Greenhouse gas fluxes from an Australian subtropical cropland under long‐term contrasting management regimes

The long‐term effects of conservation management practices on greenhouse gas fluxes from tropical/subtropical croplands remain to be uncertain. Using both manual and automatic sampling chambers, we measured N₂O and CH₄ fluxes at a long‐term experimental site (1968–present) in Queensland, Australia from 2006 to 2009. Annual net greenhouse gas fluxes (NGGF) were calculated from the 3‐year mean N₂O and CH₄ fluxes and the long‐term soil organic carbon changes. N₂O emissions exhibited clear daily, seasonal and interannual variations, highlighting the importance of whole‐year measurement over multiple years for obtaining temporally representative annual emissions. Averaged over 3 years, annual N₂O emissions from the unfertilized and fertilized soils (90 kg N ha^?1 yr^?1 as urea) amounted to 138 and 902 g N ha^?1, respectively. The average annual N₂O emissions from the fertilized soil were 388 g N ha^?1 lower under no‐till (NT) than under conventional tillage (CT) and 259 g N ha^?1 higher under stubble retention (SR) than under stubble burning (SB). Annual N₂O emissions from the unfertilized soil were similar between the contrasting tillage and stubble management practices. The average emission factors of fertilizer N were 0.91%, 1.20%, 0.52% and 0.77% for the CT‐SB, CT‐SR, NT‐SB and NT‐SR treatments, respectively. Annual CH₄ fluxes from the soil were very small (?200–300 g CH₄ ha^?1 yr^?1) with no significant difference between treatments. The NGGF were 277–350 kg CO₂‐e ha^?1 yr^?1 for the unfertilized treatments and 401–710 kg CO₂‐e ha^?1 yr^?1 for the fertilized treatments. Among the fertilized treatments, N₂O emissions accounted for 52–97% of NGGF and NT‐SR resulted in the lowest NGGF (401 kg CO₂‐e ha^?1 yr^?1 or 140 kg CO₂‐e t^?1 grain). Therefore, NT‐SR with improved N fertilizer management practices was considered the most promising management regime for simultaneously achieving maximal yield and minimal NGGF.     

Interannual variation in nitrous oxide emissions from perennial ryegrass/white clover grassland used for dairy production

Nitrous oxide (N₂O) emissions are subject to intra‐ and interannual variation due to changes in weather and management. This creates significant uncertainties when quantifying estimates of annual N₂O emissions from grazed grasslands. Despite these uncertainties, the majority of studies are short‐term in nature (<1 year) and as a consequence, there is a lack of data on interannual variation in N₂O emissions. The objectives of this study were to (i) quantify annual N₂O emissions and (ii) assess the causes of interannual variation in emissions from grazed perennial ryegrass/white clover grassland. Nitrous oxide emissions were measured from fertilized and grazed perennial ryegrass/white clover grassland (WC) and from perennial ryegrass plots that were not grazed and did not receive N input (GB), over 4 years from 2008 to 2012 in Ireland (52°51′N, 08°21′W). The annual N₂O‐N emissions (kg ha^?1; mean ± SE) ranged from 4.4 ± 0.2 to 34.4 ± 5.5 from WC and from 1.7 ± 0.8 to 6.3 ± 1.2 from GB. Interannual variation in N₂O emissions was attributed to differences in annual rainfall, monthly (December) soil temperatures and variation in N input. Such substantial interannual variation in N₂O emissions highlights the need for long‐term studies of emissions from managed pastoral systems.     

Long‐term impacts of agricultural practices and climatic variability on carbon storage in a permanent pasture

Intra‐ and interannual variability of precipitation can lead to major modifications of grassland production and carbon storage capacity. Greater understanding of how climatic variability affects net CO₂ exchange [i.e. net ecosystem exchange (NEE)] of grazed grasslands is important to adapt grassland management and reduce risks of carbon losses. Since 2002, we continuously measured NEE (i.e. eddy covariance technique) on an upland grassland site (7 ha), divided in two paddocks grazed by heifers (intensive: 1 LSU ha^?1 yr^?1, 213 kg N ha^?1 yr^?1 and extensive: 0.5 LSU ha^?1 yr^?1, no fertilization). For years with dry and warm growing seasons (i.e. 2003, 2005 and 2008), absolute annual NEE was higher in the intensive paddock compared with the extensive paddock. The opposite was observed during years of ample seasonal rainfall and soil moisture (i.e. 2004, 2006 and 2007). Contrasted management led to two distinct plant communities being different in leaf area index (LAI), soil bulk density and soil water holding capacity. Differences in annual NEEs could thus be assigned to interactions between in carbon and water fluxes during dry and wet growth periods. Dry growth periods led to a reduction in weekly gross primary productivity (GPP) in the extensively managed paddock, whereas the GPP was maintained in the intensive paddock. In turn, during wet growth periods, GPP was similar in both paddocks, whereas N amendment and frequent defoliation significantly increased ecosystem respiration in the intensive paddock, presumably through a higher heterotrophic respiration following on a better C substrate quality and availability (rhizodeposition and senescent fine roots). In the extensive paddock, where plant cover was denser (reducing soil temperature) and less decomposable, C losses through heterotrophic respiration were comparatively smaller under wet conditions. Our results demonstrate that grassland subjected to a moderately intensive management could be more resilient in terms of carbon storage during drought and heat waves, presumably because of a trade‐off between heterotrophic and autotrophic respiration.     

Nitrogen balance in the Trachypogon grasslands of Central Venezuela

The nitrogen balance of a Trachypogon grassland in Calabozo, Venezuela, is calculated for average conditions using biomass accumulation, nitrogen content, and turnover rates of organic matter. Burning Trachypogon grasslands results in losses of 8.5 kg N ha^?1 yr^?1, while rainfall inputs average 2.6 kg N ha^?1 yr^?1. Uptake of N by vegetation is 14.8 kg N ha^?1 yr^?1, but the total N required to build new tissue during a growing season is about 30 kg N ha^?1 yr^?1, so that about 50% of the nitrogen in the vegetation is recycled internally. Nitrogen lossesvia fire are probably balanced by biological N₂-fixation, but no data are available for N-fixation in these savannas. The calculations presented in this paper are based on few data and more measurements are needed to develop a conclusive picture of the N-balance of Trachypogon grasslands.     

Nitrogen loss from grassland on peat soils through nitrous oxide production

Nitrous oxide (N₂O) in soils is produced through nitrification and denitrification. The N₂O produced is considered as a nitrogen (N) loss because it will most likely escape from the soil to the atmosphere as N₂O or N₂. Aim of the study was to quantify N₂O production in grassland on peat soils in relation to N input and to determine the relative contribution of nitrification and denitrification to N₂O production. Measurements were carried out on a weekly basis in 2 grasslands on peat soil (Peat I and Peat II) for 2 years (1993 and 1994) using intact soil core incubations. In additional experiments distinction between N₂O from nitrification and denitrification was made by use of the gaseous nitrification inhibitor methyl fluoride (CH₃F).Nitrous oxide production over the 2 year period was on average 34 kg N ha^-1 yr^-1 for mown treatments that received no N fertiliser and 44 kg N ha^-1 yr^-1 for mown and N fertilised treatments. Grazing by dairy cattle on Peat I caused additional N₂O production to reach 81 kg N ha^-1 yr^-1. The sub soil (20–40 cm) contributed 25 to 40% of the total N₂O production in the 0–40 cm layer. The N₂O production:denitrification ratio was on average about 1 in the top soil and 2 in the sub soil indicating that N₂O production through nitrification was important. Experiments showed that when ratios were larger than l, nitrification was the major source of N₂O. In conclusion, N₂O production is a significant N loss mechanism in grassland on peat soil with nitrification as an important N₂O producing process.     

Soil‐derived trace gas fluxes from different energy crops – results from a field experiment in Southwest Germany

Willow coppice, energy maize and Miscanthus were evaluated regarding their soil‐derived trace gas emission potential involving a nonfertilized and a crop‐adapted slow‐release nitrogen (N) fertilizer scheme. The N application rate was 80 kg N ha^?1 yr^?1 for the perennial crops and 240 kg N ha^?1 yr^?1 for the annual maize. A replicated field experiment was conducted with 1‐year measurements of soil fluxes of CH₄, CO₂ and N₂O in weekly intervals using static chambers. The measurements revealed a clear seasonal trend in soil CO₂ emissions, with highest emissions being found for the N‐fertilized Miscanthus plots (annual mean: 50 mg C m^?² h^?1). Significant differences between the cropping systems were found in soil N₂O emissions due to their dependency on amount and timing of N fertilization. N‐fertilized maize plots had highest N₂O emissions by far, which accumulated to 3.6 kg N₂O ha^?1 yr^?1. The contribution of CH₄ fluxes to the total soil greenhouse gas subsumption was very small compared with N₂O and CO₂. CH₄ fluxes were mostly negative indicating that the investigated soils mainly acted as weak sinks for atmospheric CH₄. To identify the system providing the best ratio of yield to soil N₂O emissions, a subsumption relative to biomass yields was calculated. N‐fertilized maize caused the highest soil N₂O emissions relative to dry matter yields. Moreover, unfertilized maize had higher relative soil N₂O emissions than unfertilized Miscanthus and willow. These results favour perennial crops for bioenergy production, as they are able to provide high yields with low N₂O emissions in the field.     

Simulation of Effects of Soils,Climate and Management on N<Subscript>2</Subscript>O Emission from Grasslands

Nitrous oxide (N₂O) is a potent greenhouse gas with a high contribution from agricultural soils and emissions that depend on soil type, climate, crops and management practices. The N₂O emissions therefore need to be included as an integral part of environmental assessments of agricultural production systems. An algorithm for N₂O production and emission from agricultural soils was developed and included in the FASSET whole-farm model. The model simulated carbon and nitrogen (N) turnover on a daily basis. Both nitrification and denitrification was included in the model as sources for N₂O production, and the N₂O emissions depended on soil microbial and physical conditions. The model was tested on experimental data of N₂O emissions from grasslands in UK, Finland and Denmark, differing in climatic conditions, soil properties and management. The model simulated the general time course of N₂O emissions and captured the observed effects of fertiliser and manure management on emissions. Scenario analyses for grazed and cut grasslands were conducted to evaluate the effects of soil texture, climatic conditions, grassland management and N fertilisation on N₂O emissions. The soils varied from coarse sand to sandy loam and the climatic variation was taken to represent the climatic variation within Denmark. N fertiliser rates were varied from 0 to 500 kg N ha⁻¹. The simulated N₂O emissions showed a non-linear response to increasing N rates with increasing emission factors at higher N rates. The simulated emissions increased with increasing soil clay contents. N₂O emissions were slightly increased at higher temperatures, whereas increasing annual rainfall generally lead to decreasing emissions. Emissions were slightly higher from grazed grasslands compared with cut grasslands at similar rates of total N input (fertiliser and animal excreta). The results indicate higher emission factors and thus higher potentials for reducing N₂O emissions for intensively grazed grasslands on fine textured soils than for extensive cut-based grasslands on sandy soils.     

Nitrogen inputs and losses from New Zealand dairy farmlets, as affected by nitrogen fertilizer application: year one

Inputs and losses of nitrogen (N) were determined in dairy cow farmlets receiving 0, 225 or 360 kg N ha^-1 (in split applications as urea) in the first year of a large grazing experiment near Hamilton, New Zealand. Cows grazed perennial ryegrass/white clover pastures all year round on a free-draining soil. N₂ fixation was estimated (using ¹⁵N dilution) to be 212, 165 and 74 kg N ha^-1 yr^-1 in the 0, 225 and 360 N treatments, respectively. The intermediate N rate had little effect on clover growth during spring but favoured more total pasture cover in summer and autumn, thereby reducing overgrazing and resulting in 140% more clover growth during the latter period.Removal of N in milk was 76,89 and 92 kg N ha^-1 in the 0, 225 and 360 N treatments, respectively. Denitrification losses were low (7–14 kg N ha^-1 yr^-1), increased with N application, and occurred predominantly during winter. Ammonia volatilization was estimated by micrometeorological mass balance at 15, 45 and 63 kg N ha^-1 yr^-1 in the 0, 225 and 360 N treatments, respectively. Most of the increase in ammonia loss was attributed to direct loss after application of the urea fertilizer.Leaching of nitrate was estimated (using ceramic cup samplers at 1 m soil depth, in conjunction with lysimeters) to be 13, 18 and 31 kg N ha^-1 yr^-1 in a year of relatively low rainfall (990 mm yr^-1) and drainage (170–210 mm yr^-1). Drainage was lower in the N fertilized treatments and this was attributed to enhanced evapotranspiration associated with increased grass growth.Nitrate-N concentrations in leachates increased gradually over time to 30 mg L^-1 in the 360 N treatment whereas there was little temporal variation evident in the 0 (mean 6.4 mg L^-1) and 225 (mean 10.1 mg L^-1) N treatments. Thus, the 360 N treatment had a major effect by greatly reducing N₂ fixation and increasing N losses, whereas the 225 N treatment had little effect on N₂ fixation or on nitrate leaching. However, these results refer to the first year of the experiment and further measurements over time will determine the longer-term effects of these treatments on N inputs, transformations and losses.     

Bi-directional soil/atmosphere N2O exchange over two mown grassland systems with contrasting management practices

Nitrous oxide (N₂O) fluxes from soil under mown grassland were monitored using static chambers over three growing seasons in intensively and extensively managed systems in Central Switzerland. Emissions were largest following the application of mineral (NH₄NO₃) fertilizer, but there were also substantial emissions following cattle slurry application, after grass cuts and during the thawing of frozen soil. Continuous flux sampling, using automatic chambers, showed marked diurnal patterns in N₂O fluxes during emission peaks, with highest values in the afternoon. Net uptake fluxes of N₂O and subambient N₂O concentrations in soil open pore space were frequently measured on both fields. Flux integration over 2.5 years yields a cumulated emission of +4.7 kgN₂O‐N ha^?1 for the intensively managed field, equivalent to an average emission factor of 1.1%, and a small net sink activity of ?0.4 kg N₂O‐N ha^?1 for the unfertilized system. The data suggest the existence of a consumption mechanism for N₂O in dry, areated soil conditions, which cannot be explained by conventional anaerobic denitrification. The effect of fertilization on greenhouse gas budgets of grassland at the ecosystem level is discussed.     

Nitrous oxide emissions and nitrate leaching from a rain-fed wheat-maize rotation in the Sichuan Basin, China

Aims

A 3-year field experiment (October 2004–October 2007) was conducted to quantify N₂O fluxes and determine the regulating factors from rain-fed, N fertilized wheat-maize rotation in the Sichuan Basin, China.

Methods

Static chamber-GC techniques were used to measure soil N₂O fluxes in three treatments (three replicates per treatment): CK (no fertilizer); N150 (300 kg N fertilizer ha^?1 yr^?1 or 150 kg N?ha^?1 per crop); N250 (500 kg N fertilizer ha^?1 yr^?1 kg or 250 kg N?ha^?1 per crop). Nitrate (NO ₃ ^? ) leaching losses were measured at nearby sites using free-drained lysimeters.

Results

The annual N₂O fluxes from the N fertilized treatments were in the range of 1.9 to 6.7 kg N?ha^?1 yr^?1 corresponding to an N₂O emission factor ranging from 0.12 % to 1.06 % (mean value: 0.61 %). The relationship between monthly soil N₂O fluxes and NO ₃ ^- leaching losses can be described by a significant exponential decaying function.

Conclusions

The N₂O emission factor obtained in our study was somewhat lower than the current IPCC default emission factor (1 %). Nitrate leaching, through removal of topsoil NO ₃ ^? , is an underrated regulating factor of soil N₂O fluxes from cropland, especially in the regions where high NO ₃ ^- leaching losses occur.     

Aboveground plant biomass,carbon, and nitrogen dynamics before and after burning in a seminatural grassland of Miscanthus sinensis in Kumamoto,Japan

Although fire has been used for several thousand years to maintain Miscanthus sinensis grasslands in Japan, there is little information about the nutrient dynamics in these ecosystems immediately after burning. We investigated the loss of aboveground biomass; carbon (C) and nitrogen (N) dynamics; surface soil C change before and after burning; and carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) fluxes 2 h after burning in a M. sinensis grassland in Kumamoto, Japan. We calculated average C and N accumulation rates within the soil profile over the past 7300 years, which were 58.0 kg C ha^?1 yr^?1 and 2.60 kg N ha^?1 yr^?1, respectively. After burning, 98% of aboveground biomass and litter were consumed. Carbon remaining on the field, however, was 102 kg C ha^?1. We found at least 43% of C was possibly lost due to decomposition. However, remaining C, which contained ash and charcoal, appeared to contribute to C accumulation in soil. There was no difference in the amount of 0–5 cm surface soil C before and after burning. The amount of remaining litter on the soil surface indicated burning appeared not to have caused a reduction in soil C nor did it negatively impact the sub‐surface vegetative crown of M. sinensis. Also, nearly 50 kg N ha^?1 of total aboveground biomass and litter N was lost due to burning. Compared with before the burning event, postburning CO₂ and CH₄ fluxes from soil appeared not to be directly affected by burning. However, it appears the short time span of measurements of N₂O flux after burning sufficiently characterized the pattern of increasing N₂O fluxes immediately after burning. These findings indicate burning did not cause significant reductions in soil C nor did it result in elevated CO₂ and CH₄ emissions from the soil relative to before the burning event.     

Effects of clover density on N2O emissions and plant-soil N transfers in a fertilised upland pasture

Legumes have the potential to alter nitrous oxide (N₂O) emissions in grass-legume mixtures via changes in soil N availability, but the influence of legume abundance on N₂O fluxes in grazed multi-species grasslands has faced little attention to date. In this paper, a combination of ¹⁵N-labelled fertilizer application and automatic chamber measurements was used to investigate N₂O fluxes and soil-plant N transfers for high- and low-density clover patches in an intensively-managed, upland pasture (Auvergne, France) over the course of one growing season. During the six-month study period, N₂O fluxes were highly variable. Maximum daily N₂O emission was 52 g N₂O-N ha^?1, and was associated with fertilizer application early in the growing season. Smaller peaks of N₂O emission occured in response to cutting events and fertilizer application later in the growing season. Nitrous oxide fluxes derived from ¹⁵N-labelled fertilizer peaked at 40% shortly after fertilizer application, but the dominant source of N₂O fluxes was the soil N pool. Contrary to expectations, clover density had no significant effects on N content or patterns of ¹⁵N recovery in plant or soil mineral N pools. Nevertheless, we found a tendency for increased N₂O-N losses from the low clover treatment. Furthermore, ¹⁵N recovery in N₂O was higher in the low- compared to the high-density clover treatment during favorable growing conditions, suggesting transient shifts in plant/soil competition for N depending on legume abundance. Multiple regression analysis revealed that water-filled pore space (WFPS) and clover dry mass were the main factors driving cumulative N₂O emissions in the high clover treatment, whereas variation in cumulated N₂O emissions in the low clover treatment was best explained by WFPS and grass mass. We hypothesize that clover density had indirect effects on the sensitivity of N₂O emissions to abiotic and biotic factors possibly via changes in soil pH. Overall, our results suggest that spatial heterogeneity in clover abundance may have relatively little impact on field-scale N₂O emissions in fertilized grasslands.     

Gaseous nitrogen emmissions from undisturbed terrestrial ecosystems: An assessment of their impacts on local and global nitrogen budgets

There is increasing interest in the importance of nitrogen gas emissions from natural (non-agricultural) ecosystems with respect to local as well as global nitrogen budgets and with respect to the effects of nitrogen oxides on atmospheric ozone levels and global warming. The volatile forms of nitrogen of common interest are ammonia (NH₃), nitrous oxide, (N₂O), dinitrogen (N₂), and NO_x (principally NO + NO₂). It is often difficult to attribute emissions of these compounds from soils to a single process because they are produced by a variety of common biogeochemical mechanisms. Although environmental conditions in the soil often appear to favor nitrogen gas emissions, the potential nitrogen gas emission rate from undisturbed ecosystems is rarely approached. The best estimates to date suggest that nitrogen gas emission rates from undisturbed ecosystems typically range from > 1 to perhaps 10 or 20 kg N ha^-1 yr^-1. Under certain conditions, however, emission rates may be much higher. For example, excreta from animals in grasslands may elevate ammonia volatilization up to 100 kg N ha^-1 yr^-1 depending on grazer density; tidal input of nutrients to coastal wetlands may support denitrification rates of several hundred kg N ha^-1 yr^-1 . Excepting such cases, gaseous nitrogen losses are probably a small component of the local nitrogen budget in most undisturbed ecosystems. However, emissions from undisturbed soils are an important component of the global source strengths for (N₂O + N₂), N₂O and NO_x (50%, 21%, and 10% respectively). Emission rates of N₂O from natural ecosystems are higher than assumed previously by perhaps 10 times. Large-scale disturbance may have a stimulatory effect on nitrogen emission rates which could have important effects on global nitrogen budgets. There is a need for more sophisticated methods to account for natural temporal and spatial variations of emissions rates, to more accurately and precisely assess their global source strengths.     

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.     

Impacts of grazing intensity on denitrification and N<Subscript>2</Subscript>O production in a semi-arid grassland ecosystem

N₂O production from denitrification in soils contributes to the enhanced greenhouse effect and the destruction of the stratospheric ozone. Ungulate grazing affects denitrification and the production of N₂O. The short-term effect of grazing on denitrification and N₂O production has been examined in several grassland ecosystems. However, the effects of long-term grazing have rarely been studied. We measured denitrification and N₂O production during the 2005 and 2006 growing seasons in a long-term (17 years) experiment that had five grazing intensities (GI; 0.00, 1.33, 2.67, 4.00 and 5.33 sheep ha⁻¹). We found that denitrification and N₂O production rates were seasonally variable during the measurement period, with higher values observed in summer and lower values found in spring and autumn. The grazed treatments resulted in decreased denitrification and N₂O production, primarily due to the reduced soil nitrate concentration and organic N content under the long-term grazing. This supported our hypothesis that long-term over-grazing suppresses denitrification and N₂O production. Although significant differences in denitrification and N₂O production were not found between the four GI, there was a general trend that cumulative denitrification and N₂O production decreased as grazing intensity increased, especially in 2006. Lower N losses via denitrification and N₂O production in the grazed plots, to some extent, may contribute to the mitigation of greenhouse gas emission and help to preserve soil N and ameliorate the negative impacts of grazing on plant growth, productivity, and ecological restoration processes in the temperate steppe in northern China.     

Recovery of soil nitrogen pools in species‐rich grasslands after 12 years of simulated pollutant nitrogen deposition: a 6‐year experimental analysis

Nitrogen (N) deposition from anthropogenic sources is a global problem that can reduce biodiversity and impair ecosystem functioning through effects on soil eutrophication and acidification. While increasing controls on emissions of oxides of nitrogen (NO_x) have reduced European N deposition rates from their peak in the late 20th Century, little is known about the legacy effects of N deposition in soils or the reversibility of N‐induced shifts in ecosystem processes. We studied species‐rich limestone and acidic grasslands, located in a highly polluted region that received over 3000 kg N deposition ha^?1 throughout the 20th Century, followed by a decline of ～50% in NO_x deposition rate in the past two decades. We investigated the effects on seasonal and annual mean concentrations of soil mineral N in experimental plots established in 1990 receiving simulated enhanced N deposition (0–140 kg N ha^?1 yr^?1) until 2002, both in the final year of treatment, and the subsequent 5 years of ‘recovery’ following cessation of treatments. Winter–summer cycles of N mineralization–immobilization were strongly amplified by simulated N deposition rates through the final year of treatments and into the first year of recovery, with winter concentrations of ammonium‐N in the acidic grassland and nitrate in the limestone grassland enhanced by up to 360% and 450%, respectively. Both the magnitude of the seasonal variations and the residual effects of the treatments on soil mineral N concentrations decreased progressively in the first 5 years after treatments ceased, although dose‐dependent trends remained in the acidic grassland. This study establishes that reducing N deposition rates in species‐rich grasslands can reverse eutrophication, even in soils that have experienced prolonged high rates of deposition. It provides new insight into the rates of recovery following, and effects of, declining N deposition rates with implications for restoration of species‐rich grasslands.     

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.     

Climate- and crop-responsive emission factors significantly alter estimates of current and future nitrous oxide emissions from fertilizer use

The current Intergovernmental Panel on Climate Change (IPCC) default methodology (tier 1) for calculating nitrous oxide (N₂O) emissions from nitrogen applied to agricultural soils takes no account of either crop type or climatic conditions. As a result, the methodology omits factors that are crucial in determining current emissions, and has no mechanism to assess the potential impact of future climate and land‐use change. Scotland is used as a case study to illustrate the development of a new methodology, which retains the simple structure of the IPCC tier 1 methodology, but incorporates crop‐ and climate‐dependent emission factors (EFs). It also includes a factor to account for the effect of soil compaction because of trampling by grazing animals. These factors are based on recent field studies in Scotland and elsewhere in the UK. Under current conditions, the new methodology produces significantly higher estimates of annual N₂O emissions than the IPCC default methodology, almost entirely because of the increased contribution of grazed pasture. Total emissions from applied fertilizer and N deposited by grazing animals are estimated at 10 662 t N₂O‐N yr^?1 using the newly derived EFs, as opposed to 6 796 t N₂O‐N yr^?1 using the IPCC default EFs. On a spatial basis, emission levels are closer to those calculated using field observations and detailed soil modelling than to estimates made using the IPCC default methodology. This can be illustrated by parts of the western Ayrshire basin, which have previously been calculated to emit 8–9 kg N₂O‐N ha^?1 yr^?1 and are estimated here as 6.25–8.75 kg N₂O‐N ha^?1 yr^?1, while the IPCC default methodology gives a maximum emission level of only 3.75 kg N₂O‐N ha^?1 yr^?1 for the whole area. The new methodology is also applied in conjunction with scenarios for future climate‐ and land‐use patterns, to assess how these emissions may change in the future. The results suggest that by 2080, Scottish N₂O emissions may increase by up to 14%, depending on the climate scenario, if fertilizer and land management practices remain unchanged. Reductions in agricultural land use, however, have the potential to mitigate these increases and, depending on the replacement land use, may even reduce emissions to below current levels.     

Denitrification in the top and sub soil of grassland on peat soils

Denitrification is an important process in the nitrogen (N) balance of intensively managed grassland, especially on poorly drained peat soils. Aim of this study was to quantify the N loss through denitrification in the top and sub soil of grassland on peat soils. Sampling took place at 2 sites with both control (0 N) and N fertilised (+ N) treatments. Main difference between the sites was the ground water level. Denitrification was measured on a weekly basis for 2 years with a soil core incubation technique using acetylene (C₂H₂) inhibition. Soil cores were taken from the top soil (0–20 cm depth) and the sub soil (20–40 cm depth) and incubated in containers for 24 hours. The denitrification rate was calculated from the nitrous oxide production between 4 and 24 hours of incubation. Denitrification capacities of the soils and the soil layers were also determined.The top soil was the major layer for denitrification with losses ranging from 9 to 26 kg N ha^–1 yr^–1 from the O N treatment. Losses from the top soil of the + N treatment ranged from 13 to 49 kg N ha^–1 yr^–1. The sub soil contributed, on average, 20% of the total denitrification losses from the 0–40 layer. Losses from the 0–40 cm layer were 2 times higher on the + N treatment than on the O N treatment and totalled up to 70 kg N ha^–1 yr^–1. Significant correlation coefficients were found between denitrification activity on the one hand, and ground water level, water filled pore space and nitrate content on the other, in the top soil but not in the sub soil. The denitrification capacity experiment showed that the availability of easily decomposable organic carbon was an important limiting factor for the denitrification activity in the sub soil of these peat soils.     

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.