Effects of Grazing Exclusion on Soil Aggregate Composition and Stability in Degraded Alpine Meadows

Grazing exclusion is a widely implemented measure for restoring degraded alpine meadows on the Qinghai-Tibetan Plateau, yet its effects on soil aggregate composition and stability remain unclear. In this study, 0-20 cm soil samples were collected from a degraded alpine meadow, an alpine meadow subjected to 4 years of grazing exclusion, and an alpine meadow subjected to 12 years of grazing exclusion in the Yangtze River source region. Dry sieving and wet sieving methods were employed to separate mechanically stable and water-stable aggregates of different sizes. Key aggregate stability indices, including mean weight diameter (MWD), geometric mean diameter (GMD), mass fraction of macroaggregates (R_0.25), and percentage of aggregate destruction (PAD), were determined to assess changes in aggregate stability following grazing exclusion. Furthermore, plant, soil, and microbial properties were measured to identify the key drivers underlying soil structural recovery. The results showed that the mass fractions of dry-sieved > 5 mm and wet-sieved > 0.25 mm aggregates increased after grazing exclusion, with significant differences observed after 12 years of enclosure, indicating that long-term grazing exclusion promoted the formation of macroaggregates. In contrast, the mass fractions of dry-sieved < 2 mm and wet-sieved < 0.25 mm aggregates significantly decreased after 12 years of enclosure, reflecting a shift in aggregate size distribution toward coarser fractions. The stability of soil aggregates followed the order GE12 > GE4 > DM. After 4 years of enclosure, water-stable aggregate stability significantly increased, while both mechanical and water-stable aggregate stability showed significant improvements after 12 years. Additionally, the PAD decreased significantly from 47.44% in DM to 15.98%-20.20% in GE4 and GE12. Random forest and partial least squares path modeling (PLS-PM) analyses revealed that soil pH, plant biomass, soil total organic carbon, and microbial biomass carbon are key drivers of soil aggregate stability. In conclusion, grazing exclusion enhanced aggregate stability by promoting the formation of macroaggregates, demonstrating its effectiveness as a management strategy for restoring soil structure in degraded alpine meadows.