Indirect N2O emissions from shallow groundwater in an agricultural catchment (Seine Basin, France)

Production and accumulation of nitrous oxide (N₂O), a major greenhouse gas, in shallow groundwater might contribute to indirect N₂O emissions to the atmosphere (e.g., when groundwater flows into a stream or a river). The Intergovernmental Panel on Climate Change (IPCC) has attributed an emission factor (EF_5g) for N₂O, associated with nitrate leaching in groundwater and drainage ditches—0.0025 (corresponding to 0.25% of N leached which is emitted as N₂O)—although this is the subject of considerable uncertainty. We investigated and quantified the transport and fate of nitrate (NO₃ ^?) and dissolved nitrous oxide from crop fields to groundwater and surface water over a 2-year period (monitoring from April 2008 to April 2010) in a transect from a plateau to the river with three piezometers. In groundwater, nitrate concentrations ranged from 1.0 to 22.7 mg NO₃ ^?–N l^?1 (from 2.8 to 37.5 mg NO₃ ^?–N l^?1 in the river) and dissolved N₂O from 0.2 to 101.0 μg N₂O–N l^?1 (and from 0.2 to 2.9 μg N₂O–N l^?1 in the river). From these measurements, we estimated an emission factor of EF_5g = 0.0026 (similar to the value currently used by the IPCC) and an annual indirect N₂O flux from groundwater of 0.035 kg N₂O–N ha^?1 year^?1, i.e., 1.8% of the previously measured direct N₂O flux from agricultural soils.     

The production and emission of nitrous oxide from headwater streams in the Midwestern United States

The emission of nitrous oxide (N₂O) from streams draining agricultural landscapes is estimated by the Intergovernmental Panel on Climate Change (IPCC) to constitute a globally significant source of this gas to the atmosphere, although there is considerable uncertainty in the magnitude of this source. We measured N₂O emission rates and potential controlling variables in 12 headwater streams draining a predominantly agricultural basin on glacial terrain in southwestern Michigan. The study sites were nearly always supersaturated with N₂O and emission rates ranged from ?8.9 to 266.8 μg N₂O‐N m^?2 h^?1 with an overall mean of 35.2 μg N₂O‐N m^?2 h^?1. Stream water NO₃^? concentrations best‐predicted N₂O emission rates. Although streams and agricultural soils in the basin had similar areal emission rates, emissions from streams were equivalent to 6% of the anthropogenic emissions from soils because of the vastly greater surface area of soils. We found that the default value of the N₂O emission factor for streams and groundwater as defined by the IPCC (EF5‐g) was similar to the value observed in this study lending support to the recent downward revision to EF5‐g. However, the EF5‐g spanned four orders of magnitude across our study sites suggesting that the IPCC's methodology of applying one emission factor to all streams may be inappropriate.     

Nitrous oxide emissions in the Shanghai river network: implications for the effects of urban sewage and IPCC methodology

Global nitrogen (N) enrichment has resulted in increased nitrous oxide (N₂O) emission that greatly contributes to climate change and stratospheric ozone destruction, but little is known about the N₂O emissions from urban river networks receiving anthropogenic N inputs. We examined N₂O saturation and emission in the Shanghai city river network, covering 6300 km², over 27 months. The overall mean saturation and emission from 87 locations was 770% and 1.91 mg N₂O‐N m^?2 d^?1, respectively. Nitrous oxide (N₂O) saturation did not exhibit a clear seasonality, but the temporal pattern was co‐regulated by both water temperature and N loadings. Rivers draining through urban and suburban areas receiving more sewage N inputs had higher N₂O saturation and emission than those in rural areas. Regression analysis indicated that water ammonium (NH₄⁺) and dissolved oxygen (DO) level had great control on N₂O production and were better predictors of N₂O emission in urban watershed. About 0.29 Gg N₂O‐N yr^?1 N₂O was emitted from the Shanghai river network annually, which was about 131% of IPCC's prediction using default emission values. Given the rapid progress of global urbanization, more study efforts, particularly on nitrification and its N₂O yielding, are needed to better quantify the role of urban rivers in global riverine N₂O emission.     

Changes in fertilizer-induced direct N2O emissions from paddy fields during rice-growing season in China between 1950s and 1990s

Nitrogen fertilizer‐induced direct nitrous oxide (N₂O) emissions depend on water regimes in paddy fields, such as seasonal continuous flooding (F), flooding–midseason drainage–reflooding (F‐D‐F), and flooding–midseason drainage–reflooding–moist intermittent irrigation but without water logging (F‐D‐F‐M). In order to estimate the changes in direct N₂O emission from paddy fields during the rice‐growing season in Mainland of China between the 1950s and the 1990s, the country‐specific emission factors of N₂O‐N under different water regimes combined with rice production data were adopted in the present study. Census statistics on rice production showed that water management and nitrogen input regimes have changed in rice paddies since the 1950s. During the 1950s–1970s, about 20–25% of the rice paddy was continuously waterlogged, and 75–80% under the water regime of F‐D‐F. Since the 1980s, about 12–16%, 77%, and 7–12% of paddy fields were under the water regimes of F, F‐D‐F, and F‐D‐F‐M, respectively. Total nitrogen input during the rice‐growing season has increased from 87.5 kg N ha⁻¹ in the 1950s to 224.6 kg N ha⁻¹ in the 1990s. The emission factors of N₂O‐N were estimated to be 0.02%, 0.42%, and 0.73% for rice paddies under the F, F‐D‐F, and F‐D‐F‐M water regimes, respectively. Seasonal N₂O emissions have increased from 9.6 Gg N₂O‐N each year in the 1950s to 32.3 Gg N₂O‐N in the 1990s, which is accompanied by the increase in rice yield over the period 1950s–1990s. The uncertainties in N₂O estimate were estimated to be 59.8% in the 1950s and 37.5% in the 1990s. In the 1990s, N₂O emissions during the rice‐growing season accounted for 8–11% of the reported annual total of N₂O emissions from croplands in China, suggesting that paddy rice development could have contributed to mitigating agricultural N₂O emissions in the past decades. However, seasonal N₂O emissions would be increased, given that saving‐water irrigation and nitrogen inputs are increasingly adopted in rice paddies in China.     

Forest streams are important sources for nitrous oxide emissions

Streams and river networks are increasingly recognized as significant sources for the greenhouse gas nitrous oxide (N₂O). N₂O is a transformation product of nitrogenous compounds in soil, sediment and water. Agricultural areas are considered a particular hotspot for emissions because of the large input of nitrogen (N) fertilizers applied on arable land. However, there is little information on N₂O emissions from forest streams although they constitute a major part of the total stream network globally. Here, we compiled N₂O concentration data from low‐order streams (~1,000 observations from 172 stream sites) covering a large geographical gradient in Sweden from the temperate to the boreal zone and representing catchments with various degrees of agriculture and forest coverage. Our results showed that agricultural and forest streams had comparable N₂O concentrations of 1.6 ± 2.1 and 1.3 ± 1.8 µg N/L, respectively (mean ± SD) despite higher total N (TN) concentrations in agricultural streams (1,520 ± 1,640 vs. 780 ± 600 µg N/L). Although clear patterns linking N₂O concentrations and environmental variables were difficult to discern, the percent saturation of N₂O in the streams was positively correlated with stream concentration of TN and negatively correlated with pH. We speculate that the apparent contradiction between lower TN concentration but similar N₂O concentrations in forest streams than in agricultural streams is due to the low pH (<6) in forest soils and streams which affects denitrification and yields higher N₂O emissions. An estimate of the N₂O emission from low‐order streams at the national scale revealed that ~1.8 × 10⁹ g N₂O‐N are emitted annually in Sweden, with forest streams contributing about 80% of the total stream emission. Hence, our results provide evidence that forest streams can act as substantial N₂O sources in the landscape with 800 × 10⁹ g CO₂‐eq emitted annually in Sweden, equivalent to 25% of the total N₂O emissions from the Swedish agricultural sector.     

Annual emissions of dissolved CO2, CH4, and N2O in the subsurface drainage from three cropping systems

Indirect emission of nitrous oxide (N₂O), associated with nitrogen (N) leaching and runoff from agricultural lands is a major source of atmospheric N₂O. Recent studies have shown that carbon dioxide (CO₂) and methane (CH₄) are also emitted via these pathways. We measured the concentrations of three dissolved greenhouse gases (GHGs) in the subsurface drainage from field lysimeter that had a shallow groundwater table. Aboveground fluxes of CH₄ and N₂O were monitored using an automated closed‐chamber system. The annual total emissions of dissolved and aboveground GHGs were compared among three cropping systems; paddy rice, soybean and wheat, and upland rice. The annual drainage in the paddy rice, the soybean and wheat, and the upland rice plots was 1435, 782, and 1010 mm yr^?1, respectively. Dissolved CO₂ emissions were highest in the paddy rice plots, and were equivalent to 1.05–1.16% of the carbon storage in the topsoil. Dissolved CH₄ emissions were also higher in the paddy rice plots, but were only 0.03–0.05% of the aboveground emissions. Dissolved N₂O emissions were highest in the upland rice plots, where leached N was greatest due to small crop biomass. In the soybean and wheat plots, large crop biomass, due to double cropping, decreased the drainage volume, and thus decreased dissolved GHG emissions. Dissolved N₂O emissions from both the soybean and wheat plots and the upland rice plots were equivalent to 50.3–67.3% of the aboveground emissions. The results indicate that crop type and rotation are important factors in determining dissolved GHG emissions in the drainage from a crop field.     

Impact of altering the water table height of an acidic fen on N2O and NO fluxes and soil concentrations

The impact of experimentally intensified summer drought and precipitation on N₂O and NO turnover and fluxes was investigated in a minerotrophic fen over a 2‐year period. On three treatment plots, drought was induced for 6 and 10 weeks by means of roofs and drainage and decreased water table levels by 0.1–0.3 m compared with three nonmanipulated control plots. When averaged over the three treatment plots, both N₂O and NO emission showed only little response to the drought. On the single plot scale, however, a clear impact of the treatment on N₂O and NO fluxes could be identified. On the plot with the weakest water table reduction hardly any response could be observed, while on the plot with the greatest drainage effect, N₂O and NO fluxes increased by 530% and 270%, respectively. Rewetting reduced NO emissions to background levels (0.05–0.15 μmol m^?2 h^?1), but heavily enhanced N₂O emission (18–36 μmol m^?2 h^?1) for several days in the plots with largest water table reduction. These peaks contributed up to 40% to the cumulative N₂O fluxes and were caused by rapid N₂O production according to isotope abundance data. According to N₂O concentrations and isotope abundance analysis N₂O was mostly produced at depths between 0.3 and 0.5 m. During water table reduction net N₂O production in 0.1 m depth steadily increased in the most effectively dried plot from 2 up to 44 pmol cm^?3 day^?1. Rewetting immediately increased net N₂O production in the topsoil of the drought plots, showing rates of 18–174 pmol cm^?3 day^?1. This study demonstrates that drought and rewetting can temporarily increase N₂O emission to levels that have to date only been reported from nutrient rich and degraded fens that have been drained for agricultural purposes.     

Night and day: short‐term variation in nitrogen chemistry and nitrous oxide emissions from streams

1. Diel variation in metabolism contributes to variation in oxygen (O₂) concentrations in streams. This variation in O₂ and other parameters (e.g. pH) can in turn affect the rates of microbial nitrogen (N) processing, concentrations of nitrogenous solutes and production of the greenhouse gas nitrous oxide (N₂O). We investigated diel variability in emissions of N₂O and the magnitude of short‐term variability in N solutes across 10 streams. 2. Nitrous oxide fluxes varied on average 2.3‐fold over diel cycles. Concentrations would be underestimated by sampling around noon, but N₂O fluxes would not show a consistent bias. Time‐weighted mean daily N₂O flux was strongly related to nitrate concentration (r² = 0.58). Diel patterns in N₂O and dissolved N species were often complex (rather than simple sinusoidal curves), probably reflecting complex underlying processes. 3. Reliance on samples obtained around noon would overestimate daily mean nitrate concentrations by 5% and underestimate ammonium by 32% (average bias across all streams and dates). 4. Dissolved organic N did not show consistent day–night variation. However, the magnitude of diel variability was similar to that observed for dissolved inorganic N. Organic and inorganic N concentrations were often similar. Both appear to be dynamic components of stream N budgets. 5. The Intergovernmental Panel on Climate Change (IPCC) relies upon an emission factor to estimate indirect agricultural N₂O emissions from streams and ground water. The measured emission factor (defined as the ratio of concentrations of N₂O‐N to ‐N) was typically below the recently revised IPCC default figure. Measured values varied on average 1.8‐fold over approximately 24‐h periods and were slightly higher at night than by day. The emission factor was actually highest in streams that were net sinks for N₂O, highlighting a conceptual problem in the current IPCC method. 6. Typical sampling programmes rely on daytime‐only sampling, which might cause bias in results. In our study streams, the bias was generally small. Diel variation in nitrate concentrations was related to mean temperature; variation in ammonium and N₂O concentrations was greatest at low concentrations of nitrite and ammonium.     

Nitrous oxide emissions from a cropped soil in a semi-arid climate

Understanding nitrous oxide (N₂O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N₂O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha^?1 yr^?1) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N₂O using gas chromatography. Daily N₂O emissions were low (?1.8 to 7.3 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 0.11 kg N₂O‐N ha^?1 from N‐fertilized soil and 0.09 kg N₂O‐N ha^?1 from nonfertilized soil. Over half (55%) the annual N₂O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N₂O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N₂O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N₂O emission, demonstrating that a proportion of N₂O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N₂O emissions.     

Diurnal fluctuations of dissolved nitrous oxide (N2O) concentrations and estimates of N2O emissions from a spring-fed river: implications for IPCC methodology

There is uncertainty in the estimates of indirect nitrous oxide (N₂O) emissions as defined by the Intergovernmental Panel on Climate Change (IPCC). The uncertainty is due to the challenge and dearth of in situ measurements. Recent work in a subtropical stream system has shown the potential for diurnal variability to influence the downstream N transfer, N form, and estimates of in‐stream N₂O production. Studies in temperate stream systems have also shown diurnal changes in stream chemistry. The objectives of this study were to measure N₂O fluxes and dissolved N₂O concentrations from a spring‐fed temperate river to determine if diurnal cycles were occurring. The study was performed during a 72 h period, over a 180 m reach, using headspace chamber methodology. Significant diurnal cycles were observed in radiation, river temperature and chemistry including dissolved N₂O‐N concentrations. These data were used to further assess the IPCC methodology and experimental methodology used. River NO₃‐N and N₂O‐N concentrations averaged 3.0 mg L⁻¹ and 1.6 μg L⁻¹, respectively, with N₂O saturation reaching a maximum of 664%. The N₂O‐N fluxes, measured using chamber methodology, ranged from 52 to 140 μg m⁻² h⁻¹ while fluxes predicted using the dissolved N₂O concentration ranged from 13 to 25 μg m⁻² h⁻¹. The headspace chamber methodology may have enhanced the measured N₂O flux and this is discussed. Diurnal cycles in N₂O% saturation were not large enough to influence downstream N transfer or N form with variability in measured N₂O fluxes greater and more significant than diurnal variability in N₂O% saturation. The measured N₂O fluxes, extrapolated over the study reach area, represented only 6 × 10⁻⁴% of the NO₃‐N that passed through the study reach over a 72 h period. This is only 0.1% of the IPCC calculated flux.     

Predicting N2O emissions from agricultural land through related soil parameters

An empirical model of nitrous oxide emission from agricultural soils has been developed. It is based on the relationship between N₂O and three soil parameters – soil mineral N (ammonium plus nitrate) content in the topsoil, soil water‐filled pore space and soil temperature – determined in a study on a fertilized grassland in 1992 and 1993. The model gave a satisfactory prediction of seasonal fluxes in other seasons when fluxes were much higher, and also from other grassland sites and from cereal and oilseed rape crops, over a wide flux range (< 1 to > 20 kg N₂O‐N ha^?1 y^?1). However, the model underestimated emissions from potato and broccoli crops; possible reasons for this are discussed. This modelling approach, based as it is on well‐established and widely used soil measurements, has the potential to provide flux estimates from a much wider range of agricultural sites than would be possible by direct measurement of N₂O emissions.     

Nitrous oxide emissions from inland waters: Are IPCC estimates too high?

Nitrous oxide (N₂O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N₂O emissions are typically estimated using emission factors (EFs), defined as the proportion of the terrestrial nitrogen (N) load to a water body that is emitted as N₂O to the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has proposed EFs of 0.25% and 0.75%, though studies have suggested that both these values are either too high or too low. In this work, we develop a mechanistic modeling approach to explicitly predict N₂O production and emissions via nitrification and denitrification in rivers, reservoirs and estuaries. In particular, we introduce a water residence time dependence, which kinetically limits the extent of denitrification and nitrification in water bodies. We revise existing spatially explicit estimates of N loads to inland waters to predict both lumped watershed and half‐degree grid cell emissions and EFs worldwide, as well as the proportions of these emissions that originate from denitrification and nitrification. We estimate global inland water N₂O emissions of 10.6–19.8 Gmol N year^?1 (148–277 Gg N year^?1), with reservoirs producing most N₂O per unit area. Our results indicate that IPCC EFs are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's EFs is kinetically improbable in most river systems. Denitrification represents the major pathway of N₂O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.     

Projecting future N2O emissions from agricultural soils in Belgium

This study analyses the spatial and temporal variability of N₂O emissions from the agricultural soils of Belgium. Annual N₂O emission rates are estimated with two statistical models, MCROPS and MGRASS, which take account of the impact of changes in land use, climate, and nitrogen‐fertilization rate. The models are used to simulate the temporal trend of N₂O emissions between 1990 and 2050 for a 10′ latitude and longitude grid. The results are also aggregated to the regional and national scale to facilitate comparison with other studies and national inventories. Changes in climate and land use are derived from the quantitative scenarios developed by the ATEAM project based on the Intergovernmental Panel on Climate Change‐Special Report on Emissions Scenarios (IPCC‐SRES) storylines. The average N₂O flux for Belgium was estimated to be 8.6 × 10⁶ kg N₂O‐N yr⁻¹ (STD = 2.1 × 10⁶ kg N₂O‐N yr⁻¹) for the period 1990–2000. Fluxes estimated for a single year (1996) give a reasonable agreement with published results at the national and regional scales for the same year. The scenario‐based simulations of future N₂O emissions show the strong influence of land‐use change. The scenarios A1FI, B1 and B2 produce similar results between 2001 and 2050 with a national emission rate in 2050 of 11.9 × 10⁶ kg N₂O‐N yr⁻¹. The A2 scenario, however, is very sensitive to the reduction in agricultural land areas (−14% compared with the 1990 baseline), which results in a reduced emission rate in 2050 of 8.3 × 10⁶ kg N₂O‐N yr⁻¹. Neither the climatic change scenarios nor the reduction in nitrogen fertilization rate could explain these results leading to the conclusion that N₂O emissions from Belgian agricultural soils will be more markedly affected by changes in agricultural land areas.     

A meta‐analysis of fertilizer‐induced soil NO and combined NO+N2O emissions

Soils are among the important sources of atmospheric nitric oxide (NO) and nitrous oxide (N₂O), acting as a critical role in atmospheric chemistry. Updated data derived from 114 peer‐reviewed publications with 520 field measurements were synthesized using meta‐analysis procedure to examine the N fertilizer‐induced soil NO and the combined NO+N₂O emissions across global soils. Besides factors identified in earlier reviews, additional factors responsible for NO fluxes were fertilizer type, soil C/N ratio, crop residue incorporation, tillage, atmospheric carbon dioxide concentration, drought and biomass burning. When averaged across all measurements, soil NO‐N fluxes were estimated to be 4.06 kg ha^?1 yr^?1, with the greatest (9.75 kg ha^?1 yr^?1) in vegetable croplands and the lowest (0.11 kg ha^?1 yr^?1) in rice paddies. Soil NO emissions were more enhanced by synthetic N fertilizer (+38%), relative to organic (+20%) or mixed N (+18%) sources. Compared with synthetic N fertilizer alone, synthetic N fertilizer combined with nitrification inhibitors substantially reduced soil NO emissions by 81%. The global mean direct emission factors of N fertilizer for NO (EF_NO) and combined NO+N₂O (EF_c) were estimated to be 1.16% and 2.58%, with 95% confidence intervals of 0.71–1.61% and 1.81–3.35%, respectively. Forests had the greatest EF_NO (2.39%). Within the croplands, the EF_NO (1.71%) and EF_c (4.13%) were the greatest in vegetable cropping fields. Among different chemical N fertilizer varieties, ammonium nitrate had the greatest EF_NO (2.93%) and EF_c (5.97%). Some options such as organic instead of synthetic N fertilizer, decreasing N fertilizer input rate, nitrification inhibitor and low irrigation frequency could be adopted to mitigate soil NO emissions. More field measurements over multiyears are highly needed to minimize the estimate uncertainties and mitigate soil NO emissions, particularly in forests and vegetable croplands.     

Comparison of measured and EF5-r-derived N2O fluxes from a spring-fed river

There is considerable uncertainty in the estimates of indirect N₂O emissions as defined by the Intergovernmental Panel on Climate Change's (IPCC) methodology. Direct measurements of N₂O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N₂O budget. The objectives of this research were to measure the N₂O fluxes from a spring‐fed river, relate these fluxes to the dissolved N₂O concentrations and NO₃‐N loading of the river, and to try to define the indirect emission factor (EF5‐r) for the river. Gas bubble ebullition was observed at the river source with bubbles containing 7.9 μL N₂O L^?1. River NO₃‐N and dissolved N₂O concentrations ranged from 2.5 to 5.3 mg L^?1 and 0.4 to 1.9 μg N₂O‐N L^?1, respectively, with N₂O saturation reaching 404%. Floating headspace chambers were used to sample N₂O fluxes. N₂O‐N fluxes were significantly related to dissolved N₂O‐N concentrations (r²=0.31) but not to NO₃‐N concentrations. The N₂O‐N fluxes ranged from 38 to 501 μg m^?2 h^?1, averaging 171 μg m^?2 h^?1 (±SD 85) overall. The measured N₂O‐N fluxes equated to an EF5‐r of only 6.6% of that calculated using the IPCC methodology, and this itself was considered to be an overestimate because of the degassing of antecedent dissolved N₂O present in the groundwater that fed the river.     

Afforestation does not necessarily reduce nitrous oxide emissions from managed boreal peat soils

Pristine peatlands have generally low nitrous oxide (N₂O) emissions but drainage and management practices enhance the microbial processes and associated N₂O emissions. It is assumed that leaving peat soils from intensive management, such as agriculture, will decrease their N₂O emissions. In this paper we report how the annual N₂O emission rates will change when agricultural peat soil is either left abandoned or afforested and also N₂O emissions from afforested peat extraction sites. In addition, we evaluated a biogeochemical model (DNDC) with a view to explaining GHG emissions from peat soils under different land uses. The abandoned agricultural peat soils had lower mean annual N₂O emissions (5.5?±?5.4?kg?N?ha^?1) than the peat soils in active agricultural use in Finland. Surprisingly, N₂O emissions from afforested organic agricultural soils (12.8?±?9.4?kg?N?ha^?1) were similar to those from organic agricultural soils in active use. These emissions were much higher than those from the forests on nutrient rich peat soils. Abandoned and afforested peat extraction sites emitted more N₂O, (2.4?±?2.1?kg?N?ha^?1), than the areas under active peat extraction (0.7?±?0.5?kg?N?ha^?1). Emissions outside the growing season contributed significantly, 40% on an average, to the annual emissions. The DNDC model overestimated N₂O emission rates during the growing season and indicated no emissions during winter. The differences in the N₂O emission rates were not associated with the age of the land use change, vegetation characteristics, peat depth or peat bulk density. The highest N₂O emissions occurred when the soil C:N ratio was below 20 with a significant variability within the measured C:N range (13–27). Low soil pH, high nitrate availability and water table depth (50–70?cm) were also associated with high N₂O emissions. Mineral soil has been added to most of the soils studied here to improve the fertility and this may have an impact on the N₂O emissions. We infer from the multi-site dataset presented in this paper that afforestation is not necessarily an efficient way to reduce N₂O emissions from drained boreal organic fields.     

Comparison of measured and EF5-r-derived N2O fluxes from a spring-fed river

There is considerable uncertainty in the estimates of indirect N₂O emissions as defined by the intergovernmental panel on climate change's (IPCC) methodology. Direct measurements of N₂O yields and fluxes in aquatic river environments are sparse and more data are required to determine the role that rivers play in the global N₂O budget. The objectives of this research were to measure the N₂O fluxes from a spring‐fed river, relate these fluxes to the dissolved N₂O concentrations and NO₃‐N loading of the river, and to try and define the indirect emission factor (EF5‐r) for the river. Gas bubble ebullition was observed at the river source with bubbles containing 7.9 μL N₂O L^?1. River NO₃‐N and dissolved N₂O concentrations ranged from 2.5 to 5.3 mg L^?1 and 0.4 to 1.9 μg N₂O‐N L^?1, respectively, with N₂O saturation reaching 404%. Floating headspace chambers were used to sample N₂O fluxes. N₂O‐N fluxes were significantly related to dissolved N₂O‐N concentrations (r²=30.6) but not to NO₃‐N concentrations. The N₂O‐N fluxes ranged from 38–501 μg m^?2 h^?1, averaging 171 μg m^?2 h^?1 (±SD 85) overall. The measured N₂O‐N fluxes equated to an EF5‐r of only 6.6% of that calculated using the IPCC methodology, and this itself was considered to be an overestimate because of the degassing of antecedent dissolved N₂O present in the groundwater that fed the river.     

Nitrous oxide fluxes in estuarine environments: response to global change

Nitrous oxide is a powerful, long‐lived greenhouse gas, but we know little about the role of estuarine areas in the global N₂O budget. This review summarizes 56 studies of N₂O fluxes and associated biogeochemical controlling factors in estuarine open waters, salt marshes, mangroves, and intertidal sediments. The majority of in situ N₂O production occurs as a result of sediment denitrification, although the water column contributes N₂O through nitrification in suspended particles. The most important factors controlling N₂O fluxes seem to be dissolved inorganic nitrogen (DIN) and oxygen availability, which in turn are affected by tidal cycles, groundwater inputs, and macrophyte density. The heterogeneity of coastal environments leads to a high variability in observations, but on average estuarine open water, intertidal and vegetated environments are sites of a small positive N₂O flux to the atmosphere (range 0.15–0.91; median 0.31; Tg N₂O‐N yr^?1). Global changes in macrophyte distribution and anthropogenic nitrogen loading are expected to increase N₂O emissions from estuaries. We estimate that a doubling of current median NO₃^? concentrations would increase the global estuary water–air N₂O flux by about 0.45 Tg N₂O‐N yr^?1 or about 190%. A loss of 50% of mangrove habitat, being converted to unvegetated intertidal area, would result in a net decrease in N₂O emissions of 0.002 Tg N₂O‐N yr^?1. In contrast, conversion of 50% of salt marsh to unvegetated area would result in a net increase of 0.001 Tg N₂O‐N yr^?1. Decreased oxygen concentrations may inhibit production of N₂O by nitrification; however, sediment denitrification and the associated ratio of N₂O:N₂ is expected to increase.     

Annual emissions of nitrous oxide and nitric oxide from rice-wheat rotation and vegetable fields: a case study in the Tai-Lake region, China

Background and aims

Knowledge on nitrous oxide (N₂O) and nitric oxide (NO) emissions from typical cropping systems in the Tai-Lake region is important for estimating regional inventory and proposing effective N₂O and NO mitigation options. This study aimed at a) characterizing the seasonal and annual emissions of both gases from the major cropping systems, and b) determining their direct emission factors (EF_ds) as the key parameters for inventory compilation.

Methods

Measurements of N₂O and NO emissions were conducted year-round in the Tai-Lake region using a static opaque chamber method. The measurements involved a typical rice-wheat rotation ecosystem and a vegetable field. The two types of croplands were subjected to both a fertilized treatment and a control treatment without nitrogen addition. In the rice-wheat ecosystem, N₂O emissions were measured throughout an entire year-round rotation spanning from June 2003 to June 2004, whereas NO emissions were measured only during the non-rice period. In the vegetable field, both N₂O and NO emissions were measured from November 2003 to November 2004.

Results

During the investigation period, the average cumulative N₂O and NO emissions under the fertilized conditions amounted to 3.80 and 0.80 (during the non-rice period for NO) kg?N?ha^?1, respectively, in the rice-wheat field, and 20.81 and 47.13?kg?N ha^?1, respectively, in the vegetable field. The average total N₂O and NO emissions under the control conditions were 1.39 and 0.29 (during the non-rice period for NO) kg?N?ha^?1, respectively, in the rice?wheat rotation, and 2.98 and 0.80?kg?N ha^?1, respectively, in the vegetable field. The direct emission factor (EF_d, which is defined as the loss rate of applied nitrogen via N₂O or NO emissions in the current season or year) of N₂O was annually determined to be 0.56?% in the rice-wheat field, while the seasonal EF_d of NO was 0.34?% during the non-rice period of the rotation cycle. In the vegetable field, the seasonal EF_ds of N₂O and NO varied from 0.15?% to 14.50?% and 0.80?% to 28.21?%, respectively, among different crop seasons; and the annual EF_ds were 1.38?% and 3.59?%, respectively.

Conclusions

This study suggests that conventional vegetable fields associated with intensive synthetic nitrogen application, as well as addition of manure slurry, may substantially contribute to the regional N₂O and NO emissions though they account for a relatively small portion of the farmlands in the Tai-Lake region. However, further studies to be conducted at multiple field sites with conventional vegetable and rice-based fields are needed to test this conclusion.     

Using nitrification inhibitors to mitigate agricultural N2O emission: a double‐edged sword?

Nitrification inhibitors show promise in decreasing nitrous oxide (N₂O) emission from agricultural systems worldwide, but they may be much less effective than previously thought when both direct and indirect emissions are taken into account. Whilst nitrification inhibitors are effective at decreasing direct N₂O emission and nitrate (NO₃^–) leaching, limited studies suggest that they may increase ammonia (NH₃) volatilization and, subsequently, indirect N₂O emission. These dual effects are typically not considered when evaluating the inhibitors as a climate change mitigation tool. Here, we collate results from the literature that simultaneously examined the effects of nitrification inhibitors on N₂O and NH₃ emissions. We found that nitrification inhibitors decreased direct N₂O emission by 0.2–4.5 kg N₂O‐N ha^?1 (8–57%), but generally increased NH₃ emission by 0.2–18.7 kg NH₃‐N ha^?1 (3–65%). Taking into account the estimated indirect N₂O emission from deposited NH₃, the overall impact of nitrification inhibitors ranged from ?4.5 (reduction) to +0.5 (increase) kg N₂O‐N ha^?1. Our results suggest that the beneficial effect of nitrification inhibitors in decreasing direct N₂O emission can be undermined or even outweighed by an increase in NH₃ volatilization.