Effect of nonbiodegradable microplastics on soil respiration and enzyme activity: A meta-analysis

Nonbiodegradable microplastics (MPs) are emerging contaminants in the environment and potentially threaten soil health. In recent years, the impact of MPs on soil ecology has attracted widespread attention, but the responses of soil respiration and enzyme activity to MPs exposure remain unclear. Here, a meta-analysis including 1980 observations was used to assess the effects of MPs on soil microbial activity. MPs exposure significantly (p < 0.05) increased soil respiration by 18.2 % but did not significantly (p > 0.05) affect soil enzyme activity; moreover, these effects varied with MP type, concentration, size, and exposure period. The amendment of polypropylene (PP) MP increased soil respiration and enzyme activity by 58.8 % and 10.2 %, respectively, whereas exposure to polyethylene terephthalate (PET), polyethylene (PE) and polystyrene (PS) MPs reduced soil enzyme activities by 13.0 %, 6.8 % and 5.0 %, respectively. The soil respiration was unaffected and increased when the MPs concentrations were below and above 5 %, respectively, whereas soil enzyme activity was stimulated and inhibited when the MPs concentrations were less and >10 %, respectively. The size of MPs only significantly (p < 0.05) affected the response of soil respiration to MPs, as small (<500 mu m) and large (>= 500 mu m) sizes of MPs increased and reduced soil respiration by 53.4 % and 5.8 %, respectively. Short-term (<= 30 days) exposure to MPs increased soil respiration by 50.2 %, whereas the presence of MPs inhibited soil enzyme activity by 3.3 % when the incubation period ranged from 30 to 100 days. In addition, MPs exposure significantly (p < 0.05) increased soil respiration by 77.9 % in alkaline soil (pH > 7.5) and by 41.6 % in the absence of plants. The amendment of MPs significantly (p < 0.05) increased and reduced soil enzyme activities in acidic and alkaline soils by 4.3 % and 5.5 %, respectively, and significantly (p < 0.05) improved soil enzyme activity by 4.5 % in the presence of plants. Specifically, MPs significantly (p < 0.05) increased the activities of acid phosphatase and fluorescein diacetate hydrolase by 8.3 % and 17.1 %, respectively, but did not significantly (p > 0.05) influence urease, beta-glucosidase, and catalase activities. Overall, our results suggested that MPs have nonnegligible impacts on soil microbial activity, and it is urgently necessary to explore the long-term effects of MPs on soil ecology in the natural environment.