While food production must increase in response to global population expansion, a reduction of agricultural greenhouse gas (GHG) emissions and nitrogen (N) losses is of paramount importance. One agricultural source responsible for various N losses is ruminant excretal returns in grasslands, which is particularly concerning in countries where grazing systems are an important sector, such as Ireland. Accurate estimation of these losses, including nitrous oxide (N₂O) emissions, ammonia (NH₃) volatilisation, nitric oxide (NO) emissions and nitrates (NO₃^-) leaching are essential to monitor the trends and facilitate agricultural management decisions. However, due to several factors influencing N cycling, estimation of these losses is uncertain and may vary significantly between distinct grazing systems. There is greater uncertainty associated with losses from sheep excreta compared to cattle. Therefore, the main goal of this thesis was to estimate and compare various N losses from sheep excreta (principally urine) deposited on pasture soils representative of two typical sheep grazing systems in Ireland. Specific objectives were i) to establish N₂O emission factors associated with sheep excretal returns (EF_3PRP) on two distinct temperate grasslands, ii) to decipher if inherent soil properties could explain differences of N₂O emissions observed in situ, iiii) to determine urine N fate on these two pasture soils by a quantification of various N losses and pools, and iv) to assess the effect of sheep urine on microbial communities involved in N cycling. To address these objectives, three experiments were carried out. Firstly, a long-term field experiment was established on a lowland managed grassland and an extensively grazed upland pasture characterised by mineral and acid peat soil, respectively. Static chamber methodology was used to measure N₂O fluxes following sheep urine and dung applied during different seasons. Soil of these pastures were then incubated under denitrifying conditions to quantify N₂O, NO and dinitrogen (N₂) emissions following urine application using the He/O₂ gas flow method. The same soils were incubated for another short-term experiment to assess NH₃ volatilisation, N₂O emissions, NO₃^- leaching, microbial biomass N and nitrification/denitrification gene abundances. The field experiment showed lower N₂O emissions from sheep excreta on the upland pasture, although EF_3PRPs were very low on both grasslands, indicating that current estimations in national inventory may be overestimated. The incubation studies showed a different fate of urine N between the two pasture soils. On the peat soil, acid conditions may have inhibited nitrification leading to very low N₂O, NO and N₂ emissions. Urine derived ammonium (NH₄⁺) remained at a high level during the incubation experiment and was subject to leaching and to some NH₃ volatilisation associated with the raise of pH following urine addition, but there was no sign of immobilisation in microbial biomass. On the mineral soil, urine application led to significant losses of N gas emissions and NO₃^- leaching. Within the three weeks following application, the main loss of urine N was through NH₃ volatilisation. Therefore, low N₂O emissions on the lowland were unlikely to be due to inherent soil properties but may have been the results of a combination of low urine N application rate (127-372 kg ha^-1), high plant uptake and urinary N loss through NH₃ volatilisation. Sheep urine had no specific effect on nitrifier/denitrifier microbial community and these microbial populations size could not explain the difference in N₂O emissions between the two soils. Assessment of expression genes involved in nitrification/denitrification would be a better indicator of microbial activity. Our results represent a positive message for Irish sheep sector in Ireland given the low N₂O emissions observed, but the assessment of urine N fate in other type of grasslands is required.