[Effects of Microplastics on Soil N₂O Emission and Nitrogen Transformations from Tropical Agricultural Soils]

A widespread concern had been there regarding soil ecological and environmental problems caused by microplastic pollution in agricultural soils. A controlled laboratory incubation experiment was performed to examine the effects of different types of microplastics on soil properties, N₂O emissions, and nitrogen （N） transformations in tropical arable soils from a pepper-corn cropping system in Hainan Province. Three treatments were done： soil without microplastics （CK） and soil amended with 5% of polyethylene （PE） or with 5% of polybutylene adipate co-terephthalate （PBAT）. The results showed that both types of microplastic addition increased soil pH, soil organic carbon （SOC）, and dissolved organic carbon （DOC） contents, with stronger treatment effects observed for PBAT than those for the PE treatment. In addition, the PE and PBAT treatments increased soil ammonium nitrogen （NH₄⁺-N） contents by 66.07% and 119.65% and decreased nitrate nitrogen （NO₃^--N） contents by 8.56% and 29.68%, respectively. Compared to those in the CK treatment, the addition of PBAT significantly increased soil N₂O emissions by 254.92% （P < 0.05）, whereas that of PE produced no significant effects. Furthermore, both the PE and PBAT treatments increased soil net nitrogen mineralization rate （NMR） and decreased soil net nitrification rate （NNR）, with more obvious treatment effects observed in PBAT than in the PE treatment. PBAT addition increased the abundance of ureC, while PE had no significant effects. Microplastic addition reduced the abundance of nitrifying gene abundances （AOA-amoA, AOB-amoA, and nxrA）, with more obvious treatment effects found in the PBAT treatment. Further, PBAT addition significantly increased the gene abundances of nirK, nirS, nosZ, and fungal nirK （P < 0.05）, whereas the addition of PE had no significant effect on those gene abundances. Soil N₂O emissions had positive relationships with NH₄⁺-N intensity, pH, DOC, SOC, and nirS abundance. In conclusion, biodegradable microplastics addition produced stronger influences on soil properties and N transformations than the non-biodegradable one in tropical arable soils and aggravated soil N₂O emissions mainly by promoting denitrification.