Investigating permafrost thaw impact on nitrogen cycle
Thermokarst(opens in new window) features appear where ice-rich permafrost thaws. While the frozen soil thaws slowly, the ice wedges, often several metres wide, melt quickly. This rapid melting can make the ground collapse, causing erosion along the coast or forming many depressions that fill with water (e.g. small lakes) in inland areas.
Thermokarst processes and global warming
Thermokarst processes not only affect the Arctic landscape however – they can also have implications for climate change. “Trapped in the permafrost are dead plant remains, and thus, also organic carbon and nitrogen,” explains NITROKARST(opens in new window) project fellow Nicolas Valiente from the University of Castilla-La Mancha(opens in new window) in Spain. “Permafrost soils hold one of the world’s largest pools of organic carbon and global nitrogen. When the soils and soil-ice mixtures thaw, the complex organic matter is available and microbes become active, breaking down these compounds. As a result, they produce greenhouse gases that enhance climate warming: carbon dioxide, methane and nitrous oxide.” The NITROKARST project, coordinated by the University of Vienna(opens in new window) in Austria and supported by the Marie Skłodowska-Curie Actions(opens in new window) programme, sought to better understand the underlying mechanisms of the microbial nitrogen cycle in thermokarst-affected soils. “Nitrous oxide is a greenhouse gas with 273 times(opens in new window) the warming potential of carbon dioxide,” says Valiente. “Microbes obtain energy from nitrogen released during the decomposition of organic material, and produce nitrous oxide as a result of their metabolism. As yet, we still don’t really know how these processes take place along thermokarst landscapes.”
Collecting samples in the Canadian Arctic
To unravel this mystery, Valiente and his colleagues travelled to the Canadian Arctic, specifically to Inuvik in the Northwest Territories, for two weeks of fieldwork. During this time, soil samples – from various depths and at different stages of the thermokarst process – were collected. “At the Western Arctic Research Centre(opens in new window) in Inuvik, we thawed some of these samples over several days, to simulate natural thawing,” adds Valiente. “We then added proteins with labelled nitrogen to the samples, and incubated them in airtight containers.” After a few days, the project team analysed the samples, to identify microbial transformations. Samples were also brought back to Vienna for further analysis.
Better understanding of nitrogen recycling processes
“Our initial results indicate that nitrogen recycling processes vary with depth and the progression of thermokarst stages,” explains Valiente. “In the active layer (the top layer of soil that thaws during summer), where nitrogen availability is limited, it seems that microbes tend to uptake and immobilise it.” In contrast, in the permafrost, where there are larger nitrogen stocks, nitrogen is primarily used for mineralisation(opens in new window). “Although we did not detect significant nitrous oxide production in our 9-day experiments, mineralised nitrogen can be used as an energy source leading nitrous oxide production as a by-product of microbial pathways,” notes Valiente. Achieving a better understanding of the microbial processes at work here has opened the door to further research. A key next step, says Valiente, will be to identify the microorganisms responsible for nitrogen transformations, using genomic(opens in new window) techniques. “It would also be valuable to investigate whether nitrous oxide production, as a by-product of other processes, becomes significant in thawing ice-rich permafrost regions (prone to thermokarst) over the long term,” he concludes.
