Polyethylene microplastics distinctly affect soil microbial community and carbon and nitrogen cycling during plant litter decomposition.

Plant litter is an important input source of carbon and nitrogen in soil. While microplastics (MPs) and plant litter are ubiquitously present in soil, their combined impact on soil biogeochemical processes remains poorly understood. To address this gap, we examined the soil changes resulting from the coexistence of plant litter (Alfalfa) and polyethylene microplastics (PE). The soil changes included physicochemical properties, composition of soil dissolved organic matter, and structure of the soil microbial community. The results showed that the addition of polyethylene (PE) inhibited the degradation of humus-like substances and decreased the quantity of humic acid-like compounds in soil dissolved organic matter (DOM). PE negatively impacted plant litter decomposition, disrupted soil organic carbon (SOC) breakdown, interfered with the nitrogen cycle, and significantly altered microbial community structures during the process. By day 35, SOC and total nitrogen (TN) levels were reduced by 39.8% and 10.1%, respectively, in the presence of PE. Furthermore, PE significantly decreased the abundance of nitrogen-fixing microbes, including Streptomyces (43.1%) and Bacillus (45.9%), which play key roles in nitrate reduction to ammonium. This study highlights the environmental effects of MPs on plant litter decomposition and their potential implications for soil biogeochemical processes.