Nitrous Oxide Formation and Consumption in Thawing Permafrost: A Microcosm Study

Nitrous oxide (N2O) emissions contribute to stratospheric ozone depletion and global warming. Climate warming causes permafrost thawing and decomposition of the dormant nitrogenous compounds, releasing N2O; however, understanding of the microbial formation and consumption of N2O in permafrost is still limited. Permafrost soils collected at two depths (5.4 m and 16.9 m) from the East Siberian Sea coast of Russia were used to establish microcosms assessing N2O formation and consumption in the presence of either nitrate (NO3-, 1 mM) or N2O (1 mM), respectively, during incubation at 4 and 20{degrees}C. Rapid N2O formation was observed in NO3--amended microcosms, but N2O consumption was slow and incomplete over a 1-year incubation period in all microcosms. Twenty-six quality-filtered metagenome-assembled genomes (MAGs) harboring genes involved in the reduction of NO3- and/or N2O were recovered from 16 metagenomes obtained from duplicate NO3-- and N2O-amended microcosms. None of the MAGs carried a complete set of genes to perform canonical denitrification (i.e., NO3-[->]N2) indicating N2O formation and consumption is likely driven by non-denitrifying bacteria. While coastal permafrost microbiomes harbor nosZ genes, activity monitored in the microcosms indicates N2O formation exceeds N2O consumption, emphasizing the need for integrated approaches to assess and predict N turnover in thawing permafrost.