Microplastics stimulated nitrous oxide emissions primarily through denitrification: A meta-analysis

Microplastics can profoundly alter nitrogen cycling. However, it remains poorly understood how microplastics impact soil nitrogen processes and generate NO. A meta-analysis was conducted for this investigation based on 60 published studies to elucidate the effects of microplastics on soil nitrogen cycling, from genes to processes. Under microplastic exposure, the emissions of soil NO was significantly increased (140.6%), while the nitrate reductase activities increased by 4.8%. The denitrification rate and number of denitrifier genes were increased by 17.8% and 10.6%, respectively. Meanwhile, the nitrification rate and nitrifier genes were not significantly altered, so did the nitrogen immobilization and mineralization rates. The additional emission of soil NO might primarily from stimulated denitrification. Soil NO emission and denitrification genes were always increased, regardless of the concentrations of microplastic or experiment duration. As a result, the nitrite was increased by 38.8% and nitrate was decreased by 22.4%, respectively. Interestingly, the NO emission increments and copy number of denitrifiers genes diminished over time. This study revealed divergent changes in soil nitrogen processes and highlighted NO emissions with a greater denitrification rate under microplastic exposure. The negative impacts of microplastics on soil health were revealed from the perspective of soil nitrogen availability and NO emissions.