Coupled methane and nitrous oxide biotransformation in freshwater wetland sediment microcosms

Anaerobic oxidation of methane (AOM) coupled to denitrification is becoming the focus of scientific inquiry due to its potential contribution to global carbon and nitrogen cycles. AOM has been previously reported to proceed with nitrate (NO₃^-) or nitrite (NO₂^-). However, little research has been conducted on the simultaneous use of methane (CH₄) and nitrous oxide (N₂O). Here, coupled CH₄ and N₂O biotransformation in a freshwater wetland sediment was obtained in a 7-day anaerobic sediment incubation assay. The significant CO₂ accumulation and decrease of CH₄ emission in sediment microcosms was attributed to two mechanisms: inhibition of methanogenesis and N₂O-dependent AOM. To further confirm the coupled CH₄ and N₂O transformation, a ¹³C-labelled stable isotope tracer assay after anaerobic incubation was conducted with N₂O and/or CH₄ amendments. The N₂O-dependent AOM rate was 3.41 ± 0.13 nmol CO₂ g^-1 dry sediment·day^-1. According to metagenomic analysis, addition of N₂O stimulated AOM by increasing the activity and abundance of methanotrophic bacteria and by increasing enzymatic activities in the electron transport chain. Based on these results, we propose coupled CH₄ and N₂O biotransformation in the sediment microcosms for the first time, carried out by unidentified methanotroph(s) via intra‑oxygen produced in the presence of N₂O. Such a process has the potential to reduce the emission of two highly potent greenhouse gases and makes a significant contribution to the link of global carbon and nitrogen cycles in anoxic environments.