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

Abstract

Simple Summary Mixed cropping serves as a promising restoration approach for degraded alpine artificial grasslands. However, the effects of mixed-cropping plant assemblages on soil fertility over a single growing season, and the underlying plant–soil interaction mechanisms, remain poorly elucidated. This field experiment was conducted in Haiyan County, Haibei Prefecture, Qinghai Province, on the Qinghai–Xizang Plateau in 2024, where we established monoculture and mixed cropping treatments using Elymus breviaristatus, Medicago sativa, and Tortula subulata. We quantified above-and belowground biomass, soil physicochemical properties, microbial nutrients, and fertility indices to clarify how interspecific interactions modulate soil conditions. Mixed cropping enhanced plant biomass, optimized soil physical microhabitats, and elevated soil organic carbon and microbial biomass relative to monocultures and the unseeded control. A strong positive correlation between the facilitation index and the fertility index (R2 = 0.757) indicated that interspecific facilitation was closely associated with the improvement of soil fertility. This work provides preliminary, single-season evidence that mixed cropping is associated with accelerated soil restoration, offering a candidate strategy for the rehabilitation of degraded alpine grasslands.