In UK upland ecosystems nutrient limitation and a cool, wet climate restrict plant growth and decomposition, resulting in high soil carbon (C) storage. The aim of this thesis was to study the effect of atmospheric pollution, particularly nitrogen (N) deposition, on ecosystem C storage, and the role of phosphorus (P) availability in this relationship. A survey of 513 Scottish grasslands demonstrated impacts of SOx and N deposition, temperature and grazing pressure on soil chemical properties. In Nardus and wet grasslands, there was a negative relationship between N deposition and soil C stock at low soil P, and a positive relationship between N deposition and soil C stock at high soil P. In a two-year field experiment, small rates of P (0-7.5 kg P ha-1) were added to an upland dry heath, acid grassland and wet heath receiving elevated background N deposition. Net C assimilation was stimulated by P addition in wet and dry heathlands, but not in the grassland; there were no effects on C losses. Long term effects of P addition were studied in a 23-year-old factorial N and P fertiliser experiment on upland heath. Moderate additions of N and P to upland heath did not change soil C stocks but did influence vegetation composition, which reduced vegetation C stocks. Vegetation composition recovered from application of P, but not from P plus N. In the short term experiment, added P was likely to be retained in vegetation, and in the long term P was strongly retained in soils. These results show the importance of considering P when assessing and predicting N deposition effects on C dynamics, and suggest that adding small amounts of P to upland heathlands experiencing elevated N deposition has the potential to stimulate C assimilation in the short term, without any negative side-effects.