Objectives The application of milk vetch (Astragalus sinicus L.) combined with reduced nitrogen fertilizer is an effective measure to ensure stable and increased yield of rice. This study investigated the effects of milk vetch combined with a 40% reduction of chemical N fertilizer on grain yield, brown rice amino acid content and composition across different rice-producing regions.

Methods Field experiments were established in Anhui, Henan, Jiangxi and Hunan provinces; those in Henan and Hunan began in 2008, whereas Anhui and Jiangxi started in 2017 and 2016, respectively. Cropping systems were milk vetch-single rice or milk vetch-early rice-late rice. All sites included five treatments: winter fallow without fertilization (CK); winter fallow with 100% chemical N (F100); milk vetch without fertilization (MV); milk vetch combined with 60% chemical N (MF60); milk vetch combined with 100% chemical N (MF100). Grain yield, protein concentration, total amino acid content, essential amino acidc ontent, and non-essential amino acid content in brown rice were determined in 2020.

Results There was no significant difference in rice yield and grain protein content among MF60, MF100 and F100. Compared with F100, the total amino acid content of brown rice under MF100, MF60 and MV treatments increased by 32.8%, 62.6% and 35.7%, respectively, and the essential amino acid content of MF60 treatment increased by 30.7%. Among the essential amino acids, the content of isoleucine and phenylalanine in MF60 treatment increased by 290.6% and 37.1% respectively compared with F100 treatment, and the content of phenylalanine in MV treatment increased by 45.1% compared with F100 treatment. Among the non-essential amino acids, the contents of proline, cysteine and tyrosine in MF60 treatment increased by 309.3%, 80.1% and 279.5% respectively compared with F100 treatment, and the contents of histidine, proline and cysteine in MV treatment increased significantly by 139.7%, 180.5% and 103.1% compared with F100 treatment. Among the sampling sites, the green manure treatment in Anhui Province had the most significant increase in the amino acid content of brown rice. The total amino acid content and the essential amino acid content of brown rice under MF60 increased by 190.8% and 108.5%, respectively, compared with F100 treatment. Correlation analysis showed significant positive relationships between grain yield and protein content, and between total amino acid content and the essential amino acid content in brown rice.

Conclusions Milk vetch-rice rotation combined with 40% reduction in nitrogen fertilizer can maintain rice yield and improve the nutritional quality of brown rice, especially increase the amino acid content, demonstrating an important measure to improve the quality and efficiency of rice in southern China.