Decreasing DOC trends in soil solution along the hillslopes at two IM sites in southern Sweden — Geochemical modeling of organic matter solubility during acidification recovery

Abstract

This is an author produced version of a paper published in Science of The Total Environment. This paper has been peer-reviewed and is proofcorrected, but does not include the journal pagination.Citation for the published paper: Löfgren, S., Gustafsson, J. P., Bringmark, L. (2010) Recently, a hypothesis was presented on how various spatial and temporal factors affect 4 the DOC dynamics. It was concluded that declining sulphur deposition and thereby 5 increased DOC solubility, is the most important driver for the long-term DOC 6 concentration trends in surface waters. If this recovery hypothesis is correct, the DOC 7 levels should increase both in the soil solution as well as in the surrounding surface 8 waters as soil pH rises and the ionic strength decline due to the reduced input of SO 4 2-9 ions. In this project a geochemical model was set up to calculate the net humic charge and 10 DOC solubility trends in soils during the period 1996-2007 at two integrated monitoring 11 sites in southern Sweden, showing clear signs of acidification recovery. The Stockholm 12Humic Model was used to investigate whether the observed DOC solubility is related to 13 the humic charge and to examine how pH and ionic strength influence it. Soil water data 14 from recharge and discharge areas, covering both podzols and riparian soils, were used. 15The model exercise showed that the increased net charge following the pH increase was 16 in many cases counteracted by a decreased ionic strength, which acted to decrease the net 17 charge and hence the DOC solubility. Thus, the recovery from acidification does not 18 necessarily have to generate increasing DOC trends in soil solution. Depending on 19 changes in pH, ionic strength and soil Al pools, the trends might be positive, negative or 20 indifferent. Due to the high hydraulic connectivity with the streams, the explanations to 21 the DOC-trends in surface waters should be searched for in discharge areas and 22 peatlands. 23 24 25

Recovery of Soil Water, Groundwater, and Streamwater From Acidification at the Swedish Integrated Monitoring Catchments

“…DOC concentrations in soil water in southern Sweden and Norway have also been shown to be either decreasing or remaining fairly constant, especially in soil water from or below the B-horizon Wu et al 2010). Based on data from Aneboda and Kindla, it has been shown that soil water DOC concentrations are strongly influenced by pH, IS, and Al dynamics (Löfgren et al 2010b). …”

“…However, based on soil water data from southern Sweden, the postulated causal link between recovery from anthropogenic acidification and increases in surface water DOC has been challenged (Löfgren et al 2010b;Löfgren and Zetterberg 2011). Instead, the data indicate that climatic drivers, including precipitation, temperature, and changes in hydrological flow patterns, are the most important factors affecting surface water DOC concentrations (Erlandsson et al 2008;Sarkkola et al 2009).…”

Simulating Dissolved Organic Carbon Dynamics at the Swedish Integrated Monitoring Sites with the Integrated Catchments Model for Carbon, INCA-C

“…However, over-reliance on simple models and purely statistical criteria may lead to overly simplistic views of the environment. If nothing else, it can be argued that more than 30 years of research into surface water DOC, and its links to acidification, acidity, climate and hydrology (Krug and Frink 1983;Laudon et al 1999;Erlandsson et al 2008Erlandsson et al , 2010Lepistö et al 2008;Clark et al 2010;Haaland et al 2010;Haei et al 2010;Löfgren et al 2010b;Löfgren and Zetterberg 2011), has shown that numerous factors influence or covary with surface water DOC. So, we must conclude that, on the basis of our current understanding, reality is over-determined but that models can be a useful tool for environmental hypothesis testing and understanding.…”

“…At many sites, while SO 4 2-deposition has been declining, the climate has warmed and runoff rates have changed. Geochemical modeling of the DOC dynamics in soil water at two of the Swedish integrated monitoring (IM) sites has identified a complex pattern of relationships between DOC and SO 4 2-deposition-driven changes in pH, ionic strength and aluminum chemistry, which can result in both increasing and decreasing trends in soil water [DOC] (Löfgren et al 2010b).…”

“…End-member mixing analysis has been applied to simulate streamwater [DOC] as a function of inputs from catchment soils and wetlands (Easthouse et al 1992), and a physically based model of DOC production and transport in boreal mires has been presented (Yurova et al 2007). Detailed chemical modeling can also help to elucidate effects of soil water variables on surface water [DOC] (Löfgren et al 2010b).…”