Influences of Land Use/Cover Types on Nitrous Oxide Emissions during Freeze-Thaw Periods from Waterlogged Soils in Inner Mongolia

Nitrous oxide emissions during freeze/thaw periods contribute significantly to annual soil N₂O emissions budgets in middle- and high-latitude areas; however, the freeze/thaw-related N₂O emissions from waterlogged soils have hardly been studied in the Hulunber Grassland, Inner Mongolia. For this study, the effects of changes in land use/cover types on N₂O emissions during freeze–thaw cycles were investigated to more accurately quantify the annual N₂O emissions from grasslands. Soil cores from six sites were incubated at varying temperature (ranging from −15 to 10°C) to simulate freeze–thaw cycles. N₂O production rates were low in all soil cores during freezing periods, but increased markedly after soil thawed. Mean rates of N₂O production differed by vegetation type, and followed the sequence: Leymus chinensis (LC) and Artemisia tanacetifolia (AT) steppes > LC steppes ≥ Stipa baicalensis (SB) steppes. Land use types (mowing and grazing) had differing effects on freeze/thaw-related N₂O production. Grazing significantly reduced N₂O production by 36.8%, while mowing enhanced production. The production of N₂O was related to the rate at which grassland was mowed, in the order: triennially (M3) > once annually (M1) ≥ unmown (UM). Compared with the UM control plot, the M3 and M1 mowing regimes enhanced N₂O production by 57.9% and 13.0% respectively. The results of in situ year-round measurements showed that large amounts of N₂O were emitted during the freeze–thaw period, and that annual mean fluxes of N₂O were 9.21 μg N₂O-N m^-2 h^-1 (ungrazed steppe) and 6.54 μg N₂O-N m^-2 h^-1 (grazed steppe). Our results further the understanding of freeze/thaw events as enhancing N₂O production, and confirm that different land use/cover types should be differentiated rather than presumed to be equivalent, regarding nitrous oxide emission. Even so, further research involving multi-year and intensive measurements of N₂O emission is still needed.     

Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

N₂O gas is involved in global warming and ozone depletion. The major sources of N₂O are soil microbial processes. Anthropogenic inputs into the nitrogen cycle have exacerbated these microbial processes, including nitrification. Ammonia-oxidizing archaea (AOA) are major members of the pool of soil ammonia-oxidizing microorganisms. This study investigated the isotopic signatures of N₂O produced by soil AOA and associated N₂O production processes. All five AOA strains (I.1a, I.1a-associated and I.1b clades of Thaumarchaeota) from soil produced N₂O and their yields were comparable to those of ammonia-oxidizing bacteria (AOB). The levels of site preference (SP), δ¹⁵N^bulk and δ¹⁸O -N₂O of soil AOA strains were 13–30%, −13 to −35% and 22–36%, respectively, and strains MY1–3 and other soil AOA strains had distinct isotopic signatures. A ¹⁵N-NH₄⁺-labeling experiment indicated that N₂O originated from two different production pathways (that is, ammonia oxidation and nitrifier denitrification), which suggests that the isotopic signatures of N₂O from AOA may be attributable to the relative contributions of these two processes. The highest N₂O production yield and lowest site preference of acidophilic strain CS may be related to enhanced nitrifier denitrification for detoxifying nitrite. Previously, it was not possible to detect N₂O from soil AOA because of similarities between its isotopic signatures and those from AOB. Given the predominance of AOA over AOB in most soils, a significant proportion of the total N₂O emissions from soil nitrification may be attributable to AOA.     

No significant nitrous oxide emissions during spring thaw under grazing and nitrogen addition in an alpine grassland

A recent study (Wolf et al., 2010) suggests that short—lived pulses of N₂O emission during spring thaw dominate the annual N₂O budget and that grazing decreases N₂O emissions during the spring thaw. To verify this we conducted year—round N₂O flux measurements from June 2010 to May 2011 in Tianshan alpine grassland in central Asia. No pulse emissions of N₂O were found at grazing management sites and nitrogen addition sites during the spring thaw. The contribution of the spring thaw to the total annual N₂O budget was small and accounted for only 6.6% of the annual fluxes, with winter emissions accounting for 16.7% and growing season emissions accounting for 76.7%. The difference in N₂O emissions attributable to grazing management was not significant (P > 0.05). Nitrogen input tended to increase N₂O emissions at N addition sites during the grass growing season compared with those at unfertilized sites. N₂O fluxes showed a significant correlation with air temperature and also with both soil temperature and soil water content at 10 cm depth.     

Fluxes of nitrous oxide, methane and carbon dioxide during freezing–thawing cycles in an Inner Mongolian steppe

Combined measurements of nitrification activity and N₂O emissions were performed in a lowland and a montane tropical rainforest ecosystem in NE-Australia over a 18 months period from October 2001 until May 2003. At both sites gross nitrification rates, measured by the BaPS technique, showed a strong seasonal pattern with significantly higher rates of gross nitrification during wet season conditions. Nitrification rates at the montane site (1.48?±?0.24–18.75?±?2.38 mg N kg^?1 day^?1) were found to be significantly higher than at the lowland site (1.65?±?0.21–4.54?±?0.27 mg N kg^?1 day^?1). The relationship between soil moisture and gross nitrification rates could be described best by O’Neill functions having a soil moisture optimum of nitrification at app. 65% WFPS. At the lowland site, for which continuous measurements of N₂O emissions were available, nitrification was positively correlated with N₂O emission. Nitrification contributed significantly to N₂O formation during dry season (app.85%) but less (app. 30%) during wet season conditions. In average 0.19‰ of the N metabolized by nitrification was released as N₂O. The N₂O fraction loss for nitrification was positively correlated with changes in soil moisture and varied slightly between 0.15 and 0.22‰. Our results demonstrate that combined N₂O emission and microbial N turnover studies covering prolonged observation periods are needed to clarify and quantify the role of the microbial processes nitrification and denitrification for annual N₂O emissions from soils of terrestrial ecosystems.     

Soil biogeochemistry during the early spring in low arctic mesic tundra and the impacts of deepened snow and enhanced nitrogen availability

Air temperature freeze–thaw cycles often occur during the early spring period directly after snowmelt and before budbreak in low arctic tundra. This early spring period may be associated with nitrogen (N) and carbon (C) loss from soils as leachate or as trace gases, due to the detrimental impact of soil freeze–thaw cycles and a developing active layer on soil microorganisms. We measured soil and microbial pools of C and N in early spring during a period of fluctuating air temperature (ranging from ?4 to +10°C) and in midsummer, in low arctic birch hummock tundra. In addition we measured N₂O, CH₄ and CO₂ production in the early spring. All of these biogeochemical variables were also measured in long-term snowfence (deepened snow) and N-addition plots to characterize climate-change related controls on these variables. Microbial and soil solution pools of C and N, and trace gas production varied among the five early spring sample dates, but only marginally and no more than among sample dates in midsummer. N-addition greatly elevated N₂O fluxes, indicating that although denitrifiers were present their activity during early spring was strongly limited by N-availability, but otherwise trace gas production was very low in early spring. The later thaw, warmer winter and colder spring soil temperatures resulting from deepened snow did not significantly alter N pools or rates in early spring. Together, our results indicate strong stability in microbial and soil solution C and N pool sizes in the early spring period just after snowmelt when soil temperatures are close to 0°C (?1.5 to +5°C). A review of annual temperature records from this and other sites suggests that soil freeze–thaw cycles are probably infrequent in mesic tundra in early spring. We suggest that future studies concerned with temperature controls on soil and microbial biogeochemistry should focus not on soil freeze–thaw cycles per se, but on the rapid and often stepped increases in soil temperature that occur under the thawing snowpack.     

Nitrogen addition,rather than altered precipitation,stimulates nitrous oxide emissions in an alpine steppe

Anthropogenic‐driven global change, including changes in atmospheric nitrogen (N) deposition and precipitation patterns, is dramatically altering N cycling in soil. How long‐term N deposition, precipitation changes, and their interaction influence nitrous oxide (N₂O) emissions remains unknown, especially in the alpine steppes of the Qinghai–Tibetan Plateau (QTP). To fill this knowledge gap, a platform of N addition (10 g m⁻² year⁻¹) and altered precipitation (±50% precipitation) experiments was established in an alpine steppe of the QTP in 2013. Long‐term N addition significantly increased N₂O emissions. However, neither long‐term alterations in precipitation nor the co‐occurrence of N addition and altered precipitation significantly affected N₂O emissions. These unexpected findings indicate that N₂O emissions are particularly susceptible to N deposition in the alpine steppes. Our results further indicated that both biotic and abiotic properties had significant effects on N₂O emissions. N₂O emissions occurred mainly due to nitrification, which was dominated by ammonia‐oxidizing bacteria, rather than ammonia‐oxidizing archaea. Furthermore, the alterations in belowground biomass and soil temperature induced by N addition modulated N₂O emissions. Overall, this study provides pivotal insights to aid the prediction of future responses of N₂O emissions to long‐term N deposition and precipitation changes in alpine ecosystems. The underlying microbial pathway and key predictors of N₂O emissions identified in this study may also be used for future global‐scale model studies.     

Dominance of bacterial ammonium oxidizers and fungal denitrifiers in the complex nitrogen cycle pathways related to nitrous oxide emission

Organic compounds and mineral nitrogen (N) usually increase nitrous oxide (N₂O) emissions. Vinasse, a by‐product of bio‐ethanol production that is rich in carbon, nitrogen, and potassium, is recycled in sugarcane fields as a bio‐fertilizer. Vinasse can contribute significantly to N₂O emissions when applied with N in sugarcane plantations, a common practice. However, the biological processes involved in N₂O emissions under this management practice are unknown. This study investigated the roles of nitrification and denitrification in N₂O emissions from straw‐covered soils amended with different vinasses (CV: concentrated and V: nonconcentrated) before or at the same time as mineral fertilizers at different time points of the sugarcane cycle in two seasons. N₂O emissions were evaluated for 90 days, the period that occurs most of the N₂O emission from fertilizers; the microbial genes encoding enzymes involved in N₂O production (archaeal and bacterial amoA, fungal and bacterial nirK, and bacterial nirS and nosZ), total bacteria, and total fungi were quantified by real‐time PCR. The application of CV and V in conjunction with mineral N resulted in higher N₂O emissions than the application of N fertilizer alone. The strategy of vinasse application 30 days before mineral N reduced N₂O emissions by 65% for CV, but not for V. Independent of rainy or dry season, the microbial processes were nitrification by ammonia‐oxidizing bacteria (AOB) and archaea and denitrification by bacteria and fungi. The contributions of each process differed and depended on soil moisture, soil pH, and N sources. We concluded that amoA‐AOB was the most important gene related to N₂O emissions, which indicates that nitrification by AOB is the main microbial‐driven process linked to N₂O emissions in tropical soil. Interestingly, fungal nirK was also significantly correlated with N₂O emissions, suggesting that denitrification by fungi contributes to N₂O emission in soils receiving straw and vinasse application.     

Effects of drought and N-fertilization on N cycling in two grassland soils

Changes in frequency and intensity of drought events are anticipated in many areas of the world. In pasture, drought effects on soil nitrogen (N) cycling are spatially and temporally heterogeneous due to N redistribution by grazers. We studied soil N cycling responses to simulated summer drought and N deposition by grazers in a 3-year field experiment replicated in two grasslands differing in climate and management. Cattle urine and NH₄NO₃ application increased soil NH₄ ⁺ and NO₃ ^? concentrations, and more so under drought due to reduced plant uptake and reduced nitrification and denitrification. Drought effects were, however, reflected to a minor extent only in potential nitrification, denitrifying enzyme activity (DEA), and the abundance of functional genes characteristic of nitrifying (bacterial and archaeal amoA) and denitrifying (narG, nirS, nirK, nosZ) micro-organisms. N₂O emissions, however, were much reduced under drought, suggesting that this effect was driven by environmental limitations rather than by changes in the activity potential or the size of the respective microbial communities. Cattle urine stimulated nitrification and, to a lesser extent, also DEA, but more so in the absence of drought. In contrast, NH₄NO₃ reduced the activity of nitrifiers and denitrifiers due to top-soil acidification. In summary, our data demonstrate that complex interactions between drought, mineral N availability, soil acidification, and plant nutrient uptake control soil N cycling and associated N₂O emissions. These interactive effects differed between processes of the soil N cycle, suggesting that the spatial heterogeneity in pastures needs to be taken into account when predicting changes in N cycling and associated N₂O emissions in a changing climate.     

Symbiotic relationships between soil fungi and plants reduce N2O emissions from soil

N₂O is a potent greenhouse gas involved in the destruction of the protective ozone layer in the stratosphere and contributing to global warming. The ecological processes regulating its emissions from soil are still poorly understood. Here, we show that the presence of arbuscular mycorrhizal fungi (AMF), a dominant group of soil fungi, which form symbiotic associations with the majority of land plants and which influence a range of important ecosystem functions, can induce a reduction in N₂O emissions from soil. To test for a functional relationship between AMF and N₂O emissions, we manipulated the abundance of AMF in two independent greenhouse experiments using two different approaches (sterilized and re-inoculated soil and non-mycorrhizal tomato mutants) and two different soils. N₂O emissions were increased by 42 and 33% in microcosms with reduced AMF abundance compared to microcosms with a well-established AMF community, suggesting that AMF regulate N₂O emissions. This could partly be explained by increased N immobilization into microbial or plant biomass, reduced concentrations of mineral soil N as a substrate for N₂O emission and altered water relations. Moreover, the abundance of key genes responsible for N₂O production (nirK) was negatively and for N₂O consumption (nosZ) positively correlated to AMF abundance, indicating that the regulation of N₂O emissions is transmitted by AMF-induced changes in the soil microbial community. Our results suggest that the disruption of the AMF symbiosis through intensification of agricultural practices may further contribute to increased N₂O emissions.     

Management of nitrogen fertilizer application, rather than functional gene abundance, governs nitrous oxide fluxes in hydroponics with rockwool

Aims

Nitrous oxide (N₂O) is a strong greenhouse effective gas (GHG); the primary human source of N₂O is agricultural activities. Excessive nitrogen (N) fertilization of agricultural land is now widely recognized as a major contributor. In soil, the microbial processes of nitrification and denitrification are the principal sources of N₂O. However, it remains poorly understood how conventional hydroponics influences GHG emission. Here, we compared GHG fluxes from soil and rockwool used for hydroponics under identical nutrient conditions.

Methods

Tomato plants (Solanum lycopersicum, momotaro) were grown in soil or by hydroponics using rockwool. In situ emissions of CH₄, CO₂, and N₂O, and the abundance of genes involved in nitrification and denitrification were measured during cultivation.

Results

Hydroponics with rockwool mitigated CO₂ emission by decreasing the microbial quantity in the rhizosphere. Dilution of the nutrient solution significantly decreased N₂O emission from rockwool. Although proliferation of nitrifiers or denitrifiers in the rhizosphere did not induce N₂O emission, reuse or long-term use of rockwool induced a 3.8-fold increase in N₂O emission.

Conclusions

Our data suggest that hydroponics has a lower environmental impact and that adequate fertilizer application, rather than bacterial control, governs N₂O fluxes in hydroponic cultivation using rockwool.     

Mitigating N2O emissions from agricultural soils with fungivorous mites

Nitrous oxide (N₂O) is an important greenhouse gas and an ozone-depleting substance. Due to the long persistence of N₂O in the atmosphere, the mitigation of anthropogenic N₂O emissions, which are mainly derived from microbial N₂O-producing processes, including nitrification and denitrification by bacteria, archaea, and fungi, in agricultural soils, is urgently necessary. Members of mesofauna affect microbial processes by consuming microbial biomass in soil. However, how microbial consumption affects N₂O emissions is largely unknown. Here, we report the significant role of fungivorous mites, the major mesofaunal group in agricultural soils, in regulating N₂O production by fungi, and the results can be applied to the mitigation of N₂O emissions. We found that the application of coconut husks, which is the low-value part of coconut and is commonly employed as a soil conditioner in agriculture, to soil can supply a favorable habitat for fungivorous mites due to its porous structure and thereby increase the mite abundance in agricultural fields. Because mites rapidly consume fungal N₂O producers in soil, the increase in mite abundance substantially decreases the N₂O emissions from soil. Our findings might provide new insight into the mechanisms of soil N₂O emissions and broaden the options for the mitigation of N₂O emissions.Subject terms: Climate-change ecology, Climate-change ecology     

雪被和土壤水分对典型半干旱草原土壤冻融过程中CO_2和N_2O排放的影响

土壤冻融期间的温室气体排放量会显著增加,并在全年总排放量中占有重要的份额。但目前开展的土壤冻融循环模拟实验大多是在土壤冻结之前调节土壤水分含量,而忽视了雪被在整个土壤冻融过程中的作用,因此导致室内模拟研究的结果与野外原位观测的结果差异较大。为探索开展室内模拟土壤冻融实验的优化方案,采用人工浇水和覆雪两种方式调节土壤水分含量,研究了雪被和土壤水分对内蒙古典型半干旱草原土壤冻融过程中CO2和N2O排放的影响。结果表明,浇水和覆雪两种处理对冻融循环过程中土壤CO2排放影响的差异不显著,CO2排放量在消融期都会明显增加并随着冻融循环次数的增加而逐渐减小。当土壤孔隙含水率达50%左右时,浇水处理中的N2O排放量在第1次土壤冻融循环中最高并随冻融循环次数增加而降低,但在覆雪处理中,N2O在第1次冻融循环中的排放较小,而在后两次冻融循环中的排放量更为显著。造成两种处理N2O排放规律出现显著不同的原因可能是土壤剖面水分动态变化过程和微生物性状等方面的差异。土壤冻融过程中CO2和N2O排放量随土壤含水量升高而增加,但N2O在土壤含水量较低时排放不明显,这表明可能只有当土壤含水量达到一定阈值时,冻融作用才会对N2O的排放产生显著影响。这些结果显示,雪被和土壤水分显著影响土壤冻融过程中的CO2和N2O排放,室内模拟土壤冻融实验应进一步优化。     

Frozen Cropland Soil in Northeast China as Source of N2O and CO2 Emissions

Agricultural soils are important sources of atmospheric N₂O and CO₂. However, in boreal agro-ecosystems the contribution of the winter season to annual emissions of these gases has rarely been determined. In this study, soil N₂O and CO₂ fluxes were measured for 6 years in a corn-soybean-wheat rotation in northeast China to quantify the contribution of wintertime N₂O and CO₂ fluxes to annual emissions. The treatments were chemical fertilizer (NPK), chemical fertilizer plus composted pig manure (NPKOM), and control (Cont.). Mean soil N₂O fluxes among all three treatments in the winter (November–March), when soil temperatures are below −7°C for extended periods, were 0.89–3.01 µg N m⁻² h⁻¹, and in between the growing season and winter (October and April), when freeze-thaw events occur, 1.73–5.48 µg N m⁻² h⁻¹. The cumulative N₂O emissions were on average 0.27–1.39, 0.03–0.08 and 0.03–0.11 kg N₂O_–N ha⁻¹ during the growing season, October and April, and winter, respectively. The average contributions of winter N₂O efflux to annual emissions were 6.3–12.1%. In all three seasons, the highest N₂O emissions occurred in NPKOM, while NPK and Cont. emissions were similar. Cumulative CO₂ emissions were 2.73–4.94, 0.13–0.20 and 0.07–0.11 Mg CO₂-C ha⁻¹ during growing season, October and April, and winter, respectively. The contribution of winter CO₂ to total annual emissions was 2.0–2.4%. Our results indicate that in boreal agricultural systems in northeast China, CO₂ and N₂O emissions continue throughout the winter.     

A comparison between measured and modeled N2O emissions from Inner Mongolian semi-arid grassland

The objectives of this study were (1) to determine the effect of land use on N₂O emissions from Inner Mongolian semi-arid grasslands of China and (2) to evaluate the process-based DNDC model to extrapolate our field measurements from a limited number of sites to a larger temporal and spatial scale. The results suggest the following. Rainfall event was the dominant controlling factor for the seasonal variations of the N₂O fluxes. The seven selected sites exhibited a similar seasonal trend in N₂O emission, despite their different vegetation, land use and textures. In the typical steppe, N₂O fluxes generally decrease with decreasing soil organic C (SOC) and total N content, indicating that soil C and N pools are very important in determining the spatial magnitude of the N₂O flux. N₂O emissions were very small during the entire growing season, averaging only 0.76 g N₂O-N ha^–1 day^–1 for the five typical steppe sites, 0.35 g N₂O-N ha^–1 day^–1 for the mown meadow steppe site, and 0.83 g N₂O-N ha^–1 day^–1 from the cropped meadow steppe site. No enhanced effect due to overgrazing was observed for the N₂O emission from the semi-arid grasslands. This was mainly results from the decreased SOC content due to overgrazing, which may have reduced the promoting effect of increased soil bulk density by trampling and animal excreta. Except for the mown steppe site, the model predictions of the N₂O flux for the six different sites agree well with the observed values (r ² ranging from 0.35 to 0.68). It would be concluded that the DNDC model captured the key driving process for N₂O emission. Nitrification was the predominant process, contributing 64–88% to the N₂O emission. However, in terms of the magnitude of the N₂O emission, further modifications should focus on the underestimated N₂O flux during the spring and autumn periods (nitrification, freeze/thaw cycles) and the effect of topography and the mowing on N₂O emission.     

Peaks of in situ N2O emissions are influenced by N2O‐producing and reducing microbial communities across arable soils

Agriculture is the main source of terrestrial N₂O emissions, a potent greenhouse gas and the main cause of ozone depletion. The reduction of N₂O into N₂ by microorganisms carrying the nitrous oxide reductase gene (nosZ) is the only known biological process eliminating this greenhouse gas. Recent studies showed that a previously unknown clade of N₂O‐reducers (nosZII) was related to the potential capacity of the soil to act as a N₂O sink. However, little is known about how this group responds to different agricultural practices. Here, we investigated how N₂O‐producers and N₂O‐reducers were affected by agricultural practices across a range of cropping systems in order to evaluate the consequences for N₂O emissions. The abundance of both ammonia‐oxidizers and denitrifiers was quantified by real‐time qPCR, and the diversity of nosZ clades was determined by 454 pyrosequencing. Denitrification and nitrification potential activities as well as in situ N₂O emissions were also assessed. Overall, greatest differences in microbial activity, diversity, and abundance were observed between sites rather than between agricultural practices at each site. To better understand the contribution of abiotic and biotic factors to the in situ N₂O emissions, we subdivided more than 59,000 field measurements into fractions from low to high rates. We found that the low N₂O emission rates were mainly explained by variation in soil properties (up to 59%), while the high rates were explained by variation in abundance and diversity of microbial communities (up to 68%). Notably, the diversity of the nosZII clade but not of the nosZI clade was important to explain the variation of in situ N₂O emissions. Altogether, these results lay the foundation for a better understanding of the response of N₂O‐reducing bacteria to agricultural practices and how it may ultimately affect N₂O emissions.     

Emissions of nitrous oxide from the leaves of grasses

Aims

Nitrous oxide (N₂O) emissions from pastoral agriculture are considered to originate from the soil as a consequence of microbial activity during soil nitrification and denitrification. However, recent studies have identified the plant canopy as a potentially significant source of N₂O emissions to the atmosphere. Understanding the extent and mechanisms of plant emissions may provide new mitigation opportunities as current options only target soil microbial processes.

Methods

We developed an experimental apparatus and protocol to partition N₂O emissions between the leaves of grasses and the soil and measured emissions from ten common grass species found in New Zealand pastures.

Results

The chamber design enabled us to identify measurable changes in N₂O concentration over a period of 1 h and to distinguish a range of emissions from 0.001 to 0.25 mg N₂O-N/m² leaf area/h. There was a 10-fold variation among species; Holcus lanataus, Lolium perenne and Paspalum dilatatum had the highest leaf N₂O emissions and Poa annua the lowest.

Conclusions

Grasses do emit N₂O from their leaves and the rate that this occurs varies among grass species. The emission does not appear to arise from formation of N₂O in plant leaves but more likely reflects transport of N₂O from the soil. Differences in emission rates appear to arise from a plant influence on the rate of formation of N₂O in the soil rather than the rate of transportation through the plant.     

N2O emission in a Norway spruce forest due to soil frost: concentration and isotope profiles shed a new light on an old story

In mountain regions of Central Europe an increase of soil frost periods is predicted for this century due to reduced snow fall. To investigate the effects of freezing and thawing on soil N₂O fluxes in a mature Norway spruce forest in the mountainous Fichtelgebirge, Germany, the natural snow cover on three experimental plots was removed to induce soil frost. Three plots with natural snow cover served as controls. Soil N₂O fluxes were recorded in biweekly to monthly intervals during the frost and subsequent thawing period of the below-average cold winter in 2005/2006 and in the above-average warm winter in 2006/2007. In addition, N₂O concentrations and isotope signatures in soil air were measured along soil profiles in six different depths (from 6 to 70 cm). The soil of the snow removal plots was frozen down to 15 cm depth from January to April 2006 while the soil of control plots remained unfrozen under snow cover. Both soil freezing and thawing resulted in almost tenfold enhanced N₂O fluxes on snow removal plots contributing 84% to annual N₂O emissions. In the subsequent winter without soil frost no effects were observed. Vertical gradients of N₂O concentrations together with isotope abundance suggest that the subsoil of all plots was a probably weak, but continuous N₂O source throughout the year. Isotope signatures and N₂O concentration gradients in the soil profile indicate that microbial N₂O production and reduction of N₂O to N₂ did not or just marginally occur in frozen soil layers of the snow removal plots. Consequently, elevated N₂O fluxes in the late winter were attributed to the release of accumulated N₂O originating from the subsoil. At unfrozen soil, however, N₂O emissions were reduced due to a shift of the N₂O production-consumption ratio towards more consumption in the topsoil of both the control and snow removal plots. These findings contradict the general assumption that N₂O production in the organic layer is responsible for bursts of N₂O due to soil frost.