To assess the effects of increased atmospheric N input on N availability in ombrotrophic peatlands, the relative concentrations of dissolved organic nitrogen (DON) to dissolved inorganic nitrogen (DIN) were measured in bog waters along a natural gradient of atmospheric N deposition. Six European bogs were selected, spanning a range of chronic atmospheric N inputs from 0.2 to 2.0 g m-2 yr-1. DIN as well as DON concentrations increased with N deposition, the latter increasing at a sharper incline. The increase in DIN concentrations was related to the reduced capacity of the moss layer to trap atmospheric N, which in turn was a result of N saturation of the moss layer. The enhanced DON concentrations appear to be a consequence of increased leaching of organic N compounds by Sphagnum. The importance of DON on N biogeochemistry in bogs opens new perspectives in relation to nutrient limitation and organic matter turnover.