Soil resource availability rather than heterogeneity controls shrubland community structure through water–nutrient interactions in an arid mountainous region

Soil resource limitation is a key driver of plant community assembly in arid and semi-arid ecosystems. However, the relative importance and underlying mechanisms of soil resource availability and heterogeneity in regulating plant community structure in mountainous ecosystems remain poorly understood. In this study, we investigated shrub-dominated ecosystems in the shallow mountainous region of the Helan Mountains to evaluate the effects of soil resource availability and heterogeneity on shrub and herbaceous community structure and to elucidate the underlying regulatory pathways. Our results showed that both soil resource availability and heterogeneity significantly influenced shrub and herbaceous community abundance and species diversity, and significantly contributed to variations in shrub and herbaceous community composition. Among these factors, soil resource availability exhibited a stronger regulatory effect. Structural equation modeling further revealed that soil resource availability regulated shrub and herbaceous community abundance and species diversity through a hierarchical pathway, in which soil physical properties influenced water availability, subsequently affected nutrient availability, and ultimately shaped plant community abundance and species diversity. Among the soil properties, soil depth and soil moisture played critical roles. This study demonstrates that soil resource availability plays a predominant role in regulating plant community assembly in arid mountain ecosystems characterized by shallow soils and weathered bedrock. These findings enhance our understanding of soil resource-mediated mechanisms underlying plant community assembly in arid mountainous ecosystems and provide theoretical support for the restoration and management of degraded mountain ecosystems.