Reactive nitrogen (N) deposition has many negative impacts on terrestrial ecosystems including loss of biodiversity, increase in graminoid cover, soil acidification and an increase in nitrate leaching. However, despite these negative effects there is evidence that N deposition may positively contribute through its action as a fertiliser and the mitigation of increasing atmospheric CO2 by increased growth and the sequestration carbon (C) into plants. N has been shown to increase long-term C storage in forest ecosystems and modelling has predicted that similar increases in C storage in ericaceous ecosystems will also occur, though no experiment has directly measured changes in C pools in these ecosystems. The work in this study aimed to address this gap in knowledge by quantifying the effect of N deposition on ecosystem C pools, the pathways of C into and out of ecosystems and the processes that mediate soil organic matter turnover. Three different ericaceous~dominated ecosystems were used in this study, each with contrasting hydrology, climate, soil type and vegetation. The interactive effects between N and the differences between these key environmental characteristics were also considered. The study locations were a lowland heath i~ north west England, an upland heath in north Wales and an ombrotrophic bog in south east Scotland; each was the site of a long-term nitrogen (N) addition experiment where applications of N had been made for between 5 and 11 years. Nitrogen addition led to increased growth of Gal/una vulgaris in the heath lands and this led to increased litter fall, a greater amount of carbon entering the ecosystems and the formation of larger carbon (C) and N pools, particularly at the upland heath. At the I lowland heath, the response of biomass and the C pool to N addition was slower due to the poor recovery of Gal/una in the N amended plots following previous management. Little vegetation change on the bog was observed due to the prominent water table exerting greater control over ecosystem processes. Observed treatment related differences between gaseous C fluxes were few with changes in climate producing the greatest responses. Soil respiration, mineralisation rate, decomposition, plant photosynthesis and plant respiration increased with temperature at all sites, with water table draw down additionally stimulating microbial activity in the bog. Fluvial dissolved organic carbon (DOC) losses were unaffected by N iv in the heath lands but were increased by oxidised (NaN03-) and gaseous (NH3-) additions in the bog following treatment induced increases in pH. Nitrogen losses in leachate increased below threshold soil C:N values which were highest in the bog. This led to the conclusion that, in addition to soil C:N and the size of C pool, the type of organic matter was found to determine N leaching at a site through carbon limitation of the microbial community. Carbon available as DOC did not appear available to microbes, potentially due to its own recalcitrance. Sites with a deeper, microbially active Gal/una litter layer, such as the upland heath, leached less nitrogen. This was reflected in large observed increases in extractable NH/-N and total N pool. This study has shown that N deposition can increase C sequestration in heath lands, however, long-term accumulation of C will be influenced by the timing of any management interaction, which is able to quickly reset the growth cycle and keep the heathland at its most productive stage. Responses to N at the bog, which has accumulated the most C to date, were constrained by hydrology. Over the period studied, modelling predicted that the upland heath sequestered the most C, followed by the bog. The warmer lowland heath was predicted to be a net C source.