Nitrous oxide (N2O) is an important pollutant which is emitted during the biological nutrient removal (BNR) processes of wastewater treatment. Since it has a greenhouse effect which is 265 times ...higher than carbon dioxide, even relatively small amounts can result in a significant carbon footprint. Biological nitrogen (N) removal conventionally occurs with nitrification/denitrification, yet also through advanced processes such as nitritation/denitritation and completely autotrophic N-removal. The microbial pathways leading to the N2O emission include hydroxylamine oxidation and nitrifier denitrification, both activated by ammonia oxidizing bacteria, and heterotrophic denitrification. In this work, a critical review of the existing literature on N2O emissions during BNR is presented focusing on the most contributing parameters. Various factors increasing the N2O emissions either per se or combined are identified: low dissolved oxygen, high nitrite accumulation, low chemical oxygen demand to nitrogen ratio, slow growth of denitrifying bacteria, uncontrolled pH and temperature. However, there is no common pattern in reporting the N2O generation amongst the cited studies, a fact that complicates its evaluation. When simulating N2O emissions, all microbial pathways along with the potential contribution of abiotic N2O production during wastewater treatment at different dissolved oxygen/nitrite levels should be considered. The undeniable validation of the robustness of such models calls for reliable quantification techniques which simultaneously describe dissolved and gaseous N2O dynamics. Thus, the choice of the N-removal process, the optimal selection of operational parameters and the establishment of validated dynamic models combining multiple N2O pathways are essential for studying the emissions mitigation.
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•Processes related to AOB and denitrification are the major N2O contributors in BNR.•DO is the most important operational parameter affecting the N2O generation.•Emission factor calculation is affected by scale and operational design.•N2O measurement, quantification and reporting procedures should be standardized.•Full-scale modeling should include all microbial pathways and N2O stripping.
Nitrous oxide (N2O) is a prime greenhouse gas and cultivated soils are the most important anthropogenic source. N2O production and consumption are known to occur at depths below the A or Ap horizon, ...but their magnitude in situ is largely unknown. At a site in SW Michigan, USA, we measured N2O concentrations at different soil depths and used diffusivity models to examine the importance of depth-specific production and consumption. Additionally, we tested the influence of crop and management practices on subsurface N2O production in (1) till versus no-till, (2) a nitrogen fertilizer gradient, and (3) perennial crops including successional vegetation. N2O concentrations below 20 cm exceeded atmospheric concentrations by up to 900 times, and profile concentrations increased markedly with depth except immediately after fertilization when production was intense in the surface horizon, and in winter, when surface emissions were blocked by ice. Diffusivity analysis showed that N2O production at depth was especially important in annual crops, accounting for over 50% of total N2O production when crops were fertilized at recommended rates. At nitrogen fertilizer rates exceeding crop need, subsurface N2O production contributed 25–35% of total surface emissions. Dry conditions deepened the maximum depth of N2O production. Tillage did not. In systems with perennial vegetation, subsurface N2O production contributed less than 20% to total surface emissions. Results suggest that the fraction of total N2O produced in subsurface horizons can be substantial in annual crops, is low under perennial vegetation, appears to be largely controlled by subsurface nitrogen and moisture, and is insensitive to tillage.
Intensively managed pastures in subtropical Australia under dairy production are nitrogen (N) loaded agro-ecosystems, with an increased pool of N available for denitrification. The magnitude of ...denitrification losses and N2:N2O partitioning in these agro-ecosystems is largely unknown, representing a major uncertainty when estimating total N loss and replacement. This study investigated the influence of different soil moisture contents on N2 and N2O emissions from a subtropical dairy pasture in Queensland, Australia. Intact soil cores were incubated over 15 days at 80% and 100% water-filled pore space (WFPS), after the application of 15N labelled nitrate, equivalent to 50 kg N ha−1. This setup enabled the direct quantification of N2 and N2O emissions following fertilisation using the 15N gas flux method. The main product of denitrification in both treatments was N2. N2 emissions exceeded N2O emissions by a factor of 8 ± 1 at 80% WFPS and a factor of 17 ± 2 at 100% WFPS. The total amount of N-N2 lost over the incubation period was 21.27 kg ± 2.10 N2-N ha−1 at 80% WFPS and 25.26 kg ± 2.79 kg ha−1 at 100% WFPS respectively. N2 emissions remained high at 100% WFPS, while related N2O emissions decreased. At 80% WFPS, N2 emissions increased constantly over time while N2O fluxes declined. Consequently, N2/(N2+N2O) product ratios increased over the incubation period in both treatments. N2/(N2+N2O) product ratios responded significantly to soil moisture, confirming WFPS as a key driver of denitrification. The substantial amount of fertiliser lost as N2 reveals the agronomic significance of denitrification as a major pathway of N loss for sub-tropical pastures at high WFPS and may explain the low fertiliser N use efficiency observed for these agro-ecosystems.
•First study to quantify N2 from Australian pastures using the 15N gas flux method.•Nitrifier-denitrification contributed to N2O emissions at high WFPS.•N2 emitted accounted for a third of applied N fertiliser at high WFPS.•These N2 losses are a significant loss of fertiliser N from subtropical pastures.
•Mean soil background nitrous oxide emission rate in croplands China was 0.93 kg N ha−1 yr−1.•BNE rates from croplands in China varied among climatic zones.•IPCC default value would not accurately ...estimate soil BNE from croplands in China.•Correlation between soil and climatic variables with BNE was quadratic rather than linear.•Air temperature (10–20 °C) and precipitation of (600–1200 mm yr–1) were considered to be optimum for BNE rates.
Nitrous oxide (N2O) is one of the most important and persistent greenhouse gases, however, there is a lack of accurate data on soil background N2O emissions (BNE) from agricultural land. Calculations of N2O emission factors from land uses are currently based on a single, universal BNE value advocated by the International Panel on Climate Change, but BNE are thought to vary with climate and soil types. We ran a meta-analysis of 58 peer-reviewed, published data source on soil BNE from cropland fields across China, representing a range of climate and soil types. Mean soil BNE rate in croplands was estimated at 0.93 kg N ha−1 yr−1 and varied among the climatic zones, where the highest rate was in the north subtropical zone (1.66 kg N ha−1 yr−1) and the lowest in the cold temperate zone (0.53 kg N ha−1 yr−1). We estimated total national BNE for China based on the BNE rates for the different climatic zones and found it to be 129 Gg N yr−1; this estimate was higher than the estimate of 114 Gg N yr−1 derived from using only a single national average BNE rate. Stepwise multiple regression modelling of soil BNE rate (kg N ha-1 yr-1) with soil properties and climatic parameters, including soil bulk density, mean annual precipitation, soil total nitrogen, and soil pH showed that soil bulk density had the greatest influence on the BNE rate. Our study highlights that soil BNE should be included in regional N2O inventories to inform sustainable agricultural development and management strategies in the context of mitigating impacts of soil N2O emissions on climate change.
•3-yrs N2O emissions were examined in a hot pepper‒radish rotation field.•FM inconsiderably reduced N2O emission & yield-scaled emission in hot pepper season.•FM significantly increased N2O emission ...and yield-scaled emissions in radish season.•No remarkable effects of FM on EFs of N2O or yields of both hot pepper and radish.•Substantial differences in N2O emissions displayed between years & vegetable crops.
Plastic film mulching (FM) is widely applied in agro-ecosystems to improve soil hydrothermal conditions for better crop productivities. However, effects of such FM practices on nitrous oxide (N2O) emissions are not well known in high nitrogen (N) input vegetable fields. Using static chambers under no-mulching (NM) and FM with four N application rates N2O emissions were monitored in a hot pepper (Capsicum annuum)-radish (Raphanus sativus) rotation in southwest China over three years (Cycle 1, 2 and 3) from May 2014 to February 2017. These four N application rates for each vegetable crop were 0, 150, 300 and 450 kg N ha−1 and 0, 100, 200 and 300 kg N ha-1 in the respective hot pepper and radish seasons. Compared to NM, FM insignificantly reduced N2O emissions in the hot pepper season owing to lower soil moisture, while significantly increased N2O emissions in the radish season owing to higher soil NH4++NO3− and temperature. Additionally, FM had no remarkable effects on N2O EFs or crop yields of hot pepper or radish. N application exerted stronger effects on N2O emissions under FM and NM in Cycle 1 than in other two cycles during the hot pepper season owing to higher temperature and rainfall, resulting in very high N2O EFs (2.56 %–7.43 %). The average seasonal EF was 1.52 % (0.15 %–7.43 %), while the average annual EF was 1.74 % (0.16 %–5.32 %), which was comparable to the 2006 IPCC defaulted of 1.00 %. Yield-scaled N2O emissions under FM were significantly increased in the radish season, but not in the hot pepper season. Additionally, N application remarkably increased N2O emissions and crop yields in the hot pepper season, with a bigger effect on N2O emissions than on crop yields, leading to increased yield-scaled N2O emissions with increasing N application between 0 and 450 kg N ha-1. Moreover, the stimulation effect of N application on crop yield was strongest under 150 kg N ha-1 with lower yield-scaled emissions. In addition, N application remarkably increased N2O emissions and crop yields, but were more prominent on crop yields than N2O emissions during the radish season, resulting in insignificant increase of yield-scaled N2O emissions between 0 and 300 kg N ha-1. Our results demonstrated that no-mulching with 150 or 300 kg N ha-1 fertilization in the hot pepper or radish season is a more suitable agronomic practice to simultaneously mitigate N2O emissions while increasing crop yields in subtropical vegetable fields.
Denitrification, as both origins and sinks of N2O, occurs extensively, and is of critical importance for regulating N2O emissions in acidified soils. However, whether soil acidification stimulates ...N2O emissions, and if so for what reason contributes to stimulate the emissions is uncertain and how the N2O fractions from fungal (ffD) and bacterial (fbD) denitrification change with soil pH is unclear. Thus, a pH gradient (6.2, 7.1, 8.7) was set via manipulating cropland soils (initial pH 8.7) in North China to illustrate the effect of soil acidification on fungal and bacterial denitrification after the addition of KNO3 and glucose. For source partitioning, we used and compared SP/δ18O mapping approach (SP/δ18O MAP) and acetylene inhibition technique combined isotope two endmember mixing model (AIT-IEM). The results showed significantly higher N2O emissions in the acidified soils (pH 6.2 and pH 7.1) compared with the initial soil (pH 8.7). The cumulative N2O emissions during the whole incubation period (15 days) ranged from 7.1 mg N kg−1 for pH 8.7–18.9 mg N kg−1 for pH 6.2. With the addition of glucose, relative to treatments without glucose, this emission also increased with the decrement of pH values, and were significantly stimulated. Similarly, the highest N2O emissions and N2O/(N2O + N2) ratios (rN2O) were observed in the pH 6.2 treatment. But the difference was the highest cumulative N2O + N2 emissions, which were recorded in the pH 7.1 treatment based on SP/δ18O MAP. Based on both approaches, ffD values slightly increased with the acidification of soil, and bacterial denitrification was the dominant pathway in all treatments. The SP/δ18O MAP data indicated that both the rN2O and ffD were lower compared to AIT-IEM. It has been known for long that low pH may lead to high rN2O of denitrification and ffD, but our documentation of a pervasive pH-control of rN2O and ffD by utilizing combined SP/δ18O MAP and AIT-IEM is new. The results of the evaluated N2O emissions by acidified soils are finely explained by high rN2O and enhanced ffD. We argue that soil pH management should be high on the agenda for mitigating N2O emissions in the future, particularly for regions where long-term excessive nitrogen fertilizer is likely to acidify the soils.
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•Acidification of cropland soil in North China leads to escalating N2O emission.•The N2O/(N2O + N2) ratio of denitrification was negatively correlated with soil pH.•Cropland soil acidification may enhance fungal fraction producing N2O.•Result obtained by SP/δ18O MAP is more in conformity with the real situation than AIT-IEM.
Aquatic ecosystems are recognized as a source of N2O in accordance with the flux estimations of rivers and estuaries; however, limited research has been conducted on large lakes. In this study, we ...report the annual N2O dynamics of a large eutrophic freshwater lake located in the subtropical zone of East China. The dissolved N2O concentrations in Lake Chaohu were observed to be between 8.5 and 92.3 nmol L−1 with emission rates between 0.3 and 53.6 μmol m−2 d−1, exhibiting considerable spatiotemporal variability. The average seasonal N2O concentrations were obtained, with the highest value of 23.4 nmol L−1 found in winter and the lowest value of 12.7 nmol L−1 found in summer. In contrast to the N2O concentrations observed, the highest N2O emission rates occurred during summer, while the lowest emission rates occurred in autumn. The emissions of N2O were substantially high in the western part of the lake, which suffers from serious eutrophication. In addition, the hotspots of N2O emissions have been found around the inflowing mouth of the Nanfei River, which transports large amounts of nutrients into the lake. The results suggest that anthropogenically enhanced nutrient inputs may have a significant role in the production and emission of N2O. However, the negative relationship between the surface water temperature and the N2O concentration suggests that, N2O fluxes might be influenced by other inconspicuous mechanisms. In the future the nitrogen dynamics of water and sediment in the lake should be collated to reveal mechanisms controlling N2O emissions. In summary, Lake Chaohu acts as a source of N2O with its most eutrophic part contributing 54.9% of the total N2O emissions of the whole lake.
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•N2O concentrations and fluxes were firstly reported in the fifth largest lake in China.•Considerable spatial and temporal variability in N2O concentrations and fluxes have been found in Lake Chaohu.•Hotspots of N2O emission have been found around the inflow river mouth where suffered nutrient pollution.•Lake Chaohu acts as a source of N2O with the emission rate of 5.2 μmol m-2 d-1.•The heavily eutrophied part of the lake contributing 54.9% of the total N2O of the whole lake.
Fe oxides play a crucial role in the biogeochemical processes of nitrous oxide (N2O) emissions. However, the role of Fe in affecting the quantitative contribution of ammonia oxidizing bacteria (AOB) ...and ammonia oxidizing archaea (AOA) to N2O production has not been well understood. To clarify the effects of goethite on relative contribution of AOB and AOA to N2O production, acetylene and 1-octyne inhibition culture experiment was conducted in two paddy soils with contrasting pH (pH 5.5 and pH 7.9). The results showed that goethite increased cumulative N2O emissions by 13.5 %–39.9 % and 2.7 %–27.2 % in acidic and alkaline paddy soils, respectively. In both paddy soils with nitrogen (N) application, AOB contributed more to N2O emissions than AOA, while the trend was opposite in the treatment without N application. Goethite increased the relative contribution of AOB to N2O production from 41 % to 56 % in acidic paddy soil and from 54 % to 59 % in alkaline paddy soil, respectively. The main genera involved in N2O emissions in the two paddy soils were Nitrosospira-AOB and Nitrososphaera-AOA, and goethite increased their abundance in alkaline paddy soil. Overall, goethite mainly increased the N2O emissions by promoting the autotrophic nitrification processes dominated by AOB. The findings of our study provide new insights into the development of strategies to reduce N2O emissions by steering the microbiome responsible for N2O emissions by regulating soil iron oxide in paddy soils.
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•Goethite increased cumulative N2O emissions in paddy soils.•AOB, rather than AOA, was the main contributor to N2O emissions in both soils.•Goethite increased the abundance of AOB and its contribution to N2O emissions.•Goethite increased Nitrosospira-like AOB and Nitrososphaera-like AOA in alkaline paddy soil.
Nitrous oxide (N2O) is a long-lived greenhouse gas that contributes to global warming. Emissions of N2O mainly stem from agricultural soils. This review highlights the principal factors from ...peer-reviewed literature affecting N2O emissions from agricultural soils, by grouping the factors into three categories: environmental, management and measurement. Within these categories, each impact factor is explained in detail and its influence on N2O emissions from the soil is summarized. It is also shown how each impact factor influences other impact factors. Process-based simulation models used for estimating N2O emissions are reviewed regarding their ability to consider the impact factors in simulating N2O. The model strengths and weaknesses in simulating N2O emissions from managed soils are summarized. Finally, three selected process-based simulation models (Daily Century (DAYCENT), DeNitrification-DeComposition (DNDC), and Soil and Water Assessment Tool (SWAT)) are discussed that are widely used to simulate N2O emissions from cropping systems. Their ability to simulate N2O emissions is evaluated by describing the model components that are relevant to N2O processes and their representation in the model.
The lowland peatlands of south-east Asia represent an immense reservoir of fossil carbon and are reportedly responsible for 30% of the global carbon dioxide (CO₂) emissions from Land Use, Land Use ...Change and Forestry. This paper provides a review and meta-analysis of available literature on greenhouse gas fluxes from tropical peat soils in south-east Asia. As in other parts of the world, water level is the main control on greenhouse gas fluxes from south-east Asian peat soils. Based on subsidence data we calculate emissions of at least 900 g CO₂ m⁻² a⁻¹ (~250 g C m⁻² a⁻¹) for each 10 cm of additional drainage depth. This is a conservative estimate as the role of oxidation in subsidence and the increased bulk density of the uppermost drained peat layers are yet insufficiently quantified. The majority of published CO₂ flux measurements from south-east Asian peat soils concerns undifferentiated respiration at floor level, providing inadequate insight on the peat carbon balance. In contrast to previous assumptions, regular peat oxidation after drainage might contribute more to the regional long-term annual CO₂ emissions than peat fires. Methane fluxes are negligible at low water levels and amount to up to 3 mg CH₄ m⁻² h⁻¹ at high water levels, which is low compared with emissions from boreal and temperate peatlands. The latter emissions may be exceeded by fluxes from rice paddies on tropical peat soil, however. N₂O fluxes are erratic with extremely high values upon application of fertilizer to wet peat soils. Current data on CO₂ and CH₄ fluxes indicate that peatland rewetting in south-east Asia will lead to substantial reductions of net greenhouse gas emissions. There is, however, an urgent need for further quantitative research on carbon exchange to support the development of consistent policies for climate change mitigation.
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