Synergy of plastics and heavy metals weakened soil bacterial diversity by regulating microbial functions in the Qinghai-Tibet Plateau

How plastics coupled with metals regulate microbial functions-diversity relationships remain unknown in plateau soil environment. Three representative catchments in the Qinghai-Tibet Plateau, focusing on microplastics, their plasticisers, and metals in soils, were investigated. This research explores responses of bacterial diversity and functions to the co-existence of target pollutants, and pathways by which target pollutants regulate the diversity. Soil bacterial beta diversity and functional genes exhibited negative correlations with phthalate esters across three catchments (p < 0.05). Dibutyl phthalate emerged as a primary factor affecting beta diversity, rather than the quantity of microplastics. Additionally, the synergy of cadmium and fiber-shaped microplastics exacerbated the impact on diversity. Structural equation modeling further elucidated that plastics, copper, and iron influenced nirK/nirS genes and phoD gene, subsequently affected cbbL/cbbM genes, and ultimately the diversity. In this context, microplastics, phthalate esters and copper, iron exerted antagonistic effects on one another. Consequently, the co-existence of plastics and cadmium weakened soil bacterial diversity in the Qinghai-Tibet Plateau by disrupting microbial functions, but micronutrients alleviated these negative impacts. This research reveals that the co-existence of plastics and metals regulates soil bacterial diversity in the Qinghai-Tibet Plateau, providing a valuable reference for the protection of microbial ecology in plateau regions.