Permafrost-affected tundra soils are large carbon (C) and nitrogen (N) reservoirs. However, N is largely bound in soil organic matter (SOM), and ecosystems generally have low N availability. Therefore, microbial induced N-cycling processes and N losses were considered negligible. Recent studies show that microbial N processing rates, inorganic N availability, and lateral N losses from thawing permafrost increase when vegetation cover is disturbed, resulting in reduced N uptake or increased N input from thawing permafrost. In this review, we describe currently known N hotspots, particularly bare patches in permafrost peatland or permafrost soils affected by thermokarst, and their microbiogeochemical characteristics, and present evidence for previously unrecorded N hotspots in the tundra. We summarize the current understanding of microbial N cycling processes that promote the release of the potent greenhouse gas (GHG) nitrous oxide (N₂O) and the translocation of inorganic N from terrestrial into aquatic ecosystems. We suggest that certain soil characteristics and microbial traits can be used as indicators of N availability and N losses. Identifying N hotspots in permafrost soils is key to assessing the potential for N release from permafrost-affected soils under global warming, as well as the impact of increased N availability on emissions of carbon-containing GHGs.

Ice–rich Pleistocene permafrost deposits (Yedoma) store large amounts of nitrogen (N) and are susceptible to rapid thaw. In this study, we assess whether eroding Yedoma deposits are potential sources of N and gaseous carbon (C) losses. Therefore, we determined aerobic net ammonification and nitrification, as well as anaerobic production of nitrous oxide (N₂O), carbon dioxide (CO₂), and methane (CH₄) in laboratory incubations. Samples were collected from non-vegetated and revegetated slump floor (SF) and thaw mound (TM) soils of a retrogressive thaw slump in the Lena River Delta of Eastern Siberia. We found high nitrate concentrations (up to 110 µg N (g DW)⁻¹) within the growing season, a faster transformation of organic N to nitrate, and high N₂O production (up to 217 ng N₂O-N (g DW)⁻¹ day⁻¹) in revegetated thaw mounds. The slump floor was low in nitrate and did not produce N₂O under anaerobic conditions, but produced the most CO₂ (up to 7 µg CO₂-C (g DW)⁻¹ day⁻¹) and CH₄ (up to 65 ng CH₄-C (g DW)⁻¹ day⁻¹). Nitrate additions showed that denitrification was substrate limited in the slump floor. Nitrate limitation was rather caused by field conditions (moisture, pH) than by microbial functional limitation since nitrification rates were positive under laboratory conditions. Our results emphasize the relevance of considering landscape processes, geomorphology, and soil origin in order to identify hotspots of high N availability, as well as C and N losses. High N availability is likely to have an impact on carbon cycling, but to what extent needs further investigation.

Previous research was addressed to carbon emissions after permafrost thaw, but less attention was paid to changes in nitrogen availability and N2O emissions and in particular data from the Russian Arctic are scarce. Rise in water temperature and sea-level contribute to coastal erosion accelerating thaw rates and the release of dissolved nitrogen. Already 78% of the coastal regions of the Laptev Sea are affected by rapid permafrost thaw. This study estimates whether eroded Arctic coasts are hotspots for N availability and N2O emissions and to further understand the impact of NO3- leaching. Therefore, we estimated N-transformation rates and greenhouse gas (GHG) production (CO2, CH4, N2O) by incubating non-vegetated and revegetated soil samples from a retrogressive thaw slump in the Lena River Delta, Siberia. Within the thaw slump we found at exposed thaw mounds a domination of DIN over DON and an accumulation of NO3- with up 110 µg N (g DW)-1 within the growing season and in the presence of vegetation. Those results are contracting to what is normally reported in Arctic regions. Our incubations indicate that thaw mounds are hotspots for N-mineralization and N2O release (up to 390 ng N2O-N (g DW)-1) via denitrification while at the slump floor denitrification was substrate limited. Substrate limitation is rather caused by soil moisture and pH value than by functional limitation, since in our incubation N-mineralization could proceed in all samples. Simulated NO3- leaching removed the substrate limitation of the denitrification and converted the slump floor to a significant N2O hotspot (410 ng N2O-N (g DW)-1).Our results emphasise that it is necessary to consider geomorphology and landscape processes to identify hotspots of gaseous and dissolved N loss. A higher availability of inorganic nitrogen in coastal zones will have effects on marine ecosystems and more in depth-studies are needed to characterise seasonality of nitrogen leaching by melt water and eroded sediments.

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.

Arctic warming is facilitating the encroachment of trees into tundra landscapes. Trees at the forest-tundra ecotone are typically small, slow-growing and show high mortality rates. Tree necromass enters the soil as root, leaf and stem litter. This material can be decomposed or contribute to long-term soil organic matter stocks, as well as change decomposition of native soil organic matter (priming). We here tested whether decomposition processes change under dying spruce trees in tundra soils in a controlled laboratory experiment. The opportunity for addressing this question came up within a laboratory macrocosm experiment on the effect of various living tundra plants on carbon and nitrogen cycling in a tundra soil. For this experiment, root-picked soil was homogenized and filled into macrocosms, reconstructing the original horizon sequence. Plants with washed roots were placed in the soil and macrocosms watered regularly from the top. The small (ca. 50 cm) spruce trees died early in the experiment, and we kept the experiment running to assess changes in carbon and nutrient cycling resulting from the decomposition of spruce necromass compared to the plant-free control soil. Pore gas CO2 concentrations at 7 cm depth were significantly higher in spruce than in the control soils. We further observed significantly lower pH values, as well as significantly, ca. 25% lower potential activities of hydrolytic (leucine-aminopeptidase, cellobiohydrolase, N-acetyl-beta-D-glucosaminidase), but not oxidative extracellular enzymes. These findings suggest that the input of root and needle-leaf litter altered the functioning of the soil decomposer community. These differences extended into the deeper soil below the rooting zone of spruce plants, pointing at an important role of leaching. These first observations will be compared with surface CO2 fluxes, dissolved organic and microbial carbon concentrations to dissect the decomposition dynamics of spruce necromass at the forest-tundra ecotone.