Published: April 29, 2011 r2011 American Chemical Society 4733 dx.doi.org/10.1021/es104126f | Environ. Sci. Technol. 2011, 45, 4733–4739 ARTICLE pubs.acs.org/est Effect of Acid Deposition on Quantity and Quality of Dissolved Organic Matter in SoilWater Sara M. Ekstr€ om, †, * Emma S. Kritzberg, † Dan B. Kleja, ‡ Niklas Larsson, § P. Anders Nilsson, † Wilhelm Graneli, † and Bo Bergkvist ^ † Department of Biology/Aquatic Ecology, § Department of Chemistry/Organic Chemistry, and ^ Department of Biology/Plant Ecology & Systematics, Lund University, Lund, Sweden ‡ Department of Soil and Environment, Swedish University of Agricultural Science, Uppsala, Sweden b S Supporting Information ’ INTRODUCTION Numerous monitoring studies have shown increasing water color and organic matter content, henceforth called brownification, in surface waters of the Northern hemisphere. 16 The brownification trend is generally ascribed to increased import of terrestrially derived dissolved organic matter (DOM) from the catchment. Water color and DOM content have both ecological significance, in terms of for example light penetration regulating aquatic primary production and predators depending on vision, as well as societal importance associated with recreational value and treatment costs for drinking water. Several mechanisms have been proposed to explain the brownification, for example, increased decomposition due to in- creasing temperature, increased runoff, CO 2 fertilization, and reduced soil acidification due to decreased atmospheric sulfur deposition. 2,5,7 The reasoning behind the decreasing sulfur hypoth- esis is that the increase in soil pH following a reduction in sulfur deposition increases the charge 8 and consequently the solubility of the organic matter, 9 and therefore facilitates the transport from the terrestrial to the aquatic system. 24 However, a great part of the conclusions concerning the connection between decreasing acid- ification and DOM concentrations in surface waters are based on correlations of monitoring data that generally only include the period of decreasing sulfur deposition. 24 Some experimental studies have been conducted in laboratory settings 1012 and are generally in line with the sulfur hypothesis, whereas the few field experiments that have been performed are inconsistent regarding a connection between sulfuric acidification and DOM mobility. 1315 Little is known about whether there is a change in the composition of the DOM along with the increase in quantity. However, water color in surface waters have been found to increase more than DOM concentration 6 implying that the changes in the amount of DOM alone cannot explain the enhanced water color. An alteration in DOM quality may affect both the mobility and color of the organic matter. Various laboratory experiments have shown that changes in pH alter DOM quality, 10,11,16 whereas field experiments have been less clear. The shortage and inconsistency of results from both laboratory and field studies 17 call for experimental studies in natural settings focusing on effects of changed sulfur deposition on DOM quality. The aim of this study was to elucidate the effect of altered sulfuric acid deposition on O-horizon soilwater chemistry and the mobility and quality of DOM in a field manipulation experi- ment. We hypothesized that the amount, color, and molecular weight of soilwater DOM would decrease with enhanced sulfuric acid addition. ’ MATERIAL AND METHODS Site Description. The experiment was conducted at the Asa Experimental Forest and Research Station in southern Sweden (57°08  N, 14°45  E; alt. 190200 m.a.s.l.), in the Boreo-nemoral vegetation zone. The site is dominated by Norway spruce Received: December 9, 2010 Accepted: April 14, 2011 Revised: April 12, 2011 ABSTRACT: The aim of this study was to explore how acid deposition may affect the concentration and quality of dissolved organic matter (DOM) in soilwater. This was done by a small-scale acidification experiment during two years where 0.5  0.5 m 2 plots were arti ficially irrigated with water with different sulfuric acid content, and soilwater was sampled using zero-tension lysimeters under the O-horizon. The DOM was characterized using absorbance, fluorescence, and size exclusion chroma- tography analyses. Our results showed lower mobility of DOM in the high acid treatment. At the same time, there was a significant change in the DOM quality. Soilwater in the high acid treatment exhibited DOM that was less colored, less hydrophobic, less aromatic, and of lower molecular weight, compared to the low acid treatment. This supports the hypothesis that reduction in sulfur deposition is an important driver behind the ongoing brownification of surface waters in many regions.