Influence of recovery from acidification on the dynamics of dissolved organic carbon (DOC) in organic soils

Lead Research Organisation: NERC CEH (Up to 30.11.2019)


Peaty soils, which dominate the uplands of the UK, represent a significant store of carbon, containing around a third of the global terrestrial carbon, which is equivalent to the total amount of carbon stored in the atmosphere. These soils slowly release carbon into rivers and lakes. It is this dissolved organic carbon that gives upland waters their brown colour. Over the last 15 years, the colour of these waters has become darker as the amount of dissolved organic carbon draining from peatlands has increased, by on average 91%. This increase in the loss of carbon from peatlands soils has promoted fears that we have disturbed the stability of the global carbon cycle. The increase in water colour is also a problem for water companies as the dissolved organic carbon can react with the chlorine in water-treatment processes to produce potentially carcinogenic chemicals. Some scientists have suggested that increasing temperatures or decreasing rainfall due to global warming is resulting in more carbon being released from peatlands, while others have suggested that increasing atmospheric carbon dioxide is itself the cause. If these climate change mechanisms are responsible for the increasing trend in dissolved organic carbon, it suggests that peatlands are becoming unstable, with major implications for the global carbon cycle (with feedback effects on climate), water quality and water treatment. We argue, however, that recent rises in temperature and carbon dioxide have been insufficient to cause the observed changes in dissolved organic carbon. We believe that the decline in acid rain over the past 20 years has been a more important influence on dissolved organic carbon concentrations, as the solubility of organic carbon is controlled by pH: the more acid the water, the less is dissolved. If correct, this has very different consequences for the global carbon cycle and long-term water quality, as the systems are simply returning to their pre-industrial conditions. Therefore, increased dissolved organic carbon in freshwaters may represent an environmental recovery to more 'natural' conditions, and not an environmental degradation in response to climate change. The overall aim of this research project is therefore, to determine whether the decline in acid rain can account for the majority of the observed increase in DOC concentration in freshwaters across the UK since 1988. The research will be conducted in two ways. We will use laboratory experiments to measure the changes in soils subjected to different levels of pollution. We will also study evidence for these changes in soils from different locations across the UK. The research will take three years to complete and involves collaboration between scientists at the University of Leeds, Centre for Ecology and Hydrology and University College London. Finally we will integrate the experimental results with analysis of long-term monitoring data for upland catchments dominated by organic soils.


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Description Waters draining upland areas in the UK are usually acidic, coloured and low in nutrients and other solutes. However, over the last 20-30 years, the colour of surface waters derived
from the British uplands has been increasing associated with the increase in dissolved organic carbon (DOC) concentrations.
Given that DOC, and thus water colour, are controlled by a number of interacting factors, a wide range of potential driving mechanisms for the observed increase in DOC and colour have been proposed, many of which are linked to climate change. The results from this research has shown that a large part of the observed increases is due to recovery from acidification, both in the UK (Evans et al., 2006b) and internationally (Monteith et al., 2007). In the UK DOC concentrations have increased in proportion to reductions in the deposition of both sulphate and Chloride over the last two decades, and the response is sharper for waters with lower base cation concentrations (Clark et al., 2011).
Exploitation Route Widespread observations of increased DOC concentrations in surface waters across many parts of Europe and North America over the last two decades is of great interest to a range of
different stakeholders because it has raised concerns
about the provision of safe drinking water and stability of soil carbon stores. Hence, water companies, land managers, water regulatory agencies and policy formulating government departments are increasingly seeking guidance and advice from the scientific community on the role of different driving
mechanisms responsibly for this increase in stream water colour/DOC concentration, and whether there are any catchment management solutions to control or even reverse these trends. It is imperative, therefore, that we improve our understanding of the relative role of different driving mechanisms, and the scale at which they operate.

Water colour is a major problem for the water industry, particularly where the uplands represent the single most important source of potable water in the region. Deterioration in water colour results in breaches of European Union drinking water standards and an increase in water treatment costs. It also has health implications as the chlorination of highly
coloured water can result in the production of carcinogenic disinfection-by-products such as trihalomethanes (THMs).
Sectors Environment

Description Been cited in the REVIEW OF TRANSBOUNDARY AIR POLLUTION (RoTAP): Acidification, Eutrophication, Ground Level Ozone and Heavy Metals in the UK. This report was prepared for the Department for Environment, Food and Rural Affairs; Scottish Government; Welsh Assembly Government; and Department of the Environment in Northern Ireland as part of the Defra-funded contract, Review of Transboundary Air Pollution (RoTAP
First Year Of Impact 2012
Sector Environment
Impact Types Societal,Economic,Policy & public services

Geographic Reach National 
Policy Influence Type Citation in other policy documents