Significant changes in climate and perturbation from human activities have been reported over the Qinghai -Tibetan Plateau (QTP), likely altering the ecosystem nitrogen (N) cycling and thus N2O emission. So far, a number of studies have reported variabilities of N2O fluxes from background soil conditions, or conducted warming and N addition experiments to test these effects; however, a synthesized understanding of warming and N input on soil N2O emission is still lacking for the QTP. Here, based on available studies published for this region, we investigated spatiotemporal patterns of background N2O fluxes and performed a meta-analysis to examine the warming and N-addition effects on N2O emission. Annual N2O fluxes ranged from-0.33 to 2.14 kg N2O-N ha(-1) yr(-1) (mean =0.73), of which their spatial distributions across ecosystems were mainly reflected by mean annual precipitation. N2O fluxes during growing seasons were generally larger than those in non-growing seasons, but hot moments of N2O emission existed during freeze-thawing periods. Our meta-analysis showed that warming had a significantly negative but minor effect on N2O emission from non-permafrost soils, although the effect varied with warming magnitudes and methods. The negative response of N2O flux to warming could be explained by the associated decrease of soil moisture and enhancement of plant N uptake. In contrast, warming-induced thawing increases soil moisture in permafrost soils, which could stimulate N2O emission. N addition exhibited an overall positive impact on N2O emission over the QTP region, with a moderate emission factor (0.8%) lower than the IPCC value. Considering the moderate N2O emission from background soils (< 1 kg N2O-N ha(-1) yr(-1)) and common N limitation across ecosystems, our findings suggest that climate change and enhanced N inputs may not turn the QTP into a globally significant N2O source in the near future.

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 is profound in the Arctic where increased snowfall during winter and warmer growing season temperatures may accelerate soil nitrogen (N) turnover and increase inorganic N availability. Nitrous oxide (N2O) is a potent greenhouse gas formed by soil microbes and in the Arctic, the production is seen as limited mainly by low inorganic N availability. Hence, it can be hypothesized that climate change in the Arctic may increase total N2O emissions, yet this topic remains understudied. We investigated the combined effects of variable snow depths and experimental warming on soil N cycling in a factorial field study established along a natural snowmelt gradient in a low Arctic heath ecosystem. The study assessed N2O surface fluxes, gross N mineralization and nitrification rates, potential denitrification activity, and the pools of soil microbial, soil organic and soil inorganic N, carbon (C) and phosphorus (P) during two growing seasons. The net fluxes of N2O averaged 1.7 mu g N2O-N m- 2 h-1 (range -3.6 to 10.5 mu g N2O-N m- 2 h-1), and generally increased from ambient (1 m) to moderate (2-3 m) snow depths. At the greatest snow depth (4 m) where snowmelt was profoundly later, N2O fluxes decreased, likely caused by combined negative effects of low summer temperatures and high soil moisture. Positive correlations between N2O and nitrate (NO3- ) and dissolved organic N (DON) suggested that the availability of N was the main controlling variable along the snowmelt gradient. The maximum N2O fluxes were observed in the second half of August associated with high NO3- concentrations. The effect of growing season experimental warming on N2O surface flux varied along the snowmelt gradient and with time. Generally, the experimental warming stimulated N2O fluxes under conditions with increased concentrations of inorganic N. In contrast, warming reduced N2O fluxes when inorganic N was low. Experimental warming had no clear effects on soil inorganic N. The study suggests that if increased winter precipitation leads to a deeper snow cover and a later snowmelt, total emissions of N2O from low Arctic heath ecosystems may be enhanced in the future and, dependent on dissolved N availability, summer warming may stimulate or reduce total emissions.

Despite the fact that winter lasts for a third of the year in the temperate grasslands, winter processes in these ecosystems have been inadequately represented in global climate change studies. While climate change increases the snow depth in the Mongolian Plateau, grasslands in this region are also simultaneously facing high pressure from land use changes, such as grazing, mowing, and agricultural cultivation. To elucidate how these changes affect the grasslands' winter nitrogen (N) budget, we manipulated snow depth under different land use practices and conducted a(15)NH(4)(15)NO(3)-labeling experiment. The change in(15)N recovery during winter time was assessed by measuring the(15)N/N-14 ratio of root, litter, and soils (0-5 cm and 5-20 cm). Soil microbial biomass carbon and N as well as N2O emission were also measured. Compared with ambient snow, the deepened snow treatment reduced total(15)N recovery on average by 21.7% and 19.2% during the first and second winter, respectively. The decrease in(15)N recovery was primarily attributed to deepened snow increasing the soil temperature and thus microbial biomass. The higher microbial activity under deepened snow then subsequently resulted in higher gaseous N loss. The N2O emission under deepened snow (0.144 kg N ha(-1)) was 6.26 times than that of under ambient snow (0.023 kg N ha(-1)) during the period of snow cover and spring thaw. Although deepened snow reduced soil(15)N recovery, the surface soil N concentration remained unchanged after five years' deepened snow treatment because deepened snow reduced soil N loss via wind erosion by 86%.

Permafrost thawing may lead to the release of carbon and nitrogen in high-latitude regions of the Northern Hemisphere, mainly in the form of greenhouse gases. Our research aims to reveal the effects of permafrost thawing on CH4 and N2O emissions from peatlands in Xiaoxing'an Mountains, Northeast China. During four growing seasons (2011-2014), in situ CH4 and N2O emissions were monitored from peatland under permafrost no-thawing, mild-thawing, and severe-thawing conditions in the middle of the Xiaoxing'an Mountains by a static-chamber method. Average CH4 emissions in the severe-thawing site were 55-fold higher than those in the no-thawing site. The seasonal variation of CH4 emission became more aggravated with the intensification of permafrost thawing, in which the emission peaks became larger and the absorption decreased to zero. The increased CH4 emissions were caused by the expansion of the thawing layer and the subsequent increases in soil temperature, water table, and shifts of plant communities. However, N2O emissions did not change with thawing. Permafrost thawing increased CH4 emissions but did not impact N2O emissions in peatlands in the Xiaoxing'an Mountains. Increased CH4 emissions from peatlands in this region may amplify global warming.

Climate warming is expected to increasingly influence boreal peatlands and alter their greenhouse gases emissions. However, the effects of warming on N2O fluxes and the N2O budgets were ignored in boreal peatlands. Therefore, in a boreal peatland of permafrost zone in Northeast China, a simulated warming experiment was conducted to investigate the effects of warming on N2O fluxes in Betula. Fruticosa community (B. Fruticosa) and Ledum. palustre community (L. palustre) during the growing seasons from 2013 to 2015. Results showed that warming treatment increased air temperature at 1.5 m aboveground and soil temperature at 5 cm depth by 0.6 degrees C and 2 degrees C, respectively. The average seasonal N2O fluxes ranged from 6.62 to 9.34 mu g m(-2) h(-1) in the warming plot and ranged from 0.41 to 4.55 mu g m(-2) h(-1) in the control plots. Warming treatment increased N2O fluxes by 147% and transformed the boreal peatlands from a N2O sink to a source. The primary driving factors for N2O fluxes were soil temperature and active layer depth, whereas soil moisture showed a weak correlation with N2O fluxes. The results indicated that warming promoted N2O fluxes by increasing soil temperature and active layer depth in a boreal peatland of permafrost zone in Northeast China. Moreover, elevated N2O fluxes persisted in this region will potentially drive a noncarbon feedback to ongoing climate change. (c) 2017 Elsevier B.V. All rights reserved.

Seasonal soil freeze-thaw events may enhance soil nitrogen transformation and thus stimulate nitrous oxide (N2O) emissions in cold regions. However, the mechanisms of soil N2O emission during the freeze-thaw cycling in the field remain unclear. We evaluated N2O emissions and soil biotic and abiotic factors in maize and paddy fields over 20 months in Northeast China, and the structural equation model (SEM) was used to determine which factors affected N2O production during non-growing season. Our results verified that the seasonal freeze-thaw cycles mitigated the available soil nitrogen and carbon limitation during spring thawing period, but simultaneously increased the gaseous N2O-N losses at the annual time scale under field condition. The N2O-N cumulative losses during the non-growing season amounted to 0.71 and 0.55 kg N ha(-1) for the paddy and maize fields, respectively, and contributed to 66 and 18% of the annual total. The highest emission rates (199.2-257.4 mu g m(-2) h(-1)) were observed during soil thawing for both fields, but we did not observe an emission peak during soil freezing in early winter. Although the pulses of N2O emission in spring were short-lived (18 d), it resulted in approximately 80% of the non-growing season N2O-N loss. The N2O burst during the spring thawing was triggered by the combined impact of high soil moisture, flush available nitrogen and carbon, and rapid recovery of microbial biomass. SEM analysis indicated that the soil moisture, available substrates including NH4+ and dissolved organic carbon (DOC), and microbial biomass nitrogen (MBN) explained 32, 36, 16 and 51% of the N2O flux variation, respectively, during the non-growing season. Our results suggested that N2O emission during the spring thawing make a vital contribution of the annual nitrogen budget, and the vast seasonally frozen and snow-covered croplands will have high potential to exert a positive feedback on climate change considering the sensitive response of nitrogen biogeochemical cycling to the freeze-thaw disturbance.

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.

Release of greenhouse gases from thawing permafrost is potentially the largest terrestrial feedback to climate change and one of the most likely to occur; however, estimates of its strength vary by a factor of thirty. Some of this uncertainty stems from abrupt thaw processes known as thermokarst (permafrost collapse due to ground ice melt), which alter controls on carbon and nitrogen cycling and expose organic matter from meters below the surface. Thermokarst may affect 20-50% of tundra uplands by the end of the century; however, little is known about the effect of different thermokarst morphologies on carbon and nitrogen release. We measured soil organic matter displacement, ecosystem respiration, and soil gas concentrations at 26 upland thermokarst features on the North Slope of Alaska. Features included the three most common upland thermokarst morphologies: active-layer detachment slides, thermo-erosion gullies, and retrogressive thaw slumps. We found that thermokarst morphology interacted with landscape parameters to determine both the initial displacement of organic matter and subsequent carbon and nitrogen cycling. The large proportion of ecosystem carbon exported off-site by slumps and slides resulted in decreased ecosystem respiration postfailure, while gullies removed a smaller portion of ecosystem carbon but strongly increased respiration and N2O concentration. Elevated N2O in gully soils persisted through most of the growing season, indicating sustained nitrification and denitrification in disturbed soils, representing a potential noncarbon permafrost climate feedback. While upland thermokarst formation did not substantially alter redox conditions within features, it redistributed organic matter into both oxic and anoxic environments. Across morphologies, residual organic matter cover, and predisturbance respiration explained 83% of the variation in respiration response. Consistent differences between upland thermokarst types may contribute to the incorporation of this nonlinear process into projections of carbon and nitrogen release from degrading permafrost.

Soil organic matter decomposition under global warming has a potential to alter soil carbon and nitrogen storages in permafrost. The objectives of this study were to investigate the temperature sensitivity of greenhouse gas emissions from soil samples along a mountain wetland-forest ecotone in the continuous permafrost and determine its influencing mechanisms. The CO2, N2O and carbon, nitrogen substrates were measured at 5, 15 and 25 degrees C. The relation between greenhouse gas emission rates and temperature depended on substrate quality in the three ecosystems. Soil DOC, MBC, NH4+ and NO3- concentrations determined the higher CO2 and N2O emission rates in the thicket peatland and the surface soil layer. During the incubation period, the degrees of soil carbon and nitrogen losses in the thicket peatland were 0.6-4.7% and 1.0-143 (1000 x %), approximately 1.6 and 1.2 times higher than those in the forest and fen, respectively. The highest degrees of soil carbon and nitrogen losses in the thicket peatland indicated that more greenhouse gases would emit from soils when permafrost degradation induced the succession from wetlands or forest to the wetland-forest ecotone. Although the gas emission rates presented significant differences in the three ecosystems, the Q(10) values with 2.0 to 2.2 for CO2 and 2.4 to 3.0 for N2O, did not change significantly, indicating that the temperature sensitivity of gas emissions would not fluctuate much in the ecosystems along the mountain wetland-forest ecotone. However, the higher Q(10) values in the deeper soil layer in our study indicated that the decomposition of soil C and N in the deeper active layer of the permafrost region is more impressionable to global warming. As laboratory results could not actually reflect the situation in the field, more field work about temperature sensitivity of soil organic matter decomposition in different ecosystems should be encouraged in the future. (C) 2014 Elsevier B.V. All rights reserved.