Aridity represents a primary driver of desertification in semiarid and arid ecosystems, where soil microbial communities play pivotal roles in sustaining dryland ecosystem functions. However, the mechanisms by which biocrust-forming mosses influence bacterial community distribution and assembly under varying levels of aridity remain insufficiently resolved. In this study, we investigated the effects of moss crusts on bacterial community diversity and assembly processes across arid and semiarid regions of northern China using targeted metagenomic analysis of the 16S rRNA gene. Our results show that moss crusts significantly enhance bacterial alpha-diversity while reducing beta-diversity relative to adjacent bare soil across all sampled regions. Moreover, bacterial communities associated with moss crusts exhibit a stronger propensity for stochastic assembly processes compared to those in bare soils, and display attenuated spatial turnover along the aridity gradient. In contrast, bacterial assembly in bare soils is jointly constrained by aridity and soil nutrient availability, whereas moss crust-associated communities are predominantly influenced by aridity. Notably, moss crusts elevate the aridity threshold under which bacterial community assembly transitions from stochastic to deterministic dominance, from 0.83 to 0.91, thereby extending the range of aridity conditions under which stochastic processes prevail. These findings highlight the critical role of moss crusts in enhancing the functional stability of desert ecosystems under drought stress, through modulation of regional-scale bacterial diversity, heterogeneity, and assembly dynamics, and support their strategic application in climate-resilient ecological restoration.