The form of reactive nitrogen deposition affects the capacity of peatland vegetation to immobilise nitrogen: implications for the provision of ecosystem services

Peatlands represent significant carbon (C) reserves accumulated over millennia, as a consequence of slow decomposition rates conditioned by the acidity and anoxia that define these ecosystems. Such conditions are maintained largely through climate but also the activities of peatland ‘engineers’, vegetation such as Sphagnum mosses. Peatlands are hugely valued for C sequestration and the distinct communities they support. However, increased nitrogen (N) availability, from anthropogenic deposition, has been linked to detrimental changes in the vitality of Sphagnum and species active in perpetuating peatland processes. The effects of manipulating the form and dose of N to an ombrotrophic peatland, Whim bog in the Scottish Borders, UK, have been studied since 2002. Ammonia is provided by free air release, in response to wind direction and wind speed, and wet deposition, comprising nitrate or ammonium, in response to rainfall. Manipulation has increased the background deposition of 8 kg N ha−1 year−1 by 2, 4 and 8 times. Responses to the different N forms in terms of species cover, importance of component species in maintaining low nutrient availability through N immobilisation and the implications of breakdown in vegetative cover and species replacement for peatland function are discussed in relation to N fluxes. All forms of N were not equally detrimental: ammonia deposition significantly reduced the vegetative cover, removing the sink for N, leading to increased nitrate in soil pore water and nitrous oxide emission whereas effects of wet N deposition, though still detrimental, were more modest. Nitrogen driven reductions in the cover of the keystone Sphagnum species and other characteristic mosses and their ability to immobilise incoming N can affect soil chemistry and lead to changes that could compromise C sequestration.