Reactive nitrogen (N) as a result of anthropogenic activities (e.g. fossil fuel combustion, fertiliser manufacturing and intensive agricultural practices), now dominates the global nitrogen cycle. For sensitive ecosystems across the globe, perturbation of the nitrogen cycle as a result of increased inputs of reactive N, has been seen to have a profoundly detrimental impact on biodiversity and ecosystem structure and function. Concerns surrounding the conservation and protection of both natural and semi-natural habitats from the damaging effects of N deposition, have prompted widescale research, that aims to understand and quantify ecosystem responses to elevated N inputs. Much of this research is in the experimental domain, based upon the simulation of N deposition within controlled conditions. The research outlined in this thesis, is principally focused on the findings of a long-term experimental approach on a lowland heathland system in the south east of England. N additions of 30 kg N, ha−1, yr−1 were seen to cause large, persistent effects on Calluna growth, phenology and chemistry, retard the development of lower plant groups and alter soil biogeochemical processes. Interactions between N inputs and climatic stress were also observed, with N additions seen to significantly modify Calluna's response to drought conditions. Interactions between N deposition and climate were also evident at a larger scale, following a nationwide survey of UK heathlands across broad pollution, climatic and geographic gradients. Representing one of the first surveys to test both above and below ground responses to ambient N deposition across UK heathlands, the findings provide compelling evidence that many N driven changes observed within an experimental context, are also occurring at a nationwide scale. Such findings facilitate the identification of robust bioindicators of N deposition that could be successfully integrated into heathland conservation measures. Given that current emissions and deposition rates of N are predicted to double by 2050, and in regard to the future climate changes that are expected, research of the kind presented within this field of research is of the upmost importance if heathlands, and other sensitive ecosystems, are to be preserved.