Agricultural plastic films reshape soil microplastic distribution, nitrogen cycle and ecological risks in facility agriculture

Microplastic pollution in protected agriculture is intensifying due to excessive plastic film usage. However, little is known concerning how different protected agriculture practices reshape residual characteristics of microplastics, and their coupling with soil physicochemical factors and heavy metals remains unclear. This study conducted a comparative analysis of microplastic features in typical protected agriculture systems and traditional farmland soils, examining their relationships with soil physicochemical factors and heavy metals. Results revealed that the abundance of microplastics in mulched farmland (MF) was significantly (p < 0.05) higher than that in traditional farmland (TF), and greenhouse (GH), suggesting that plastic mulch, rather than greenhouse film, significantly increases the abundance of soil-residual microplastics. Dominant microplastic forms were films and fragments < 1 mm (84.25 %, with 62.66 % <0.5 mm), primarily polyethylene (PE, 36.8 %) and polypropylene (PP, 18.9 %), appearing mainly in black (45 %), brown (18.5 %), and transparent (18 %) colors. Microplastic distribution was significantly influenced by nitrate nitrogen, ammonium nitrogen, and chromium. Moreover, the abundance of genes associated with nitrogen fixation was significantly greater in the plastisphere than in the soil, with Cyanobacteriota being the major contributor. Ecological risk assessments showed GH posed the highest risks, followed by the TF, with the MF exhibiting the lowest risk levels. These findings enhance understanding of microplastic behavior in agricultural systems and provide scientific evidence for managing agricultural nonpoint source pollution, supporting the development of targeted mitigation strategies.