Biodegradable and conventional microplastics differentially affected greenhouse gas emissions from a flooded paddy soil: Insight into metagenomic analysis.

Paddy soils are important sources of greenhouse gases (GHGs), and microplastics (MPs) are increasingly widespread in paddies. However, the type-dependent effects of biodegradable and conventional MPs on methane (CH4), carbon dioxide (CO2), and nitrous oxide (N2O) emissions remain unclear. Here, an incubation experiment was conducted to evaluate the effects of polyethylene (PE), polyethylene terephthalate (PET), Polybutylene succinate (PBS), and polylactic acid (PLA) on GHG emissions and the involved mechanism was clarified. PLA significantly increased cumulative CH4 and CO2 emissions by 264% and 27.3%, respectively, whereas PE and PET inhibited CH4 and CO2 emissions. In contrast, PE significantly enhanced N2O emissions by 93.1%, while PLA had no significant effect. Mechanistically, PLA increased dissolved organic carbon (DOC), soil pH, HCl-extractable Fe(II), and soluble/exchangeable Mn contents, but decreased soil redox potential (Eh) and sulfate content, creating favorable conditions for microbial anaerobic metabolism. PLA increased the relative abundances of methanogenic taxa and genes (fwdA, fdhA, acsC, cdhC, mttB, and mtbC), but decreased those associated with anaerobic methane oxidation (mcrA, mtrH, and mer), indicating greater CH4-production potential. PLA also increased fermentation (ldh, pfl, ackA, adhE, and por), sulfate-reduction (sat, aprA, aprB, dsrA, and dsrB), and iron-reduction (feR) gene abundances, suggesting greater anaerobic carbon-transformation potential. PE and PET increased denitrifiers and related genes (narH, narI, nirK, and norB), indicating greater N2O-production potential, whereas increased nosZ abundance under PLA treatment suggested greater N2O-reduction potential. Overall, MPs differentially affected paddy GHG emissions in a type-dependent manner, and biodegradable PLA exacerbated short-term GHG emission risks from paddy soils.