Different hydrological conditions after permafrost thaw result in distinct microbial community compositions

Permafrost degradation leads to the formation of contrasting hydrological conditions such as water-saturated anoxic and dry oxic soils, which significantly influence the bacterial community structure and abundance. To investigate the bacterial abundance and diversity under these hydrological conditions, we collected soil samples from different horizons of both dry and wet degraded permafrost soils and non-degraded intact permafrost soil for comparison. The bacterial alpha diversity, measured by the Observed and Chao1 indices, was significantly greater in wet degraded permafrost soil (wet site) and intact permafrost soil (intact site) than in dry degraded permafrost soils (dry site). Notably, the wet and intact sites exhibited similar levels of alpha diversity as well as shared greater number of zOTUs. The relative proportion of most bacterial taxa was significantly differed among the sites. At the class level, dry site was dominated mainly by K-strategic bacteria like aliphatic degraders (Thermoleophilia), acidophilic and cellulolytic (Acidobacteriae), while wet site was dominated by mainly r-strategic bacteria from class Gammaproteobacteria and anoxygenic aerobic phototrophs (Gemmatimonadetes). According to Spearman correlation analysis, the relative proportion of bacterial taxa in dry and wet sites showed significant correlation with soil physicochemical parameters, whereas fewer correlations were found in intact site. The relative proportion of Pseudomonadota classes and Acidobacteriota were positively correlated with SOC, N, and C:N ratio, but were negatively correlated with pH in both dry and wet sites. In contrast, anaerobic methylotrophs (Methylomirabilota), anoxygenic photoheterotrops (Chloroflexota), Gemmatimonadota and filamentous Actinobacteriota were negatively correlated with SOC, N, and C:N. Additionally, strong correlations were observed between bacterial taxa and extracellular enzyme activities, where Alphaproteobacteria, Gammaproteobacteria, and Acidobacteriota showed positive correlations with both hydrolytic and oxidative enzymes in dry and wet sites, except for PerOx in wet site where they showed negative correlation. Conversely, Gemmatimonadota and Actinobacteriota displayed negative correlations with enzymes in dry and wet site, except for the PerOx in wet site, where they showed positive correlation.