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Adequately estimating soil nitrous oxide (N 2 O) emissions using static chambers is challenging due to the high spatial variability and episodic nature of these fluxes. We discuss how to design experiments using static chambers to better account for this variability and reduce the uncertainty of N 2 O emission estimates. This paper is part of a series, each discussing different facets of N 2 O chamber methodology. Aspects of experimental design and sampling affected by spatial variability include site selection and chamber layout, size, and areal coverage. Where used, treatment application adds a further level of spatial variability. Time of day, frequency, and duration of sampling (both individual chamber closure and overall experiment duration) affect the temporal variability captured. We also present best practice recommendations for chamber installation and sampling protocols to reduce further uncertainty. To obtain the best N 2 O emission estimates, resources should be allocated to minimize the overall uncertainty in line with experiment objectives. Sometimes this will mean prioritizing individual flux measurements and increasing their accuracy and precision by, for example, collecting four or more headspace samples during each chamber closure. However, where N 2 O fluxes are exceptionally spatially variable (e.g., in heterogeneous agricultural landscapes, such as uneven and woody grazed pastures), using available resources to deploy more chambers with fewer headspace samples per chamber may be beneficial. Similarly, for particularly episodic N 2 O fluxes, generated for example by irrigation or freeze-thaw cycles, increasing chamber sampling frequency will improve the accuracy and reduce the uncertainty of temporally interpolated N 2 O fluxes., (© 2020 The Authors. Journal of Environmental Quality © 2020 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America.)

Urine patches and dung pats from grazing livestock create hotspots for production and emission of the greenhouse gas, nitrous oxide (N 2 O), and represent a large proportion of total N 2 O emissions in many national agricultural greenhouse gas inventories. As such, there is much interest in developing country specific N 2 O emission factors (EFs) for excretal nitrogen (EF 3, pasture, range and paddock) deposited during gazing. The aims of this study were to generate separate N 2 O 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 N 2 O 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 N 2 O EFs were 0.69% and 0.19%, respectively, resulting in a combined excretal N 2 O EF (EF 3 ), of 0.49%, which is <25% of the IPCC default EF 3 for excretal returns from grazing cattle. Regression analysis suggests that urine N 2 O EFs were controlled more by composition than was the case for dung, whilst dung N 2 O EFs were more related to soil and environmental factors. The urine N 2 O 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 N 2 O EF from urine patches by an average of 46%. The significantly lower excretal EF 3 than the IPCC default has implications for the UK's national inventory and for subsequent carbon footprinting of UK ruminant livestock products., (Copyright © 2018 The Authors. Published by Elsevier B.V. All rights reserved.)