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

Nitrous oxide (N2O) emissions cause stratospheric ozone depletion and global warming. The reduction of N2O to dinitrogen by microorganisms expressing clade I or clade II N2O reductase (NosZ) is the only known biological process to mitigate N2O emissions. Climate warming causes permafrost thawing and decomposition of the dormant nitrogenous compounds, releasing N2O; however, the assessment of the microbial reduction of N2O in permafrost is still limited. Permafrost soils collected at two depths (5.4 m and 16.9 m) from a borehole situated on the East Siberian Sea coast were used to establish microcosms and assess N2O reduction in the presence of either N2O (1 mM) or nitrate (1 mM) at both 4C and 20C. Metagenome analysis of these permafrost samples revealed that they harbor N2O-reducing populations dominated by clade II type N2O reducers. Negligible N2O reduction was observed in all microcosms that received N2O; however, nitrate was effectively reduced in all nitrate-treated microcosms but the reduction was stalled at N2O as the end product. These findings suggest that the coastal permafrost would be the net N2O source if thawing persists. Genomic evidence as well as the physiology characterization are essential for the validation of the N2O reduction potential in permafrost.