Uncertainties in direct NO emissions from grazing ruminant excreta (EF) in national greenhouse gas inventories.

Excreta deposition onto pasture, range and paddocks (PRP) by grazing ruminant constitute a source of nitrous oxide (NO), 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, EF (proportion of nitrogen (N) deposited to the soil through ruminant excreta, which is emitted as NO) Depending on local data available, countries use various EFs and approaches to estimate NO 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) EFs are still widely applied although the default values were revised in 2019. Some countries have developed country-specific (Tier 2) EF based on local field studies. The accuracy of estimation can be improved through the disaggregation of EF or the application of models; two approaches including factors of variation. While a disaggregation of EF 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 EF. 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.