Plastic Mulching Residue Reshapes the Plastisphere Microenvironment and Impacts Soil Multifunctionality

Polyethylene agricultural mulching films can increase crop yields; however, long-term residue leads to the accumulation of plastic fragments and microplastics in soil, threatening soil health and food security. In this study, we conducted a field-simulated experiment in a rapeseed (Brassica napus L.) system to compare plastisphere and rhizosphere soils with respect to soil properties, soil multifunctionality (MF), and bacterial and fungal community characteristics. Our results indicate that compared with the rhizosphere, the plastisphere exhibited a significantly higher pH and lower water content. In addition, inorganic nitrogen, dissolved organic nitrogen, total nitrogen, ammonium nitrogen, net nitrogen mineralization rate, and available phosphorus were all lower in the plastisphere, while total carbon was higher. These nutrient availability shifts translated directly into ecosystem function. Soil MF was substantially lower in the plastisphere than in the rhizosphere (p < 0.001). Both bacterial and fungal richness and diversity (Chao1 and Shannon indices) were markedly reduced in the plastisphere (p < 0.01). The plastisphere also showed reduced microbial diversity and richness in both bacteria and fungi (Chao1 and Shannon indices, p < 0.01), with strong compartmental separation across communities (bacteria R2 = 0.85; fungi R2 = 0.78; p < 0.01) and simplified co-occurrence network cohesion. Random forest analyses indicated that fungal community traits were more important than bacterial traits in predicting MF. Overall, residual mulching film drives nutrient limitation and restructures microbial interaction networks, leading to decreased soil multifunctionality and potential downstream impacts on nutrient cycling and plant nutrient supply.