Uncertainties in direct N2O emissions from grazing ruminant excreta (EF3PRP) in national greenhouse gas inventories.

Excreta deposition onto pasture, range and paddocks (PRP) by grazing ruminant constitute a source of nitrous oxide (N2O), a potent greenhouse gas (GHG). These emissions must be reported in national GHG inventories, and their estimation is based on the application of an emission factor, EF3PRP (proportion of nitrogen (N) deposited to the soil through ruminant excreta, which is emitted as N2O). Depending on local data available, countries use various EF3PRPs and approaches to estimate N2O emissions from grazing ruminant excreta. Based on ten case study countries, this review aims to highlight the uncertainties around the methods used to account for these emissions in their national GHG inventories, and to discuss the efforts undertaken for considering factors of variation in the calculation of emissions. Without any local experimental data, 2006 the IPCC default (Tier 1) EF3PRPs are still widely applied although the default values were revised in 2019. Some countries have developed country-specific (Tier 2) EF3PRP based on local field studies. The accuracy of estimation can be improved through the disaggregation of EF3PRP or the application of models; two approaches including factors of variation. While a disaggregation of EF3PRP by excreta type is already well adopted, a disaggregation by other factors such as season of excreta deposition is more difficult to implement. Empirical models are a potential method of considering factors of variation in the establishment of EF3PRP. Disaggregation and modelling requires availability of sufficient experimental and activity data, hence why only few countries have currently adopted such approaches. Replication of field studies under various conditions, combined with meta-analysis of experimental data, can help in the exploration of influencing factors, as long as appropriate metadata is recorded. Overall, despite standard IPCC methodologies for calculating GHG emissions, large uncertainties and differences between individual countries' accounting remain to be addressed.

Low N2O emissions associated with sheep excreta deposition in temperate managed lowland grassland and extensively grazed hill pasture.

Nitrous oxide (N2O) is a potent greenhouse gas (GHG) whose emission from soil can be enhanced by ruminant excretal returns in grasslands. The default (Tier 1) emission factors (EF3PRP; i.e. proportion of deposited nitrogen emitted as N2O) for ruminant excreta deposition are associated with a wide range of uncertainties and the development of country-specific (Tier 2) EF3PRP is encouraged. In Ireland, a Tier 2 EF3PRP has been developed for cattle excreta but no data are available for sheep. The aim of this study was to generate data to contribute to the derivation of a Tier 2 EF3PRP for sheep excreta, while assessing the effect of excreta type, grassland type and season of deposition on N2O emissions. An experiment was carried out on two sites in the west of Ireland: a managed lowland grassland (LOW) and an extensively grazed hill pasture (HILL), characterised by mineral and acid peat soils, respectively. For each season, four treatments were applied to the soil in a fully randomized block design: control (C), sheep urine (U), sheep dung (D), and artificial urine (AU). Nitrous oxide fluxes were assessed over a full year following each application of treatments, using a static chambers methodology. Results showed a brief initial peak following each application of U/AU in LOW but not in HILL. Cumulative N2O emissions were significantly higher from the lowland site. Average EF3PRP for combined excreta was negligible on both sites, thus lower than the IPCC Tier 1 EF3PRP. Causes of low emissions are likely to depend on site characteristics (e.g. soil acidity in HILL) and season of application (i.e. ammonia volatilisation in summer). This study showed very low N2O emissions from sheep excretal returns in Irish grasslands and highlighted the importance of developing Tier 2, animal-specific EF3PRP. More experimental grasslands should be assessed to confirm these results.