Mixed sowing improves soil fertility via enhancing plant facilitation in degraded grassland system

Background By enhancing plant complementarity and facilitation, mixed sowing may accelerate soil fertility recovery and improve ecosystem resilience. Nevertheless, the short-term impacts of mixed sowing on soil fertility improvement and the underlying plant-soil interaction mechanisms remain poorly understood. We hypothesize that interspecific interactions in mixed sowing systems may cause changes in rhizosphere soil properties, thereby affecting soil fertility. Results To test this hypothesis, monoculture and mixed sowing treatments were designed using Elymus breviaristatus , Medicago sativa , and moss to investigate their ecological adaptation. Our results showed that compared with monocropping, mixed sowing significantly increased above and belowground biomass, improved soil water content, and reduced bulk density, thereby creating a more favorable soil microenvironment. Compared with monocropping, mixed sowing enhanced microbial biomass C and N by 26% and 10%, respectively, and increased soil organic C by 30%. Fertility indices were positive in all mixed sowing systems, contrasting with negative values under monocropping and control. Across treatments, the facilitation index was positively correlated with the fertility index (R ²  = 0.76), demonstrating that interspecific facilitation was the primary driver of soil fertility gains. Conclusions These findings reveal that mixed sowing enhances soil fertility restoration in degraded grasslands through interspecific facilitation that promotes biomass accumulation, improves soil physicochemical conditions, and stimulates microbial-mediated nutrient cycling. These mechanisms highlight crop diversification as an effective strategy for accelerating soil fertility restoration.