtained from the 5-year (Gaoan) and 12-year (Yujiang) long-term experiments demonstrated that compared with conventional fertilization (F100), treatments with MV markedly increased the yield of early and late rice by 2.5-8.8 % and 5.6-10.9 %, respectively, and high rates of MV (G3F80 and G4F80) performed better in grain yields. The nitrogen (N) and potassium (K) uptake of early rice in Gaoan increased by 38.1 and 34.8 kg ha(-1), respectively, and the N uptake of late rice in Gaoan and Yujiang increased by 11.7 and 62.6 kg ha(-1) in G2F80, respectively, compared to that in F100. Compared with F100, treatments with MV significantly increased phosphorus (P) fertilizer recovery efficiency by 23.1-36.4 %, and the agronomic efficiency of N, P, and K fertilizers increased by 16.1-21.1 %, 13.9-19.6 %, and 15.7-21.4 %, respectively. Partial least squares path model analysis indicated that soil available P was the most important factor affecting the nutrient uptake of rice, and available K was the most important factor affecting fertilizer use efficiency. Our results suggested that 22.5 t ha(-1) MV supplemented with 80 % chemical fertilizer is recommended for double-rice cropping sites to improve rice yield and crop uptake and the use efficiency of N, P, and K fertilizers.