Biodegradable microplastics induce more soil nitrous oxide emission than conventional in semi-arid Loess Plateau

Biodegradable plastics, proposed as sustainable alternatives of conventional plastics, pose risk to soil ecosystems and play critical roles in regulating soil greenhouse gas emission. Yet, how biodegradable microplastics (MPs) exposure affect the soil nitrous oxide (NO) emission and underlying mechanisms remains poorly unclear. This study investigated the effects of biodegradable (polylactic acid, PLA) MPs and conventional (polyethylene, PE) MPs on soil NO concentration and emissions using a soil column experiment. Soil NO concentration and emission were measured during NO "hot moment" events induced by simulated rainfall events, alongside high-frequently monitor of soil oxygen (O) concentration. Simultaneously, microbial functional genes related to nitrification and denitrification were measured to compare their contribution to NO production. We found that exposure of MPs triggered NO pulse by reducing O concentration, and cumulative NO emissions in PLA treatments increased by 2.3-2.8 folds higher than their counterparts in PE treatments. Consumptions of O caused by MPs exposure was dose-dependent, with minimum O concentration ranged 16.2-18.3 % for PE treatments and 4.0-12.0 % for PLA treatments, respectively. Denitrification emerged as the primary contributor of increased NO concentration, as evidenced by increased in nirK and nirS genes, particularly when soil O concentrations dropped to 7.2-12.3 % in PLA treatments. Overall, our findings underscore biodegradable MPs could intensity O consumption and NO production, offering critical insights for risk management strategies and guiding the application of biodegradable plastics in future agricultural contexts.