Low NO emissions associated with sheep excreta deposition in temperate managed lowland grassland and extensively grazed hill pasture.

Nitrous oxide (NO) 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 (EF; i.e. proportion of deposited nitrogen emitted as NO) for ruminant excreta deposition are associated with a wide range of uncertainties and the development of country-specific (Tier 2) EF is encouraged. In Ireland, a Tier 2 EF 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 EF for sheep excreta, while assessing the effect of excreta type, grassland type and season of deposition on NO 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 NO emissions were significantly higher from the lowland site. Average EF for combined excreta was negligible on both sites, thus lower than the IPCC Tier 1 EF. 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 NO emissions from sheep excretal returns in Irish grasslands and highlighted the importance of developing Tier 2, animal-specific EF. More experimental grasslands should be assessed to confirm these results.