Thawing permafrost in retrogressive thaw slumps leads to higher N availability

Currently, 20% of the Northern Hemisphere is affected by thermokarst, with an increase expected in future. In particular, ice-rich Yedoma sediments are susceptible to abrupt thaw, which leads to the formation of retrogressive thaw slumps (RTS). These erosion processes result in loss of vegetation, expose long-term frozen permafrost sediments at the surface, and makes soil organic matter (SOM) available for mineralization. Permafrost-affected soils of RTS exhibited higher N availability, as indicated by higher δ15N content of bulk soil, higher nitrate content and higher microbial N turnover (N mineralization especially net nitrification and denitrification) associated with high abundance of functional N genes compared to undisturbed soils. This elevated N availability results in significant emission of the greenhouse gas N2O, especially from exposed permafrost. Based on measured N2O emissions, N2O loss could be as high as 54.8 mg N2O-N per year, which is 0.14% of the initial inorganic N content of exposed Yedoma. The higher N availability of eroded permafrost-affected soils might affect C mineralization because eroded soils had lower aerobic CO2 production than undisturbed soils and CH4 production was detectable in laboratory incubations only in the absence of N2O production.