Different microplastics in anaerobic paddy soils: Altering methane emissions by influencing organic matter composition and microbial metabolic pathways

Microplastics are widespread in farmland and threaten soil ecology. However, how different microplastics affect methane emission in paddy soil are still unclear. To address this knowledge gap, three typical microplastics, polyethylene, polypropylene, and polyvinyl chloride were added to paddy soil with different doses (0.01%, 0.1%, and 1%), and a 50-day incubation experiment was conducted. Compared with polyethylene and polypropylene microplastics, 1% of polyvinyl chloride microplastic resulted in the highest increase in methane production. According to the integrated liquid phase analysis, all three microplastics contributed to the accumulation of humus in the soil solution. Differential microbial taxa analysis indicated that some denitrifying bacteria such as Gemmatimonadetes and Omnitrophicaeota were significantly enriched in 1% polyethylene and polypropylene microplastics groups on day 30, and Rice Cluster I methanogens archaea were enriched specially in the 1% polyvinyl chloride microplastic group. The predicted soil metabolic functions suggested that polyethylene and polyvinyl chloride microplastics enhanced oxidative phosphorylation and amino acid reduction pathways. Macromolecular humus induced by microplastics may play a critical role in the dynamic equilibrium of anaerobic systems in paddy soils. These findings highlighted the threat of microplastics in agricultural fields greenhouse gas emissions. The identified quinone reductive metabolic pathways may provide potential strategies to reduce methane emissions.