| Summary: | Land use intensity and reactive nitrogen (N) enrichment from atmospheric deposition are recognised as major threats to upland ecosystem function. Despite recent reductions in emissions of nitrogen oxides and ammonia from combustion and agricultural activities, N deposition rates on upland habitats from pollution remain above accepted critical loads. Increases in N availability can result in N saturation and lead to a reduced capacity for N retention which may be exacerbated by a shift in nutrient limitation status, from N limitation to phosphorus (P) limitation, which can constrain vegetation biomass production. Phosphorus limitation however is potentially an important mechanism constraining biodiversity loss associated with increasing nutrient availability and may moderate soil microbial and decomposer activity, which positively influences soil carbon (C) storage potential. This thesis aimed to elucidate the effect of current drivers of environmental change on upland ecosystem processes utilising a landscape scale survey and experimental manipulation of nutrient availability. The results presented here include investigations into the interactive effects of livestock stocking rate and N deposition (Chapter 2), and the role of N and P availability in influencing upland ecosystem components and processes, including vegetation species composition, soil chemistry (Chapter 3), soil invertebrates, vegetation productivity and tissue chemistry (Chapter 4), and soil C flux (Chapter 5). It is hypothesised that increasing livestock grazing intensity and nutrient availability from N deposition, alter upland ecosystem processes resulting in C storage reductions in upland soil. The results demonstrate a combined pressure from livestock grazing and N deposition which act in concert to modify upland vegetation and soil invertebrates and chemistry. Significant influence of N and P availability on ecosystem processes in upland habitats was also shown. Vegetation species composition was modified by the addition of P, with significant increases in cryptogam coverage. Microbial activity was enlarged, which was evident as higher soil C flux. These effects resulted in soil C content loss through changes in vegetation species composition, resulting in reductions in the direct input of organic matter from reduced root structures, and faster C cycling from increased decomposer activity. It is concluded that the availability of P in upland habitats has strong controlling effects on ecosystem processes, which can reduce the ability of upland soil to retain C, and which continue to act in the long term due to the persistent nature of P in soil. Thus, the addition of P as a management tool to alleviate the effects of chronic N enrichment is not recommended. As a consequence, there is need for alternative approaches to upland land management which reduce the pressure from grazing livestock on sensitive upland habitats, particularly for habitats in exceedance of N deposition critical loads.
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