From acquisition to recycling: Vegetation restoration reshapes microbial nitrogen-cycling strategies in a semi-arid sandy land

Reestablishing nitrogen (N) cycling is critical for stabilizing ecosystems in semi-arid sandy lands, yet the microbial mechanisms driving functional reorganization during vegetation restoration remain unclear. This study investigated the coupling of soil physicochemical properties, microbial traits, and N-cycling genes across a five-stage restoration sequence in the Mu Us Sandy Land: bare sandy land (BSL), grassland (GL), shrubland (SL), grass-shrub mixed land (GSL), and forest land (FL). Restoration significantly ameliorated edaphic conditions, with soil organic carbon (SOC) increasing approximately 7-fold and pH declining from 8.57 to 6.79. Notably, the GSL stage emerged as a “biogeochemical hotspot”, exhibiting peak microbial biomass and hydrolytic enzyme activities (urease and sucrase) linked to resource heterogeneity. Metagenomic profiling revealed a functional shift in microbial strategies: genetic potential for N-fixation ( nifH/K ) and urea hydrolysis dominated in oligotrophic BSL, indicating a strong investment in N acquisition. In contrast, nitrification ( amo ) was most prominent in GL, while denitrification and anammox pathways intensified in shrub-grass mosaics (GSL). Structural Equation Modeling (SEM) and Mantel tests further demonstrated that SOC accumulation primarily drives these functional shifts by reshaping microbial biomass and community assembly. Collectively, these findings suggest that vegetation restoration transitions the soil N-cycle from an external-acquisition-dominated system to a tighter, internal-recycling regime, with mixed vegetation stages acting as critical functional transitions.