Vegetation restoration restructures soil sulfur allocation and sulfur-cycling functional potential in the Mu Us Sandy...

Vegetation restoration in semi-arid sandy ecosystems can alter soil sulfur cycling not only through changes in sulfur stocks, but also through shifts in the partitioning between organic sulfur and sulfate and their microbial regulation. Here, we investigated soil sulfur pool allocation and sulfur-cycling functional potential along a five-stage vegetation restoration gradient in the Mu Us Sandy Land by integrating sulfur fraction measurements with metagenomic analyses. Vegetation restoration markedly reshaped the soil physicochemical and microbial context, as reflected by lower pH and higher TN, microbial biomass carbon, and enzyme activity in restored soils. In contrast, sulfur pools responded asynchronously: total sulfur and organic sulfur declined substantially from bare sandy land to restored vegetation types, whereas sulfate showed a weaker and comparatively more stable response. At the functional level, dominant sulfur-cycling genes were generally more abundant in bare sandy land, declined across restored vegetation types, and showed only partial recovery in forestland, indicating that restoration reorganized sulfur-cycling functional composition rather than uniformly enhancing sulfur-cycling potential. Taxonomically, dominant sulfur-cycling genes were consistently affiliated mainly with Actinomycetota and Pseudomonadota, but restored vegetation types exhibited more partitioned host compositions, with greater contributions from Acidobacteriota, Chloroflexota, and, for some genes, Thermoproteota. MAG-based analyses further showed that key sulfur-cycling genes were phylogenetically widespread but unevenly distributed across specific host lineages. Co-variation and Mantel analyses showed that sulfur-cycling genes formed coordinated functional modules and were most strongly associated with soil sulfur pools and fractions. Overall, vegetation restoration in the Mu Us Sandy Land primarily reshaped sulfur allocation and sulfur-cycling functional potential rather than promoting simple sulfur accumulation. These findings highlight that sulfur recovery in sandy drylands is better characterized by pool reallocation and functional reorganization.