Acidity controls on organic matter cycling and nitrogen saturation in organic soils.

Summary Peats and other organic soils provide a major global store of carbon (C), and growing peats provide a continuing sink for CO2 from the atmosphere, fixed through photosynthesis and stored as slowly-decomposing organic matter. The plants that grow in peaty ecosystems are characterised by a tolerance for low levels of the essential nutrient, nitrogen, but in many areas of world, including Europe and North America, intensive agriculture and fossil fuel burning have greatly increased the emissions of pollutant nitrogen (N) to the atmosphere, and subsequent deposition to these ecosystems. This increased nitrogen supply has been shown to lead to the displacement of species adapted for low-nitrogen conditions by other species better able to exploit increased nutrient availability, degrading biodiversity and potentially threatening the function of peatlands as a carbon sink. Over a prolonged period, 'nitrogen saturation' (nitrogen supply exceeding biological demand) can also occur, with excess nitrate leached to surface waters, where it can contribute to acidification and eutrophication. However, while organic soil ecosystems are currently accumulating nitrogen from the atmosphere, many are also undergoing dramatic chemical changes due to decreasing sulphur deposition (due to legislation to control sulphur emissions), and resulting recovery from acidification. Virtually our entire understanding of nitrogen cycling under elevated N deposition, and a significant part of our understanding of C cycling, is based on work within ecosystems that have also been impacted by acidifying sulphur deposition. We propose that both the C and N cycles of these systems are being strongly altered by acidity change, and therefore that currently observed behaviour may change in future. Specifically, we believe that rising pH will increase the loss of carbon and nitrogen as dissolved organic matter (DOM) from the system due to increases in biological production, and an increasing solubility of this DOC. This loss of N, coupled with increased demand for N by a growing microbial biomass, and increased plant productivity, will result in a tightening of the N cycle and less leakage of NO3. This tightening of the N cycle is likely reduce nitrate loss in soils subjected to high N deposition, and may even halt nitrate loss entirely from areas of lower N deposition. We will test our hypotheses with a set of realistic, field-based experiments in UK peaty ecosystems that have been exposed to relatively low and high historic levels of N pollution. In each experiment, we will manipulate soil pH over a two year period, during which time we will measure the effect of changing acidity on losses of C and N in gaseous and dissolved forms. We will also measure the effects of acidity change on a range of key ecosystem processes, such as the transformation of N between organic and inorganic forms, decomposition, and the assimilation of atmosperic CO2 into different organic matter stores, using a combination of cutting edge isotopic tracing techniques and enzyme measurements. We will use the results to support the development of a model of C and N cycling which, unlike most existing models, fully incorporates the effects of changing acidity, and this model will be used to predict future change in ecosystem carbon and nitrogen balances. We believe that this study could significantly refine our current understanding of the capacity of peaty ecosystems to store carbon and nitrogen from the atmosphere. If proven, our hypotheses would imply that ecosystems will become less susceptible to nitrogen saturation (and resulting environmental damage) as they become less acidic. At the same time, increased growth rates with rising pH (together with elevated nitrogen) could provide an enhanced sink for atmospheric CO2 sequestration within these important ecosystems.