Microplastic accumulation reshapes the assembly of bacterial and fungal communities and soil nutrient cycling in long-term mulched farmland soil

Long-term plastic film mulching promotes the accumulation of microplastics (MPs) in farmland soil, yet the underlying mechanisms through which MPs affect microbial community assembly and nutrient cycling functions remain unclear. Based on soils with different mulching durations (5, 13, and 20 years), this study revealed that MP abundance increased with mulching duration, accompanied by a progressive decrease in particle size. Polyethylene, polypropylene, and polyurethane were the predominant MP markers. As MP accumulation increased, microbial carbon limitation was alleviated, whereas widespread microbial phosphorus limitation was prevalent. Bacterial alpha diversity increased, while fungal alpha diversity decreased. Bacterial co-occurrence networks showed strengthened connectivity and significantly enhanced cooperative interactions. The bacterial genus Polynucleobacter and the fungal genus Acremonium acted as key hubs linking the two kingdoms through both cooperative and competitive interactions. Correspondingly, changes in the importance of community assembly processes were noted along the mulching duration gradient. In bacteria, homogeneous selection remained the dominant process, although its contribution decreased over time, whereas homogenizing dispersal and heterogeneous selection increased. In fungi, the dominant process shifted from homogeneous selection to drift. Partial least squares path modeling further demonstrated that MPs indirectly drove microbial community and functional dynamics primarily by altering soil nutrient conditions. Overall, this study demonstrates that MP accumulation induces nutrient stoichiometric imbalances, thereby reshaping microbial community assembly and nutrient cycling, and provides new insight into the ecological risk assessment of plastic-mulched farmland.