Polyester Microplastic Mitigated NH3 Volatilization from a Rice–Wheat Rotation System: Does Particle Size or Natural Aging Effect Matter?

Microplastics (MPs) accumulation in agricultural soils has shown serious implications for soil ecosystem functions. Ammonia (NH3) volatilization is a major pathway of nitrogen loss in agricultural systems, while its response mechanism to MPs occurrence, particularly the role of MPs size and natural aging, has been scarcely explored. In this study, polyethylene terephthalate of sizes of 10 μm (PET10) and 200 μm (PET200) were selected in a rice–wheat rotation cycle soil system. Results showed that PET10 and PET200 reduced soil NH3 volatilization cumulative amount from 58.9 to 18.6–40.3 kg N/ha and yield-scaled loss from 2.95 to 0.92–1.60 kg/t grain. Reduction of NH3 volatilization occurred in both the rice growth season and the subsequent wheat growth season, in which two MPs were aged in the initial rice season, with verification by 2.8 wt % more bulk oxygen of PET200 separated from the soil after rice harvest. Moreover, PET200 treatment showed 7.7–25.9% less cumulative NH3 volatilization than PET10, which was significant in the rice growth season. This was partially related to the higher adsorption capacity (10.6 mg/g) for free NH4+–N of PET200 and 5.2% more microaggregate (<250 μm) formation in PET200 treatment. These findings provide references for the assessment of MPs effects on soil nitrogen biogeochemical cycling.