Low legume-grass seeding ratio combined with phosphorus fertilization promotes forage yield and soil quality in managed grasslands

Legume-grass mixtures are often used to increase forage yield and soil fertility in managed grasslands, but it remains unclear whether these benefits could be further improved by utilizing optimal legume-grass seeding ratios and phosphorus (P) fertilization. Here, we conducted a 5-year field experiment across 3 sites to investigate the effects of legume-grass seeding ratio and P fertilization on forage yield and soil quality. This experiment included mixtures comprised of two legume species and two grass species at five legume-grass ratios (3:7, 4:6, 5:5, 6:4, and 7:3), and monoculture of each species. P fertilizer was applied at the rate of 0, 9, 18, or 27 kg P ha^-1 year^-1. Plant diversity effects, e.g., complementarity and selection effects, were assessed by comparing yield of mixtures with monocultures. Our results show that a legume-grass mixture with a seeding ratio of 3:7 under moderate P fertilization resulted in the highest forage yield among all monocultures and mixtures, for each of the three sites, with persistent and consistent transgressive overyielding. Notably, greater soil organic matter, total nitrogen, enzymatic activity, and microbial biomass were observed with the legume-grass ratio of 3:7, compared to monocultures or other seeding ratios. Structural equation modeling indicated that the legume-grass ratio of 3:7 achieved highest yield through directly improving complementarity effect, and indirectly promoting selection effects because of increased fungal biomass. P fertilization directly enhanced soil nutrient and enzymatic activities, and further improved complementarity effect, resulting in high forage yield and soil quality. These results indicate that forage diversification practices with low legume-grass seeding ratios and moderate P fertilization can simultaneously benefit forage production and soil quality in managed grasslands. Overall, our study suggests that low legume seeding proportion in legume-grass mixtures combined with moderate nutrient management is a useful strategy for sustainable and highly productive managed grasslands.