Plant and microbial pathways driving soil carbon sequestration in dryland leguminous shrublands

The establishment of leguminous shrubs has become a key strategy for dryland restoration, offering substantial potential to enhance soil organic carbon (SOC) sequestration. However, the mechanisms governing SOC stabilization in these systems remain insufficiently understood. Here, we investigated 30 Caragana korshinskii shrubland sites distributed across north-facing (NFS) and south-facing slopes (SFS) to identify the dominant drivers of SOC accrual within the 0–40 cm soil profile relative to adjacent croplands. Across both slope aspects, NFS shrublands exhibited a 77 % increase in particulate organic carbon (POC) and a 28 % increase in mineral-associated organic carbon (MAOC), whereas SFS shrublands showed a markedly larger enhancement in POC (257 %) but a comparable increase in MAOC (22 %). Shrubland soils accumulated substantially greater quantities of lignin phenols and microbial residue carbon (MRC), increasing by 43 % and 49 % on NFS and by 183 % and 75 % on SFS, respectively, compared with croplands. Isotopic signatures (δ¹⁵N and δ¹³C) were consistently more depleted in shrublands across both slope aspects, indicating shifts in organic matter sources and transformation pathways. Structural equation modelling revealed that C. korshinskii establishment enhanced both POC and MAOC pools by increasing root biomass and soil total nitrogen, thereby stimulating microbial activity and the accumulation of lignin phenols and MRC. Slope aspect exerted an indirect influence on POC and MAOC through its effects on root biomass, soil moisture, and mineral properties. Our findings provide direct evidence that leguminous shrubs promote SOC sequestration through the coordinated action of plant and microbial pathways, highlighting a key mechanism by which topography modulates the formation and stabilization of SOC under changing land use regimes in dryland ecosystems.