NITROKARST · Effects of permafrost thaw on the global nitrogen cycle: the role of thermokarst systems

Effects of permafrost thaw on the global nitrogen cycle: the role of thermokarst systems

The Arctic is warming more rapidly than any other region in the world. There, permafrost soils cover ~25% of terrestrial surface and hold the world's largest soil organic carbon (C) and global nitrogen (N) pools. Rising temperatures are increasing the magnitude of permafrost thaw, impacting biogeochemistry, hydrology and ecology. Permafrost with low ice content suffers a gradual top-down thawing process during seasonal freeze-thaw period. However, thaw of ice-rich permafrost results in thermokarst processes, which occur abruptly and lead to ground surface collapse. Its widespread occurrence affects large areas (~40% of the northern permafrost region) contributing to develop ecosystems like ponds and lakes. In such ecosystems, the presence of anaerobic environments enhances microbial activity. As Arctic warms, both active layer deepening and thermokarst processes will increase, releasing soluble N into the environment and enhancing microbial decomposition of soil organic matter (SOM). So far, many studies have addressed the importance of permafrost thaw in the C cycle. However, little attention has been paid to the N cycle, despite nitrous oxide (N2O) is a powerful greenhouse gas (GHG), an ozone-depleting agent and may create unaccounted permafrost-climate feedback. Processes such as mineralization, nitrification and denitrification rates are expected to increase, and thus, N2O emissions to the atmosphere. The goal of NITROKARST was to explore the underlying mechanisms of the N cycle in thermokarst systems, looking at how microbial pathways promote N transformation. We hypothesized that distinct redox environments can be found along a thermokarst transect, and thus, differences in microbial communities and N cycling processes, including those leading to N2O production. To determine the effect of permafrost thaw and thermokarst development on microbial soil N cycle, we used a combination of isotope tracing assays and molecular tools (i.e., DNA metabarcoding). Our project conducted an intensive field campaign in August-September 2022 in the Canadian Arctic, more specifically between the Inuvik and Tuktoyaktuk (Northwest Territories). We used state of the art methods to assess whether depolymerization, mineralization and nitrification increase in seasonally thawed active layer compared to permafrost and thermokarst sediments, whether that leads to higher rates of SOM decomposition and/or N2O production, and which microorganisms are responsible for performing such processes. This multidisciplinary approach increases our knowledge about the importance of thermokarst-affected permafrost soils in the global N cycle.

Data: CORDIS, © European Union