Mixed Seeding of Annual Ryegrass–Chinese Milk Vetch Sustains High Aboveground Biomass and Soil Fertility in Southern China

Winter fallow fields are widely distributed in southern China, where legume–grass mixtures are considered a useful approach for improving land-use efficiency and forage productivity while maintaining agroecosystem functions. Previous studies on the use of winter fallow fields have generally focused either on monoculture or on crops subjected to a single cut. These studies have primarily assessed aboveground productivity, whereas soil bacterial community responses have received comparatively little attention. Consequently, limited information is available on how different seeding combinations and cutting times affect aboveground biomass and soil bacterial community structure in mixed-cropping systems. A fixed-site field experiment involving annual ryegrass (Lolium multiflorum L.) and Chinese milk vetch (Astragalus sinicus L.) was conducted on yellow clay soil. Five annual ryegrass-Chinese milk vetch seeding combinations (100:0, 75:25, 50:50, 25:75, and 0:100) were established as proportions of their respective monoculture seeding rates. The same plots were maintained for five consecutive winter growing seasons, from autumn 2021 to spring 2026. Plant and soil samples were collected only during the final growing season (2025–2026), at four cutting times in January, March, April, and May. These samples were used to assess the effects of seeding combination and cutting time on forage productivity, root traits, soil fertility, and soil bacterial community structure. Among all groups, the 75% annual ryegrass + 25% Chinese milk vetch group (R75M25) exhibited the best overall performance, with cumulative yields after four cuttings of 17,274.21 kg·ha−1 dry matter, 2717.02 kg·ha−1 crude protein, and 12,220.46 kg·ha−1 digestible dry matter. In addition, the R75M25 group maintained relatively high contents of alkali-hydrolyzable nitrogen, available phosphorus, and available potassium in soil. The top three phyla of soil samples were Proteobacteria, Actinobacteria, and Acidobacteria, with a total relative abundance of >60%. Among these, Proteobacteria showed the greatest relative abundance in monoculture, whereas the seeding combination had a reduced proportion of it. In addition, its relative abundance rose steadily with cutting time. At the genus level, RB41, Gemmatimonas, and Sphingomonas dominated the soil bacterial community. The seeding combinations did not alter soil bacterial alpha diversity. Cutting times significantly reduced the phylogenetic diversity index and drove the temporal differentiation of taxa such as Actinobacteria and Bacteroidetes. The inclusion of an appropriate proportion of Chinese milk vetch improved forage nutritional value while maintaining high biomass accumulation in southern China, with the R75M25 group showing the best overall performance in winter fallow fields.