Discrepant impact of polyethylene microplastics on methane emissions from different paddy soils

Microplastics (MPs) can affect soil organic carbon (C) cycling in paddy soil by changing microbial function and soil properties. In this study, a laboratory incubation experiment was applied to explore the effect of polyethylene (PE) MP on methane (CH4) and carbon dioxide (CO2) emissions from paddy soils after rice straw addition. Moreover, the microbial functional genes encoding CH4 oxidation and production and organic C degradation were measured. MP addition significantly (p < 0.05) affected the release of CH4, but this effect varied with soil type. The amendment of PE MPs significantly (p < 0.05) reduced CH4 emission by 16.9 % for the acidic soil (pH = 5.0) and 16.1 % for the alkaline soil (pH = 8.2), which was probably due to these plastic particles inhibiting CH4 production by reducing the abundance of the mcrA gene or stimulating CH4 oxidation with the increasing number of pmoA gene copies, respectively. For the neutral soils, the presence of PE MPs did not significantly (p > 0.05) alter CH4 emissions and their associated microbial functional genes. Across the experimental paddy soils, the response of CH4 emissions to MPs was negatively and positively correlated with those of soil ammonium (NH4+) content and mcrA gene abundance, respectively, suggesting that the change in these variables may control the release of CH4 in the presence of MPs. In addition, PE MPs had a negligible effect on soil CO2 emissions and the abundances of all functional genes encoding starch, hemicellulose, and lignin degradation. Our results demonstrated that the presence of MPs affects the release of CH4, probably by regulating NH4+ substrates and methanogens, and this emerging environmental factor should be considered when estimating CH4 emissions from paddy fields.