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Urine patches and dung pats from grazing livestock create hotspots for production and emission of the greenhouse gas, nitrous oxide (N2O), and represent a large proportion of total N2O emissions in many national agricultural greenhouse gas inventories. As such, there is much interest in developing country specific N2O emission factors (EFs) for excretal nitrogen (EF3, pasture, range and paddock) deposited during gazing. The aims of this study were to generate separate N2O emissions data for cattle derived urine and dung, to provide an evidence base for the generation of a country specific EF for the UK from this nitrogen source. The experiments were also designed to determine the effects of site and timing of application on emissions, and the efficacy of the nitrification inhibitor, dicyandiamide (DCD) on N2O losses. This co-ordinated set of 15 plot-scale, year-long field experiments using static chambers was conducted at five grassland sites, typical of the soil and climatic zones of grazed grassland in the UK. We show that the average urine and dung N2O EFs were 0.69% and 0.19%, respectively, resulting in a combined excretal N2O EF (EF3), of 0.49%, which is <25% of the IPCC default EF3 for excretal returns from grazing cattle. Regression analysis suggests that urine N2O EFs were controlled more by composition than was the case for dung, whilst dung N2O EFs were more related to soil and environmental factors. The urine N2O EF was significantly greater from the site in SW England, and significantly greater from the early grazing season urine application than later applications. Dycandiamide reduced the N2O EF from urine patches by an average of 46%. The significantly lower excretal EF3 than the IPCC default has implications for the UK's national inventory and for subsequent carbon footprinting of UK ruminant livestock products.

The global warming potential of nitrous oxide (N 2 O) and its long atmospheric lifetime mean its presence in the atmosphere is of major concern, and that methods are required to measure and reduce emissions. Large spatial and temporal variations means, however, that simple extrapolation of measured data is inappropriate, and that other methods of quantification are required. Although process-based models have been developed to simulate these emissions, they often require a large amount of input data that is not available at a regional scale, making regional and global emission estimates difficult to achieve. The spatial extent of organic soils means that quantification of emissions from these soil types is also required, but will not be achievable using a process-based model that has not been developed to simulate soil water contents above field capacity or organic soils. The ECOSSE model was developed to overcome these limitations, and with a requirement for only input data that is readily available at a regional scale, it can be used to quantify regional emissions and directly inform land-use change decisions. ECOSSE includes the major processes of nitrogen (N) turnover, with material being exchanged between pools of SOM at rates modified by temperature, soil moisture, soil pH and crop cover. Evaluation of its performance at sitescale is presented to demonstrate its ability to adequately simulate soil N contents and N 2 O emissions from cropland soils in Europe. Mitigation scenarios and sensitivity analyses are also presented to demonstrate how ECOSSE can be used to estimate the impact of future climate and land-use change on N 2 O emissions. C max A constant (set at 50 kg N ha -1 ) that adjusts the maximum rate of nitrification possible [this occurs at high levels of NH 4 ? and will be dependent on soil composition (Parton et al. 1996)] D p Potential denitrification rate (kg N ha -1 layer -1 day -1 ) k nitrif A rate constant for nitrification [set at 0.6 (Bradbury et al. (1993)] m b Biological activity rate modifier m NO 3 Modifies the amount of denitrification depending on soil NO 3 -content m pH A rate modifier due to soil pH m t A rate modifier due to soil temperature m w Soil water rate modifier for decomposition m w0 Soil water rate modifier for decomposition at permanent wilting point and saturation = 0.2 m 0 w Soil water rate modifier for denitrification N d The amount of N emitted from the soil during denitrification (kg N ha -1 layer -1 ) N d;N 2The amount of N 2 gas lost by denitrification (kg N ha -1 day -1 ) N d;N 2 O The amount of N 2 O gas lost by denitrification (kg N ha -1 day -1 ) N d50The soil nitrate content at which denitrification is 50% of its full potential (kg N ha -1 layer -1 ) N FERT N in NH 4 ? and urea in the added fertiliser (kg N ha -1 ) N n Nitrification rate (kg N ha -1 layer -1 ) N n;N 2 O The amount of N 2 O gas released during nitrification (kg N ha -1 day -1 ) N NH 4

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SUMMARYIncreasing recognition of the extent to which nitrous oxide (N2O) contributes to climate change has resulted in greater demand to improve quantification of N2O emissions, identify emission sources and suggest mitigation options. Agriculture is by far the largest source and grasslands, occupying c. 0·22 of European agricultural land, are a major land-use within this sector. The application of mineral fertilizers to optimize pasture yields is a major source of N2O and with increasing pressure to increase agricultural productivity, options to quantify and reduce emissions whilst maintaining sufficient grassland for a given intensity of production are required. Identification of the source and extent of emissions will help to improve reporting in national inventories, with the most common approach using the IPCC emission factor (EF) default, where 0·01 of added nitrogen fertilizer is assumed to be emitted directly as N2O. The current experiment aimed to establish the suitability of applying this EF to fertilized Scottish grasslands and to identify variation in the EF depending on the application rate of ammonium nitrate (AN). Mitigation options to reduce N2O emissions were also investigated, including the use of urea fertilizer in place of AN, addition of a nitrification inhibitor dicyandiamide (DCD) and application of AN in smaller, more frequent doses. Nitrous oxide emissions were measured from a cut grassland in south-west Scotland from March 2011 to March 2012. Grass yield was also measured to establish the impact of mitigation options on grass production, along with soil and environmental variables to improve understanding of the controls on N2O emissions. A monotonic increase in annual cumulative N2O emissions was observed with increasing AN application rate. Emission factors ranging from 1·06–1·34% were measured for AN application rates between 80 and 320 kg N/ha, with a mean of 1·19%. A lack of any significant difference between these EFs indicates that use of a uniform EF is suitable over these application rates. The mean EF of 1·19% exceeds the IPCC default 1%, suggesting that use of the default value may underestimate emissions of AN-fertilizer-induced N2O loss from Scottish grasslands. The increase in emissions beyond an application rate of 320 kg N/ha produced an EF of 1·74%, significantly different to that from lower application rates and much greater than the 1% default. An EF of 0·89% for urea fertilizer and 0·59% for urea with DCD suggests that N2O quantification using the IPCC default EF will overestimate emissions for grasslands where these fertilizers are applied. Large rainfall shortly after fertilizer application appears to be the main trigger for N2O emissions, thus applicability of the 1% EF could vary and depend on the weather conditions at the time of fertilizer application.

The contribution of soil organic carbon (SOC) to atmospheric greenhouse gas (GHG) concentrations could increase due to rising temperatures, agricultural land‐management, and land‐use change. Here the results of a modeling study are presented, which reviews the changing patterns of UK land‐use from 1925 to 2007, and estimates the contribution that these changes have had toward UK GHG emissions. The study uses a large database of SOC concentrations from which SOC stocks are estimated for land‐uses typical of the UK, and combines this with literature values of transition times for SOC to adjust to a new concentration following land‐use change. The model was designed to be used with limited input data, allowing the impacts of historical land‐use change, lacking in site specific soil and vegetation change data to be assessed. This study suggests that from 1925 to 2007 the UK's soils have acted as a net carbon sink as a result of land‐use change, sequestering a total of 102 Tg C. This represents a 5% net gain in total SOC stocks, and an average increase of 1.9 Tg C/year (inter‐quartile range: 0.19–3.12 Tg C/yr). When the reported losses of SOC due to climate change are compared to the gains resulting from land‐use change the UK's soils are a sink of carbon, with the gains from land‐use change offsetting those due to climate change. This overall sink is the result of an increase in the area of woodland, and conversion of arable land to permanent grassland. The greatest sequestration in any one year occurred in 1993 and coincides with the introduction of set‐aside. The largest SOC flux to the atmosphere occurred in 1942 following arable expansion, emitting 12.3 Tg C in one year. This flux is equivalent to almost 10% of the UK's current total GHG emissions, indicating that such land‐use change should be avoided in the future if targets to reduce GHG emissions are to be met.