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

Nitrous oxide (N 2 O) emissions contribute to stratospheric ozone depletion and global warming. Climate warming causes permafrost thawing and decomposition of the dormant nitrogenous compounds, releasing N 2 O; however, understanding of the microbial formation and consumption of N 2 O 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 N 2 O formation and consumption in the presence of either nitrate (NO 3 - , 1 mM) or N 2 O (1 mM), respectively, during incubation at 4 and 20°C. Rapid N 2 O formation was observed in NO 3 - -amended microcosms, but N 2 O 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 NO 3 - and/or N 2 O were recovered from 16 metagenomes obtained from duplicate NO 3 - - and N 2 O-amended microcosms. None of the MAGs carried a complete set of genes to perform canonical denitrification (i.e., NO 3 - →N 2 ) indicating N 2 O formation and consumption is likely driven by non-denitrifying bacteria. While coastal permafrost microbiomes harbor nosZ genes, activity monitored in the microcosms indicates N 2 O formation exceeds N 2 O consumption, emphasizing the need for integrated approaches to assess and predict N turnover in thawing permafrost.