Methane-associated micro-ecological processes crucially improve the self-purification of lindane-polluted paddy soil

Reductive dechlorination, an efficient pathway for complete removal of organic chlorinated pollutants (OCPs), is commonly reported to be coupled to oxidation of methane (CH₄) or methanogenesis in anaerobic environments. However, the relationship between dechlorination and CH₄-associated bioprocesses is unclear. Based on the hypothesis that CH₄ supplementation could facilitate OCP dechlorination, we investigated the role of CH₄-associated bioprocesses in the self-purification of flooded lindane-spiked paddy soils. Four treatments were conducted for up to 28 days: sterilized soil (S), sterilized soil + CH₄ (SC), non-sterilized soil (NS), and non-sterilized soil + CH₄ (NSC). Results indicated that both sterilization and addition of CH₄ promoted lindane degradation and CH₄ emissions in the flooded paddy soils. In the NS treatment, lindane had the lowest degradation rate when CH₄ emissions were barely detected; while in the SC treatment, lindane had the highest degradation rate when CH₄ achieved its highest emissions from anaerobic soil. Also, sterilization led to microbial diversity loss and functional recession, but increased ferrous ion [Fe(II)] concentrations compared to non-sterilized soils. Methanogenic communities and mcrA gene recovered faster than the majority of microorganisms (e.g., Fe bacteria, Bdellovibrionaceae, Rhizobiaceae, Dehalogenimonas) or functional genes (e.g., Dhc, Geo, narG, nirS). Collectively, we assume the enhanced removal of lindane may partly be due to both abiotic dechlorination promoted by chemical Fe redox processes and methanogenesis-derived biotic dechlorination. Revealing the coupling between dechlorination and CH₄-associated bioprocesses is helpful to resolve both pollution remediation and mitigation of CH₄ emissions in anaerobic contaminated sites.