Abstract
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We report results from a long-term experiment in which additional nitrogen has been deposited on a peat bog in central Scotland for over fourteen years, in three different forms: as ammonia (NH\({}_{3}\)) gas, as ammonium (NH\({}_{4}^{+}\)) solution or nitrate (NO\({}_{3}^{-}\)) solution. The automated experiment was designed to apply nitrogen in such a way that the nitrogen concentration in solution on the leaf surface mimics that in real-world nitrogen deposition.
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Observations of cover for \(>\)60 species were made, comprising more than 2.2 million records at the sub-quadrat level. We analysed the change in six common species in relation to nitrogen dose and form.
The responses differed among species and nitrogen forms, but all six species declined, and NH\({}_{3}\) produced the biggest change in cover per unit of nitrogen addition. The exception was the graminoid sedge Eriophorum vaginatum , which increased dramatically in the NH\({}_{3}\) treatment.
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Multivariate analyses identified coherent community-level responses to nitrogen dose, using the principal response curves and partial least squares regression methods. Multivariate approaches combine information from all species present, and so can detect more subtle changes.
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We surmised that the larger experimental response to nitrogen observed in the NH\({}_{3}\) treatment (cf. the NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) treatments) was because of the higher nitrogen concentrations at the vegetation surface produced by dry deposition. NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) were sprayed in solution, but much of this will enter the soil water, and be further diluted. NH\({}_{3}\), however, is deposited as a gas, directly through the stomata to the leaf apoplast, and via sorption through the leaf surface (particularly in mosses), having dissolved in water on the leaf surface. Because NH\({}_{3}\) deposits directly to the leaf, it stays contained within the small volume of water on and in the leaf, producing a high internal concentration of nitrogen ions.
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The magnitude of the effects in the NH\({}_{4}^{+}\) and NO\({}_{3}^{-}\) treatments was also smaller compared with some other long-term experiments using NH\({}_{4}\)NO\({}_{3}\), probably because the application in many small doses avoids high nitrogen concentrations.
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Synthesis . Although NH\({}_{3}\) has a greater impact per g of nitrogen deposited, considering the actual deposition rates of the different forms on UK peat bogs, we estimate that NH\({}_{4}^{+}\) deposition has the largest impact on Sphagnum capillifolium followed by NO\({}_{3}^{-}\) and NH\({}_{3}\).
Keywords: Global change ecology, plant community, air pollution, nitrogen deposition, peatlands, Sphagnum, ammonia, nitrate, multivariate analysis