Methanotrophy-driven accumulation of organic carbon in four paddy soils of Bangladesh

Biological methane oxidation is a crucial process in the global carbon cycle that reduces methane emissions from paddy fields and natural wetlands into the atmosphere. However, soil organic carbon accumulation associated with microbial methane oxidation is poorly understood. Therefore, to investigate methane-derived carbon incorporation into soil organic matter, paddy soils originated from different parent materials (Inceptisol, Entisol, and Alfisol) were collected after rice harvesting from four major rice-producing regions in Bangladesh. Following microcosm incubation with 5% (volume/volume) 13CH2, soil 13C-atom abundances significantly increased from background level of 1.08% to 1.88%–2.78%, leading to a net methane-derived accumulation of soil organic carbon ranging from 120 to 307 mg kg-1. Approximately 23.6%–60.0% of the methane consumed was converted to soil organic carbon during microbial methane oxidation. The phylogeny of 13C-labeled pmoA (enconding the alpha subunit of the particulate methane monooxygenase) and 16S rRNA genes further revealed that canonical α (type II) and γ (type I) Proteobacteria were active methane oxidizers. Members within the Methylobacter- and Methylosarcina-affiliated type Ia lineages dominated active methane-oxidizing communities that were responsible for the majority of methane-derived carbon accumulation in all three paddy soils, while Methylocystis-affiliated type IIa lineage was the key contributor in one paddy soil of Inceptisol origin. These results suggest that methanotroph-mediated synthesis of biomass plays an important role in soil organic matter accumulation. This study thus supports the concept that methanotrophs not only consume the greenhouse gas methane but also serve as a key biotic factor in maintaining soil fertility.