Improving and disaggregating N₂O emission factors for ruminant excreta on temperate pasture soils

Cattle excreta deposited on grazed grasslands are a major source of the greenhouse gas (GHG) nitrous oxide (N₂O). Currently, many countries use the IPCC default emission factor (EF) of 2% to estimate excreta-derived N₂O emissions. However, emissions can vary greatly depending on the type of excreta (dung or urine), soil type and timing of application. Therefore three experiments were conducted to quantify excreta-derived N₂O emissions and their associated EFs, and to assess the effect of soil type, season of application and type of excreta on the magnitude of losses. Cattle dung, urine and artificial urine treatments were applied in spring, summer and autumn to three temperate grassland sites with varying soil and weather conditions. Nitrous oxide emissions were measured from the three experiments over 12 months to generate annual N₂O emission factors. The EFs from urine treated soil was greater (0.30–4.81% for real urine and 0.13–3.82% for synthetic urine) when compared with dung (− 0.02–1.48%) treatments. Nitrous oxide emissions were driven by environmental conditions and could be predicted by rainfall and temperature before, and soil moisture deficit after application; highlighting the potential for a decision support tool to reduce N₂O emissions by modifying grazing management based on these parameters. Emission factors varied seasonally with the highest EFs in autumn and were also dependent on soil type, with the lowest EFs observed from well-drained and the highest from imperfectly drained soil. The EFs averaged 0.31 and 1.18% for cattle dung and urine, respectively, both of which were considerably lower than the IPCC default value of 2%. These results support both lowering and disaggregating EFs by excreta type. - Highlights: • N₂O emissions were measured from cattle excreta applied to pasture. • N₂O was universally higher from urine compared with dung. • N₂O was driven by rainfall, temperature and soil moisture deficit. • Emission factors were highest in autumn and from imperfectly-drained soil • N₂O emission factors were lower than the 2% IPCC default value.