Soil nutrient levels regulate the effect of soil microplastic contamination on microbial element metabolism and carbon use efficiency

Microplastics (MPs) are emerging environmental contaminants in soil ecosystems that disrupt the soil carbon (C) pool. Therefore, the response of microbial metabolism to MP-contaminated soil is crucial for soil-C stabilization. We undertook factorial experiments in a greenhouse with three types of soil microplastics with three levels of soil nutrients and undertook soil physiochemical analyses after 60 days. The present study revealed how the presence of degradable polylactic acid (PLA) and non-degradable polyethylene (PE) MPs affects soil microbial nutrient limitation and C use efficiency (CUE) at varying nutrient concentrations. The presence of PLA in soil with low nutrient levels led to a significant increase (29%) in the activities of nitrogen (N)-acquiring enzymes. In contrast, the presence of MPs had no effect on C- and N-acquiring enzymes. The occurrence of PE caused a 41% reduction in microbial C limitation in high-nutrient soils, and microbial nutrient metabolism was limited by the occurrence of MPs in soils amended with nutrients. A strong positive correlation between microbial C and nutrient limitation in the soil indicates that addressing C limitation followed by amendment of soil with MPs could potentially intensify microbial N limitation in soils with varying nutrients. In comparison, the microbial CUE increased by 10% with the application of degradable MPs (PLA) to soils with a low nutrient status. These findings highlight the significant influence of both degradable PLA and non-degradable PE MPs on soil microbial processes and C dynamics. In conclusion, PLA enhances metabolic efficiency in nutrient-rich soils, potentially aiding C utilization, whereas PE reduces microbial C limitation, offering promise for soil C sequestration strategies. Our findings underscore the importance of considering MPs in soil ecosystem studies and in broader sustainability efforts.