Effect of Acid Deposition on Quantity and Quality of Dissolved Organic Matter in Soil–Water

Published: April 29, 2011

r 2011 American Chemical Society 4733 dx.doi.org/10.1021/es104126f | Environ. Sci. Technol. 2011, 45, 4733–4739

ARTICLE

pubs.acs.org/est

Effect of AcidDeposition onQuantity andQuality of DissolvedOrganicMatter in Soil!WaterSara 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

bS Supporting Information

’ INTRODUCTION

Numerousmonitoring studies have shown increasing water colorand organic matter content, henceforth called browni!cation, insurface waters of the Northern hemisphere.1!6 The browni!cationtrend is generally ascribed to increased import of terrestrially deriveddissolved organic matter (DOM) from the catchment. Water colorand DOM content have both ecological signi!cance, in terms of forexample light penetration regulating aquatic primary productionand predators depending on vision, as well as societal importanceassociated with recreational value and treatment costs for drinkingwater. Several mechanisms have been proposed to explain thebrowni!cation, for example, increased decomposition due to in-creasing temperature, increased runo", CO2 fertilization, andreduced soil acidi!cation due to decreased atmospheric sulfurdeposition.2,5,7 The reasoning behind the decreasing sulfur hypoth-esis is that the increase in soil pH following a reduction in sulfurdeposition increases the charge 8 and consequently the solubility ofthe organic matter,9 and therefore facilitates the transport from theterrestrial to the aquatic system.2!4 However, a great part of theconclusions concerning the connection between decreasing acid-i!cation and DOM concentrations in surface waters are based oncorrelations of monitoring data that generally only include theperiod of decreasing sulfur deposition.2!4 Some experimentalstudies have been conducted in laboratory settings 10!12 and aregenerally in line with the sulfur hypothesis, whereas the few !eldexperiments that have been performed are inconsistent regarding aconnection between sulfuric acidi!cation and DOM mobility.13!15

Little is known about whether there is a change in thecomposition of the DOM along with the increase in quantity.

However, water color in surface waters have been found toincrease more than DOM concentration 6 implying that thechanges in the amount of DOM alone cannot explain theenhanced water color. An alteration in DOM quality may a"ectboth the mobility and color of the organic matter. Variouslaboratory experiments have shown that changes in pH alterDOM quality,10,11,16 whereas !eld experiments have been lessclear. The shortage and inconsistency of results from bothlaboratory and !eld studies 17 call for experimental studies innatural settings focusing on e"ects of changed sulfur depositionon DOM quality.

The aim of this study was to elucidate the e"ect of alteredsulfuric acid deposition on O-horizon soil!water chemistry andthe mobility and quality of DOM in a !eld manipulation experi-ment. We hypothesized that the amount, color, and molecularweight of soil!water DOM would decrease with enhanced sulfuricacid addition.

’MATERIAL AND METHODS

Site Description. The experiment was conducted at the AsaExperimental Forest and Research Station in southern Sweden(57!08"N, 14!45"E; alt. 190!200 m.a.s.l.), in the Boreo-nemoralvegetation zone. The site is dominated by Norway spruce

Received: December 9, 2010Accepted: April 14, 2011Revised: April 12, 2011

ABSTRACT:The aimof this studywas to explore how acid depositionmay a"ect theconcentration andquality of dissolved organicmatter (DOM) in soil!water. Thiswasdone by a small-scale acidi!cation experiment during two years where 0.5 " 0.5 m2

plots were arti!cially irrigated with water with di"erent sulfuric acid content, andsoil!water was sampled using zero-tension lysimeters under the O-horizon. TheDOM was characterized using absorbance, #uorescence, and size exclusion chroma-tography analyses. Our results showed lower mobility of DOM in the high acidtreatment. At the same time, there was a signi!cant change in the DOM quality.Soil!water in the high acid treatment exhibited DOM that was less colored, lesshydrophobic, less aromatic, and of lower molecular weight, compared to the low acidtreatment. This supports the hypothesis that reduction in sulfur deposition is animportant driver behind the ongoing browni!cation of surfacewaters inmany regions.

4734 dx.doi.org/10.1021/es104126f |Environ. Sci. Technol. 2011, 45, 4733–4739

Environmental Science & Technology ARTICLE

(Picea abies (L.) Karst.) with the present stand planted in 1967after a clear-cut in 1966.18 The soil is Podzol with an O-horizonof 3!10 cm depth with a total carbon pool of 4.3 kg m!2 andpH(H2O) between 3.7 and 4.2.

19 Annual mean precipitation andtemperature in the area is 688 mm and 5.5 !C respectively andthe average length of the growing season is 190 days.18 Theposition of the topographically homogeneous experimental site(10 " 30 m2) was selected subjectively based on absence or aminimum of field and bottom layer vegetation and homogeneityof the O-horizon with a leveled-out microtopography.Experimental Design. To simulate percolation, a computer

controlled irrigation system was set up with 12 watering devices.Each device consisted of a 0.5" 0.5 m2 square frame, positioned0.5 m above ground, with 100 pressure compensated drippers(Netafim 2 L/H) connected by tubes in grid formation. Within awide pressure range the drippers allowed the water to drip-feed,with a well-defined drip volume and rate, where each water pulselasted for one second and gave 56 ( 4 mL of water. The 12watering devices, in series of 6 belonging either to a high sulfuricacid (HS) or a low sulfuric acid (LS) treatment, were connectedby tubes to 2 pumps with buffered pressure systems (1.5 bar) thatfed the devices with water to the respective treatment from acubic meter tank. A solenoid valve on each device controlled theirrigation. Watering was conducted every second day with waterpulses every 5 minutes, in total 66 pulses per day giving a total 15mm water. Watering began in October 2007 and ended inSeptember 2009, with intermissions between November 2007to April 2008 andNovember 2008 toMay 2009 when the groundwas frozen and no irrigation could be performed. The totalirrigated volume corresponded to approximately 1300 mmprecipitation over a period of 6 months, approximately twicethe average natural annual precipitation in the area to obtainsufficient soil!water for analysis. At the given irrigation rate,7.6% of the samples were empty whereas overflow of the bottlesoccurred in 18.4% of the samples. In both the HS and the LStreatment, the irrigated water consisted of deionized water withadditions of salts to a final concentration of Ca, 10.3; K, 54.0; Na,66.0; Mg, 5.0; NH4!N, 14.0; NO3!N, 14.0; and Cl, 160.5!Eq L!1 aiming to mimic natural throughfall. The only differencebetween the HS and the LS treatment water was the amount ofH2SO4 added, resulting in aH2SO4 concentration of 6.0!EqL

!1 inthe LS treatment and 180 !Eq L!1 in the HS treatment. Meanconcentrations of SO4

2! in bulk precipitation in Southern Sweden1962!1982 exceeded 80 !Eq L!1.20 However, as throughfall fluxesin spruce stand can be two to three times greater than the bulkdeposition,21 the applied concentration in the HS treatment washigh but within the range of what has occurred. pH was 6.74 in theLS treatment, and 3.35 in the HS treatment, which is within therange of pH measured in throughfall in southern Sweden spruceforest during the peak acidification.13

Sampling and Analyses. O-horizon leachates were sampledusing 0.3 " 0.3 m2 zero-tension lysimeters (Plexiglas andpolyethylene netting), installed November 2002 in the intersec-tion between the O-horizon and the mineral soil, directly underthe watering devices. Leachates were collected in 1 L polyethy-lene bottles (Nalgene) placed below ground, that is in darknessand at low temperature. Sampling of the bottles was conductedbiweekly. The water volume collected in each bottle was mea-sured in the field before the water was transported cooled and indarkness for analysis. HS treatment data from only five replicatesare reported from 2007!2009 as one lysimeter ceased to func-tion. Soil!waters were collected at biweekly intervals during a

pretreatment period from July to October 2003 and analyzed forDOC concentration and absorbance (260 nm). The lysimetersthat were later assigned to the LS and HS treatments did notdiffer in DOC concentrations or absorbance (Wilks’ " = 0.490,F2,4 = 2.083, p = 0.240, LS lysimeters ! DOC: 18.6!103.4mg C L!1, absorbance: 0.52!3.59; HS lysimeters ! DOC:19.0!127.2 mg C L!1, absorbance: 0.45!3.76). Hence, differ-ences between LS and HS in the current study should be theresult of treatment effects rather than spatial heterogeneity.Analyses of chemical characteristics of the soil!water such as

SO4!S concentration, pH, major ions, DOC concentration andabsorbance were performed throughout the whole experimentalperiod. The soil!waters were !ltered through Acrodisc PFSyringe Filter 0.8/0.2 !m Supor Membrane (PALL Corp. PN4187) and analyzed for SO4!S, Cl, NO3!N, and PO4!P usingan 861 Advanced Compact ion chromatograph with a MetrosepA Supp 5 column (Metrohm). pH measurements were per-formed with a Microprocessor Ionanalyzer/901 pH meterequipped with a combination electrode (Orion Research). Foranalysis of Ca, Mg, K, Al, Mn, and Na, un!ltered samples wereacidi!ed with concentrated nitric acid (BDH Aristar) to 1% (v:v)and analyzed with Inductively Coupled Plasma Atomic EmissionSpectroscopy using an Optima 3000DV (PerkinElmer). Forammonium, the samples were !ltered as above and analyzed incompliance with ISO 11732:2005 on a FIASTAR 5000 (FOSSAS). DOC concentration (nonpurgeable organic carbon method)wasmeasured on !ltered samples using a TOC-VCHP total organiccarbon analyzer (Shimadzu). Absorbance measurements wereperformed on !ltered samples using a Lambda 11 UV!vis spectro-photometer with a 1 cm cuvette (PerkinElmer) where the absor-bance at a wavelength of 420 nm is representative of water color(abs420).Analyses of soil!water DOM quality, such as #uorescence

index (FI), speci!c UV absorbance (SUVA260), carbon speci!cabsorbance (CSA420), and size exclusion chromatography (SEC)weremade in 2009 on soil!water !ltered through precombustedGF/F !lters (Whatman). To exclude the possibility that di"er-ences between treatments were due to direct e"ects of di"erentpH on #uorescence and absorbance, the pH of the samples wasadjusted to 4 by dilution (1:2) with a bu"er (1% acetic acid and200 mM sodium acetate) prior to these measurements. Fluores-cence was determined using a RF-1501 spectro#uorophotometer(Shimadzu). Fluorescence index (FI), a re#ection of the hydro-phobicity of the DOM, was calculated as the ratio of the emissionat 520 and 470 nm at an excitation wavelength of 370 nm.22 Thecorrection coe$cient for inner !lter e"ect deviated from 1 bymaximum 0.03 and was therefore disregarded. Absorbance forSUVA260 and CSA420 were measured on a Beckman DU 800Coulter spectrophotometer. SUVA260, a measure of the aroma-ticity of the DOM, and CSA420 were calculated by dividing theabsorbance at 260 and 420 nm respectively by the DOCconcentration.23

SEC separates DOM primarily by molecular size, even thoughlack of authentic calibration compounds makes SEC of DOMbetter suited for comparative than exact studies.24 Here, ablended method based primarily on 25!27 was used. SEC wasperformed with a BioSep-SEC-S 3000 Phenomenox column(300mm"7.8 mm ID) in an Agilent 1100 liquid chromatographsystem with a diode array detector operating at 254 nm and usinga Phenomenox analytical guard column with 4 " 3.0 GFC 3000cartridges. Separations were performed at 27!31 !C with10 mM sodium acetate adjusted to pH 7.0 with acetic acid 25,26



at a #ow rate of 0.9 mL min!1. Before injection (80 !L) sampleswere spiked with 5% of 200 mM sodium acetate to decreasedi"erences in ionic strength between samples and the eluent.Molecular weight (MW) calibration was performed with poly-styrene sulfonates 25!27 with MW of 4300, 6800, and 13 000 Da(Fluka). Apparent MW of DOM fractions at peak height werecalculated from the linear correlation (r = 0.998) betweenMWofthe standards and the elution volume. Peaks eluting between8!12 min (corresponding to a distribution coe$cient of0.25!0.86 between total exclusion and total permeation) wereselected for further analyses. The retention time of the highestpeak in this time window (SECtime) was used as measurement ofthe relative average size of the UV-absorbing (254 nm) fractionof DOM, taking into account the MW calibration. The areafraction of the highest peak of the total chromatogram area(SECarea%) was used as a complement, showing approximatelyhow much of the DOM in the sample that constituted the givenmass of SECtime. SECarea% is a simpli!cation because di"erentsize fractions of DOM may have di"erent absorptive capacity.The anionic charge de!cit was calculated as the charge

di"erence between inorganic cations (H#, Na#, K#, Ca2#,Mg2#, Mn2#, NH4

#, Al3#, Al(OH)2#, Al(OH)2#, AlSO4

#)and anions (Cl!, NO3

!, SO42!, H2PO4

!). Charge of inorganicAl species was obtained using the geochemical computer pro-gramVisual MINTEQ ver. 3.0. The standard database was used inthe calculations and temperature was set to 10 !C. Complexationof Al by dissolved organic ligands was accounted for by assuming85% of DOC being fulvic acids with a carbon content of 50%.These !gures are justi!ed by van Schaik et al. (2010),28 whoisolated three fractions of DOM from O-horizon soil!waters atthe Asa site. Fulvic acids constituted the major fraction of DOM(59%), whereas 23% of DOM was assigned to be macromole-cular hydrophilic acids. The copper and proton binding proper-ties of these fractions were similar, supporting the use of onefulvic acid analogue for the active fraction of DOM in ourcalculations. The net charge of the fulvic acid analogues obtainedfrom the program was used as a measure for the net chargeg!1 DOC.Control of Storage Effect. As the water was sampled

biweekly, part of the soil!water was stored in the ground upto 14 days. An experiment was conducted to test if the chemicalcharacteristics of the water were altered during this period oftime. Changes in DOC concentration and DOM quality para-meters were small during the two weeks of storage and did notaffect the evaluation of the data (Figure S1 of the SupportingInformation).Statistical Analysis. Statistical analyses were divided into one

analysis of the variables describing chemical characteristics of thesoil!water for the whole sampling period and one analysis of thevariables describing soil!water DOMquality of 2009. Because ofthe intermission in sampling during winter seasons and con-sidering that not all lysimeters gave water at all samplingoccasions, a randomized block (rb) MANOVA, rather thanrepeated measures ANOVA, was used for statistical analysis.This statistical approach also considers potential autocorrela-tions between dependent variables. Sampling date (time) wasused as a blocking factor to allow for evaluation of treatmenteffects in spite of seasonal variations in the dependent variables.When using the separate replicates in the rbMANOVA, the datacould not be transformed to meet with the normality assump-tions. Mean values for each treatment and sampling occasionwere therefore used in the model. Dependent variables in the

statistical analysis of soil!water chemical characteristics wereconcentration of SO4!S, pH, base cations (Ca, K, Mg), DOCconcentration, and abs420. Because of missing values of abs420from 2007, only data from 2008 and onward were included in thestatistical model. For the analysis of DOM quality variables, thesame statistical approach was used with mean values for eachtreatment and sampling date of FI, SUVA260, CSA420, SECtime,SECarea%, and net charge g!1 DOC. In this analysis, theassumptions of normality could be met when using all replicatesin the model. However, as the outcome of the statistical test wasunaffected, results based on mean values are reported forconsistency. All statistical analyses were performed in PASWStatistics ver. 18.

’RESULTS

Both SO4 and hydrogen ions were present at higher concen-trations in theHS treatment soil!water than in the LS treatment,as illustrated in Figure 1. Moreover, higher concentrations ofbase cations and lower charge de!cits of anions were observed inHS (Figure 1). The same di"erences were observed throughoutthe sampling period as showed by the rbMANOVA (Wilks’ " =0.032, F5,17 = 103.196, p < 0.001) supporting the hypothesis ofthe e"ect of acid treatment on the chemical characteristics of thequantitative variables measured. Testing for between-subjecte"ects showed that all variables included were signi!cantlya"ected by treatment (p < 0.001). The blocking factor timehad signi!cant e"ects (Wilks’ " = 0.008, F105,87.849 = 1.419, p =0.046) on DOC concentration (p = 0.036), whereas all othervariables were una"ected (p > 0.05). Already from the !rstsampling occasion and onward, the SO4!S concentration in thesoil!water was markedly higher in HS than in LS (part a ofFigure 2). pH was similarly a"ected by the treatment, with pH inHS consistently lower than in LS (part b of Figure 2). The e"ectof the treatment could also be clearly seen in the concentration ofbase cations, which was consistently higher in the HS treatment

Figure 1. Ion charge balance in the soil!water, mean concentration ofeach lysimeter from 2009, grouped by treatment. HS = high sulfuric acidtreatment, LS = low sulfuric acid treatment. Mn, NO3!N and PO4!Pare too small to be shown in the graph.



(part c of Figure 2). The e"ect of treatment on DOC concentra-tion and abs420 was not so apparent in 2008, but in 2009 bothDOC and abs420 were lower in the HS than in the LS treatment(parts d and e of Figure 2). The average di"erence between thetreatments in 2009 was greater for abs420 (58%) than for DOC(42%).

DOM quality di"ered between the two treatments as veri!edby the rbMANOVA (Wilks’ " = 0.006, F6,2 = 52.070, p = 0.019).The test of between-subject e"ects showed that all includedvariables di"ered signi!cantly between treatments (p < 0.005).The blocking factor time was not signi!cant (p = 0.556). DOM inthe HS treatment was less aromatic and hydrophobic, than DOMin the LS treatment, as indicated by higher FI and lower SUVA260(parts a and b of Figure 3), and had lower CSA420 (part c ofFigure 3). In the SEC analyses, the major peak eluted after anaverage of 9.37 min in the HS treatment and after 9.24 min in theLS treatment (SECtime, part d of Figure 3). This corresponds to aMW of about 1950 and 2280 Da respectively relative to the PSSstandards. The average fraction of the total area accounted for bythe dominating peak (SECarea%) was smaller for HS (56%) thanfor LS (64%, part e of Figure 3), indicating that the average MWwas smaller in HS than in LS. The selected window included92.7!99.8% of the total area of the chromatograms. Themodeled net charge g!1 DOC was lower in the HS treatment(part f of Figure 3) and a strong correlation between pH and netcharge g!1 DOC that could be observed (r = !0.984, n = 22,p < 0.001, Figure 4).

’DISCUSSION

This !eld experiment with manipulation of sulfuric acidaddition demonstrates a clear e"ect on soil!water chemistryand DOM dynamics and is in agreement with the prevailingtheory that reversed acidi!cation is a signi!cant mechanismbehind the browni!cation of surface waters.4 Although monitor-ing data from many regions supports the notion that decreasingsulfur deposition explains increasing DOC levels, a limitation ofthe monitoring studies is that they tend to include only theperiod of decreasing sulfur deposition.1!4 Moreover, correla-tions found in monitoring studies can be coincidental, and maynot re#ect causal relations. While monitoring studies are incon-clusive with regard to pinpointing casual mechanisms, thecorrelative evidence can be used to formulate hypotheses thatcan be tested. In our study, higher sulfuric acid addition rapidlyincreased the concentrations of SO4!S and base cations,whereas pH in the soil!water was signi!cantly lowered. BothDOC concentration and abs420 were lower in the HS treatmentthan in the LS treatment, in agreement with previous laboratory 10!12

and small-scale !eld experiments.13 Similarly, soil!water DOCdecreased in response to acidi!cation in a whole-catchment experi-ment (HUMEX),29 implying a reduction in mobility of the DOMwith higher acid load. The underlying mechanism for this reductioninmobility of DOM is that a greater concentration of hydrogen ionsboth increase the leakage of base cations through cation exchangeand reduce the net charge ofDOMthrough protonation of functional

Figure 2. Observed chemical characteristics of lysimeter soil!water from Oct 2007 ! Sep 2009, all replicates. A. SO4!S; B. pH; C. base cations;D. dissolved organic carbon (DOC) and E. Absorbance at 420 nm (abs420). HS = high sulfuric acid treatment, LS = low sulfuric acid treatment. Errorbars (1 SD. No sampling was conducted during winter seasons.



groups (e.g., carboxylic groups), which reduces the solubility andthe mobility of the DOM.8,17,30 This was shown in our resultsthrough the strong correlation between pH and net charge of the

organic matter in the soil!water. In monitoring data, alteredcharge of the organic matter has been indenti!ed as a plausiblemechanism explaining the correlation between changes in aciddeposition and concentrations of organic matter in surfacewaters.3

As previously observed for surface waters,6 there was a greaterdi"erence in water color (abs420) than in DOC concentrationbetween the two treatments. This implies that changes in DOMconcentration are insu$cient in explaining the entire change inwater color, and that there is a concurrent change in DOMquality. The analyses of DOM quality during 2009 showed aparallel change in the characteristics of the organic matter.SUVA260 was lower in the HS treatment, and together with ahigher FI this indicates that the DOM was less aromatic and lesshydrophobic than in the LS treatment. Hydrophobic fractions ofDOM require a greater net charge to become soluble thanhydrophilic fraction.30 Decreasing pH therefore lowers thesolubility of the hydrophobic acids, leading to a soil!water lessdominated by hydrophobic compounds.10,16 Hydrophobicityalso a"ects the color of organic matter, that is the color ofDOM decreases with decreasing ratio of hydrophobic to hydro-philic acids.31 This was shown quite clearly in our experimentwith less color per DOC molecule (lower CSA420) in the HStreatment. SEC showed that the molecular weight of the UV-absorbing fraction of HS DOM was lower than that of the LSDOM, which is in agreement with earlier laboratory experiments

Figure 3. Observed DOM quality of lysimeter soil!water from May!Sep 2009, all replicates. A. Fluorescence Index (FI); B. Speci!c UV absorbance(SUVA260); C. Carbon speci!c absorbance (CSA420); D. Size exclusion chromatography, elution time (SECtime); E. Size exclusion chromatography,area % (SECarea%); and F. Net charge g

!1 DOC. HS = high sulfuric acid treatment, LS = low sulfuric acid treatment. Error bars (1 SD.

Figure 4. Correlation between pH andmodeled net charge g!1 DOC inthe soil!water (r =!0.984, n = 22, p < 0.001), based onmean values foreach treatment and sampling date. HS = high sulfuric acid treatment,LS = low sulfuric acid treatment. HSmean net charge:!3.04 mmol g!1,LS mean net charge: !3.59 mmol g!1.



where positive correlations between pH and molecular weight oforganic matter released to soil!water have been observed.11,32

As the high sulfuric acid addition reduced the solubility,hydrophobicity, molecular weight, and color of the organicmatter, the ongoing decrease of acidi!cation ought to result inthe opposite: a more mobile, hydrophobic and aromatic DOM,with larger and more colored molecules. These alterations in soilDOM quality may considerably impact the reactivity of theorganic matter as it enters the aquatic system, and may thereforea"ect the turnover of organic carbon. Freshwaters form a signif-icant component of the global carbon cycle and turnover greatamounts of terrestrially originated carbon.33,34 It is thereforeimportant to clarify how changes in the quantity and quality ofDOM from the drainage a"ect the reactivity and biodegradabilityof the organic carbon.

The e"ect of the acid treatment was observed already from the!rst sampling date and was consistent other than three samplingoccasions in 2008 when the treatment means shifted for DOCand abs420. For all sampling dates, there was great within-treatment variation, which is to be expected in an in situexperiment. Throughout these three dates, we believe that theDOC and absorbance values were a"ected by dry conditions thatresulted in very small sample volumes. The virtually instanta-neous and consistent e"ect demonstrates that the response to theacidity changes is of chemical nature, with an altered soilchemical steady state, rather than a biological response thatshould be more progressive in nature.

Alternations in DOM quality might have great impact uponthe reactivity of the matter entering surface waters. The threeprocesses through which DOM in aquatic systems is turnedover ! photochemical decomposition, bacterial decomposition,and #occulation with subsequent sedimentation ! may all bea"ected by DOM quantity and quality. Our results indicate thatthe reduced acidi!cation leads to increased aromaticity, hydro-phobicity, and molecular weight; all characteristics consistentlypointing toward decreasing susceptibility to both photochemicaland bacterial decomposition, 35!37 whereas #occulation andsedimentation rises with increasing concentrations of terrestrialoriginated DOM.38 Thus, #occulation and sedimentation oforganic matter may become relatively more important in theprocessing of terrestrial DOM in the aquatic system.

This study suggests that the reduction in sulfur deposition ledto increased color of soil!water, due to an increase in dissolvedorganic matter content, but also to changes in DOM quality.With higher pH, the DOM has a higher net charge and is hencemore soluble. Parallel to the change in DOM mobility, there is achange in DOM quality toward a larger sized matter with morearomatic, hydrophobic, and colored characteristics. This changein quality of the DOM may a"ect its fate as it moves fromthe terrestrial and into the aquatic system.We therefore concludethat reduced acidi!cation may be an important driver of thebrowni!cation of surface waters and that altered DOM qualitymay greatly impact the biogeochemical turnover of the organicmatter.

’ASSOCIATED CONTENT

bS Supporting Information. Description of control of sto-rage e"ect, statistical results for testing for between-subjecte"ects, and an example size exclusion chromatography chroma-togram. This material is available free of charge via the Internet athttp://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*Tel: #46 462228433, fax: #46 462224563, e-mail: [email protected].

’ACKNOWLEDGMENT

This study was funded by the Swedish Research Council forEnvironment, Agriculture Science and Spatial Planning (217-2006-547).We thank the sta" at the Asa Experimental Forest andResearch Station for assistance with !eld equipment and sam-pling. This manuscript has been greatly improved by commentsfrom four anonymous reviewers.

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Effect of Acid Deposition on Quantity and Quality of Dissolved Organic Matter in Soil–Water

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