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.

Throughout most of the northern hemisphere, snow cover decreased in almost every winter month from 1967 to 2012. Because snow is an effective insulator, snow cover loss has likely enhanced soil freezing and the frequency of soil freeze-thaw cycles, which can disrupt soil nitrogen dynamics including the production of nitrous oxide (N2O). We used replicated automated gas flux chambers deployed in an annual cropping system in the upper Midwest US for three winters (December-March, 2011-2013) to examine the effects of snow removal and additions on N2O fluxes. Diminished snow cover resulted in increased N2O emissions each year; over the entire experiment, cumulative emissions in plots with snow removed were 69% higher than in ambient snow control plots and 95% higher than in plots that received additional snow (P < 0.001). Higher emissions coincided with a greater number of freeze-thaw cycles that broke up soil macroaggregates (250-8000 A mu m) and significantly increased soil inorganic nitrogen pools. We conclude that winters with less snow cover can be expected to accelerate N2O fluxes from agricultural soils subject to wintertime freezing.

Short-term N2O emission occurs in relation to snowmelt within seasonally frozen soil. To understand the effects of changing winter climates on the N2O flux, snow cover manipulation experiments are useful. In Japan, snow cover manipulation is practiced by farmers to improve agricultural yield and is executed either by applying a broadcast of blackish agent onto the snow cover, which leads to faster snow-melting thereby extending the crop-growing season, or by snow cover removal/re-accumulation, leading to an enhanced soil frost depth for weed management. Implementation of these practices involves using an amount of fossil fuel, in addition to influencing soil-derived N2O emissions, therefore, the load factors of snow cover management practices per unit area of agricultural field were estimated in this study. Field data including micrometeorological conditions, ground surface flux of N2O, and amount of fossil fuel consumed during machinery operation for management practices, were obtained at two sites in Hokkaido over 2 years (2008-2010). Fuel consumption for the field spreading was found to be unexpectedly small (0.017 Mg CO2 eq ha(-1)). It was therefore suggested that acceleration of snowmelt may have the potential to reduce net greenhouse gas emissions if the agent used is a low-degradable C-rich material, such as charcoal. For soil frost control, the fossil fuel consumption during a set of snow cover removal/re-accumulation (estimated as 0.052 Mg CO2 eq ha(-1)) is discussed, together with the relationship between possible mechanisms causing stimulation of N2O production in frozen soil and inherent large differences in N2O flux among sites.