Mechanisms Associated with Lower Methane Emissions from Paddy Soil by Aged Polylactic Acid Microplastics

The widespread contamination of paddy soils with microplastics (MPs) has the potential to significantly influence the global carbon (C) cycle. While there are reports of MPs affecting methane (CH4) emissions from paddy soil, the mechanisms, especially associated with the aging of different types of MPs, remain unknown. To address this paucity in knowledge and better understand and quantify the role of MPs and aged MPs on CH4 emissions, we conducted a microcosm experiment using paddy soil amended with pristine and aged polyethylene (PE) and polylactic acid (PLA). Our results showed that MPs inhibited CH4 emissions with pristine PLA lowering cumulative CH4 emissions from 6.80 to 4.90 μM/g soil over a 50 day incubation, while pristine PE was less effective by lowering emissions to 5.50 μM/g soil (P < 0.05). Interestingly, aging of PE increased the efficacy of CH4 mitigation, while aging of PLA lowered its efficacy. Mechanistic analyses revealed that aged PE, in contrast to pristine PE, decreased the ratio of methanogenesis to CH4 oxidation gene abundance. In comparison, the aging of PLA confers its role as an electron shuttle, facilitating the transfer of electrons from Geobacteraceae to nitrates and iron minerals, thereby competing with the methanogenic process for electrons and substrates, and thus inhibiting methanogenesis. Our study provides key evidence with supporting mechanisms showing that aging of MPs influences the emissions of CH4 from paddy soils, with the results having importance in national greenhouse gas inventories and their abatement.