This article relates to the published paper Fine-scale temporal characterization of trends in soil water dissolved organic carbon and potential drivers.

This paper, published in a Special Issue of the journal Ecological Indicators to mark 20 years of data collection at ECN terrestrial sites, presents an analysis of the temporal relationships between soil solution chemistry and parameters thought to regulate Dissolved Organic Carbon (DOC) production. Data from both ECN and Forest Level II plots were used.


Reference

Sawicka, K., Monteith, DT. and et al. (2016). Fine-scale temporal characterization of trends in soil water dissolved organic carbon and potential drivers. Ecological Indicators68, 36-51. DOI: 10.1016/j.ecolind.2015.12.028


Abstract

Long-term monitoring of surface water quality has shown increasing concentrations of dissolved organic carbon (DOC) across a large part of the Northern Hemisphere. Several drivers have been implicated including climate change, land management change, nitrogen and sulphur deposition and CO2 enrichment. Analysis of stream water data, supported by evidence from laboratory studies, indicates that an effect of declining sulphur deposition on catchment soil chemistry is likely to be the primary mechanism, but there are relatively few long term soil water chemistry records in the UK with which to investigate this, and other, hypotheses directly. In this paper, we assess temporal relationships between soil solution chemistry and parameters that have been argued to regulate DOC production and, using a unique set of co-located measurements of weather and bulk deposition and soil solution chemistry provided by the UK Environmental Change Network and the Intensive Forest Monitoring Level II Network. We used statistical non-linear trend analysis to investigate these relationships at 5 forested and 4 non-forested sites from 1993 to 2011. Most trends in soil solution DOC concentration were found to be non-linear. Significant increases in DOC occurred mostly prior to 2005. The magnitude and sign of the trends was associated qualitatively with changes in acid deposition, the presence/absence of a forest canopy, soil depth and soil properties. The strongest increases in DOC were seen in acidic forest soils and were most clearly linked to declining anthropogenic acid deposition, while DOC trends at some sites with westerly locations appeared to have been influenced by shorter-term hydrological variation. The results indicate that widespread DOC increases in surface waters observed elsewhere, are most likely dominated by enhanced mobilization of DOC in surficial organic horizons, rather than changes in the soil water chemistry of deeper horizons. While trends in DOC concentrations in surface horizons have flattened out in recent years, further increases may be expected as soil chemistry continues to adjust to declining inputs of acidity.


Why this research matters

This paper is relevant to the following issues:

  • Acidification ('acid rain') and recovery from acidification
  • Water quality and treatment
  • Climate change and carbon stocks

In brief

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In more detail

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.