Divergent responses in microbial metabolic limitations and carbon use efficiency to variably sized polystyrene microplastics in soil

Microplastics, considered emerging contaminants, have been accumulating excessively within soil ecosystems, conferring potentially detrimental effects with respect to soil carbon turnover and pools. As a major participant in soil carbon processes, microplastics affecting microorganisms may be one of the main agents affecting soil carbon dynamics. However, the microbial metabolism processes through which microplastics affect soil carbon dynamics remain unclear. Therefore, this study aimed to assess the impact of variously sized (1300, 800, 100, and 0.1 μm) polystyrene microplastics on soil microbial metabolism. Soil microplastics of all sizes invariably and consistently affected microbial metabolism, though nanomicroplastics (0.1 μm) stressed soil microorganisms more than micron-microplastics. Furthermore, microplastics inhibited both microbial carbon use efficiency (CUE) and respiration, the exception being the CUE in the 1300 μm microplastics treatment. As microplastic particle size decreased, the suppressive influence of microplastics on microbial respiration was gradually lost, the inhibitory effect on microbial CUE increased steadily, and the impact on microorganisms shifted from extracellular to intracellular, with intracellular microplastics exhibiting higher toxicity than extracellular microplastics. We used stoichiometric models to provide precise projections of microbial metabolism features associated with microplastic contamination, thereby enhancing the understanding of the effects of microplastic contamination on the carbon cycle and soil ecosystem.