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.     

Nitrous oxide fluxes from a grain–legume crop (narrow‐leafed lupin) grown in a semiarid climate

Understanding nitrous oxide (N₂O) fluxes from grain–legume crops in semiarid and arid regions is necessary if we are to improve our knowledge of global terrestrial N₂O losses resulting from biological N₂ fixation. N₂O fluxes were measured from a rain‐fed soil, cropped to a grain–legume in a semiarid region of southwestern Australia for 1 year on a subdaily basis. The site included plots planted to narrow‐leafed lupin (Lupinus angustifolius; ‘lupin’) and plots left bare (no lupin). Fluxes were measured using soil chambers connected to a fully automated system that measured N₂O by gas chromatography. Daily N₂O fluxes were low (?0.5 to 24 g N₂O‐N ha^?1 day^?1) and not different between treatments, culminating in an annual loss of 127 g N₂O‐N ha^?1. Greatest daily N₂O fluxes occurred from both treatments in the postharvest period, and following a series of summer and autumn rainfall events. At this time of the year, soil conditions were conducive to soil microbial N₂O production: elevated soil water contents, increased inorganic nitrogen (N) and dissolved organic carbon concentrations, and soil temperatures generally > 25 °C; furthermore, there was no active plant growth to compete for mineralized N. N₂O emissions from the decomposition of legume crop residue were low, and approximately half that predicted using the currently recommended IPCC methodology. Furthermore, the contribution of the biological N₂ fixation process to N₂O emissions appeared negligible in the present study, supporting its omission as a source of N₂O from the IPCC methodology for preparing national greenhouse gas inventories.     

我国养殖水体N2O排放特征及其影响因素研究进展

养殖水体作为不可忽视的温室气体排放源,已引起社会广泛关注。本文通过文献综述分析了国内养殖水体N₂O溶存浓度、饱和度及排放通量的时空分布特征,并结合N₂O产生机制探讨了养殖水体N2O排放的影响因素。我国养殖水体N₂O介于饱和与过饱和状态,水-气界面排放通量范围为-9.82×10^-3~143.25×10^-3mg·m^-2·h^-1,养殖水体成为N₂O的重要排放源;空间变异特征表现为南方地区的排放通量显著高于北方地区,北方地区养殖水体N2O排放通量整体高于本区自然水体,而南方地区相反;从时间变化特征看,各养殖塘排放通量变化规律较为相似,大致表现为养殖中期排放量最高,养殖末期和养殖初期较低;季节变化上,夏季最高、冬春季较低。养殖水体N₂O的产生机制与其他水生生态系统相似,主要为硝化和反硝化作用,影响养殖水体N₂O在排放的因素除温度、pH、营养盐、盐度、叶绿素a等环境因素外,还与曝气活动、饵料投放、排水活动和养殖种类等人为因素有关。     

Estimation of nitrous oxide, nitric oxide and ammonia emissions from croplands in East, Southeast and South Asia

Agricultural activities have greatly altered the global nitrogen (N) cycle and produced nitrogenous gases of environmental significance. More than half of all chemical N fertilizer produced globally is used in crop production in East, Southeast and South Asia, where rice is central to nutrition. Emissions of nitrous oxide (N₂O), nitric oxide (NO) and ammonia (NH₃) from croplands in this region were estimated by considering background emission and emissions resulting from N added to croplands, including chemical N, animal manure, biologically fixed N and N in crop residues returned to fields. Background emission fluxes of N₂O and NO from croplands were estimated to be 1.22 and 0.57 kg N ha^?1 yr^?1, respectively. Separate fertilizer‐induced emission factors were estimated for upland fields and rice fields. Total N₂O emission from croplands in the study region was estimated to be 1.19 Tg N yr^?1, with 43% contributed by background emissions. The average fertilizer‐induced N₂O emission, however, accounts for only 0.93% of the applied N, which is less than the default IPCC value of 1.25%, because of the low emission factor from paddy fields. Total NO emission was 591 Gg N yr^?1 in the study region, with 40% from background emissions. The average fertilizer‐induced NO emission factor was 0.48%. Total NH₃ emission was estimated to be 11.8 Tg N yr^?1. The use of urea and ammonium bicarbonate and the cultivation of rice led to a high average NH₃ loss rate from chemical N fertilizer in the study region. Emissions were displayed at a 0.5° × 0.5° resolution with the use of a global landuse database.     

Effects of an experimental drought on soil emissions of carbon dioxide, methane, nitrous oxide, and nitric oxide in a moist tropical forest

Changes in precipitation in the Amazon Basin resulting from regional deforestation, global warming, and El Niño events may affect emissions of carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and nitric oxide (NO) from soils. Changes in soil emissions of radiatively important gases could have feedback implications for regional and global climates. Here we report results of a large‐scale (1 ha) throughfall exclusion experiment conducted in a mature evergreen forest near Santarém, Brazil. The exclusion manipulation lowered annual N₂O emissions by >40% and increased rates of consumption of atmospheric CH₄ by a factor of >4. No treatment effect has yet been detected for NO and CO₂ fluxes. The responses of these microbial processes after three rainy seasons of the exclusion treatment are characteristic of a direct effect of soil aeration on denitrification, methanogenesis, and methanotrophy. An anticipated second phase response, in which drought‐induced plant mortality is followed by increased mineralization of C and N substrates from dead fine roots and by increased foraging of termites on dead coarse roots, has not yet been detected. Analyses of depth profiles of N₂O and CO₂ concentrations with a diffusivity model revealed that the top 25 cm soil is the site of most of the wet season production of N₂O, whereas significant CO₂ production occurs down to 100 cm in both seasons, and small production of CO₂ occurs to at least 1100 cm depth. The diffusivity‐based estimates of CO₂ production as a function of depth were strongly correlated with fine root biomass, indicating that trends in belowground C allocation may be inferred from monitoring and modeling profiles of H₂O and CO₂.     

Nonlinear nitrous oxide (N2O) response to nitrogen fertilizer in on‐farm corn crops of the US Midwest

Row‐crop agriculture is a major source of nitrous oxide (N₂O) globally, and results from recent field experiments suggest that significant decreases in N₂O emissions may be possible by decreasing nitrogen (N) fertilizer inputs without affecting economic return from grain yield. We tested this hypothesis on five commercially farmed fields in Michigan, USA planted with corn in 2007 and 2008. Six rates of N fertilizer (0–225 kg N ha^?1) were broadcast and incorporated before planting, as per local practice. Across all sites and years, increases in N₂O flux were best described by a nonlinear, exponentially increasing response to increasing N rate. N₂O emission factors per unit of N applied ranged from 0.6% to 1.5% and increased with increasing N application across all sites and years, especially at N rates above those required for maximum crop yield. At the two N fertilizer rates above those recommended for maximum economic return (135 kg N ha^?1), average N₂O fluxes were 43% (18 g N₂O–N ha^?1 day^?1) and 115% (26 g N₂O–N ha^?1 day^?1) higher than were fluxes at the recommended rate, respectively. The maximum return to nitrogen rate of 154 kg N ha^?1 yielded an average 8.3 Mg grain ha^?1. Our study shows the potential to lower agricultural N₂O fluxes within a range of N fertilization that does not affect economic return from grain yield.     

Snow depth, soil freezing, and fluxes of carbon dioxide, nitrous oxide and methane in a northern hardwood forest

Soil–atmosphere fluxes of trace gases (especially nitrous oxide (N₂O)) can be significant during winter and at snowmelt. We investigated the effects of decreases in snow cover on soil freezing and trace gas fluxes at the Hubbard Brook Experimental Forest, a northern hardwood forest in New Hampshire, USA. We manipulated snow depth by shoveling to induce soil freezing, and measured fluxes of N₂O, methane (CH₄) and carbon dioxide (CO₂) in field chambers monthly (bi-weekly at snowmelt) in stands dominated by sugar maple or yellow birch. The snow manipulation and measurements were carried out in two winters (1997/1998 and 1998/1999) and measurements continued through 2000. Fluxes of CO₂ and CH₄ showed a strong seasonal pattern, with low rates in winter, but N₂O fluxes did not show strong seasonal variation. The snow manipulation induced soil freezing, increased N₂O flux and decreased CH₄ uptake in both treatment years, especially during winter. Annual N₂O fluxes in sugar maple treatment plots were 207 and 99 mg N m⁻² yr⁻¹ in 1998 and 1999 vs. 105 and 42 in reference plots. Tree species had no effect on N₂O or CO₂ fluxes, but CH₄ uptake was higher in plots dominated by yellow birch than in plots dominated by sugar maple. Our results suggest that winter fluxes of N₂O are important and that winter climate change that decreases snow cover will increase soil:atmosphere N₂O fluxes from northern hardwood forests.     

Latitudinal differentiated water table control of carbon dioxide, methane and nitrous oxide fluxes from hydromorphic soils: feedbacks to climate change

The possibility of carbon (C) being locked away from the atmosphere for millennia is given in hydromorphic soils. However, the water-table-dependent feedback from soil organic matter (SOM) decomposition to the climate system is less clear. At least three greenhouse gases are produced: carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). These gases show emission peaks at different water table positions and have different global warming potentials (GWP), for example a factor of 23 for CH₄ and 296 for N₂O as compared with the equivalent mass of CO₂ on a 100-year time horizon. This review of available annual data on all three gases revealed that the radiative forcing effect of SOM decomposition is principally dictated by CO₂ despite its low GWP. Anaerobic SOM decomposition generally has a lower potential feedback to the climatic system than aerobic SOM decomposition. Concrete values are constrained by a lack of data from tropical and subarctic regions. Furthermore, data on N₂O and on plant effects are generally rare. However, there is a clear latitudinal differentiation for the GWP of soils under anaerobic conditions compared with aerobic conditions when looking at CO₂ and CH₄: in the tropical and temperate regions, the anaerobic GWP showed a range of 25–60% of the aerobic value, but values varied between 80% and 110% in the boreal zone. Hence, particularly in the vulnerable boreal zone, the feedback from ecosystems to climate change will highly depend on plant responses to changing water tables at elevated temperatures.     

Elevated UV-B radiation alters fluxes of methane and carbon dioxide in peatland microcosms

Increases in solar ultraviolet‐B radiation (UV‐B; 280–320 nm) reaching the earth have been estimated to continue until 2050s in the boreal and subarctic regions with an abundant peatland cover. Peatlands are significant sinks for carbon dioxide (CO₂) and sources for methane (CH₄). To assess whether the future increases in UV‐B could affect the fluxes of CO₂ and CH₄ in peatlands via an impact on vegetation, we exposed peatland microcosms to modulated 30% supplementation of erythemally weighted UV‐B at an outdoor facility for one growing season. The experimental design included appropriate controls for UV‐A and ambient radiation. The UV‐B caused a significant reduction in gross photosynthesis, net ecosystem CO₂ exchange, and CH₄ emission of the peatland microcosms. These changes in the carbon gas cycling can be partly explained by UV‐B‐induced morphological changes in Eriophorum vaginatum which acts as a conduit for CH₄. Leaf cross section and the percentage of CH₄‐conducting aerenchymatous tissue in E. vaginatum were significantly reduced by UV‐B. Methanol‐extractable UV‐B absorbing compounds decreased under both UV‐B and UV‐A in Sphagnum angustifolium, and tended to accumulate under UV‐B in S. papillosum. Membrane permeability to magnesium (Mg) and calcium (Ca) ions was higher in UV‐B exposed S. angustifolium. Amount of chlorophyll and carotenoid pigments was increased by UV‐A in S. magellanicum. The observed changes in Sphagnum mosses did not coincide with those in carbon gas fluxes but occurred at the time of the highest UV intensity in the mid summer. Our findings indicate that increasing UV‐B may have more substantial effects on gas exchange in peatlands than previously thought.     

Soil environmental variables affecting the flux of methane from a range of forest, moorland and agricultural soils

Measurements of the net methane exchange over a range of forest, moorland, and agricultural soils in Scotland were made during the period April to June 1994 and 1995. Fluxes of CH₄ ranged from oxidation –12.3 to an emission of 6.8 ng m^–2 s^–1. The balance between CH₄ oxidation and emission depended on the physical conditions of the soil, primarily soil moisture. The largest oxidation rates were found in the mineral forest soils, and CH₄ emission was observed in several peat soils. The smallest oxidation rate was observed in an agricultural soil. The relationship between CH₄ flux and soil moisture observed in peats (Flux_{CH 4} = 0.023 × %H₂O (dry weight) – 7.44, p > 0.05) was such that CH₄ oxidation was observed at soil moistures less than 325%( ± 80%). CH₄ emission was found at soil moistures exceeding this value. A large range of CH₄ oxidation rates were observed over a small soil moisture range in the mineral soils. CH₄ oxidation in mineral soils was negatively correlated with soil bulk density (Flux_{CH 4} = –37.35 × bulk density (g cm^–3) + 48.83, p > 0.05). Increased nitrogen loading of the soil due to N fixation, atmospheric deposition of N, and fertilisation, were consistently associated with decreases in the soil sink for CH₄, typically in the range 50 to 80%, on a range of soil types and land uses.     

Black alder (Alnus Glutinosa (L.) Gaertn.) trees mediate methane and nitrous oxide emission from the soil to the atmosphere

Three-year-old seedlings of black alder (Alnus glutinosa (L.) Gaertn.), a common European wetland tree species, were grown in native soil taken from an alder swamp. Fluxes of methane (CH₄) and nitrous oxide (N₂O) between the tree stem and the atmosphere were determined under controlled conditions. Both CH₄ and N₂O were emitted through the bark of the stem into the atmosphere when the root zone exhibited higher-than-ambient CH₄ and N₂O gas mixing ratios. Flooding of the soil caused a decreased N₂O emission but an increased CH₄ efflux from the stem. Immediately after flooding of the soil, N₂O was emitted from the seedlings' bark at a rate of 350 mol N₂O m^-2 h^-1 whereas CH₄ flux could not be detected. After more than 40 days of flooding CH₄ fluxes up to 3750 mol CH₄ m^-2 h^-1 from the stem were measured, while N₂O emission had decreased below the limit of detection. Gas efflux decreased with increasing stem height and correlated with gas mixing ratios in the soil, indicating diffusion through the aerenchyma as the major path of gas transport. From these results it is assumed that woody species with aerenchyma can serve as conduits for soil-derived trace gases into the atmosphere, to date only shown for herbaceous plants. This, yet unidentified, woody plant pathway contributes to the total greenhouse gas source strength of wetlands.     

Fluxes of nitrous oxide and methane from nitrogen-amended soils in a Colorado alpine ecosystem

In order to determine the effect of increased nitrogen inputs on fluxed of N₂O and CH₄ from alpine soils, we measured fluxes of these gases from fertilized and unfertilized soils in wet and dry alpine meadows. In the dry meadow, the addition of nitrogen resulted in a 22-fold increase in N₂O emissions, while in the wet meadow, we observed a 45-fold increase in N₂O emission rates. CH₄ uptake in the dry meadow was reduced 52% by fertilization; however, net CH₄ production occurred in all the wet meadow plots and emission rates were not significantly affected by fertilization. Net nitrification rates in the dry meadow were higher in fertilized plots than in non-fertilized plots throughout the growing season; net mineralization rates in fertilized dry meadow pots were higher than those in non-fertilized plots during the latter half of the growing season.     

Rates and pathways of nitrous oxide production in a shortgrass steppe

Most of the small external inputs of N to the Shortgrass steppe appear to be conserved. One pathway of loss is the emission of nitrous oxide, which we estimate to account for 2.5–9.0% of annual wet deposition inputs of N. These estimates were determined from an N₂O emission model based on field data which describe the temporal variability of N₂O produced from nitrification and denitrification from two slope positions. Soil water and temperature models were used to translate records of air temperature and precipitation between 1950 and 1984 into variables appropriate to drive the gas flux model, and annual N₂O fluxes were estimated for that period. The mean annual fluxes were 80 g N ha^–1 for a midslope location and 160 g N ha^–1 for a swale. Fluxes were higher in wet years than in dry, ranging from 73 to 100 g N ha^–1y^–1at the midslope, but the variability was not high. N₂O fluxes were also estimated from cattle urine patches and these fluxes while high within a urine patch, did not contribute significantly to a regional budget. Laboratory experiments using C₂H₂ to inhibit nitrifiers suggested that 60–80% of N₂O was produced as a result of nitrification, with denitrification being less important, in contrast to our earlier findings to the contrary. Intrasite and intraseasonal variations in N₂O flux were coupled to variations in mineral N dynamics, with high rates of N₂O flux occurring with high rates of inorganic N turnover. We computed a mean flux of 104 g N ha^–1 y^–1 from the shortgrass landscape, and a flux of 2.6 × 109 g N y^– from all shortgrass steppe (25 × 106 ha).     

18O composition of CO2 and H2O ecosystem pools and fluxes in a tallgrass prairie: Simulations and comparisons to measurements

In this paper we describe measurements and modeling of ¹⁸O in CO₂ and H₂O pools and fluxes at a tallgrass prairie site in Oklahoma. We present measurements of the δ¹⁸O value of leaf water, depth‐resolved soil water, atmospheric water vapor, and Keeling plot δ¹⁸O intercepts for net soil‐surface CO₂ and ecosystem CO₂ and H₂O fluxes during three periods of the 2000 growing season. Daytime discrimination against C¹⁸OO, as calculated from measured above‐canopy CO₂ and δ¹⁸O gradients, is also presented. To interpret the isotope measurements, we applied an integrated land‐surface and isotope model (ISOLSM) that simulates ecosystem H₂¹⁸O and C¹⁸OO stocks and fluxes. ISOLSM accurately predicted the measured isotopic composition of ecosystem water pools and the δ¹⁸O value of net ecosystem CO₂ and H₂O fluxes. Simulations indicate that incomplete equilibration between CO₂ and H₂O within C₄ plant leaves can have a substantial impact on ecosystem discrimination. Diurnal variations in the δ¹⁸O value of above‐canopy vapor had a small impact on the predicted δ¹⁸O value of ecosystem water pools, although sustained differences had a large impact. Diurnal variations in the δ¹⁸O value of above‐canopy CO₂ substantially affected the predicted ecosystem discrimination. Leaves dominate the ecosystem ¹⁸O‐isoflux in CO₂ during the growing season, while the soil contribution is relatively small and less variable. However, interpreting daytime measurements of ecosystem C¹⁸OO fluxes requires accurate predictions of both soil and leaf ¹⁸O‐isofluxes.     

Monoterpene emissions from rubber trees (Hevea brasiliensis) in a changing landscape and climate: chemical speciation and environmental control

Emissions of biogenic volatile organic compounds (VOCs) have important roles in ecophysiology and atmospheric chemistry at a wide range of spatial and temporal scales. Tropical regions are a major global source of VOC emissions and magnitude and chemical speciation of VOC emissions are highly plant-species specific. Therefore it is important to study emissions from dominant species in tropical regions undergoing large-scale land-use change, for example, rubber plantations in South East Asia. Rubber trees ( Hevea brasiliensis ) are strong emitters of light-dependent monoterpenes. Measurements of emissions from leaves were made in the dry season in February 2003 and at the beginning of the wet season in May 2005. Major emitted compounds were sabinene, α -pinene and β -pinene, but β -ocimene and linalool also contributed significantly at low temperature and light. Cis -ocimene was emitted with a circadian course independent of photosynthetic active radiation (PAR) and temperature changes with a maximum in the middle of the day. Total isoprenoid VOC emission potential at the beginning of the wet season (94 μg gdw⁻¹ h⁻¹) was almost two orders of magnitude higher than measured in the dry season (2 μg g dw⁻¹ h⁻¹). Composition of total emissions changed with increasing temperature or PAR ramps imposed throughout a day. As well as light and temperature, there was evidence that assimilation rate was also a factor contributing to seasonal regulating emission potential of monoterpenes from rubber trees. Results presented here contribute to a better understanding of an important source of biogenic VOC associated with land-use change in tropical South East Asia.     

Controls on Soil Carbon Dioxide and Methane Fluxes in a Variety of Taiga Forest Stands in Interior Alaska

CO₂ and CH₄ fluxes were monitored over 4 years in a range of taiga forests along the Tanana River in interior Alaska. Floodplain alder and white spruce sites and upland birch/aspen and white spruce sites were examined. Each site had control, fertilized, and sawdust amended plots; flux measurements began during the second treatment year. CO₂ emissions decreased with successional age across the sites (alder, birch/aspen, and white spruce, in order of succession) regardless of landscape position. Although CO₂ fluxes showed an exponential relationship with soil temperature, the response of CO₂ production to moisture fit an asymptotic model. Of the manipulations, only N fertilization had an effect on CO₂ flux, decreasing flux in the floodplain sites but increasing it in the birch/aspen site. Landscape position was the best predictor of CH₄ flux. The two upland sites consumed CH₄ at similar rates (approximately 0.5 mg C m⁻² d⁻¹), whereas the floodplain sites had lower consumption rates (0–0.3 mg C m⁻² d⁻¹). N fertilization and sawdust both inhibited CH₄ consumption in the upland birch/aspen and floodplain spruce sites but not in the upland spruce site. The biological processes driving CO₂ fluxes were sensitive to temperature, moisture, and vegetation, whereas CH₄ fluxes were sensitive primarily to landscape position and biogeochemical disturbances. Hence, climate change effects on C-gas flux in taiga forest soils will depend on the relationship between soil temperature and moisture and the concomitant changes in soil nutrient pools and cycles. Received 10 March 1998; accepted 29 December 1999.     

Phosphorus availability and elevated CO2 affect biological nitrogen fixation and nutrient fluxes in a clover-dominated sward

The response of biological nitrogen fixation (BNF) to elevated CO(2) was examined in white clover (Trifolium repens)-dominated swards under both high and low phosphorus availability. Mixed swards of clover and buffalo grass (Stenotaphrum secundatum) were grown for 15 months in 0.2 m2 sand-filled mesocosms under two CO2 treatments (ambient and twice ambient) and three nutrient treatments [no N, and either low or high P (5 or 134 kg P ha(-1)); the third nutrient treatment was supplied with high P and N (240 kg N ha(-1))]. Under ambient CO2, high P increased BNF from 410 to 900 kg ha(-1). Elevated CO2 further increased BNF to 1180 kg ha(-1) with high P, but there was no effect of CO2 on BNF with low P. Allocation of N belowground increased by approx. 50% under elevated CO2 irrespective of supplied P. The results suggest that where soil P availability is low, elevated CO2 will not increase BNF, and pasture quality could decrease because of a reduction in aboveground N.     

Changes of methane and nitrous oxide emissions in a transition bog in central Germany (German National Park Harz Mountains) after rewetting

During the last decades, various renaturation programmes have been initialized to recover nutrient sink and ecological functions of peatlands by rewetting. Rewetting, however, often results in the formation of hotspots for methane (CH₄) emissions and in temporal dieback of local vegetation. The present study aimed at quantifying changes of CH₄ and nitrous oxide (N₂O) emissions in a peatland currently under continuous rewetting conditions. Emissions where studied at a permanently flooded site and a non-flooded peat site with fluctuating water tables by using common closed chamber method. The permanently flooded site revealed extremely high CH₄ emissions (up to 1195 mg C m^?2 d^?1) which were positively correlated with temperature, nutrient content, dissolved organic carbon and nitrogen concentration of the peat soil water. In contrast, the non-flooded peat site, with lower and fluctuating water tables (WT), showed significantly lower CH₄ emissions and an increasing trend of CH₄ release associated with a generally increasing WT caused by the progressing rewetting process. Lower N₂O emissions (<24 µg N m^?2 d^?1) were observed at the flooded site. By contrast, the non-flooded peat site with fluctuating WT showed significantly higher N₂O emissions (up to 4178 µg N m^?2 d^?1), in particular at high temperatures during summer time. The present results indicate that permanently flooded conditions during rewetting processes might cause higher CH₄ emissions compared to fluctuating WT which in contrast might enhance N₂O emissions. In total, however, no decreasing trend for CH₄ emissions throughout the five-year renaturation period could be found. At least for N₂O we observed a decreasing trend during rewetting.     

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.