Coexistence of microplastics and Cd alters soil N transformation by affecting enzyme activity and ammonia oxidizer abundance

Interactions between heavy metal and microplastics represent a serious threat to ecosystems and human health, but the effect of their coexistence on the soil N transformation processes is unclear. The mechanism in which metal-polluted soil reacts to additional microplastics stress and their toxicology interactions on soil N transformation were determined by investigating the dynamics of soil microbial N transformation in response to Cd stress and different doses of polythene (PE) microplastics by conducting a 14 days aerobic 15N microcosmic incubation experiment. The gross nitrification rates (n_gross) were decreased by 7.47% and 12.5% in the 1% and 2% (w/w) PE groups, respectively, through the direct effect on enzyme activity (β-glucosidase, N-acetylglucosaminidase, and leucine-aminopeptidase) and the abundance and community composition of ammonia oxidizer. It also exerted indirect effect by reducing nitrification substrate concentrations. PE microplastics (>1% [w/w]) significantly increased the gross N immobilization rate, and this change could have been driven by C/N stoichiometry. Cd stress alone led to a rapid short-term mineralization–immobilization turnover (1.67 times of the control). However, such effect was offset when Cd coexisted with PE microplastics, possibly because Cd was directly adsorbed by PE microplastics, and/or microplastics satisfied the C demand by microorganisms under Cd stress. Our findings demonstrated that the coexistence of microplastics and Cd significantly altered soil N nitrification and immobilization, which would change the N bioavailability in soil and alter the effect N cycling on the ecological environment.