Wet N deposition comprises oxidised (nitrate) and reduced (ammonium) N forms in proportions that vary spatially with source and topography. Field evidence of long-term N form effects on semi-natural ecosystems and how these are modified by phosphorus and potassium availability are lacking. This study describes cover changes for some key peatland species and litter chemistry from Sphagnum capillifolium, Calluna and Eriophorum vaginatum, and peat in response to 9 years of N treatment. Ammonium and nitrate as NH4Cl or NaNO3 were provided to replicate plots in rainwater spray at +8 (low) or +56 (high) kg N ha(-1) year, with and without PK via an automated system coupled to site meteorological conditions. Reduced N caused greater N accumulation in all key species than oxidised N, especially at higher doses, but cover declined more, though not significantly so, with oxidised than reduced N at the high N dose. Overall the detrimental effects of high N on Sphagnum and Calluna cover were significant but small. By comparison PK inclusion with 56 kg N ha(-1) year as oxidised N, not reduced N, had devastating effects on cover, causing both S. capillifolium and Calluna to decrease 3-5-fold, facilitating invasion and expansion of nitrophiles, non-characteristic bog plants e.g. Epilobium angustifolium, Epilobium palustre, Juncus effusus, Digitalis purpurea and Dryopteris dilatata. N form appears to be significant for peatlands because of its effects on pH. The significance of changes in plant cover for peat chemistry and decomposition for biogeochemistry is discussed.