The contribution of cattle urine and dung to nitrous oxide emissions: Quantification of country specific emission factors and implications for national inventories

Abstract

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 (EF_3, 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.

Keywords: Cattle; Dicyandiamide; Dung pat; Grassland; Greenhouse gas; Inventory; Nitrification inhibitor; Nitrous oxide; Urine patch.