A comparison between measured and modeled N2O emissions from Inner Mongolian semi-arid grassland

The objectives of this study were (1) to determine the effect of land use on N₂O emissions from Inner Mongolian semi-arid grasslands of China and (2) to evaluate the process-based DNDC model to extrapolate our field measurements from a limited number of sites to a larger temporal and spatial scale. The results suggest the following. Rainfall event was the dominant controlling factor for the seasonal variations of the N₂O fluxes. The seven selected sites exhibited a similar seasonal trend in N₂O emission, despite their different vegetation, land use and textures. In the typical steppe, N₂O fluxes generally decrease with decreasing soil organic C (SOC) and total N content, indicating that soil C and N pools are very important in determining the spatial magnitude of the N₂O flux. N₂O emissions were very small during the entire growing season, averaging only 0.76 g N₂O-N ha^–1 day^–1 for the five typical steppe sites, 0.35 g N₂O-N ha^–1 day^–1 for the mown meadow steppe site, and 0.83 g N₂O-N ha^–1 day^–1 from the cropped meadow steppe site. No enhanced effect due to overgrazing was observed for the N₂O emission from the semi-arid grasslands. This was mainly results from the decreased SOC content due to overgrazing, which may have reduced the promoting effect of increased soil bulk density by trampling and animal excreta. Except for the mown steppe site, the model predictions of the N₂O flux for the six different sites agree well with the observed values (r ² ranging from 0.35 to 0.68). It would be concluded that the DNDC model captured the key driving process for N₂O emission. Nitrification was the predominant process, contributing 64–88% to the N₂O emission. However, in terms of the magnitude of the N₂O emission, further modifications should focus on the underestimated N₂O flux during the spring and autumn periods (nitrification, freeze/thaw cycles) and the effect of topography and the mowing on N₂O emission.     

Annual nitrous oxide flux and soil nitrogen characteristics in sagebrush steppe ecosystems

Soil nitrogen transformations and nitrous oxide fluxes were measured in a range of sagebrush steppe ecosystems in south-central Wyoming. Net nitrate production, measured in laboratory incubations, was highest in the ecosystem type dominated by Artemisia tridentata ssp. vaseyana, especially early in the growing season. Fluxes of nitrous oxide, measured in closed chambers and analyzed by gas chromatography, also tended to be higher in the same type, but only for short periods in the spring. Thereafter, all nitrous oxide fluxes were low and did not differ consistently among types. Estimated average annual fluxes for three Artemisia ecosystem types (dominated by Artemisia tridentata ssp. vaseyana, Artemisia tridentata ssp. wyomingensis, and Artemisia nova) were 0.32, 0.23 and 0.13 kg N₂O-N ha^–1 y^–1 repsectively. Average annual flux, weighted by the areal extent of these and other vegetation types in the region, was approximately 0.21 kg N₂O-N ha^–1y^–1. Assuming this landscape is representative of sagebrush steppe, we calculate a flux of 9.5 × 10⁹ g y^–1 of N₂O-N from U.S. sagebrush steppe, and a flux of 1.1 × 10¹¹ g y^–1 of N₂0-N from analogous desert and semi-desert shrublands of the world.     

Estimation of annual nitrous oxide emissions from a transitional grassland-forest region in Saskatchewan, Canada

The increasing atmospheric N₂O concentration and the imbalance in its global budget have triggered the interest in quantifying N₂O fluxes from various ecosystems. This study was conducted to estimate the annual N₂O emissions from a transitional grassland-forest region in Saskatchewan, Canada. The study region was stratified according to soil texture and land use types, and we selected seven landscapes (sites) to cover the range of soil texture and land use characteristics in the region. The study sites were, in turn, stratified into distinguishable spatial sampling units (i.e., footslope and shoulder complexes), which reflected the differences in soils and soil moisture regimes within a landscape. N₂O emission was measured using a sealed chamber method. Our results showed that water-filled pore space (WFPS) was the variable most correlated to N₂O fluxes. With this finding, we estimated the total N₂O emissions by using regression equations that relate WFPS to N₂O emission, and linking these regression equations with a soil moisture model for predicting WFPS. The average annual fluxes from fertilized cropland, pasture/hay land, and forest areas were 2.00, 0.04, and 0.02 kg N₂O-N ha^–1 yr^–1, respectively. The average annual fluxes for the medium- to fine-textured and sandy-textured areas were 1.40 and 0.04 kg N₂O-N ha^–1 yr^–1, respectively. The weighted-average annual flux for the study region is 0.95 kg N₂O-N ha^–1yr^–1. The fertilized cropped areas covered only 47% of the regional area but contributed about 98% of the regional flux. We found that in the clay loam, cropped site, 2% and 3% of the applied fertilizer were emitted as N₂O on the shoulders and footslopes, respectively.Contribution no. R824 of Saskatchewan Center for Soil Research, Saskatoon, Saskatchewan, Canada     

Methane and nitrous oxide exchange in differently fertilised grassland in southern Germany

We examined the effect of fertilisation (200 kg cattle slurry-N ha^–1 year^–1) on the exchange of N₂O and CH₄ in the soil–plant system of meadow agroecosystems in southern Germany. From 1996 to 1998, we regularly determined the gas fluxes (closed chamber method) and associated environmental parameters. N₂O and CH₄ fluxes were not significantly affected by fertilisation. N₂O fluxes at the unfertilised and fertilised plots were small, generally between 50 and –20 g N m^–2 h^–1. We identified some incidents of N₂O uptake. CH₄-C fluxes ranged from 1.3 to –0.2 mg m^–2 h^–1 and were not significantly different from 0 at both plots. We budgeted an annual net emission of 15.5 and 29.6 mg m^–2 N₂O-N and an annual CH₄-C net emission of 184.2 and 122.7 mg m^–2 at the unfertilised and fertilised plots, respectively. Apparently, rapid N mineralization and uptake in the densely rooted topsoil prevents N losses and the inhibition of CH₄ oxidation.     

Nitrous Oxide Emission from Grazed Grassland Under Different Management Systems

Nitrous oxide (N₂O) emissions from grazed grasslands are estimated to be approximately 28% of global anthropogenic N₂O emissions. Estimating the N₂O flux from grassland soils is difficult because of its episodic nature. This study aimed to quantify the N₂O emissions, the annual N₂O flux and the emission factor (EF), and also to investigate the influence of environmental and soil variables controlling N₂O emissions from grazed grassland. Nitrous oxide emissions were measured using static chambers at eight different grasslands in the South of Ireland from September 2007 to August 2009. The instantaneous N₂O flux values ranged from -186 to 885.6 μg N₂O-N m⁻² h⁻¹ and the annual sum ranged from 2 ± 3.51 to 12.55 ± 2.83 kg N₂O-N ha⁻¹ y⁻¹ for managed sites. The emission factor ranged from 1.3 to 3.4%. The overall EF of 1.81% is about 69% higher than the Intergovernmental Panel on Climate Change (IPCC) default EF value of 1.25% which is currently used by the Irish Environmental Protection Agency (EPA) to estimate N₂O emission in Ireland. At an N applied of approximately 300 kg ha⁻¹ y⁻¹, the N₂O emissions are approximately 5.0 kg N₂O-N ha⁻¹ y⁻¹, whereas the N₂O emissions double to approximately 10 kg N ha⁻¹ for an N applied of 400 kg N ha⁻¹ y⁻¹. The sites with higher fluxes were associated with intensive N-input and frequent cattle grazing. The N₂O flux at 17°C was five times greater than that at 5°C. Similarly, the N₂O emissions increased with increasing water filled pore space (WFPS) with maximum N₂O emissions occurring at 60–80% WFPS. We conclude that N application below 300 kg ha⁻¹ y⁻¹ and restricted grazing on seasonally wet soils will reduce N₂O emissions.     

Soil nitrous oxide emissions from a typical semiarid temperate steppe in inner Mongolia: effects of mineral nitrogen fertilizer levels and forms

Nitrous oxide (N₂O) emissions can be significantly affected by the amounts and forms of nitrogen (N) available in soils, but the effect is highly dependent on local climate and soil conditions in specific ecosystem. To improve our understanding of the response of N₂O emissions to different N sources of fertilizer in a typical semiarid temperate steppe in Inner Mongolia, a 2-year field experiment was conducted to investigate the effects of high, medium and low N fertilizer levels (HN: 200 kg N?ha^-1y^-1, MN: 100 kg N ha^-1y^-1, and LN: 50 kg N ha^-1y^-1) respectively and N fertilizer forms (CAN: calcium ammonium nitrate, AS: ammonium sulphate and NS: sodium nitrate) on N₂O emissions using static closed chamber method. Our data showed that peak N₂O fluxes induced by N treatments were concentrated in short periods (2 to 3 weeks) after fertilization in summer and in soil thawing periods in early spring; there were similarly low N₂O fluxes from all treatments in the remaining seasons of the year. The three N levels increased annual N₂O emissions significantly (P?<?0.05) in the order of MN > HN > LN compared with the CK (control) treatment in year 1; in year 2, the elevation of annual N₂O emissions was significant (P?<?0.05) by HN and MN treatments but was insignificant by LN treatments (P?>?0.05). The three N forms also had strong effects on N₂O emissions. Significantly (P?<?0.05) higher annual N₂O emissions were observed in the soils of CAN and AS fertilizer treatments than in the soils of NS fertilizer treatments in both measured years, but the difference between CAN and AS was not significant (P?>?0.05). Annual N₂O emission factors (EF) ranged from 0.060 to 0.298% for different N fertilizer treatments in the two observed years, with an overall EF value of 0.125%. The EF values were by far less than the mean default EF proposed by the Intergovernmental Panel on Climate Change (IPCC).     

Nitrogen cycling in an acid forest ecosystem in the Netherlands under increased atmospheric nitrogen input

Within a long-term research project studying the biogeochemical budget of an oak-beech forest ecosystem in the eastern part of the Netherlands, the nitrogen transformations and solute fluxes were determined in order to trace the fate of atmospherically deposited NH₄ ⁺ and to determine the contribution of nitrogen transformations to soil acidification.The oak-beech forest studied received an annual input of nitrogen via throughfall and stemflow of 45 kg N ha^–1 yr^–1, mainly as NH₄ ⁺, whereas 8 kg N ha^–1 yr^–1 was taken up by the canopy. Due to the specific hydrological regime resulting in periodically occurring high groundwater levels, denitrification was found to be the dominant output flux (35 kg N ha^–1 yr^–1). N₂0 emmission rate measurements indicated that 57% of this gaseous nitrogen loss (20 kg N ha^–1 yr^–1) was as N₂O. The forest lost an annual amount of 11 kg N ha^–1 yr^–1 via streamwater output, mainly as N0₃ ^–.Despite the acid conditions, high nitrification rates were measured. Nitrification occurred mainly in the litter layer and in the organic rich part of the mineral soil and was found to be closely correlated with soil temperature. The large amount of NH₄ ⁺ deposited on the forest floor via atmospheric deposition and produced by mineralization was to a large extent nitrified in the litter layer. Almost no NH₄ ⁺ reached the subsurface soil horizons. The N0₃ ^– was retained, taken up or transformed mainly in the mineral soil. A small amount of N0₃ ^– (9 kg N ha^–1 yr^–1) was removed from the system in streamwater output. A relatively small amount of nitrogen was measured in the soil water as Dissolved Organic Nitrogen.On the basis of these data the proton budget of the system was calculated using two different approaches. In both cases net proton production rates were high in the vegetation and in the litter layer of the forest ecosystem. Nitrogen transformations induced a net proton production rate of 2.4 kmol ha^–1 yr^–1 in the soil compartment.     

Nitrogen transformations and nitrous oxide flux in a tropical deciduous forest in México

Summary Emissions of nitrous oxide and soil nitrogen pools and transformations were measured over an annual cycle in two forests and one pasture in tropical deciduous forest near Chamela, México. Nitrous oxide flux was moderately high (0.5–2.5 ng cm^–2 h^–1) during the wet season and low (<0.3 ng cm^–2 h^–1) during the dry season. Annual emissions of nitrogen as nitrous oxide were calculated to be 0.5–0.7 kg ha^–1 y^–1, with no substantial difference between the forests and pasture. Wetting of dry soil caused a large but short-lived pulse of N₂O flux that accounted for <2% of annual flux. Variation in soil water through the season was the primary controlling factor for pool sizes of ammonium and nitrate, nitrogen transformations, and N₂O flux.     

Exchange of N-gases at the Höglwald Forest – A summary

During 4 years continuous measurements of N-trace gas exchange were carried out at the forest floor-atmosphere interface at the Höglwald Forest that is highly affected by atmospheric N-deposition. The measurements included spruce control, spruce limed and beech sites. Based on these field measurements and on intensive laboratory measurements of N₂-emissions from the soils of the beech and spruce control sites, a total balance of N-gas emissions was calculated. NO₂-deposition was in a range of –1.6 –2.9 kg N ha^–1 yr^–1 and no huge differences between the different sites could be demonstrated. In contrast to NO₂-deposition, NO- and N₂O-emissions showed a huge variability among the different sites. NO emissions were highest at the spruce control site (6.4–9.1 kg N ha^–1 yr^–1), lowest at the beech site (2.3–3.5 kg N ha^–1 yr^–1) and intermediate at the limed spruce site (3.4–5.4 kg N ha^–1 yr^–1). With regard to N₂O-emissions, the following ranking between the sites was found: beech (1.6–6.6 kg N ha^–1 yr^–1) >> spruce limed (0.7–4.0 kg N ha^–1 yr^–1) > spruce control (0.4–3.1 kg N ha^–1 yr^–1). Average N-trace gas emissions (NO, NO₂, N₂O) for the years 1994–1997 were 6.8 kg N ha^–1 yr^–1 at the spruce control site, 3.6 kg N ha^–1 yr^–1 at the limed spruce site and 4.5 kg N ha^–1 yr^–1 at the beech site. Considering N₂-losses, which were significantly higher at the beech (12.4 kg N ha^–1 yr^–1) than at the spruce control site (7.2 kg N ha^–1 yr^–1), the magnitude of total gaseous N losses, i.e. N₂-N + NO-N + NO₂-N + N₂O-N, could be calculated for the first time for a forest ecosystem. Total gaseous N-losses were 14.0 kg N ha^–1 yr^–1 at the spruce control site and 15.5 kg N ha^–1 yr^–1 at the beech site, respectively. In view of the huge interannual variability of N-trace gas fluxes and the pronounced site differences in N-gas emissions it is concluded that more research is needed in order to fully understand patterns of microbial N-cycling and N-gas production/emission in forest ecosystems and mechanisms of reactions of forest ecosystems to the ecological stress factor of atmospheric N-input.     

Soil nitrous oxide and methane fluxes are low from a bioenergy crop (canola) grown in a semi-arid climate

Understanding nitrous oxide (N₂O) and methane (CH₄) fluxes from agricultural soils in semi‐arid climates is necessary to fully assess greenhouse gas emissions from bioenergy cropping systems, and to improve our knowledge of global terrestrial gaseous exchange. Canola is grown globally as a feedstock for biodiesel production, however, resulting soil greenhouse gas fluxes are rarely reported for semi‐arid climates. We measured soil N₂O and CH₄ fluxes from a rain‐fed canola crop in a semi‐arid region of south‐western Australia for 1 year on a subdaily basis. The site included N fertilized (75 kg N ha^?1 yr^?1) and nonfertilized plots. Daily N₂O fluxes were low (?1.5 to 4.7 g N₂O‐N ha^?1 day^?1) and culminated in an annual loss of 128 g N₂O‐N ha^?1 (standard error, 12 g N₂O‐N ha^?1) from N fertilized soil and 80 g N₂O‐N ha^?1 (standard error, 11 g N₂O‐N ha^?1) from nonfertilized soil. Daily CH₄ fluxes were also low (?10.3 to 11.9 g CH₄‐C ha^?1 day^?1), and did not differ with treatments, with an average annual net emission of 6.7 g CH₄–C ha^?1 (standard error, 20 g CH₄–C ha^?1). Greatest daily N₂O fluxes occurred when the soil was fallow, and following a series of summer rainfall events. Summer rainfall increased soil water contents and available N, and occurred when soil temperatures were >25 °C, and when there was no active plant growth to compete with soil microorganisms for mineralized N; conditions known to promote N₂O production. The proportion of N fertilizer emitted as N₂O, after correction for emissions from the no N fertilizer treatment, was 0.06%; 17 times lower than IPCC default value for the application of synthetic N fertilizers to land (1.0%). Soil greenhouse gas fluxes from bioenergy crop production in semi‐arid regions are likely to have less influence on the net global warming potential of biofuel production than in temperate climates.     

Rate of accumulation and emission of N2, N2O and CH4 from a flooded rice soil

In a field experiment using microplots, a flooded Crowley silt loam (Typic Albaqualfs) rice soil was fertilized with ¹⁵N labelled (60–74 atom %) urea and KNO₃. Emission of N₂, N₂O and CH₄ and accumulation in soil were measured for 21 d after fertilizer application.Emission of ¹⁵N₂-N measured from the urea and KNO₃ treated plots ranged from <15 to 570 and from 330 to 3,420 g ha^–1 d^–1, respectively. Entrapped ¹⁵N₂-N in the urea treated microplots was significantly lower (<15 g to 2.1 kg ha^–1) on all sampling dates compared to the ¹⁵N₂-N gas accumulation in the KNO₃ treated plots (6.4 to 31.5 kg ha^–1). Emissions of N₂O-N were low and did not exceed 4 g ha^–1 d^–1. Fluxes of CH₄ from the fertilizer and control plots were low and never exceeded 33 g ha^–1 d^–1. Maximum accumulation of CH₄ in the flooded soil measured 460 and 195 g ha^–1 for the urea and KNO₃ treatments, respectively.     

Microbial N Turnover and N-Oxide (N<Subscript>2</Subscript>O/NO/NO<Subscript>2</Subscript>) Fluxes in Semi-arid Grassland of Inner Mongolia

Gross rates of N mineralization and nitrification, and soil–atmosphere fluxes of N₂O, NO and NO₂ were measured at differently grazed and ungrazed steppe grassland sites in the Xilin river catchment, Inner Mongolia, P. R. China, during the 2004 and 2005 growing season. The experimental sites were a plot ungrazed since 1979 (UG79), a plot ungrazed since 1999 (UG99), a plot moderately grazed in winter (WG), and an overgrazed plot (OG), all in close vicinity to each other. Gross rates of N mineralization and nitrification determined at in situ soil moisture and soil temperature conditions were in a range of 0.5–4.1 mg N kg⁻¹ soil dry weight day⁻¹. In 2005, gross N turnover rates were significantly higher at the UG79 plot than at the UG99 plot, which in turn had significantly higher gross N turnover rates than the WG and OG plots. The WG and the OG plot were not significantly different in gross ammonification and in gross nitrification rates. Site differences in SOC content, bulk density and texture could explain only less than 15% of the observed site differences in gross N turnover rates. N₂O and NO _x flux rates were very low during both growing seasons. No significant differences in N trace gas fluxes were found between plots. Mean values of N₂O fluxes varied between 0.39 and 1.60 μg N₂O-N m⁻² h⁻¹, equivalent to 0.03–0.14 kg N₂O-N ha⁻¹ y⁻¹, and were considerably lower than previously reported for the same region. NO _x flux rates ranged between 0.16 and 0.48 μg NO _x -N m⁻² h⁻¹, equivalent to 0.01–0.04 kg NO _x -N ha⁻¹ y⁻¹, respectively. N₂O fluxes were significantly correlated with soil temperature and soil moisture. The correlations, however, explained only less than 20% of the flux variance.     

Whole-system estimation of denitrification in a plains river: a comparison of two methods

Whole-system denitrification in the South Platte River was measured over a 13-month period using an open-channel N₂ method and mass-balance measurements. Concentrations of dissolved N₂ were measured with high precision by membrane-inlet mass spectrometry and estimates of denitrification were based on the mass flux of N₂, after correction for reaeration and groundwater flux. Open-channel estimates of denitrification ranged from 0 to 3.08 g N m^–2 d^–1 and the mean annual rate was 1.62 g N m^–2 d^–1, which corresponds to removal of approximately 34% of the nitrate transported by the river over a distance of 18.5 km. Over the same period of time, estimates of denitrification based on mass-balance measurements ranged from 0.29 to 5.25 g N m^–2 d^–1 and the mean annual rate was 2.11 g N m^–2 d^–1. The two methods revealed similar seasonal patterns of denitrification the highest rates were measured from late April to August and the lowest rates were in winter. Both methods provide whole-system estimates of denitrification in running waters; where reaeration rate coefficients are low and flux of groundwater is well quantified, the open-channel method has fewer sources of uncertainty and is easier to implement.     

The role of cattle in the volatile loss of nitrogen from a shortgrass steppe

The cycling and volatile loss of N derived from cattle urine at upland and lowland sites within the shortgrass steppe of eastern Colorado was studied, using¹⁵N-labelled urea as an N source. Losses of NH ₀ ³ were determined by direct measurement and by difference. Losses were higher from coarse (27% summer, 12% winter) than from fine textured (0–2%) soils. Immobilization and plant uptake of N accounted for significant amounts of added N. Extrapolating our plot measurements to a typical pasture, using spatially and temporally stratified urine deposition data, losses from upland sites were calculated to be 0.016 g N · m^-2 · y^-1, while losses from lowland sites were negligible. This resulted in an average loss of 0.011 g N · m^-2 · y^-1 for a pasture divided 70:30 between uplands and lowlands. The loss of urine N calculated assuming no spatial stratification would be sevenfold higher (0.076 g N · m^-2 · y^-1). Losses of NH ₀ ³ from urine, animal biomass removal, and NH₂O loss totaled only 0.07 g N · m^-2 · y^-1 , or about 25% of wet deposition input. We calculated a potential loss of NH ₀ ³ from senescing vegetation of 0.26 g N · m^-2 · y^-1, an order of magnitude larger than all other losses combined.     

Frozen Cropland Soil in Northeast China as Source of N2O and CO2 Emissions

Agricultural soils are important sources of atmospheric N₂O and CO₂. However, in boreal agro-ecosystems the contribution of the winter season to annual emissions of these gases has rarely been determined. In this study, soil N₂O and CO₂ fluxes were measured for 6 years in a corn-soybean-wheat rotation in northeast China to quantify the contribution of wintertime N₂O and CO₂ fluxes to annual emissions. The treatments were chemical fertilizer (NPK), chemical fertilizer plus composted pig manure (NPKOM), and control (Cont.). Mean soil N₂O fluxes among all three treatments in the winter (November–March), when soil temperatures are below −7°C for extended periods, were 0.89–3.01 µg N m⁻² h⁻¹, and in between the growing season and winter (October and April), when freeze-thaw events occur, 1.73–5.48 µg N m⁻² h⁻¹. The cumulative N₂O emissions were on average 0.27–1.39, 0.03–0.08 and 0.03–0.11 kg N₂O_–N ha⁻¹ during the growing season, October and April, and winter, respectively. The average contributions of winter N₂O efflux to annual emissions were 6.3–12.1%. In all three seasons, the highest N₂O emissions occurred in NPKOM, while NPK and Cont. emissions were similar. Cumulative CO₂ emissions were 2.73–4.94, 0.13–0.20 and 0.07–0.11 Mg CO₂-C ha⁻¹ during growing season, October and April, and winter, respectively. The contribution of winter CO₂ to total annual emissions was 2.0–2.4%. Our results indicate that in boreal agricultural systems in northeast China, CO₂ and N₂O emissions continue throughout the winter.     

Nonlinear response of N2O flux to incremental fertilizer addition in a continuous maize (Zea mays L.) cropping system

The relationship between nitrous oxide (N₂O) flux and N availability in agricultural ecosystems is usually assumed to be linear, with the same proportion of nitrogen lost as N₂O regardless of input level. We conducted a 3‐year, high‐resolution N fertilizer response study in southwest Michigan USA to test the hypothesis that N₂O fluxes increase mainly in response to N additions that exceed crop N needs. We added urea ammonium nitrate or granular urea at nine levels (0–292 kg N ha^?1) to four replicate plots of continuous maize. We measured N₂O fluxes and available soil N biweekly following fertilization and grain yields at the end of the growing season. From 2001 to 2003 N₂O fluxes were moderately low (ca. 20 g N₂O‐N ha^?1 day^?1) at levels of N addition to 101 kg N ha^?1, where grain yields were maximized, after which fluxes more than doubled (to >50 g N₂O‐N ha^?1 day^?1). This threshold N₂O response to N fertilization suggests that agricultural N₂O fluxes could be reduced with no or little yield penalty by reducing N fertilizer inputs to levels that just satisfy crop needs.     

Denitrification and N2O emission from urine-affected grassland soil

Denitrification and N₂O emission rates were measured following two applications of artificial urine (40 g urine-N m^–2) to a perennial rye-grass sward on sandy soil. To distinguish between N₂O emission from denitrification or nitrification, urine was also applied with a nitrification inhibitor (dicyandiamide, DCD). During a 14 day period following each application, the soil was frequently sampled, and incubated with and without acetylene to measure denitrification and N₂O emission rates, respectively.Urine application significantly increased denitrification and N₂O emission rates up to 14 days after application, with rates amounting to 0.9 and 0.6 g N m^–2 day^–1 (9 and 6 kg N ha^–1 day^–1), respectively. When DCD was added to the urine, N₂O emission rates were significantly lower from 3 to 7 days after urine application onwards. Denitrification was the main source of N₂O immediately following each urine application. 14 days after the first application, when soil water contents dropped to 15% (v/v) N₂O mainly derived from nitrification.Total denitrification losses during the 14 day periods were 7 g N m^–2, or 18% of the urine-N applied. Total N₂O emission losses were 6.5 and 3 g N m^–2, or 16% and 8% of the urine-N applied for the two periods. The minimum estimations of denitrification and N₂O emission losses from urine-affected soil were 45 to 55 kg N ha^–1 year^–1, and 20 to 50 kg N ha^–1 year^–1, respectively.     

Effect of tree distance on N2O and CH4-fluxes from soils in temperate forest ecosystems

Complete annual cycles of N₂O and CH₄ flux in forest soils at a beech and at a spruce site at the Höglwald Forest were followed in 1997 by use of fully automatic measuring systems. In order to test if on a microsite scale differences in the magnitude of trace gas exchange between e.g. areas in direct vicinity of stems and areas in the interstem region at both sites exist, tree chambers and gradient chambers were installed in addition to the already existing interstem chambers at our sites. N₂O fluxes were in a range of –4.6–473.3 g N₂O-N m^–2 h^–1 at the beech site and in a range of –3.7–167.2 g N₂O-N m^–2 h^–1 at the spruce site, respectively. Highest N₂O emissions were observed during and at the end of a prolonged frost period, thereby further supporting previous findings that frost periods are of crucial importance for controlling annual N₂O losses from temperate forests. Fluxes of CH₄ were in a range of +10.4––194.0 g CH₄ m^–2 h^–1 at the beech site and in a range of –4.4––83.5 g CH₄ m^–2 h^–1 at the spruce site. In general, both N₂O-fluxes as well as CH₄-fluxes were higher at the beech site. On a microsite scale, N₂O and CH₄ fluxes at the beech site were highest within the stem area (annual mean: 49.6±3.3 g N₂O-N m^–2 h^–1; –77.2±3.1 g CH₄ m^–2 h^–1), and significantly lower within interstem areas (18.5±1.4 g N₂O-N m^–2 h^–1; –60.2±1.8 g CH₄ m^–2 h^–1). Significantly higher values of total N, C and pH in the organic layer, as well as increased soil moisture, especially in spring, in the stem areas, are likely to contribute to the higher N₂O fluxes within the stem area of the beech. Also for the spruce site, such differences in trace gas fluxes could be demonstrated to exist (mean annual N₂O emission within (a) stem areas: 9.7±0.9 g N₂O-N m^–2 h^–1 and (b) interstem areas: 6.2±0.6 g N₂O-N m^–2 h^–1; mean annual CH₄ uptake within (a) stem areas: –26.1±0.6 g CH₄ m^–2 h^–1 and (b) interstem areas: –38.4±0.8 g CH₄ m^–2 h^–1), though they were not as pronounced as at the beech site.     

Regional Patterns in Carbon Cycling Across the Great Plains of North America

The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C m^–2 y^–1) to tallgrass prairie (180, 1221, and 208 g C m^–2 y^–1 for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.     

Soil-atmosphere exchange of CH4, CO2, NOx,and N2O in the Colorado shortgrass steppe under elevated CO2

In late March 1997, an open-top-chamber (OTC) CO₂ enrichment study was begun in the Colorado shortgrass steppe. The main objectives of the study were to determine the effect of elevated CO₂ (720 mol mol^–1) on plant production, photosynthesis, and water use of this mixed C₃/C₄ plant community, soil nitrogen (N) and carbon (C) cycling and the impact of changes induced by CO₂ on trace gas exchange. From this study, we report here our weekly measurements of CO₂, CH₄, NO_x and N₂O fluxes within control (unchambered), ambient CO₂ and elevated CO₂ OTCs. Soil water and temperature were measured at each flux measurement time from early April 1997, year round, through October 2000. Even though both C₃ and C₄ plant biomass increased under elevated CO₂ and soil moisture content was typically higher than under ambient CO₂ conditions, none of the trace gas fluxes were significantly altered by CO₂ enrichment. Over the 43 month period of observation NO_x and N₂O flux averaged 4.3 and 1.7 in ambient and 4.1 and 1.7 g N m^–2 hr ^–1 in elevated CO₂ OTCs, respectively. NO_x flux was negatively correlated to plant biomass production. Methane oxidation rates averaged –31 and –34 g C m^–2 hr^–1 and ecosystem respiration averaged 43 and 44 mg C m^–2 hr^–1 under ambient and elevated CO₂, respectively, over the same time period.