Enhanced reactive nitrogen deposition may compromise the sustainability and functioning of bogs, with respect to carbon sequestration and greenhouse gas production. Since 2002, three N forms have been applied to an ombrotrophic bog growing Calluna, Sphagnum capillifolium and Eriophorum vaginatum in order to test this. Significant changes in species cover and soil chemistry, especially in response to elevated ammonia concentrations, have been recorded. Ammonia deposition has also increased nitrous oxide emissions, while the same N dose as ammonium or nitrate had a far smaller effect. Methane emissions were increased by nitrate additions, although fluxes were equally responsive to water table and temperature. The significance of N form, and ’natural’ drivers, is discussed.