Nitrous oxide production in a forest soil at low temperatures – processes and environmental controls

Effects of freeze-thaw cycles on methanogenic hydrocarbon degradation: Experiment and modeling

Cold regions are warming much faster than the global average, resulting in more frequent and intense freeze-thaw cycles (FTCs) in soils. In hydrocarbon-contaminated soils, FTCs modify the biogeochemical and physical processes controlling petroleum hydrocarbon (PHC) biodegradation and the associated generation of methane (CH₄) and carbon dioxide (CO₂). Thus, understanding the effects of FTCs on the biodegradation of PHCs is critical for environmental risk assessment and the design of remediation strategies for contaminated soils in cold regions. In this study, we developed a diffusion-reaction model that accounts for the effects of FTCs on toluene biodegradation, including methanogenic biodegradation. The model is verified against data generated in a 215 day-long batch experiment with soil collected from a PHC contaminated site in Ontario, Canada. The fully saturated soil incubations with six different treatments were exposed to successive 4-week FTCs, with temperatures oscillating between -10 °C and +15 °C, under anoxic conditions to stimulate methanogenic biodegradation. We measured the headspace concentrations and ¹³C isotope compositions of CH₄ and CO₂ and analyzed the porewater for pH, acetate, dissolved organic and inorganic carbon, and toluene. The numerical model represents solute diffusion, volatilization, sorption, as well as a reaction network of 13 biogeochemical processes. The model successfully simulates the soil porewater and headspace concentration time series data by representing the temperature dependencies of microbial reaction and gas diffusion rates during FTCs. According to the model results, the observed increases in the headspace concentrations of CH₄ and CO₂ by 87% and 136%, respectively, following toluene addition are explained by toluene fermentation and subsequent methanogenesis reactions. The experiment and the numerical simulation show that methanogenic degradation is the primary toluene attenuation mechanism under the electron acceptor-limited conditions experienced by the soil samples, representing 74% of the attenuation, with sorption contributing to 11%, and evaporation contributing to 15%. Also, the model-predicted contribution of acetate-based methanogenesis to total produced CH₄ agrees with that derived from the ¹³C isotope data. The freezing-induced soil matrix organic carbon release is considered as an important process causing DOC increase following each freezing period according to the calculations of carbon balance and SUVA index. The simulation results of a no FTC scenario indicate that, in the absence of FTCs, CO₂ and CH₄ generation would decrease by 29% and 26%, respectively, and that toluene would be biodegraded 23% faster than in the FTC scenario. Because our modeling approach represents the dominant processes controlling PHC biodegradation and the associated CH₄ and CO₂ fluxes, it can be used to analyze the sensitivity of these processes to FTC frequency and duration driven by temperature fluctuations.

Responses of Nitrous Oxide Emissions and Bacterial Communities to Experimental Freeze–Thaw Cycles in Contrasting Soil Types

Nitrous oxide (N2O) pulse emissions are detected in soils subjected to freeze–thaw cycles in both laboratory and field experiments. However, the mechanisms underlying this phenomenon are poorly understood. In this study, a laboratory incubation experiment that included freeze–thaw cycles (FTC), freezing (F) and control (CK) treatments was performed on three typical Chinese upland soils, namely, fluvo-aquic soil (FS), black soil (BS) and loess soil (LS). A higher similarity in soil properties and bacterial community structure was discovered between FS and LS than between FS and BS or LS and BS, and the bacterial diversity of FS and LS was higher than that of BS. FTC significantly increased the denitrification potential and the proportion of N2O in the denitrification gas products in FS and LS but decreased the denitrification potential in BS. Accordingly, with the increasing number of freeze–thaw cycles, the bacterial community composition in the FTC treatments in FS and LS diverged from that in CK but changed little in BS. Taxa that responded to FTC or correlated with denitrification potential were identified. Taken together, our results demonstrated that the effects of FTC on N2O emissions are soil-type-dependent and that the shift in the microbial community structure may contribute to the elevated N2O emissions.

Contribution of the nongrowing season to annual N2O emissions from the permafrost wetland in Northeast China

Permafrost regions store large amounts of soil organic carbon and nitrogen, which are major sources of greenhouse gas. With climate warming, permafrost is thawing and releasing an abundance of greenhouse gases into the atmosphere and contributing to climate warming. Numerous studies have shown the mechanism of nitrous oxide (N₂O) emissions from the permafrost region during the growing season. However, little is known about the temporal pattern and drivers of nongrowing season N₂O emissions from the permafrost region. In this study, N₂O emissions from the permafrost region were investigated from June 2016 to June 2018 using the static opaque chamber method. We aimed to quantify the seasonal dynamics of nongrowing season N₂O emissions and their contribution to the annual budget. The results showed that the N₂O emissions ranged from - 35.75 to 74.16 μg m^-2 h^-1 with 0.89 to 1.44 kg ha^-1 being released into the atmosphere during the nongrowing season in the permafrost region. The permafrost wetland types had no significant influence on the nongrowing season N₂O emissions due to the nitrate content. The cumulative N₂O emissions during the nongrowing season contributed to 41.96-53.73% of the annual budget, accounting for almost half of the annual emissions in the permafrost region. The driving factors of N₂O emissions were different among the nongrowing season, growing season, and entire period. The N₂O emissions from the nongrowing season and total 2-year observation period were mainly affected by soil temperature, which could explain 3.01-9.54% and 6.07-14.48% of the temporal variation in N₂O emissions, respectively. In contrast, the N₂O emissions from the growing season were controlled by soil temperature, water table level, pH, NH₄⁺-N, NO₃^--N, total nitrogen, total organic carbon, and C/N ratio, which could explain 14.51-45.72% of the temporal variation of N₂O emissions. Nongrowing season N₂O emissions are an essential component of annual emissions and cannot be ignored in the permafrost region.

Roots and other residues from leys with or without red clover: Quality and effects on N2O emission factors in a partly frozen soil following autumn ploughing

Revised IPCC guidelines assume that a constant share of N in decomposing crop residues is directly emitted as N₂O (emission factor: EF_N2O), and calculate the amount of nitrogen (N) in non-removable residues of temporary grasslands proportionally to the average annual herbage yield. However, EF_N2O depends on the intrinsic quality of the residues and their interactions with environmental conditions. Only a few field studies on N₂O emissions from grassland renewal are available, and none have simultaneously quantified the N amount and quality of non-removable residues (roots and stubble). To gain insight into the effect of non-removable residue quality on EF_N2O, we studied the amount and quality of roots and stubble and their effect on EF_N2O following the ploughing of three-year-old swards. The measured amount of N in non-removable residues was approximately 20, 25, and 31 kg N per 1 Mg average annual dry matter yield in grass, red clover-grass, and red clover, and 70-83% of it was below ground. However, the EF_N2O of non-removable residues measured over 252 days was lower (0.24%, SE = 14% for grass and red clover-grass) than the IPCC default value (0.6%, CV: 50%) for wet regions, although within the uncertainty margin, and was significantly lower than the EF_N2O of incorporated herbage, which was related to differences in EF_CO2. We advocate for more specific studies that separate the effects of belowground and aboveground residues (AGR), considering the possibility of simplifying the accounting of N₂O emissions from belowground residues while improving that of non-removable AGR from temporary grasslands and other green crops. We observed the accumulation of N₂O in the frozen soil under snow, which was released during diurnal percolation of meltwater. N₂O emissions from frozen soil accounted for 30% or more of the total emissions.

Soil pH-increase strongly mitigated N2O emissions following ploughing of grass and clover swards in autumn: A winter field study

Emissions from crop residues contribute largely to the total estimated N₂O emissions from agriculture. Since low soil pH increases N₂O production by impairing the last denitrification step, liming has been suggested as a mitigation strategy; however, it may also increase N₂O emissions by enhancing mineralization and nitrification. To gain field-based empirical knowledge, we measured N₂O fluxes with an autonomous field-flux robot in limed and control plots before and after autumn ploughing of 3-year-old grass, clover grass or red clover swards under different N fertilization regimes. Dolomite applied before establishing the swards raised soil pH_CaCl2 from ~4.8 to ~5.8 in limed plots. Higher pH halved emissions from ploughed leys despite higher soil mineral N contents. It also reduced emissions before ploughing. We observed substantial N₂O fluxes after ploughing, with peaks during a relatively warm wet period after freezing and higher peaks during diurnal snow melt over frozen soil. Average N₂O fluxes were strongly positively correlated with high herbage yields in the preceding growing seasons rather than with the presence of clover. The yield-scaled average N₂O fluxes were strongest in low pH soils at all yield levels; this was a true effect of soil pH on N₂O, as herbage yields were not increased by liming. Here, yield-scaled flux was defined as the average N₂O flux after ploughing divided by the dry matter. Fluxes in red clover plots were similar to those in grass plots, despite the lower C/N ratio and higher total amount of N in clover residues. However, clover in mixtures with grass increased yields and N₂O emissions. This suggests that higher ley production enhanced microbial activity, including nitrifiers and denitrifiers, and that the pH effect on facilitating complete denitrification to N₂ overrode any effect on mineralization and nitrification, thus resulting in N₂O mitigation.

Soil Moisture but Not Warming Dominates Nitrous Oxide Emissions During Freeze–Thaw Cycles in a Qinghai–Tibetan Plateau Alpine Meadow With Discontinuous Permafrost

Large quantities of organic matter are stored in frozen soils (permafrost) within the Qinghai–Tibetan Plateau (QTP). The most of QTP regions in particular have experienced significant warming and wetting over the past 50 years, and this warming trend is projected to intensify in the future. Such climate change will likely alter the soil freeze–thaw pattern in permafrost active layer and toward significant greenhouse gas nitrous oxide (N2O) release. However, the interaction effect of warming and altered soil moisture on N2O emission during freezing and thawing is unclear. Here, we used simulation experiments to test how changes in N2O flux relate to different thawing temperatures (T5–5°C, T10–10°C, and T20–20°C) and soil volumetric water contents (VWCs, W15–15%, W30–30%, and W45–45%) under 165 F–T cycles in topsoil (0–20 cm) of an alpine meadow with discontinuous permafrost in the QTP. First, in contrast to the prevailing view, soil moisture but not thawing temperature dominated the large N2O pulses during F–T events. The maximum emissions, 1,123.16–5,849.54 μg m–2 h–1, appeared in the range of soil VWC from 17% to 38%. However, the mean N2O fluxes had no significant difference between different thawing temperatures when soil was dry or waterlogged. Second, in medium soil moisture, low thawing temperature is more able to promote soil N2O emission than high temperature. For example, the peak value (5,849.54 μg m–2 h–1) and cumulative emissions (366.6 mg m–2) of W30T5 treatment were five times and two to four times higher than W30T10 and W30T20, respectively. Third, during long-term freeze–thaw cycles, the patterns of cumulative N2O emissions were related to soil moisture. treatments; on the contrary, the cumulative emissions of W45 treatments slowly increased until more than 80 cycles. Finally, long-term freeze–thaw cycles could improve nitrogen availability, prolong N2O release time, and increase N2O cumulative emission in permafrost active layer. Particularly, the high emission was concentrated in the first 27 and 48 cycles in W15 and W30, respectively. Overall, our study highlighted that large emissions of N2O in F–T events tend to occur in medium moisture soil at lower thawing temperature; the increased number of F–T cycles may enhance N2O emission and nitrogen mineralization in permafrost active layer.

Comparison of Soil Greenhouse Gas Fluxes during the Spring Freeze–Thaw Period and the Growing Season in a Temperate Broadleaved Korean Pine Forest, Changbai Mountains, China

Soils in mid-high latitudes are under the great impact of freeze–thaw cycling. However, insufficient research on soil CO2, CH4, and N2O fluxes during the spring freeze–thaw (SFT) period has led to great uncertainties in estimating soil greenhouse gas (GHG) fluxes. The present study was conducted in a temperate broad-leaved Korean pine mixed forest in Northeastern China, where soils experience an apparent freeze–thaw effect in spring. The temporal variations and impact factors of soil GHG fluxes were measured during the SFT period and growing season (GS) using the static-chamber method. The results show that the soil acted as a source of atmospheric CO2 and N2O and a sink of atmospheric CH4 during the whole observation period. Soil CO2 emission and CH4 uptake were lower during the SFT period than those during the GS, whereas N2O emissions were more than six times higher during the SFT period than that during the GS. The responses of soil GHG fluxes to soil temperature (Ts) and soil moisture during the SFT and GS periods differed. During the SFT period, soil CO2 and CH4 fluxes were mainly affected by the volumetric water content (VWC) and Ts, respectively, whereas soil N2O flux was influenced jointly by Ts and VWC. The dominant controlling factor for CO2 was Ts during the GS, whereas CH4 and N2O were mainly regulated by VWC. Soil CO2 and N2O fluxes accounted for 97.3% and 3.1% of the total 100-year global warming potential (GWP100) respectively, with CH4 flux offsetting 0.4% of the total GWP100. The results highlight the importance of environmental variations to soil N2O pulse during the SFT period and the difference of soil GHG fluxes between the SFT and GS periods, which contribute to predicting the forest soil GHG fluxes and their global warming potential under global climate change.

Effects of short-term freezing on nitrous oxide emissions and enzyme activities in a grazed pasture soil after bovine-urine application

Nitrous oxide (N₂O) emissions from soils applied with livestock excreta have been widely reported previously. The highest N₂O emissions from soils are also often reported during thawing periods in cold regions where soil freezing is common. However, the combined effects of cow urine application and freeze-thaw events on N₂O emissions and the related enzyme activities are still not clear. Thus, we simulated a freeze-thaw event at -3 °C for 7 days, and then increased to 3 °C for 46 days using intact soil cores with cow urine (392 kg N ha^-1). We compared the factors influencing the magnitudes of N₂O emissions through soil microbial processes with and without the freeze-thaw event. Dicyandiamide (DCD), an inhibitor of nitrification, was added to investigate the significance of nitrification on N₂O emissions. The N₂O emission rates from the urine-applied soils peaked to approximately 1000 μg N₂O-N m^-2 h^-1 immediately after the soils thawed. Soil freezing with urine application was significantly higher cumulative N₂O emissions (537 mg N₂O-N m^-2), compared to non-frozen soils with urine (247 mg N₂O-N m^-2) during the incubation period (54 days). The effect of DCD application on N₂O emissions was not clear during the freeze-thaw event, although nitrate production rates were reduced. After the freezing event, soil moisture (water-filled pore space) was significantly higher in the non-frozen soils compared to the frozen soils, due to a 9% decline in bulk density of frozen soils. Additionally, the impact of thawing on urease and denitrification enzyme activities was influenced by the urine application. Urine application increased the urease activity, while the freezing event decreased the magnitudes. The physical changes in the soils were also important controlling factors of the N₂O emissions from cow urine-applied soils in cold regions.

Nitrous oxide emissions from the urine of beef cattle as regulated by dietary crude protein and gallic acid1

Two consecutive trials were carried out to study the effects of dietary CP and adding gallic acid (GA) in basal rations on nitrogen (N) metabolism and nitrous oxide (N2O) emissions from the urine of beef cattle. In Trial I, eight Simmental castrated male cattle with initial liveweight of 310.5 ± 21.5 kg were used as experimental animals. Two levels of dietary CP (113.5 and 150.8 g/kg DM) and two levels of GA (0.0 and 15.2 g/kg DM) were used as experimental treatments in a 2 × 2 reversal design. Two cattle received each treatment in each of two experimental periods. Each experimental period lasted 19 d, of which the first 14 d were for adaptation and the last 5 d were for sampling. In Trial II, the urine samples collected from Trial I were used for measuring N2O-N emissions using static incubation technique. Glass jars containing soil were used as the incubation vessels. Three jars were used for each of the urine samples as replicates and two jars without urine samples were used as blanks. The incubation lasted 15 d, and the daily N2O-N emission from each jar was determined using gas chromatography. The results showed that no effects of interactions were found between dietary CP and GA on the N metabolism of beef cattle and the estimated cattle N2O-N emissions (P > 0.05). Increasing dietary CP from 113.5 to 150.8 g/kg DM increased the excretions of total N, urinary N, and urea (P < 0.001), whereas adding GA at 15.2 g/kg DM in ration did not affect these parameters (P > 0.05). Increasing dietary CP from 113.5 to 150.8 g/kg DM increased the estimated cattle urine N2O-N emissions by 36.8% (without adding GA) and 32.3% (adding GA at 15.2 g/kg DM) (P < 0.01), whereas adding GA at 15.2 g/kg DM in ration decreased the estimated cattle urine N2O-N emissions by 28.5% (dietary CP 113.5 g/kg DM) and 30.9% (dietary CP 150.8 g/kg DM) (P < 0.01). The inhibiting effects of GA on decreasing the N2O-N emissions of urine could have been resulted from the effects of GA metabolites including pyrogallol and resorcinol excreted in urine. Feeding cattle with relatively low dietary CP or adding GA in ration is effective to decrease the N2O-N emissions from the urine patches of beef cattle applied to soil.

Sunlight stimulates methane uptake and nitrous oxide emission from the High Arctic tundra

Many environmental factors affecting methane (CH₄) and nitrous oxide (N₂O) fluxes have been investigated during the processes of carbon and nitrogen transformation in the boreal tundra. However, effects of sunlight on CH₄ and N₂O fluxes and their budgets were neglected in the boreal tundra. Here, summertime CH₄ and N₂O fluxes in the presence and total absence of sunlight were investigated at the six tundra sites (DM1-DM6) on Ny-Ålesund in the High Arctic. The mean CH₄ fluxes at the tundra sites ranged from -4.7 to -158.6μg CH₄ m^-2h^-1 in the presence of light, indicating that a large CH₄ sink occurred in the tundra soils. However, enhanced CH₄ emission in total absence of light occurred at all the tundra sites. The mean N₂O fluxes ranged from 7.4 to 14.6μg N₂O m^-2h^-1 in the presence of light, whereas in the absence of light all the tundra sites generally released less N₂O, and even significant N₂O uptake occurred there. Soil temperature, chamber temperature and soil moisture showed no significant correlations with tundra CH₄ and N₂O flux. The presence of sunlight increased tundra CH₄ uptake by 114.2μg CH₄ m^-2h^-1 and N₂O emission by 10.9μg N₂O m^-2h^-1 compared with total absence of light. Overall our results showed that tundra ecosystem switched from CH₄ sink and N₂O emission source in the presence of light to CH₄ emission source and N₂O sink in the absence of light. Therefore sunlight had an important effect on CH₄ and N₂O budgets in the High Arctic tundra. The exclusion of sunlight might overestimate CH₄ budgets, but underestimate N₂O budgets in the Arctic tundra ecosystem.

The subzero microbiome: microbial activity in frozen and thawing soils

Most of the Earth's biosphere is characterized by low temperatures (<5°C) and cold-adapted microorganisms are widespread. These psychrophiles have evolved a complex range of adaptations of all cellular constituents to counteract the potentially deleterious effects of low kinetic energy environments and the freezing of water. Microbial life continues into the subzero temperature range, and this activity contributes to carbon and nitrogen flux in and out of ecosystems, ultimately affecting global processes. Microbial responses to climate warming and, in particular, thawing of frozen soils are not yet well understood, although the threat of microbial contribution to positive feedback of carbon flux is substantial. To date, several studies have examined microbial community dynamics in frozen soils and permafrost due to changing environmental conditions, and some have undertaken the complicated task of characterizing microbial functional groups and how their activity changes with changing conditions, either in situ or by isolating and characterizing macromolecules. With increasing temperature and wetter conditions microbial activity of key microbes and subsequent efflux of greenhouse gases also increase. In this review, we aim to provide an overview of microbial activity in seasonally frozen soils and permafrost. With a more detailed understanding of the microbiological activities in these vulnerable soil ecosystems, we can begin to predict and model future expectations for carbon release and climate change.

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.

Denitrification and N2O:N2 production in temperate grasslands: processes, measurements, modelling and mitigating negative impacts

In this review we explore the biotic transformations of nitrogenous compounds that occur during denitrification, and the factors that influence denitrifier populations and enzyme activities, and hence, affect the production of nitrous oxide (N2O) and dinitrogen (N2) in soils. Characteristics of the genes related to denitrification are also presented. Denitrification is discussed with particular emphasis on nitrogen (N) inputs and dynamics within grasslands, and their impacts on the key soil variables and processes regulating denitrification and related gaseous N2O and N2 emissions. Factors affecting denitrification include soil N, carbon (C), pH, temperature, oxygen supply and water content. We understand that the N2O:N2 production ratio responds to the changes in these factors. Increased soil N supply, decreased soil pH, C availability and water content generally increase N2O:N2 ratio. The review also covers approaches to identify and quantify denitrification, including acetylene inhibition, (15)N tracer and direct N2 quantification techniques. We also outline the importance of emerging molecular techniques to assess gene diversity and reveal enzymes that consume N2O during denitrification and the factors affecting their activities and consider a process-based approach that can be used to quantify the N2O:N2 product ratio and N2O emissions with known levels of uncertainty in soils. Finally, we explore strategies to reduce the N2O:N2 product ratio during denitrification to mitigate N2O emissions. Future research needs to focus on evaluating the N2O-reducing ability of the denitrifiers to accelerate the conversion of N2O to N2 and the reduction of N2O:N2 ratio during denitrification.

Effects of temperatures near the freezing point on N2O emissions, denitrification and on the abundance and structure of nitrifying and denitrifying soil communities

Climate warming in temperate regions may lead to decreased soil temperatures over winter as a result of reduced snow cover. We examined the effects of temperatures near the freezing point on N(2)O emissions, denitrification, and on the abundance and structure of soil nitrifiers and denitrifiers. Soil microcosms supplemented with NO3 - and/or NO3 - plus red clover residues were incubated for 120 days at -4 °C, -1 °C, +2 °C or +5 °C. Among microcosms amended with residues, N(2)O emission and/or denitrification increased with increasing temperature on Days 2 and 14. Interestingly, N(2)O emission and/or denitrification after Day 14 were the greatest at -1 °C. Substantial N(2) O emissions were only observed on Day 2 at +2 °C and +5 °C, while at -1 °C, N(2)O emissions were consistently detected over the duration of the experiment. Abundances of ammonia oxidizing bacteria (AOB) and archaea (AOA), Nitrospira-like bacteria and nirK denitrifiers were the lowest in soils at -4 °C, while abundances of Nitrobacter-like bacteria and nirS denitrifiers did not vary among temperatures. Community structures of nirK and nirS denitrifiers and Nitrobacter-like bacteria shifted between below-zero and above-zero temperatures. Structure of AOA and AOB communities also changed but not systematically among frozen and unfrozen temperatures. Results indicated shifts in some nitrifier and denitrifier communities with freezing and a surprising stimulation of N(2)O emissions at -1 °C when NO3 - and C are present.

Nitrous oxide emissions from Phragmites australis-dominated zones in a shallow lake

Nitrous oxide (N(2)O) emissions from Phragmites australis (reed)--dominated zones in Baiyangdian Lake, the largest shallow lake of Northern China, were investigated under different hydrological conditions with mesocosm experiments during the growing season of reeds. The daily and monthly N(2)O emissions were positively correlated with air temperature and the variation of aboveground biomass of reeds (p < 0.05), respectively. The N(2)O emissions from reeds were about 45.8-52.8% of that from the sediments. In terms of the effect of hydrological conditions, N(2)O emissions from the aquatic-terrestrial ecotone were 9.4-26.1% higher than the submerged zone, inferring that the variation of water level would increase N(2)O emissions. The annual N(2)O emission from Baiyangdian Lake was estimated to be about 114.2 t. This study suggested that N(2)O emissions from shallow lakes might be accelerated by the climate change as it has increased air temperature and changed precipitation, causing the variation of water level.

Both catabolic and anabolic heterotrophic microbial activity proceed in frozen soils

A large proportion of the global soil carbon pool is stored in soils of high-latitude ecosystems in which microbial processes and production of greenhouse gases proceed during the winter months. It has been suggested that microorganisms have limited ability to sequester substrates at temperatures around and below 0 °C and that a metabolic shift to dominance of catabolic processes occurs around these temperatures. However, there are contrary indications that anabolic processes can proceed, because microbial growth has been observed at far lower temperatures. Therefore, we investigated the utilization of the microbial substrate under unfrozen and frozen conditions in a boreal forest soil across a temperature range from -9 °C to +9 °C, by using gas chromatography-isotopic ratio mass spectrometry and (13)C magic-angle spinning NMR spectroscopy to determine microbial turnover and incorporation of (13)C-labeled glucose. Our results conclusively demonstrate that the soil microorganisms maintain both catabolic (CO(2) production) and anabolic (biomass synthesis) processes under frozen conditions and that no significant differences in carbon allocation from [(13)C]glucose into [(13)C]CO(2) and cell organic (13)C-compounds occurred between +9 °C and -4 °C. The only significant metabolic changes detected were increased fluidity of the cell membranes synthesized at frozen conditions and increased production of glycerol in the frozen samples. The finding that the processes in frozen soil are similar to those in unfrozen soil has important implications for our general understanding and conceptualization of soil carbon dynamics in high-latitude ecosystems.

Nitrifiers and denitrifiers respond rapidly to changed moisture and increasing temperature in a pristine forest soil

Complete cycling of mineral nitrogen (N) in soil requires the interplay of microorganisms performing nitrification and denitrification, whose activity is increasingly affected by extreme rainfall or heat brought about by climate change. In a pristine forest soil, a gradual increase in soil temperature from 5 to 25 degrees C in a range of water contents stimulated N turnover rates, and N gas emissions were determined by the soil water-filled pore space (WFPS). NO and N(2)O emissions dominated at 30% WFPS and 55% WFPS, respectively, and the step-wise temperature increase resulted in a threefold increase in the NO(3)(-) concentrations and a decrease in the NH(4)(+) concentration. At 70% WFPS, NH(4)(+) accumulated while NO(3)(-) pools declined, indicating gaseous N loss. AmoA- and nirK-gene-based analysis revealed increasing abundance of bacterial ammonia oxidizers (AOB) with increasing soil temperature and a decrease in the abundance of archaeal ammonia oxidizers (AOA) in wet soil at 25 degrees C, suggesting the sensitivity of the latter to anaerobic conditions. Denitrifier (nirK) community structure was most affected by the water content and nirK gene abundance rapidly increased in response to wet conditions until the substrate (NO(3)(-)) became limiting. Shifts in the community structure were most pronounced for nirK and most rapid for AOA, indicating dynamic populations, whereas distinct adaptation of the AOB communities required 5 weeks, suggesting higher stability.

Use of stable nitrogen isotope fractionation to estimate denitrification in small constructed wetlands treating agricultural runoff

Constructed wetlands (CWs) in the agricultural landscape reduce non-point source pollution through removal of nutrients and particles. The objective of this study was to evaluate if measurements of natural abundance of (15)NO(3)(-) can be used to determine the fate of NO(3)(-) in different types of small CWs treating agricultural runoff. Nitrogen removal was studied in wetland trenches filled with different filter materials (T1--sand and gravel; T3--mixture of peat, shell sand and light-weight aggregates; T8--barley straw) and a trench formed as a shallow pond (T4). The removal was highest during summer and lowest during autumn and winter. Trench T8 had the highest N removal during summer. Measurements of the natural abundance of (15)N in NO(3)(-) showed that denitrification was not significant during autumn/winter, while it was present in all trenches during summer, but only important for nitrogen removal in trench T8. The (15)N enrichment factors of NO(3)(-) in this study ranged from -2.5 to -5.9 per thousand (T3 and T8, summer), thus smaller than enrichment factors found in laboratory tests of isotope discrimination in denitrification, but similar to factors found for denitrification in groundwater and a large CW. The low enrichment factors compared to laboratory studies was attributed to assimilation in plants/microbes as well as diffusion effect. Based on a modified version of the method presented by Lund et al. [Lund LJ, Horne AJ, Williams AE, Estimating denitrification in a large constructed wetland using stable nitrogen isotope ratios. Ecol Engineer 2000; 14: 67-76], denitrification and assimilation were estimated to account for 53 to 99 and 1 to 47%, respectively, of the total N removal during summer. This method is, however, based on a number of assumptions, and there is thus a need for a better knowledge of the effect of plant uptake, microbial assimilation as well as nitrification on N isotopic fractionation before this method can be used to evaluate the contribution of dinitrification in CWs.

Nitrous oxide emission from Deyeuxia angustifolia freshwater marsh in northeast china

Here we report N(2)O emission results for freshwater marshes isolated from human activities at the Sanjiang Experimental Station of Marsh Wetland Ecology in northeastern China. These results are important for us to understand N(2)O emission in natural processes in undisturbed freshwater marsh. Two adjacent plots of Deyeuxia angustifolia freshwater marsh with different water regimes, i.e., seasonally waterlogged (SW) and not- waterlogged (NW), were chosen for gas sampling, and soil and biomass studies. Emissions of N(2)O from NW plots were obviously higher than from the SW plots. Daily maximum N(2)O flux was observed at 13 o'clock and the seasonal maximum occurred in end July to early August. The annual average N(2)O emissions from the NW marsh were 4.45 microg m(-2) h(-1) in 2002 and 6.85 microg m(-2) h(-1) in 2003 during growing season. The SW marsh was overall a sink for N(2)O with corresponding annual emissions of -1.00 microg m(-2) h(-1) for 2002 and -0.76 microg m(-2) h(-1) for 2003. There were significant correlations between N(2)O fluxes and temperatures of both air and 5-cm-depth soil. The range of soil redox potential 200-400 mV appeared to be optimum for N(2)O flux. Besides temperature and plant biomass, the freeze-thaw process is also an important factor for N(2)O emission burst. Our results show that the freshwater marsh isolated from human activity in northeastern China is not a major source of N(2)O.

Influence of freeze-thaw stress on the structure and function of microbial communities and denitrifying populations in soil

Microbial N2O release during the course of thawing of soil was investigated in model experiment focusing on denitrification, since freeze-thaw has been shown to cause significant physical and biological changes in soil, including a surge of N2O and CO2. The origin of these is still controversially discussed. The increase in denitrification after thawing may be attributed to the diffusion of organic substrates newly available to denitrifiers from disrupted soil aggregates, leading to an increase in microbial activity. Laboratory experiments with upper soil layer of a grassland were conducted in microcosms for real-time gas measurements during the entire phase of freeze and thaw. Shifts in microbial communities were evident on resolution of 16S and 18S rRNA genes and transcripts by denaturing gradient gel electrophoresis (DGGE). Microbial expression profiles were compared by RNA-arbitrarily primed PCR technique and subsequent resolution of amplified products on acrylamide gels. Differences in expression levels of periplasmic nitrate reductase gene (napA) and cytochrome cd1 nitrite reductase (nirS) were observed by most-probable-number-reverse transcription-PCR, with higher levels of expression occurring just after thawing began, followed by a decrease. napA and nirS DGGE profiles showed no change in banding patterns with fingerprints derived from DNA, whereas those derived from cDNA showed a clear succession of denitrifying bacteria, with the most complex pattern being observed at the end of the N2O surge. This study provides insight into the structural community changes and expression dynamics of denitrifiers as a result of freeze-thaw stress. Also, the results presented here support the belief that the gas fluxes observed during thawing is a result of freezing initiated high microbial activity.

Comparison of temperature effects on soil respiration and bacterial and fungal growth rates

Temperature is an important factor regulating microbial activity and shaping the soil microbial community. Little is known, however, on how temperature affects the most important groups of the soil microorganisms, the bacteria and the fungi, in situ. We have therefore measured the instantaneous total activity (respiration rate), bacterial activity (growth rate as thymidine incorporation rate) and fungal activity (growth rate as acetate-in-ergosterol incorporation rate) in soil at different temperatures (0-45 degrees C). Two soils were compared: one was an agricultural soil low in organic matter and with high pH, and the other was a forest humus soil with high organic matter content and low pH. Fungal and bacterial growth rates had optimum temperatures around 25-30 degrees C, while at higher temperatures lower values were found. This decrease was more drastic for fungi than for bacteria, resulting in an increase in the ratio of bacterial to fungal growth rate at higher temperatures. A tendency towards the opposite effect was observed at low temperatures, indicating that fungi were more adapted to low-temperature conditions than bacteria. The temperature dependence of all three activities was well modelled by the square root (Ratkowsky) model below the optimum temperature for fungal and bacterial growth. The respiration rate increased over almost the whole temperature range, showing the highest value at around 45 degrees C. Thus, at temperatures above 30 degrees C there was an uncoupling between the instantaneous respiration rate and bacterial and fungal activity. At these high temperatures, the respiration rate closely followed the Arrhenius temperature relationship.