Biodegradable and conventional microplastics affect soil-plant systems through altering microbial community assembly and functions

Microplastics pose a serious threat to the stability of agricultural ecosystems by disrupting soil element cycling and plant growth. However, the extent and mechanisms of these impacts depend on microplastic biodegradability and remain poorly understood. Here, we conducted a mesocosm experiment to evaluate the influences of a biodegradable microplastic (poly(butylene adipate- co -butylene terephthalate), PBAT) and three conventional microplastics with distinct side-chain structures (polystyrene, PS; polyethylene, PE; polypropylene, PP) on soil–plant systems. We found that PBAT exerted more severe phytotoxic effects than conventional microplastics, leading to pronounced reductions in shoot height and root length, alongside the greatest declines in soil nitrate nitrogen. While conventional microplastics increased bacterial diversity and network robustness, PBAT fundamentally reshaped community assembly by reducing the importance of drift and homogeneous selection while intensifying dispersal limitation, thereby narrowing the ecological niche breadth. These assembly shifts were accompanied by distinct bacterial colonizers: PBAT enriched putative plastic-degrading taxa, whereas conventional microplastics favored oligotrophic groups with diminished element-cycling capacities. Structural equation modelling further confirmed that microplastic biodegradability exerted direct negative effects on soil nutrients and plant growth. Collectively, our findings uncover type-dependent microbial mechanisms linking microplastic properties to soil–plant ecosystem function, highlighting that biodegradable microplastics can exert stronger negative effects than conventional ones through distinct microbial assembly pathways. • PB altered bacterial assembly via dispersal limitation and reduced niche breadth. • Conventional plastics enhanced bacterial diversity and network robustness. • Conventional microplastics favor oligotrophic groups with weaker element cycling functions. • The biodegradability of microplastics is negatively correlated with plant growth.