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

Periodic Reporting for period 1 - NITROKARST (Effects of permafrost thaw on the global nitrogen cycle: the role of thermokarst systems)

Reporting period: 2022-02-01 to 2024-01-31

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.
We sampled 9 similar thermokarst transects between 15.08.2022 and 07.09.2022 in a catchment adjacent to the Dempster Highway, 140 km north from Inuvik, with thermokarst features appropriate for our research plan. There, we sampled 2 soil cores and 1 sediment core in each transect. These cores covered the heterogeneity of the different sites from upland soils (i.e. “stable”) to lowland (i.e. unstable, and patterned ground). From each soil core, 3 different sample layers were collected including horizons A, B, and the upper part of the permafrost. At the end of each transect (i.e. lake), a core was collected (surface sediment, upper 15 cm) plus surface water. The total of soil/sediment samples was 62 (7 samples/transect, one core had only two depths). For core sampling, permafrost samples were collected using a SIPRE corer and transported to the laboratories at the Western Arctic Research Centre (Inuvik, Northwest Territories, Canada). There, K2SO4 extracts were performed for further analyses (nutrients and isotopes), while aliquots were dried for further elemental analyses. For genomic analyses, samples were preserved in tubes with RNA-later. Aliquots for each sample were collected for further isotope-tracing incubations.
Incubations consisted of a 15N labelling approach by using 20 g of fresh material each in 60 mL incubation vials. Incubations were performed at two different temperatures, with active layer samples incubated at 10ºC, while permafrost and sediment samples were incubated at 4ºC. Few active layer samples were incubated at both temperatures in order to calculate the temperature coefficient (Q10). Samples were incubated in replicates and time points were sampled by sacrificing a vial at each time point (i.e. after 3, 5, 7, and 9 days). The labelling approach consisted of adding algal crude protein extract-15N (98 at%15N). Headspace gas samples and K2SO4 extracts were obtained at each time point to track the 15N added along the different pools. The results from such fieldwork campaign and incubations are being currently obtained. They are expected to be published in scientific journals and conferences in the coming months.
NITROKARST has so far provided for the first time an overview of the effect of permafrost thaw and thermokarst development on microbial soil N cycle. Once the results are fully analysed and published, we expect to provide recommendations for future scientific research to close our knowledge gaps, and increased the evidence that we need to include the role of microorganisms when considering the impacts of global N cycle in permafrost-affected soils. Our results will also determine a step forward in the social and economic context by addressing one of the main issues related to climate change at high latitudes (i.e. permafrost thaw). In this sense, the project goals clearly fall within the scope of the United Nations (UN) Decade on Ecosystem Restoration, aiming to massively scale up the restoration of degraded and destroyed ecosystems. Therefore, these measures aim to improve the productivity and capacity of ecosystems to meet the needs of society, which is fundamental to achieve the Sustainable Development Goals (SDG), and therefore aligned with the European policy objectives on climate change (SDG 13) and biodiversity conservation (SDGs 14 and 15). Due to their prevalence in northern countries, the behavior and consequences of permafrost thaw have been particularly studied in these regions for many decades, in some cases with the direct participation of the stakeholders. However, the escalating effects of climate change are causing thawing to increase at higher latitudes. Thus, our results will increase the knowledge about how to manage and restore permafrost-affected areas.
Conceptual model of a thermokarst transect. Credits: Nicolas Valiente Parra
Detail of a ice-rich permafrost core. Credits: Nicolas Valiente Parra
Microbial processes of the N cycle assessed in NITROKARST. Credits: Nicolas Valiente Parra
Fieldwork conducted during NITROKARST in August 2022. Credits: Nicolas Valiente Parra