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Fine-scale temporal characterization of trends in soil water dissolved organic carbon and potential drivers

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This paper is relevant to the following issues:

    • Acidification ('acid rain') and recovery from acidification

    • Water quality and treatment

    • Climate change and carbon stocks

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Pollution and subsequent climate change have led to unprecedented environmental changes across the globe, including decreases in air, soil and water quality. Among many indicators, levels of dissolved organic carbon (DOC) concentrations (primarily derived from soil organic matter) in soils and surface waters inform us about water and soil quality.

In recent decades, concentrations of DOC have tended to increase in soils and surface waters in many northern hemisphere regions. This has raised concerns for public health and the fate of our soil carbon stocks, since DOC plays several important roles in the environment:

  • it is a significant conveyor of pollutants from soil to surface waters, it is a nutrient in aquatic food webs and it affects the aquatic light environment and energy balance.
  • it is sometimes regarded as a water contaminant, causing the brown colour of some water. When water containing DOC is treated with chlorine in conventional water treatment processes, potentially harmful 'disinfection bi-products' can result. DOC concentrations, therefore, must be reduced to safe levels by expensive treatment processes before chlorination.
  • Last, but not least, soils represent one of the largest global carbon stores. Globally, soils contain about three times the amount of carbon in vegetation and twice that in the atmosphere. Carbon released from soils in the form of DOC may eventually re-enter the atmosphere as carbon dioxide or methane, both greenhouse gases. Hence, safeguarding natural carbon stores is an important strategy in our response to climate change.

It is therefore important to understand what controls long-term changes in DOC concentrations and if these concentrations may be subject to further change in the future. Climate changeland use change, and nitrogen deposition have all been offered as explanations for rising DOC concentrations. An alternative hypothesis, rapidly gaining acceptance, is that the DOC trends are linked to recovery from human-caused acidification ('acid rain' from the burning of fossil fuels).

Long-term monitoring of soil water chemistry provided by the Environmental Change Network and the UK's Forest Monitoring Level II programme provide unique opportunities to explore linkages between different DOC drivers and ecosystem properties and management.

In this study, scientists collated up to 18 years of data on soil water DOC concentrations, and corresponding soil water chemistry, atmospheric deposition and weather indicators from nine different ecosystems with varying soil types, vegetation types and land management. They applied state-of-the-art statistical techniques to identify periods of significant changes in DOC and corresponding candidate drivers.

The study confirmed that grassland and forest soils in the UK are in the process of recovering from acidification. Large reductions in sulphur deposition correlated with soils becoming less acidic and DOC concentrations increasing in surface organic layersAlthough long-term DOC trend patterns varied between sites, the strongest increases in DOC were seen in acidic forest soils and were most clearly linked to declining acid deposition.

The analysis also suggested that increases of DOC in upper soil horizons due to declining acid deposition are the most likely sources of increased DOC in stream waters. This has important implications for how recent trends are perceived and the extent to which they might be managed or mitigated.



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