Soil aggregate stability and its influencing factors in Picea Schrenkiana var. Tianschanica forests of the Central Tianshan Mountains

Soil aggregate stability (SAS) is a key indicator of soil structural quality that underpins erosion resistance, water regulation, nutrient retention, and ecosystem functioning. We examined how topography and forest stand structure were associated with soil aggregate-size distribution and stability in Picea schrenkiana var. tianschanica forests in the central Tianshan Mountains, China. Water-stable aggregates and soil physicochemical properties were measured in 188 plots at two soil depths (0–20 and 20–60 cm). Linear mixed-effects models (MEMs), redundancy analysis (RDA), and structural equation modeling (SEM) were used to evaluate complementary univariate, multivariate, and hypothesized pathway-level relationships. Fine aggregates (0.25–0.053 mm) constituted the largest fraction (21.2–49.3%). Descriptively, aggregate stability tended to be higher on 25–35° slopes, in valley and downslope positions, on sunny aspects, and at elevations of 2300–2700 m. After accounting for plot-level dependence and sampling year, exposure and stand age were significant predictors of MWD, whereas slope, slope position, altitude, and canopy density were not. SOM showed the strongest positive association with SAS in the SEM (standardized β = 0.49), whereas bulk density showed a negative association (β = −0.41). The combined analyses indicate that terrain and stand structure are associated with SAS partly through variation in SOM, soil water content, and bulk density; because the study was observational, the modeled paths should not be interpreted as proof of causation. These findings identify soil organic matter retention and avoidance of compaction as priorities for maintaining soil structural resilience in high-altitude spruce forests.