Sinks Of Nitrous Oxide Research Articles

Microbial Sources and Sinks of Nitrous Oxide during Organic Waste Composting.

Composting is widely used for organic waste management and is also a major source of nitrous oxide (N2O) emission. New insight into microbial sources and sinks is essential for process regulation to reduce N2O emission from composting. This study used genome-resolved metagenomics to decipher the genomic structures and physiological behaviors of individual bacteria for N2O sources and sinks during composting. Results showed that several nosZ-lacking denitrifiers in feedstocks drove N2O emission at the beginning of the composting. Such emission became negligible at the thermophilic stage, as high temperatures inhibited all denitrifiers for N2O production except for those containing nirK. The nosZ-lacking denitrifiers were notably enriched to increase N2O production at the cooling stage. Nevertheless, organic biodegradation limited energy availability for chemotaxis and flagellar assembly to restrain nirKS-containing denitrifiers for nitrate reduction toward N2O sources but insignificantly interrupt norBC- and nosZ-containing bacteria (particularly nosZ-containing nondenitrifiers) for N2O sinks by capturing N2O and nitric oxide (NO) for energy production, thereby reducing N2O emission at the mature stage. Furthermore, nosZII-type bacteria included all nosZ-containing nondenitrifiers and dominated N2O sinks. Thus, targeted strategies can be developed to restrict the physiological behaviors of nirKS-containing denitrifiers and expand the taxonomic distribution of nosZ for effective N2O mitigation in composting.

Contribution of Microorganisms with the Clade II Nitrous Oxide Reductase to Suppression of Surface Emissions of Nitrous Oxide.

The sources and sinks of nitrous oxide, as control emissions to the atmosphere, are generally poorly constrained for most environmental systems. Initial depth-resolved analysis of nitrous oxide flux from observation wells and the proximal surface within a nitrate contaminated aquifer system revealed high subsurface production but little escape from the surface. To better understand the environmental controls of production and emission at this site, we used a combination of isotopic, geochemical, and molecular analyses to show that chemodenitrification and bacterial denitrification are major sources of nitrous oxide in this subsurface, where low DO, low pH, and high nitrate are correlated with significant nitrous oxide production. Depth-resolved metagenomes showed that consumption of nitrous oxide near the surface was correlated with an enrichment of Clade II nitrous oxide reducers, consistent with a growing appreciation of their importance in controlling release of nitrous oxide to the atmosphere. Our work also provides evidence for the reduction of nitrous oxide at a pH of 4, well below the generally accepted limit of pH 5.

Greenhouse gases fluxes and carbon cycle in agroecosystems under humid continental climate conditions

Few complex studies of greenhouse gas (GHG: carbon dioxide, CO2; methane, CH4; nitrous oxide, N2O) fluxes and carbon cycle components were carried out on the geographical territory of Eastern Europe and represented in world databases. Most of the investigations had focused on particular problems of carbon and nitrogen pools and fluxes, or had covered a narrow range of ecosystems. Therefore, the idea of this research was to carry out a multi-disciplinary study of carbon fluxes and pools in the sector of agriculture and land use – one of the main sources of GHG – in regions with humid continental climate that are the most affected by current climate change. Our work was aimed at solving the problem of quantifying GHG fluxes and carbon pools in crop and livestock production, analyzing the reasons for their formation and possible absorption pathways at three locations in the European part of Russia in the cold continental climate zone. The study examined both agricultural and natural ecosystems; field measurements, laboratory analyses, and simulation modelling techniques were applied. The sites included seven types of ecosystems: croplands, pastures, hayfields, fallows, forests, stockyards, and compost piles. Livestock facilities are potent sources of three main biogenic GHG (0.42 − 9.27 g C-CO2 m-2 h-1; 2.43 − 783.51 mg C-CH4 m-2 h-1; 0.01 − 1.49 mg N-N2O m-2 h-1); whereas croplands, hayfields and forests can absorb methane (from −1.10 to −118.2 μg C-CH4 m-2 h-1), as well as pastures, hayfields, fallows and forests are weak sinks of nitrous oxide (from −0.10 to −4.57 μg N-N2O m-2 h-1). Ecosystems were ranked using a non-parametric test by increasing rate of CO2 emission from soil in the following order: croplands (0.06 − 0.24 g C-CO2 m-2 h-1) ≤ hayfields (0.06 − 0.25 g C-CO2 m-2 h-1) ≤ pastures (0.06 − 0.22 g C-CO2 m-2 h-1) = fallows (0.13 − 0.26 g C-CO2 m-2 h-1) = forests (0.16 − 0.30 g CO2 m-2 h-1) ≤ stockyards (0.42 − 1.57 g CO2 m-2 h-1) < compost piles (1.95 − 9.27 g CO2 m-2 h-1). According to the correlation analysis, the most important dynamic environmental factor on which the intensity of soil respiration depends is the soil temperature at the depth of 10 cm (r = 0.55 − 0.97, p < 0.05). As the regression analysis shows, hydrothermal and agrochemical controls explain the 46 − 89% variance of CO2 emission from soil. The content of total carbon (1.31−7.95%) as well as of total nitrogen (0.12 − 0.62%) in soil follows a latitudinal pattern and increases from north to south regardless of the type of land use and farming intensity. The amount of carbon incorporated into the arable soil layer by root residues decreases in the following sequence: barley > winter wheat > spring wheat > oats > soybean from 402 to 198 kg C ha-1 yr-1. Field crop stubble is an additional source of soil carbon at 118 − 368 kg C ha-1 yr-1 in the following sequence: spring wheat > winter wheat > oats > soybean > barley. According to simulation modelling (DeNitrification-DeComposition model, DNDC; and Rothamsted Carbon model, RothC), accumulation of soil organic carbon is possible under winter wheat (270 kg C ha-1 yr-1), while under other crops its stocks decrease (from −190 to −423 kg C ha-1 yr-1). According to DNDC modelling, agrolandscapes are net sinks of carbon to a varying degree (at least 500 kg C ha-1 yr-1), with phytomass being the main pool of carbon sink, most of which is removed with yields.

Prairie wetlands as sources or sinks of nitrous oxide: Effects of land use and hydrology

National and global greenhouse gas (GHG) budgets are continually being refined as data become available. Primary sources of the potent GHG nitrous oxide (N2O) include agricultural soil management and burning of fossil fuels, but comprehensive N2O budgets also incorporate less prominent factors such as wetlands. Freshwater wetland GHG flux estimates, however, have high uncertainty, and wetlands have been identified as both sources and sinks. Here, we analyzed a regional database of >26,000 N2O chamber flux measurements sampled across >150 wetlands from the Prairie Pothole Region (PPR) in the Great Plains of North America. Our goal was to identify important land use and hydrologic drivers of N2O flux to help reduce uncertainty in N2O models, and to incorporate these drivers into an upscaled estimate of wetland N2O emissions from the U.S. portion of the PPR. Within individual wetlands, exposed soils with no standing water, such as along wetland edges, were hotspots that accounted for greater than 90% of wetland N2O emissions. In contrast wet (i.e., ponded) areas had minimal or negative N2O flux. N2O flux from wetlands nested within croplands (16.3–17.3 μg N2O m−2 hr−1) was, in some instances, nearly double that from wetlands within grasslands (9.2–14.4 μg N2O m−2 h−1). We estimated that seasonal N2O flux from PPR wetlands equated to roughly 0.2% (1.04 Tg CO2 equivalents) of the U.S. N2O budget (c. 2019). Overall, even though PPR wetlands are a small net source of N2O to the atmosphere, their emissions are negligible relative to agricultural soil management. Policy and management to restore wetland hydrology and surrounding uplands from cropland to grasslands can reduce landscape N2O fluxes. Future activities focused on wetland N2O flux would benefit from inclusion of adjacent land use and hydrologic factors, as well as from incorporation of temporally dynamic ponded wetland areas.

Soil moisture determines nitrous oxide emission and uptake

Soils are major sources and sinks of nitrous oxide (N2O). The main pathway of N2O emission is performed through soil denitrification; however, the uptake phenomenon in denitrification is overlooked, leading to an underestimation of N2O production. Soil moisture strongly influences denitrification rates, but exact quantifications coupled with nosZ, nirK, and nirS gene analysis remain inadequately unaccounted for. In this study, a 15N-N2O pool dilution (15N2OPD) method was used to measure N2O production rates under different soil moisture levels. Therefore, 20%, 40%, 60%, 80% and 100% soil water holding capacity (WHC) were used. The results revealed that N2O uptake rates increased proportionally with soil moisture content and peaked at 80% WHC with 4.17 ± 2.74 μg N kg−1 soil h−1. The N2O production and net emission rates similarly peaked at 80% WHC, reading at 32.50 ± 4.86 and 27.63 ± 3.09 μg N kg−1 soil h−1 during the incubation period (18 days). Soil moisture content increased the gene copy number of the nosZ, NH4+ content, and denitrification potential in soil. N2O uptake at WHC 80–100% was significantly greater than that at WHC 20–60%. It was attributed to a decrease in O2 and the high NO3− concentration inhibition (> 50 mg N kg−1 of soil NO3−-N content). Principal components analysis (PCA) indicated that the number of nosZ genes was the major driver of N2O uptake, especially nosZ clade II. Thus, the results of this study deepen our understanding of the mechanisms underpinning N2O sources and sinks in soils and provide a useful gene-based indicator to estimate N2O uptake.

Nitrogen addition turns a temperate peatland from a near-zero source into a strong sink of nitrous oxide

Peatlands, as important global nitrogen (N) pools, are potential sources of nitrous oxide (N&lt;sub&gt;2&lt;/sub&gt;O) emissions. We measured N&lt;sub&gt;2&lt;/sub&gt;O flux dynamics in Hani peatland in a growing season with simulating warming and N addition for 12 years in the Changbai Mountains, Northeastern China, by using static chamber-gas chromatography. We hypothesised that warming and N addition would accelerate N&lt;sub&gt;2&lt;/sub&gt;O emissions from the peatland. In a growing season, the peatland under natural conditions showed near-zero N&lt;sub&gt;2&lt;/sub&gt;O fluxes and warming increased N&lt;sub&gt;2&lt;/sub&gt;O emissions but N addition greatly increased N&lt;sub&gt;2&lt;/sub&gt;O absorption compared with control. There was no interaction between warming and N addition on N&lt;sub&gt;2&lt;/sub&gt;O fluxes. Pearson correlation analysis showed that water table depth was one of the main environmental factors affecting N&lt;sub&gt;2&lt;/sub&gt;O fluxes and a positive relationship between them was observed. Our study suggests that the N&lt;sub&gt;2&lt;/sub&gt;O source function in natural temperate peatlands maybe not be so significant as we expected before; warming can increase N&lt;sub&gt;2&lt;/sub&gt;O emissions, but a high dose of N input may turn temperate peatlands to be strong sinks of N&lt;sub&gt;2&lt;/sub&gt;O, and global change including warming and nitrogen deposition can alter N&lt;sub&gt;2&lt;/sub&gt;O fluxes via its indirect effect on hydrology and vegetation in peatlands.

Negligible Greenhouse Gas Release from Sediments in Oyster Habitats.

After centuries of decline, oyster populations are now on the rise in coastal systems globally following aquaculture development and restoration efforts. Oysters regulate the biogeochemistry of coastal systems in part by promoting sediment nutrient recycling and removing excess nitrogen via denitrification. Less clear is how oysters alter sediment greenhouse gas (GHG) fluxes-an important consideration as oyster populations grow. Here, we show that sediments in oyster habitats produce carbon dioxide (CO2), with highest rates in spring (2396.91 ± 381.98 μmol CO2 m-2 h-1) following deposition of seasonal diatom blooms and in summer (2795.20 ± 307.55 μmol CO2 m-2 h-1) when temperatures are high. Sediments in oyster habitats also consistently released methane to the water column (725.94 ± 150.34 nmol CH4 m-2 h-1) with no seasonal pattern. Generally, oyster habitat sediments were a sink for nitrous oxide (N2O; -36.11 ± 7.24 nmol N2O m-2 h-1), only occasionally releasing N2O in spring. N2O release corresponded to high organic matter and dissolved nitrogen availability, suggesting denitrification as the production pathway. Despite potential CO2 production increases under aquaculture in some locations, we conclude that in temperate regions oysters have an overall negligible impact on sediment GHG cycling.

Greenhouse gas emissions from the water\u2013air interface of a grassland river: a case study of the Xilin River

Greenhouse gas (GHG) emissions from rivers and lakes have been shown to significantly contribute to global carbon and nitrogen cycling. In spatiotemporal-variable and human-impacted rivers in the grassland region, simultaneous carbon dioxide, methane and nitrous oxide emissions and their relationships under the different land use types are poorly documented. This research estimated greenhouse gas (CO2, CH4, N2O) emissions in the Xilin River of Inner Mongolia of China using direct measurements from 18 field campaigns under seven land use type (such as swamp, sand land, grassland, pond, reservoir, lake, waste water) conducted in 2018. The results showed that CO2 emissions were higher in June and August, mainly affected by pH and DO. Emissions of CH4 and N2O were higher in October, which were influenced by TN and TP. According to global warming potential, CO2 emissions accounted for 63.35% of the three GHG emissions, and CH4 and N2O emissions accounted for 35.98% and 0.66% in the Xilin river, respectively. Under the influence of different degrees of human-impact, the amount of CO2 emissions in the sand land type was very high, however, CH4 emissions and N2O emissions were very high in the artificial pond and the wastewater, respectively. For natural river, the greenhouse gas emissions from the reservoir and sand land were both low. The Xilin river was observed to be a source of carbon dioxide and methane, and the lake was a sink for nitrous oxide.

Nitrate removal and greenhouse gas production of woodchip denitrification walls under a humid subtropical climate

Denitrification walls are a low-cost technology with the capability to reduce nitrogen (N) loading in shallow groundwater beneath agricultural systems. The aims of this study were to quantify the effect of different carbon (C) substrates on nitrate removal rate (NRR) and greenhouse gas (GHG) production in two soil-capped denitrification walls (volume ≈ 27 m3) under subtropical climate conditions. The relative performance of softwood and hardwood woodchips to promote denitrification was tested over a 2-year program of weekly monitoring, during which water samples were collected for nitrate (NO3−) and dissolved GHG analysis. Both the softwood and the hardwood wall had similar average NRR (2.0 and 1.6 g N m−3 d−1, respectively) but were NO3− limited, and acted as a sink for nitrous oxide (N2O) produced in the walls and dissolved in the aquifer. Both walls produced carbon dioxide (CO2) and methane (CH4), with the hardwood producing respectively 3-fold and 2.5-fold higher fluxes compared to the softwood. Calculation of the Global Warming Potential (GWP) permitted a comparison of the GHG emissions within the walls in terms of CO2 equivalents (CO2-eq). Both the walls emitted CO2-eq lower than natural environments, with the softwood producing null emissions and the hardwood emitting 65-fold higher than softwood. The results of the present study suggest that woodchip bioreactors may be used to reduce nutrient loading from agricultural areas into surrounding aquatic environments as well as to decrease GHG emissions under subtropical climates, with softwood being a preferable substrate.

Nitrogen Cycling in Soybean Rhizosphere: Sources and Sinks of Nitrous Oxide (N2O).

Nitrous oxide (N2O) is the third most important greenhouse gas after carbon dioxide and methane, and a prominent ozone-depleting substance. Agricultural soils are the primary anthropogenic source of N2O because of the constant increase in the use of industrial nitrogen (N) fertilizers. The soybean crop is grown on 6% of the world’s arable land, and its production is expected to increase rapidly in the future. In this review, we summarize the current knowledge on N-cycle in the rhizosphere of soybean plants, particularly sources and sinks of N2O. Soybean root nodules are the host of dinitrogen (N2)-fixing bacteria from the genus Bradyrhizobium. Nodule decomposition is the main source of N2O in soybean rhizosphere, where soil organisms mediate the nitrogen transformations that produce N2O. This N2O is either emitted into the atmosphere or further reduced to N2 by the bradyrhizobial N2O reductase (N2OR), encoded by the nos gene cluster. The dominance of nos– indigenous populations of soybean bradyrhizobia results in the emission of N2O into the atmosphere. Hence, inoculation with nos+ or nos++ (mutants with enhanced N2OR activity) bradyrhizobia has proved to be promising strategies to reduce N2O emission in the field. We discussed these strategies, the molecular mechanisms underlying them, and the future perspectives to develop better options for global mitigation of N2O emission from soils.

A seasonal source and sink of nitrous oxide in mangroves: Insights from concentration, isotope, and isotopomer measurements

Mangrove forests can be a source or a sink of nitrous oxide (N2O) to the atmosphere, but seasonal mangrove N2O dynamics are not well understood. We used continuous high-resolution N2O concentration data to construct seasonal budgets of N2O in two tropical mangrove-lined creeks for both the wet and dry seasons. Furthermore we used N stable isotope analysis to constrain the pathways of N2O production and consumption within these creeks. Calculated water-air fluxes were within a range typical of undisturbed mangroves, however obvious seasonal differences in N2O dynamics were observed at the two sites. The mangrove systems were a net source of N2O to the atmosphere during the dry season and a net sink during the wet season. In the wet season we propose that increased allochthonous carbon enhances N2O consumption by denitrification. Combining the seasonal fluxes, the Fitzroy mangrove system, which received a continual input of allochthonous nitrate (NO3−), was a net source of N2O, while the Burdekin mangrove system was close to a net N2O balance. Interpreted in the context of DIN and N2O concentrations, the N2O isotopic values observed in the dry season point to fungal denitrification as the most likely pathway of N2O production. This study shows that subtle shifts in seasonal nutrient supply regulate mangrove N2O production and consumption.

Hyporheic Source and Sink of Nitrous Oxide

AbstractNitrous oxide (N2O) is a potent greenhouse gas with an estimated 10% of anthropogenic N2O coming from the hyporheic zone of streams and rivers. However, difficulty in making accurate fine‐scale field measurements has prevented detailed understanding of the processes of N2O production and emission at the bedform and flowline scales. Using large‐scale, replicated flume experiments that employed high‐density chemical concentration measurements, we have been able to refine the current conceptualization of N2O production, consumption, and emission from the hyporheic zone. We present a predictive model based on a Damköhler‐type transformation (τ̃) in which the hyporheic residence times (τ) along the flowlines are multiplied by the dissolved oxygen consumption rate constants for those flowlines. This model can identify which bedforms have the potential to produce and emit N2O, as well as the portion and location from which those emissions may occur. Our results indicate that flowlines with τ̃up (τ̃ as the flowline returns to the surface flow) values between 0.54 and 4.4 are likely to produce and emit N2O. Flowlines with τ̃up values of less than 0.54 will have the same N2O as the surface water and those with values greater than 4.4 will likely sink N2O (reference conditions: 17C, surface dissolved oxygen 8.5 mg/L). N2O production peaks approximately at τ̃ = 1.8. A cumulative density function of τ̃up values for all flowlines in a bedform (or multiple bedforms) can be used to estimate the portion of flowlines, and in turn the portion of the streambed, with the potential to emit N2O.

Nitrous oxide (N2O)-reducing denitrifier-inoculated organic fertilizer mitigates N2O emissions from agricultural soils

The only known sink for nitrous oxide (N2O) is biochemical reduction to dinitrogen (N2) by N2O reductase (N2OR). We hypothesized that the application of N2O-reducing denitrifier-inoculated organic fertilizer could enhance soil N2O consumption while the disruption of nosZ genes could result in inactivation of N2O consumption. To test such hypotheses, a denitrifier-inoculated granular organic fertilizer was applied to both soil microcosms and fields. Of 41 denitrifier strains, 38 generated 30N2 in the end products of denitrification (30N2 and 46N2O) after the addition of Na15NO3 in culture condition, indicating their high N2O reductase activities. Of these 41 strains, 18 were screened in soil microcosms after their inoculation into the organic fertilizer, most of which were affiliated with Azospirillum and Herbaspirillum. These 18 strains were nutritionally starved to improve their survival in soil, and 14 starved and/or non-starved strains significantly decreased N2O emissions in soil microcosms. However, the N2O emission had not been decreased in soil microcosms after inoculating with a nosZ gene-disruptive strain, suggesting that N2O reductase activity might be essential for N2O consumption. Although the decrease of N2O was not significant at field scales, the application of organic fertilizer inoculated with Azospirillum sp. TSH100 and Herbaspirillum sp. UKPF54 had decreased the N2O emissions by 36.7% in Fluvisol and 23.4% in Andosol in 2014, but by 21.6% in Andosol in 2015 (H. sp. UKPF54 only). These results suggest that the application of N2O-reducing denitrifier-inoculated organic fertilizer may enhance N2O consumption or decrease N2O emissions in agricultural soils.

Who contributes more to N2O emission during sludge bio-drying with two different aeration strategies, nitrifiers or denitrifiers?

Global warming effects have drawn more and more attention to studying all sources and sinks of nitrous oxide (N2O). Sludge bio-drying, as an effective sludge treatment technology, is being adopted worldwide. In this study, two aeration strategies (piles I and II) were compared to investigate the primary contributors to N2O emission during sludge bio-drying through studying the evolution of functional genes involved in nitrification (amoA, hao, and nxrA) and denitrification (narG, nirS, nirK, norB, and nosZ) by quantitative PCR (qPCR). Results showed that the profile of N2O emission can be divided into three stages, traditional denitrification contributed largely to N2O emission at stage I (days 1-5), but N2O emission mainly happened at stage II (days 5-14) due to nitrifier denitrification and NH2OH accumulation by ammonia-oxidizing bacteria (AOB), accounting for 51.4% and 58.2% of total N2O emission for piles I and II, respectively. At stage III (days 14-21), nitrifier denitrification was inhibited because sludge bio-drying proceeded mainly by the physical aeration, thus N2O emission decreased and changed little. The improved aeration strategy availed pile I to reduce N2O emission much especially at stages II and III, respectively. These results indicated that nitrifier denitrification by AOB and biological NH2OH oxidation due to AOB made more contribution to N2O emission, and aeration strategy was crucial to mitigate N2O emission during sludge bio-drying.

Pristine mangrove creek waters are a sink of nitrous oxide.

Nitrous oxide (N2O) is an important greenhouse gas, but large uncertainties remain in global budgets. Mangroves are thought to be a source of N2O to the atmosphere in spite of the limited available data. Here we report high resolution time series observations in pristine Australian mangroves along a broad latitudinal gradient to assess the potential role of mangroves in global N2O budgets. Surprisingly, five out of six creeks were under-saturated in dissolved N2O, demonstrating mangrove creek waters were a sink for atmospheric N2O. Air-water flux estimates showed an uptake of 1.52 ± 0.17 μmol m−2 d−1, while an independent mass balance revealed an average sink of 1.05 ± 0.59 μmol m−2 d−1. If these results can be upscaled to the global mangrove area, the N2O sink (~2.0 × 108 mol yr−1) would offset ~6% of the estimated global riverine N2O source. Our observations contrast previous estimates based on soil fluxes or mangrove waters influenced by upstream freshwater inputs. We suggest that the lack of available nitrogen in pristine mangroves favours N2O consumption. Widespread and growing coastal eutrophication may change mangrove waters from a sink to a source of N2O to the atmosphere, representing a positive feedback to climate change.

Environmental and microbial factors influencing methane and nitrous oxide fluxes in Mediterranean cork oak woodlands: trees make a difference.

Cork oak woodlands (montado) are agroforestry systems distributed all over the Mediterranean basin with a very important social, economic and ecological value. A generalized cork oak decline has been occurring in the last decades jeopardizing its future sustainability. It is unknown how loss of tree cover affects microbial processes that are consuming greenhouse gases in the montado ecosystem. The study was conducted under two different conditions in the natural understory of a cork oak woodland in center Portugal: under tree canopy (UC) and open areas without trees (OA). Fluxes of methane and nitrous oxide were measured with a static chamber technique. In order to quantify methanotrophs and bacteria capable of nitrous oxide consumption, we used quantitative real-time PCR targeting the pmoA and nosZ genes encoding the subunit of particulate methane mono-oxygenase and catalytic subunit of the nitrous oxide reductase, respectively. A significant seasonal effect was found on CH4 and N2O fluxes and pmoA and nosZ gene abundance. Tree cover had no effect on methane fluxes; conversely, whereas the UC plots were net emitters of nitrous oxide, the loss of tree cover resulted in a shift in the emission pattern such that the OA plots were a net sink for nitrous oxide. In a seasonal time scale, the UC had higher gene abundance of Type I methanotrophs. Methane flux correlated negatively with abundance of Type I methanotrophs in the UC plots. Nitrous oxide flux correlated negatively with nosZ gene abundance at the OA plots in contrast to that at the UC plots. In the UC soil, soil organic matter had a positive effect on soil extracellular enzyme activities, which correlated positively with the N2O flux. Our results demonstrated that tree cover affects soil properties, key enzyme activities and abundance of microorganisms and, consequently net CH4 and N2O exchange.

Denitrification at two nitrogen-polluted, ombrotrophic Sphagnum bogs in Central Europe: Insights from porewater N2O-isotope profiles

Measurements of nitrogen isotope ratios (δ15N) along vertical porewater profiles are instrumental in localizing sources and sinks of nitrous oxide (N2O) in wetland ecosystems. We present the first N2O concentration and isotope data for porewaters of ombrotrophic, mountain-top bogs. Our two study sites, situated in the Czech Republic at elevations higher than 1000 m, have been affected by atmospheric N pollution for more than 100 years. In recent decades, the northeastern site KB received over 30 kg Nha−1yr−1via atmospheric deposition, the southwestern site BS was 3 times less polluted. Both peat bogs were surrounded by defoliated, dead spruce stands. We hypothesized that elevated nitrate (NO3−) inputs in rain-fed bogs may cause sizeable N2O emissions. Porewater N2O concentrations in the studied ombrotrophic bogs were 400–800 times higher, compared to fens in nearby Bavaria (Germany). The highest N2O concentrations and the lowest δ15N–N2O values were found at the bottom of the sampled porewater profiles, 40–60 cm below surface. Low δ15N–N2O values resulted from a N isotope fractionation associated with denitrification. The site with the lower historical N pollution, BS, exhibited up to 7 times higher porewater N2O concentrations than KB, possibly due to a higher denitrification potential determined as abundance of nirK and nirS genes. Upcore, N2O abundance decreased and δ15N–N2O increased, indicating N2O reduction. Processes of N2O formation and consumption at different depths were integrated by measuring N2O fluxes across the peat – atmosphere interface. These fluxes were minute (<0.02 μmol N2O m−2h−1). We even observed scavenging of air-borne N2O by the bog substrate. Nitrogen pollution of ombrotrophic bogs led to vigorous N cycling, manifested by extremely high peat porewater N2O concentrations. However, effective in-situ consumption of the produced N2O greatly reduced its warming potential.

Denitrification alternates between a source and sink of nitrous oxide in the hypolimnion of a thermally stratified reservoir

Nitrogen loading from developed watersheds to aquatic ecosystems can stimulate microbial denitrification, a process that reduces nitrate (NO−3) to dinitrogen (N2) or nitrous oxide (N2O), the latter a potent greenhouse gas. While aquatic ecosystems are a globally significant source of N2O to the atmosphere, the relationship between denitrification and N2O production is not well known. Until recently, this field of research has been limited by the technical challenges of simultaneously measuring denitrification and N2O production or consumption in situ at the ecosystem scale. Here we use membrane inlet mass spectrometry, an analytical method providing precise and accurate measurements of dissolved N2, and gas chromatography to directly measure N2 and N2O concentrations in the hypolimnion of a stratified reservoir draining an agricultural watershed. Denitrification resulted in a consistent increase in dissolved N2 and decrease in NO−3 concentrations in the hypolimnion during the 5 month period of stratification, though temporal patterns in dissolved N2O concentrations were less consistent. Denitrification alternated between an N2O source and sink, with the N2O yield, defined as the relative production of N2O to N2 via denitrification, ranging from −3.4% (i.e., net N2O consumption) to 19.5% (mean = −0.03%). Whereas denitrification in the hypolimnion functioned as a small N2O sink during the stratified period, the reservoir was an N2O source on an annual time scale. Additional studies across reservoirs of different sizes, trophic status, and ages are needed to resolve the role of reservoirs in the global N2O budget.

An estimate of the global sink for nitrous oxide in soils

A literature survey of studies reporting nitrous oxide uptake in the soils of natural ecosystems is used to suggest that the median uptake potential is 4 μg m(-2) h(-1). The highest values are nearly all associated with soils of wetland and peatland ecosystems. Globally, the consumption of nitrous oxide in soils is not likely to exceed 0.3 TgN yr(-1), indicating that the projected sink is not more than 2% of current estimated sources of N(2)O in the atmosphere.

Biological sources and sinks of nitrous oxide and strategies to mitigate emissions.

Nitrous oxide (N(2)O) is a powerful atmospheric greenhouse gas and cause of ozone layer depletion. Global emissions continue to rise. More than two-thirds of these emissions arise from bacterial and fungal denitrification and nitrification processes in soils, largely as a result of the application of nitrogenous fertilizers. This article summarizes the outcomes of an interdisciplinary meeting, 'Nitrous oxide (N(2)O) the forgotten greenhouse gas', held at the Kavli Royal Society International Centre, from 23 to 24 May 2011. It provides an introduction and background to the nature of the problem, and summarizes the conclusions reached regarding the biological sources and sinks of N(2)O in oceans, soils and wastewaters, and discusses the genetic regulation and molecular details of the enzymes responsible. Techniques for providing global and local N(2)O budgets are discussed. The findings of the meeting are drawn together in a review of strategies for mitigating N(2)O emissions, under three headings, namely: (i) managing soil chemistry and microbiology, (ii) engineering crop plants to fix nitrogen, and (iii) sustainable agricultural intensification.