Diazotrophic Community Structure and Environmental Correlates in BSCs Under Sand-Fixing Plantations in Alpine Sandy Land.

Biological soil crusts (BSCs) are critical components of alpine sandy land ecosystems, yet how their diazotrophic communities respond to plantation type and successional stage remains largely unknown. This study examined diazotrophic community differentiation in BSCs under four sand-fixing plantations (Salix psammophila SL, Caragana korshinskii NT, Salix cheilophila WL, Populus simonii XYY) across three successional stages in an alpine sandy land. Soil properties, microbial biomass, and enzyme activities were measured, along with nifH gene sequencing (Illumina), to characterize community composition and its relationships with environmental factors along BSCs succession. The results indicate that Skermanella, Mastigocladus, and Nostoc were the dominant diazotrophic groups in the study area. The Nostoc was the dominant genus across all plantation types, with the highest average relative abundance (37.92%) recorded in moss crusts under the XYY plantations. Diazotrophic α-diversity increased with BSCs succession, and β-diversity analysis (PCoA with ANOSIM) demonstrated significant community differentiation among plantation types (variance explanation = 50.91%, R = 0.59458, p = 0.001). Spearman's and redundancy analysis (RDA) showed that sand-fixing plantations drive the differentiation of diazotrophic communities by regulating soil physicochemical properties and microbial metabolism. Furthermore, the diazotrophic communities in moss crusts exhibited stronger environmental responsiveness than those in algae crusts. In the COG functional categories, J (translation and ribosomal biogenesis, 7.92%), E (amino acid metabolism, 10.14%), and C (energy production, 7.11%) were identified as the core foundational functions of BSCs in the study area. The relative abundances of these functional categories showed convergence. In summary, different plantations were associated with distinct diazotrophic community compositions, and these associations were largely mediated by soil environmental factors. Concurrently, BSC succession covaried with enhanced differentiation in community structure and functional traits. These patterns are consistent with a stepwise response cascade linking vegetation and soil properties to diazotrophic communities and nitrogen metabolism functions. This study provides a theoretical basis for enhancing the stability and long-term restoration of artificial ecosystems by regulating diazotrophic communities in BSCs in alpine sandy land.