Mixed Cropping Improves Soil Fertility via Enhancing Plant Facilitation in Alpine Artificial Grassland

By enhancing plant complementarity and facilitation, mixed cropping may accelerate soil fertility recovery and improve grassland ecosystem resilience. Nevertheless, the impacts of mixed cropping on soil fertility over a single growing season, and the underlying plant-soil interaction mechanisms, remain poorly understood. We hypothesized that mixed grass-legume-moss sowing would increase above- and belowground biomass, aggregate stability (mean weight diameter), and soil organic carbon and total nitrogen relative to monocultures, partly through relief of the nitrogen limitation of the grass by the nitrogen-fixing legume. To test this hypothesis, seven monoculture and mixed-cropping treatments using Elymus breviaristatus, Medicago sativa, and Tortula subulata were established in May 2024 with three replicate plots per treatment (n = 3); plant and soil variables were measured at the end of the growing season (late September 2024), and differences among treatments were tested by one-way ANOVA followed by Tukey's HSD post hoc test, with significance accepted at p < 0.05. Our results showed that compared with monocropping, mixed cropping 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 cropping 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-cropping systems, contrasting with the negative values observed for two of the three monocultures and for the unseeded control. Across treatments, the facilitation index was positively correlated with the fertility index (R² = 0.757), indicating that interspecific facilitation was closely associated with soil fertility gains. These findings suggest that mixed cropping can enhance soil fertility restoration in alpine artificial grassland and that this effect is consistent with interspecific facilitation promoting biomass accumulation, improving soil physicochemical conditions, and supporting soil microbial biomass. These results highlight species diversification as a promising strategy for accelerating soil fertility restoration, although multi-year and multi-site validation is still required.