Tire microplastics rather than polystyrene microplastics reduce soil microbial diversity and network complexity and stability, and induce microbial homogenization

Microplastics (MPs) pollution poses escalating threats to soil biodiversity, yet its impacts on microbial community structure, stability, and assembly are far from fully understood, limiting the comprehensive assessment of MPs risks. This study investigated effects of polystyrene (PS) and tire particle (TP) MPs (0, 1 %, 5 %; w/w) on soil microbial communities in a maize-planted system, evaluating shifts in diversity, network architecture, and assembly processes. Our results demonstrated that high-concentration (5 %) PS MPs significantly enhanced bacterial α-diversity by promoting some taxa (e.g., Planctomycetes, Betaproteobacteria), and increased bacterial network complexity. In contrast, 5 % TP MPs reduced bacterial and fungal diversity, destabilized bacterial networks, and induced taxonomic homogenization. TP MPs amplified deterministic assembly processes by elevating homogeneous selection contribution while reducing stochastic drift, thereby driving microbial community convergence. Bacterial and fungal community structure shifts under TP MPs correlated with soil stoichiometric alterations, including depleted nitrate nitrogen and available phosphorus, and elevated pH, contents of dissolved organic carbon, ammonium nitrogen, and total carbon. These findings highlight the divergent ecological risks posed by PS and TP MPs, and underscore the urgent need for prioritized mitigation of TP MPs pollution in agroecosystems to preserve microbial functional integrity.