Stochasticity dominated adaptive assembly of benthic microbiota across oasis-to-desert sections of an arid inland river.

Sediment microbial communities in arid inland rivers are important for biogeochemical cycling, but the assembly mechanisms and network responses across the transition from irrigated oases to desert margins remain unclear. We investigated bacterial and fungal communities in Yarkand River sediments along salinity and nutrient gradients using 16S rRNA and ITS sequencing, sediment chemistry, iCAMP null-model analysis, and co-occurrence networks. Downstream increases in electrical conductivity, total nitrogen, and pH were associated with reduced bacterial α-diversity and turnover from nutrient-responsive Pseudomonadota in the oasis midstream to stress-tolerant Chloroflexota and Actinomycetota in the desert downstream. Fungal diversity showed high inter-site variability without a linear decline. Despite strong physicochemical gradients, ecological drift dominated assembly in both kingdoms, while desert networks showed higher connectivity. These findings support a hierarchical assembly model in which salinity filters community composition, but stochastic processes and network densification shape benthic microbiota in arid river sediments.