Soil Nutrients Drive Microbial Changes to Alter Surface Soil Aggregate Stability in Typical Grasslands

The composition and stability of soil aggregates are important indexes for evaluating soil quality. They are influenced by many factors in the soil system to varying degrees. The management of soil health is necessary to understand the importance and mechanism of the key factors in the composition and stability of soil aggregates. In this study, topsoil (0–10 cm) in typical grassland with different degradation gradients (non-degradation, light degradation, moderate degradation, and heavy degradation) caused by grazing was taken. The changes in soil aggregates and their related factors and the main driving mechanisms affecting the changes of topsoil aggregates were analyzed. With the increase in degradation gradient, the content of large macro-aggregates (> 2 mm) decreased, while the content of micro-aggregates (0.25–0.053 mm) increased. The composition of silt + clay size fraction (< 0.053 mm) changed parabolically with the increase in degradation gradient. The soil aggregate stability index (mean weight diameter (MWD), geometric mean diameter (GMD), and > 0.25 mm aggregate content (R0.25) tended to decrease with the increase in degradation gradient. Soil aggregate stability was significantly positively correlated with main soil physicochemical properties and microorganisms. The main nutrients of topsoil affected the change of microorganisms, which explained the variance of 75.0% of soil aggregate stability. Our results indicated that soil nutrient-driven microbial changes might be the dominant factor that caused the changes in the stability of surface soil aggregate.