Vegetation-Mediated Carbon Inputs and Erosion Protection Shape Soil Carbon Dynamics across Aridity Thresholds

Intensified aridity beyond a critical threshold could disrupt vegetation, microbial, and soil processes, reshaping the mechanisms controlling soil carbon (C) storage in drylands. However, the aridity threshold at which the transition occurs and how the controls over different soil C fractions shift remain unclear. Here, we conducted a 2400 km transect survey across 45 sites spanning a broad aridity gradient in temperate grasslands of China. We identified a pronounced shift in the dominant drivers of soil C storage at an aridity threshold of 0.749. Below this threshold, complex vegetation structures enhanced soil C by promoting microbial activity and mineral abundance, which stimulated the accumulation of both POM and MAOM, with a stronger effect on POM. Above the threshold, fine roots dominated soil biochemical processes, sustaining microbial activity and mineral formation that indirectly stabilized SOC, particularly via MAOM. Across the entire aridity gradient, vegetation structure mediated surface soil susceptibility to wind erosion with complex structures providing effective protection, while simpler structures offered limited buffering. These findings highlight the dual role of vegetation-mediated C input and wind erosion protection in sustaining soil stocks in drylands, underscoring the need to account for canopy and root structure when species are selected for dryland restoration.