Divergent effects of biodegradable and conventional microplastics on denitrification in soils: Mechanisms governing...

Microplastics (MPs) are increasingly prevalent in agroecosystems, yet their impacts on soil denitrification rate and nitrous oxide (N₂O) emissions remain poorly understood, particularly regarding how MPs type influences the partitioning between N₂O and dinitrogen (N₂). We integrated the gas-flow-soil-core (GFSC) technique, ¹⁵N tracing technique, and a global meta-analysis of 162 pairwise observations to demonstrate how biodegradable and conventional MPs affect soil denitrification and N₂O production. The GFSC analysis showed that the highest N₂ emission rates was observed in biodegradable MPs-amended soils (3.9 μg N kg soil^-1 h^-1); while the N₂O emission rates was higher in conventional MPs-amended soils (0.46 μg N kg soil^-1 h^-1) than in biodegradable MPs-amended and no-MPs soils. The N₂O/(N₂O+N₂) ratio in conventional MPs-amended soils was much higher than that in other treatments, indicating a high N₂O emission potential. The meta-analysis revealed that MPs globally increase soil N₂O emissions, with contrasting effects between biodegradable and conventional types under different moisture regimes. Mechanistically, biodegradable MPs released labile carbon, stimulating microbial activity, enhancing gross N mineralization and nitrification, and increasing N availability for plant and microbial uptake, as shown by ¹ ⁵N tracing. They also upregulated key denitrifying genes (nirK, nirS, nosZI and nosZII), promoting complete denitrification and lowering the N₂O/(N₂O+N₂) ratio despite greater total (N₂O+N₂) flux. In contrast, recalcitrant conventional MPs failed to activate these pathways, resulting in incomplete denitrification and elevated N₂O emissions. Notably, in soils with high moisture, the N₂O emission rates increased following addition of biodegradable MPs, indicating a potential risk of N₂O emissions under regions with intensified future rainfall. Collectively, our integrated approach demonstrates that the net impact of MPs on greenhouse gas emissions is substantially modulated by their polymer type, which differentially regulates microbial N cycling and the ultimate partitioning of denitrification products. Our findings provide novel insights into the mechanisms of MPs on microbe-mediated N₂O emissions from the perspective of N transformation.