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Statistical and dynamic modelling of long-term trends of dissolved organic carbon in soil water

Katarzyna Sawicka1, Don Monteith2, Ed Rowe3, Elena Vanguelova4, Andrew Wade1, Joanna Clark1
1 Department of Geography and Environmental Science, University Of Reading
2 Centre for Ecology and Hydrology, Lancaster
3 Centre for Ecology and Hydrology, Bangor
4 Forest Research

O 1.9 in Long term trends in the functioning of ecosystems

15.07.2014, 10:00-10:20, H18

Long-term monitoring of surface water quality has shown increasing concentrations of Dissolved Organic Carbon (DOC) across large part of the Northern Hemisphere. Several drivers have been proposed to influence these trends, including: climate change, changes in land management and use, nutrient deposition and CO2 enrichment. Analyses of stream water data, supported by evidence from laboratory studies, have shown that declining sulphur deposition could be responsible for DOC increases, but the hypothesis is yet to be confirmed. Plausible causes of observed increases in DOC concentrations in streams and lakes all have their origin in soils and are soil process dependent. Here, using statistical non-linear trends analysis we investigated relationships between long-term trends of DOC in soil solution and a range of potential climatic (i.e. temperature and effective rainfall), chemical (acid deposition loading and concentrations of anions and cations in soil solution) and land characteristics (vegetation type and soil type) drivers using DOC data from 15 sites across the UK, from 1992 to 2010. Most trends in DOC concentration were found to be non-linear and were not monotonic. DOC in soil water increased mostly before 2000, when the greatest declines in anthropogenic sulphate emissions occurred but did not continue to increase after 2005, while simulations using the dynamic model MADOC (Model of Acidity Dynamics and Organic Carbon) indicated that current DOC concentrations are returning to preindustrial levels. Analysis of DOC sensitivity to acid deposition in MADOC showed that DOC trends are dependent on the acid anion loading and the soil type. For example, acid deposition appeared to dominate DOC trends in podzols receiving annually large acid anion loading, but had little influence in gleysoils regardless of the level of acid anion loading. Our analysis provides some insight into reconciling the differences in observed DOC trends between specific sites, and supports the argument that concerns over mounting terrestrial carbon losses could be overly pessimistic and that current trends mostly reflect a return in soil biogeochemistry to a more natural state.



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