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.

Climate change-related increases in winter temperatures and precipitation, as predicted for eastern Canada, may alter snow cover, with consequences for soil temperature and moisture, nitrogen cycling, and greenhouse gas fluxes. To assess the effects of snow depth in a humid temperate agricultural ecosystem, we conducted a two-year field study with (1) snow removal, (2) passive snow accumulation (via snow fence), and (3) ambient snow treatments. We measured in situ N2O and CO2 fluxes and belowground soil gas concentration, and conducted denitrification and potential nitrification laboratory assays, from November through May. Snow manipulation significantly affected winter N2O dynamics. In the first winter, spring thaw N2O fluxes in snow removal plots were 31 and 48 times greater than from ambient snow and snow accumulation plots respectively. Mid-winter soil N2O concentration was also highest in snow removal plots. These effects may have been due to increased substrate availability due to greater soil frost, along with moderate gas diffusivities facilitating N2O production, in snow removal plots. In the second winter, spring thaw N2O fluxes and soil N2O concentration were greatest for ambient snow plots. Peak fluxes in ambient snow plots were 19 and 24 times greater than in snow accumulation and snow removal plots, respectively. Greater soil moisture in ambient snow plots overwinter could have facilitated denitrification both through decreased O-2 availability and increased disruption of soil aggregates during freeze-thaw cycles. Overall, results suggest that effects of changing snow cover on N cycling and N2O fluxes were not solely a direct effect of snow depth; rather, effects were mediated by both soil water content and temperature. Furthermore, the fact that treatments with greatest mid-winter belowground N2O accumulation also had greatest spring thaw N2O fluxes in both years suggests the hypothesis that high spring thaw fluxes were due not only to spring soil conditions, but also to an effect of soil conditions in frozen soils that had facilitated N2O production throughout winter.

Increasing temperatures have been shown to impact soil biogeochemical processes, although the corresponding changes to the underlying microbial functional communities are not well understood. Alterations in the nitrogen (N) cycling functional component are particularly important as N availability can affect microbial decomposition rates of soil organic matter and influence plant productivity. To assess changes in the microbial component responsible for these changes, the composition of the N-fixing (nifH), and denitrifying (nirS, nirK, nosZ) soil microbial communities was assessed by targeted pyrosequencing of functional genes involved in N cycling in two major biomes where the experimental effect of climate warming is under investigation, a tallgrass prairie in Oklahoma (OK) and the active layer above permafrost in Alaska (AK). Raw reads were processed for quality, translated with frameshift correction, and a total of 313,842 amino acid sequences were clustered and linked to a nearest neighbor using reference datasets. The number of OTUs recovered ranged from 231 (NifH) to 862 (NirK). The N functional microbial communities of the prairie, which had experienced a decade of experimental warming were the most affected with changes in the richness and/or overall structure of NifH, NirS, NirK and NosZ. In contrast, the AK permafrost communities, which had experienced only 1 year of warming, showed decreased richness and a structural change only with the nirK-harboring bacterial community. A highly divergent nirK-harboring bacterial community was identified in the permafrost soils, suggesting much novelty, while other N functional communities exhibited similar relatedness to the reference databases, regardless of site. Prairie and permafrost soils also harbored highly divergent communities due mostly to differing major populations.