Coupled methane and nitrous oxide biotransformation in freshwater wetland sediment microcosms

Abstract

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.