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Nitric and nitrous oxide are products of the microbial processes of nitrification and denitrification in soils; they are both important gases in that they contribute to stratospheric ozone depletion and climate change. The effects of land management as fertiliser and lime addition on the function and microorganisms contributing to nitrification and denitrification is poorly understood. It is therefore important to understand the factors affecting both the microbial processes themselves and in turn how this influences their activity in the field. Soil microcosm experiments were used to quantify the effects of changing soil moisture and temperature as an environmental factor on the dominant production of nitric and nitrous oxide in soil sampled from an un-improved upland grassland field in the Scottish borders (Sourhope). Microcosms were amended with NH₄NO₃ at the field application rate of 126 kg N ha⁻¹ yr⁻¹, nitric oxide flux increased significantly at soil moisture contents below 60% WFPS (Water Filled Pore Space), the maximum flux was 0.134 µg NO -N g⁻¹ h⁻¹ at 40% WFPS. Nitrous oxide flux however, only increased at 80% WFPS with a flux of 9.942 x 10⁻³ μg N₂0 -N g⁻¹ h⁻¹ and decreased over a period of 27 days to 2.09 x 10⁻⁶ µg N₂0 -N g⁻¹ h⁻¹, similar to that of the other soil WFPS microcosms. The effect of changing soil temperature on nitric and nitrous oxide emissions at 40 and 80% WFPS and 1,5,10,15,20 and 25 °C were quantified. At low soil moisture contents the maximum flux of nitric and nitrous oxide, 0.17 µg NO-N g soil⁻¹ h⁻¹ and 4.9 x 10 ⁻⁵ µg N₂O -N g soil⁻¹ h⁻¹ respectively, was at 15 °C. The flux of both decreased with increasing temperature at 40% WFPS. A collaborative long term incubation study using soil microcosms with soil sampled from the control treatments at Sourhope was set up to quantify the relationship between function and diversity. This study showed that land management regimes in upland agricultural grassland soils as inorganic N fertiliser addition and lime, may result in increased N loss as nitric and nitrous oxide via both nitrification and denitrification at 60% WFPS. Increases in both nitric and nitrous oxide emissions could not be correlated with increases in gross nitrification rates due to the large variation within treatments, however the proportion of nitrous oxide produced via nitrification was greater than that produced via denitrification for all treatments. There were shifts in the number of bands for all three functional groups measured using DGGE 16sRNA, although changes in the eubacterial DGGE profiles were greater indicating that both the AMO and Nitrobacter sp. ammonium and nitrite oxidisers were relatively stable. Following ¹⁵N analyses of the nitrous oxide and N₂ produced was predominantly via nitrification with no ¹⁵N labelled N₂ produced indicating that nitrification was the predominant source of nitrous oxide. Inhibition work in re- packed soil microcosms with soil from the Sourhope field treatments was carried out using DCD (an autotrophic inhibitor) and cyclohexamide (a heterotrophic inhibitor). The results from these studies showed a reduction in the nitrous oxide emission in lime amended soils relative to control soils without inhibitor, by 37% in the lime only and 80% in the nitrogen and lime treatments. Since the ¹⁵N labelling of the nitrous oxide produced indicated that more than 99% was due to nitrification it would seem that decreased emissions as a result of liming are due to changes in the nitrifier rather than denitrifier populations. Field flux measurements were made throughout 2000/2001 with a period of reduced activity due to the foot and mouth epidemic in the UK during 2001. Field measurements have shown that nitric and nitrous emissions and soil mineral N concentrations in the four week period following the addition of ammonium nitrate were significantly greater than those in control plots. Emission of nitric and nitrous oxide were further increased to 0.26 µg NO-N m⁻² h⁻¹ and 0.4 µg N₂O-N m⁻² h⁻¹ respectively by addition of nitrogen combined with lime. Control soils and soils treated with lime only acted as a net sink for nitrous oxide. Following stepwise multiple regression analyses rainfall, air temperature and soil ammonium concentrations were important factors for modelling the emissions of nitrous oxide whereas soil WFPS and soil nitrate concentrations were important for nitric oxide emissions, these findings are in agreement with regression equations in the DNDC model.