Enhancement of Ecosystem Multifunctionality in Altay Natural Mowing Grasslands by Mixed Grass Species Overseeding

Under the combined influence of climate change and long-term mowing pressure, natural mowing grasslands in the Altai Mountain meadow region of Xinjiang have undergone degradation, primarily manifested as a decline in the proportion of high-quality forage species and an increase in forbs, which has severely constrained grassland-based livestock production and regional ecological security. For the restoration of degraded natural mowing grasslands, systematic assessments of the effects of legume–grass mixture overseeding on ecosystem multifunctionality (EMF) are still lacking; existing studies have mostly focused on single ecological functions, and the understanding of how different species mixtures drive synergistic vegetation–soil system recovery and the underlying mechanisms remains unclear. This study targeted degraded natural mowing grasslands in Altai and selected seven species: Onobrychis viciifolia cv. Qitai, Medicago sativa cv. Xinmu No. 4, Trifolium pratense cv. Minshan, Dactylis glomerata, Poa pratensis, Bromus inermis cv. Wusu No. 1, and Elymus dahuricus. Overseeding mixtures with different species compositions were established under a uniform legume–grass ratio of 2:8. Through a fixed-point field observation experiment conducted from 2024 to 2025, we integrated indicators of quantitative community characteristics, forage nutritional quality, soil physical properties, and soil chemical properties to construct aboveground EMF (AEMF), belowground EMF (BEMF), and overall EMF indices. The effects of different legume–grass mixtures on the restoration of degraded natural mowing grasslands were evaluated, candidate mixtures suitable for different restoration goals were screened, and the driving mechanisms were elucidated. The results showed that: (1) The restoration effects of different legume–grass mixtures on degraded natural mowing grasslands differed markedly. Community composition changed after overseeding, and some mixtures rapidly formed a grass-dominated community structure. (2) Superior mixtures significantly increased community cover and aboveground biomass, improved forage quality, and enhanced soil fertility to a certain extent. (3) Overseeding resulted in a greater improvement in aboveground EMF than in belowground EMF. In the first year, EMF exhibited synchronous enhancement across all functions, whereas in the second year, the system shifted to a phase of functional reorganization. (4) Based on the 2024–2025 field trial results, candidate legume–grass mixtures suitable for different restoration objectives were preliminarily identified: for comprehensive ecological restoration, a mixture of 5% Onobrychis viciifolia cv. Qitai + 15% Trifolium pratense cv. Minshan + 15% Dactylis glomerata + 15% Poa pratensis + 50% Bromus inermis cv. Wusu No. 1 is recommended; for rapid productivity recovery, a mixture of 10% Trifolium pratense cv. Minshan + 10% Medicago sativa cv. Xinmu No. 4 + 30% Poa pratensis + 50% Bromus inermis cv. Wusu No. 1 is recommended; and for producing high-quality forage, a mixture of 10% Medicago sativa cv. Xinmu No. 4 + 10% Trifolium pratense cv. Minshan + 30% Dactylis glomerata + 50% Bromus inermis cv. Wusu No. 1 is recommended. This study clarifies the goal-specific suitability of different legume–grass mixtures in terms of productivity enhancement, quality improvement, and soil function recovery, and provides a reference for the ecological restoration and subsequent regional verification of degraded natural mowing grasslands in the Altai Mountain meadow area.