Competing drivers of soil microbial community assembly in a cold arid zone and their implications on constraints of n-alkane proxies

Soil microbial assembly in cold arid deserts is under dispute, while its possible constraints on n-alkane proxies have been posited without direct evidence. To further explore their underlying mechanisms, we analyzed the 16S rRNA data along two salinity and one precipitation gradient from the Junggar Basin of northwest China. The null-model phylogenetic composition analysis shows homogeneous selection of deterministic processes assembled prokaryotic (dominated by bacterial) communities, while stochastic processes dominated the archaeal community assembly. Multivariate linear regression analyses show precipitation-related variables and salinity are leading predictors of the prokaryotic beta diversity in the low- and high-salinity settings, respectively, but can only explain limited variance, while reliable predictors for alpha diversity are lacking along different gradients. We suggest instead of previously reported a primary one, multiple competing environmental variables assembled the prokaryotic communities. Further comparison with n-alkane proxies reveals multiple species could have potentially contributed to, but not controlled the previously reported negative n-alkane ACL (Average Chain Length)-salinity correlation in high-salinity soils, while the reported positive n-alkane CPI (Carbon Preference Index)-precipitation correlation is attributed to precipitation-regulated combined plant-biome input in low-salinity settings. This provided the first direct microbial evidence validating CPI's application in paleo-vegetation/precipitation reconstruction in low-salinity dry land settings.