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Modelling impacts of atmospheric deposition and temperature on long-term DOC trends

ECN and the Forest Level II monitoring network are the UK’s only source of information on long-term change in soil water chemistry. In this study, soil solution chemistry data from three upland ECN sites and three UK ICP Forest Level II sites were modelled using the dynamic soil chemistry model, MADOC, which simulates long-term changes in carbon and nitrogen cycling and soil acidity. Results suggest that future dissolved organic carbon (DOC) concentrations may exceed preindustrial levels due to nitrogen pollution, which has implications for drinking water catchment management.
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K. Sawicka, E.C. Rowe, C.D. Evans, D.T. Monteith, E.I. Vanguelova, A.J. Wade, J.M. Clark, Modelling impacts of atmospheric deposition and temperature on long-term DOC trends, In Science of The Total Environment, Volume 578, 2017, Pages 323-336, ISSN 0048-9697

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Substantial increases in Dissolved Organic Matter (DOM) in surface waters in recent decades have burdened the drinking water industry and consumers with rising water treatment costs, as well as leading to concerns over possible destabilisation of the soils from which most of this DOM originates. It has been demonstrated that these changes correlate with reductions in acid rain, suggesting that soil organic matter is becoming more soluble as soils begin to recover from the long-term impacts of acid rain. One implication of this hypothesis is that the recent changes in DOM reflect soils and soil water chemistry returning to its pre-acidification, i.e. pre-industrial, condition, and that as sulphur deposition is beginning to level out, DOM concentrations will do likewise. However it has also been proposed that trends are at least partly influenced by 1) increased plant production in nutrient poor upland environments, as a consequence of the enrichment of soils by man-made nitrogen deposited from the atmosphere, and 2) increased soil microbial activity in response to gradually rising soil temperatures – linked to global warming. Process based models, such as the MADOC model, have been developed in an attempt to simulate these effects, and hence gain a better understanding of the likely future changes in DOM in response to all of these drivers. 

ECN and Forest Level II monitoring networks provide the UK’s only source of information on long-term change in soil water chemistry, including changes in concentrations of Dissolved Organic Carbon – a measure of DOM. Of equal importance, collection of other measurements including weather and atmospheric deposition allows direct links to be made between drivers and biogeochemical responses – a fundamental attribute of ECN. In this paper, the authors ran the MADOC model to predict the effects of the various potential drivers. They concluded that while recovery from acidification appeared to be the predominant control on the observed soil water DOC trends, the model suggested that over the long term, the effects of nitrogen deposition on nitrogen limited soils may have been sufficient to raise the “acid recovery DOC baseline” significantly, so that in some catchments DOM concentrations may begin to surpass levels ever experienced in the past. These observations are now being used to help the water industry to plan for the anticipated further increases in DOM.