C H A P T E R O N EDissolved Organic Matter:Biogeochemistry,Dynamics,and Environmental Significance in SoilsNanthi S.Bolan,*,†Domy C.Adriano,‡Anitha Kunhikrishnan,*,†Trevor James,§Richard McDowell,}and Nicola Senesi #Contents1.Introduction32.Sources,Pools,and Fluxes of Dissolved Organic Matter in Soils 53.Properties and Chemical Composition of Dissolved Organic Matter in Soils133.1.Structural components133.2.Fulvic acid—The dominant component 153.3.Elemental composition204.Mechanisms Regulating Dynamics of Dissolved Organic Matter in Soils204.1.Sorption/complexation 234.2.Biodegradation 274.3.Photodegradation 284.4.Leaching295.Factors Influencing Dynamics of Dissolved Organic Matter in Soils 305.1.Vegetation and land use 315.2.Cultivation325.3.Soil amendments 335.4.Soil pH366.Environmental Significance of Dissolved Organic Matter in Soils 376.1.Soil aggregation and erosion control 376.2.Mobilization and export of nutrients386.3.Bioavailability and ecotoxicology of heavy metals43Advances in Agronomy,Volume 110#2011Elsevier Inc.ISSN 0065-2113,DOI:10.1016/B978-0-12-385531-2.00001-3All rights reserved.*Centre for Environmental Risk Assessment and Remediation (CERAR),University of South Australia,Australia {Cooperative Research Centre for Contaminants Assessment and Remediation of the Environment (CRC CARE),University of South Australia,Australia {University of Georgia,Savannah River Ecology Laboratory,Drawer E,Aiken,South Carolina,USA }AgResearch,Ruakura Research Centre,Hamilton,New Zealand }AgResearch,Invermay Agricultural Centre,Mosgiel,New Zealand #Department of Agroforestal and Environmental Biology and Chemistry,University of Bari,Bari,Italy 12Nanthi S.Bolan et al.6.4.Transformation and transport of organic contaminants506.5.Gaseous emission and atmospheric pollution587.Summary and Research Needs607.1.Macroscale(landscape to global)617.2.Microscale(water bodies and soil profile)617.3.Molecular scale(carbon fractions,organic acids,andmicroorganisms)61 Acknowledgments62 References62“Dissolved organic matter comprises only a small part of soil organicmatter;nevertheless,it affects many processes in soil and water includ-ing the most serious environmental problems like soil and waterpollution and global warming.”(Kalbitz and Kaiser,2003)AbstractDissolved organic matter(DOM)is defined as the organic matter fraction in solution that passes through a0.45m m filter.Although DOM is ubiquitous in terrestrial and aquatic ecosystems,it represents only a small proportion of the total organic matter in soil.However,DOM,being the most mobile and actively cycling organic matter fraction,influences a spectrum of biogeochemical pro-cesses in the aquatic and terrestrial environments.Biological fixation of atmo-spheric CO2during photosynthesis by higher plants is the primary driver of global carbon cycle.A major portion of the carbon in organic matter in the aquatic environment is derived from the transport of carbon produced in the terrestrial environment.However,much of the terrestrially produced DOM is consumed by microbes,photo degraded,or adsorbed in soils and sediments as it passes to the ocean.The majority of DOM in terrestrial and aquatic environ-ments is ultimately returned to atmosphere as CO2through microbial respira-tion,thereby renewing the atmospheric CO2reserve for photosynthesis.Dissolved organic matter plays a significant role in influencing the dynamics and interactions of nutrients and contaminants in soils and microbial functions, thereby serving as a sensitive indicator of shifts in ecological processes.This chapter aims to highlight knowledge on the production of DOM in soils under different management regimes,identify its sources and sinks,and integrate its dynamics with various soil processes.Understanding the significance of DOM in soil processes can enhance development of strategies to mitigate DOM-induced environmental impacts.This review encourages greater interactions between terrestrial and aquatic biogeochemists and ecologists,which is essential for unraveling the fundamental biogeochemical processes involved in the synthesis of DOM in terrestrial ecosystem,its subsequent transport to aquatic ecosystem, and its role in environmental sustainability,buffering of nutrients and pollutants (metal(loid)s and organics),and the net effect on the global carbon cycle.Dissolved Organic Matter31.IntroductionThe total organic matter(TOM)in terrestrial and aquatic environ-ments consists of two operationally defined phases:particulate organic matter(POM)and dissolved organic matter(DOM).For all practical purposes,DOM is defined as the organic matter fraction in solution that passes through a0.45m m filter(Thurman,1985;Zsolnay,2003).Some workers have used finer filter paper(i.e.,0.2m m)in an effort to separate “true”DOM from colloidal materials,but0.45m m filtration appears to be standard(Buffle et al.,1982;Dafner and Wangersky,2002).In some litera-ture,the term dissolved organic carbon(DOC)is used,which represents total organic carbon in solution that passes through a0.45m m filter (Zsolnay,2003).Since carbon represents the bulk of the elemental compo-sition of the organic matter(ca.67%),DOM is often quantified by its carbon content and referred to as DOC.In the case of studies involving soils,the term water-soluble organic matter(WSOM)or water-extractable organic matter(WEOM)is also used when measuring the fraction of the soil organic matter(SOM)extracted with water or dilute salt solution(e.g.,0.5 M K2SO4)that passes through a0.45m m filter(Bolan et al.,1996;Herbert et al.,1993).Recently,the distinction between POM and DOM in the marine environment is being replaced by the idea of an organic matter continuum of gel-like polymers,replete with colloids and crisscrossed by “transparent”polymer strings,sheets,and bundles,from a few to hundreds of micrometers—referred to as oceanic“dark matter”(Dafner and Wangersky,2002).Dissolved organic matter is ubiquitous in terrestrial and aquatic ecosys-tems,but represents only a small proportion of the total organic matter in soil(McGill et al.,1986).However,it is now widely recognized that because DOM is the most mobile and actively cycling organic matter fraction,it influences a myriad of biogeochemical processes in aquatic and terrestrial environments as well as key environmental parameters (Chantigny,2003;Kalbitz et al.,2000;McDowell,2003;Stevenson, 1994;Zsolnay,2003).Dissolved organic carbon has been identified as one of the major components responsible for determining the drinking water quality.For example,DOM leads to the formation of toxic disinfection by-products(DBPs),such as trihalomethanes,after reacting with disinfectants (e.g.,chlorine)during water treatment.Similarly,DOM can be related to bacterial proliferation within the drinking water distribution system.There-fore,the control of DOM has been identified as an important part of the operation of drinking water plants and distribution systems(Volk et al., 2002).In aquatic environments,the easily oxidizable compounds in the DOM can act as chemical and biological oxygen demand compounds, thereby depleting the oxygen concentration of aquifers and influencing4Nanthi S.Bolan et al. aquatic biota(Jones,1992).Dissolved organic carbon can act as a readily available carbon source for anaerobic soil organisms,thereby inducing the reduction of nitrate(denitrification)resulting in the release of green house gases,such as nitrous oxide(N2O)and nitric oxide(NO),which are implicated in ozone depletion(Siemens et al.,2003).Organic pesticides added to soil and aquifers are partitioned preferentially onto DOM,which can act as a vehicle for the movement of pesticide residues to groundwater (Barriuso et al.,1992).Similarly,the organic acids present in the DOM can act as chelating agents,thereby enhancing the mobilization of toxic heavy metals and metalloids[metal(loid)s](Antoniadis and Alloway,2002).The release and retention of DOM are the driving forces controlling a number of pedological processes including podzolization(Hedges,1987).Biological fixation of atmospheric CO2by higher plants during photo-synthesis is the primary driver of global carbon cycle.A major portion of the carbon in aquatic environments is derived from the transport of carbon produced on land.It has been estimated that worldwide about210Mt DOM and170Mt POM are transported annually to oceans from land. Carbon in the ocean is recognized as one of the three main reservoirs of organic material on the planet,equal to the carbon stored in terrestrial plants or soil humus(Hedges,1987).The terrestrially produced DOM is subject to microbial-and photodegradation and adsorption by soil and sediments.The majority of DOM in terrestrial and aquatic environments is returned to the atmosphere as CO2through microbial respiration,thereby ultimately replenishing the atmospheric CO2reserve for photosynthesis and reinvi-gorating the global carbon cycle.Dissolved organic carbon can be envisioned both as a link and bottle-neck among various ecological bined with its dynamic nature,this enables DOM to serve as a sensitive indicator of shifts in ecological processes,especially in aquatic systems.Recently,the significance of DOM in the terrestrial environment has been realized and attempts have been made to extend this knowledge to DOM dynamics in aquatic envir-onments.However,DOM dynamics on land are fundamentally different from those in water,where biomass of primary producers is relatively small, allochthonous sources of DOM are dominant,the surface area of reactive solid particles(i.e.,sediments)is smaller,and the fate of DOM is strongly influenced by photolysis and other light-mediated reactions.In contrast,the dynamics of DOM on land are largely controlled by its interactions with abiotically and biotically reactive solid components.Although there have been a number of reviews on the individual components of DOM in soils(e.g.,sources and sink—Kalbitz et al. (2000);microbial degradation—Marschner and Kalbitz(2003);sorption by soils—Kaiser et al.(1996)),there has been no comprehensive review linking the dynamics of DOM to its environmental significance.This chapter aims to elaborate on the production and degradation of DOM inDissolved Organic Matter5 soils under different landscape conditions,identify its sources and sinks,and integrate its dynamics with environmental impacts.Understanding the long-term control on DOM production and flux in soils will be particularly important in predicting the effects of various environmental changes and management practices on soil carbon dynamics.Improved knowledge on the environmental significance of DOM can enhance the development of strategies to mitigate DOM-induced environmental impacts.It is hoped that this chapter will encourage greater interaction between terrestrial and aquatic biogeochemists and ecologists and stimulate the unraveling of fundamental biogeochemical processes involved in the synthesis and trans-port of DOM in terrestrial and aquatic ecosystems.2.Sources,Pools,and Fluxes of DissolvedOrganic Matter in SoilsNearly all DOM in soils comes from photosynthesis.This represents the various C pools including recent photosynthates,such as leaf litter, throughfall and stemflow(in the case of forest ecosystems),root exudates, and decaying fine roots,as well as decomposition and metabolic by-pro-ducts and leachates of older,microbiologically processed SOM(Figure1) (Guggenberger,et al.,1994a;McDowell,2003;McDowell,et al.,1998). The majority of DOM in soils and aquifers originates from the solubilization of SOM accumulated through vegetation and the addition of biological waste materials(Guggenberger,et al.,1994b;McDowell,2003;McDowell, et al.,1998;Tate and Meyer,1983).The addition of biological waste materials,such as poultry and animal manures and sewage sludges,increases the amount of DOM in soils either by acting as a source of DOM or by enhancing the solubilization of the SOM.Most biological waste materials of plant origin contain large amounts of DOM(Table1)and the addition of certain organic manures such as poultry manure increases the pH and thereby enhances the solubilization of SOM(Schindler et al.,1992).The concentrations of DOM in soils and aquifers are highly susceptible to changes induced by humans,such as cultivation,fire,clear-cutting, wetland drainage,acidic precipitation,eutrophication,and climate change (Kreutzweiser et al.,2008;Laudon et al.,2009;Martinez-Mena et al.,2008; Mattsson et al.,2009;Yallop and Clutterbuck,2009).Dissolved organic matter in environmental samples,such as soils and manures,is often extracted with water or dilute aqueous salt solutions.Various methods have been used to measure the concentration of DOM in extracts (Table2).These methods are grouped into three categories(Moore, 1985;Sharp et al.,2004;Stewart and Wetzel,1981;Tue-Ngeun et al., 2005).The most frequently used method involves the measurement ofabsorption of light by the DOM using a spectrophotometer (Stewart and Wetzel,1981).The second method involves wet oxidation of samples containing DOM and the subsequent measurement of the CO 2released or the amount of oxidant consumed (Ciavatta et al.,1991).This method is often referred to as chemical oxygen demand (COD).Dichromates or permanganates are the most common oxidizing agents used in the wet oxidation of DOM,and the amount of oxidant consumed in the oxidation of DOM is measured either by titration with a reducing agent or by calorimetric methods.The third method involves dry oxidation of DOM to CO 2at high temperature in the presence of a stream of oxygen.The amount of CO 2produced is measured either by infrared (IR)detector or by titration after absorbing in an alkali,or by weight gain after absorbing in ascarite (Bremner and Tabatabai,1971).The most commonly used dry combustion techniques include LECO TM combustion and total organic carbon (TOC)analyzer.B horizonA horizonDOMDOMLitter layer Crop residueC horizonAquiferAgricultural soilForest soil 1111101099886677CO 2CO 2PhotosynthesisPhotosynthesis554433212Parent/geologicmaterialFigure 1Pathways of inputs and outputs of dissolved organic matter (DOM)in forest and agricultural soils.Inputs:1,throughfall and stemflow;2,root exudates;3,microbial lysis;4,humification;5,litter/and crop residue decomposition;6,organic amendments;outputs;7,microbial degradation;8,microbial assimilation;9,lateral flow;10,sorp-tion;11,leaching.6Nanthi S.Bolan et al.Plant litter and humus are the most important sources of DOM in soil,which is confirmed by both field and laboratory (including greenhouse)studies (Kalbitz et al.,2000;Kalbitz et al.,2007;Muller et al.,2009;Table 1Sources of dissolved organic matter input to soilsSourcesTotal organic matter (g C kg À1)Dissolvedorganic matterReference(g C kg À1)(%of total organic matter)Pasture leys Brome grass 13.30.0410.31Shen et al .(2008)Clover 15.10.0390.26Shen et al .(2008)Crowtoe10.40.0360.35Shen et al .(2008)Lucerne Cv.Longdong 11.40.0380.32Shen et al .(2008)Lucerne Cv.Saditi 10.90.0360.33Shen et al .(2008)Sainfoin 13.80.0400.29Shen et al .(2008)Sweet pea 10.20.0340.33Shen et al .(2008)SoilForest soil—litter leachate 60.00.0260.04Jaffrain et al.(2007)Arable soil12.00.150 1.25Gonet et al.(2008)Soil under bermuda grass turf 8.100.300 3.70Provin et al.(2008)Pasture soil 32.0 1.02 3.18Bolan et al.(1996)Pasture soil82.5 3.12 3.80Bolan et al.(1996)Organic amendments Sewage sludge 420 2.420.58Hanc et al.(2009)Sewage sludge 321 6.00 1.87Bolan et al.(1996)Paper sludge 2817.19 2.56Bolan et al.(1996)Poultry manure 4258.18 1.92Bolan et al.(1996)Poultry litter a37775.720.1Guo et al.(2009)Mushroom compost 3857.10 1.84Bolan et al.(1996)Fresh spent mushroom substrate28813346.2Marin-Benito et al.(2009)Composted spentmushroom substrate 27443.415.8Marin-Benito et al.(2009)Separated cow manure 4569.80 2.15Zmora-Nahuma et al.(2005)Poultry manure 4258.18 1.92Bolan et al.(1996)Pig manure2966.132.07Bolan et al.(1996)aBisulfate amended,phytase-diet Delmarva poultry litter.Dissolved Organic Matter 7Table2Selected references on methods of extraction and analysis of DOM in environmental samplesSamples Extraction of DOM Measurement of DOM ReferenceVolcanic ash soils Soil solutions collected by centrifugation ofcores at7200rpm;filtration(0.45m mfilters)DOC by Shimadzu TOC-5000analyzerKawahigashi et al.(2003)Peat—moorsh soil Soil samples were crushed an passed througha1mm sieve,then heated in a redistilledwater at100 C for2h under a reflexcondenser;filtration(0.45m mfilters)DOC by Shimadzu TOC5050A analyzerSzajdak et al.(2007)Soils(medial,amorphic thermic,Humic Haploxerands)Extraction with0.5mol LÀ1K2SO4solution1:5(w/v);filtration(AdvantecMFS Nº5C paper).TOC by combustion at675 Cin an analyzer(Shimadzu—model TOC-V CPN)Undurraga et al.(2009)Moss,litter and topsoil (0–5cm)Aqueous samples were estimated for DOCby oxidation of the sample with asulfochromic mixture(4.9g dmÀ3K2Cr2O7and H2SO4,1:1,w/w)withcolorimetric detection of the reduced Cr3þColorimeter KFK-3at590nm Prokushkin et al.(2006)Soil solutions from forested watersheds of North Carolina Samples werefiltered through a WhatmanG/F glassfiberfilters.Wet combustion persulfatedigestion followed byTOC analyzerQualls and Haines(1991)Organic fertilizer Extracted DOC by0.01M CaCl2solutionwith a solid to solution ratio of1:10(w/v),mixed for30min at200rpm;filtration(0.45m mfilter)Shimadzu TOC-5000ATOC analyzerLi et al.(2005)Soil solution and stream waters along a natural soil catena Soil solution collected by tension-freelysimetersDOC by infrared detectionfollowing persulfateoxidationPalmer et al.(2004)Liquid and solid sludge,farm slurry,fermented straw,soil, and drainage water Water extraction followed by centrifugation(40,000Âg)andfiltration(0.45m mfilter)Dry combustion(DhormannCarbon Analyzer DC-80)Barriuso et al.(1992)Soils,peat extract,sludge,pig and poultry manure and mushroom compost Extracted with water(1:3solid:solution ratio);centrifugation(12,000rpm)andfiltration(0.45m mfilter)Wet chemical oxidation withdichromate followed byback titrationBaskaran et al.(1996)Soil(Entic Haplothord)Extraction with deionized water(1:10solid:solution ratio);filtered through0.45m mpolysulfore membrane Dry combustion(TOCanalyzer Shimadzu5050)Kaiser et al.(1996)Pig manure Extracted with water(1:3solid:solution ratio);shaken at200rpm for16h at4o C;centrifugation(12,000rpm)andfiltration(0.45m mfilter)DOC by Shimadzu TOC-5000A TOC analyzerCheng and Wong(2006)Cow manure slurryfiltered through0.45m m polysulforemembrane TOC analyzer using UVabsorbanceAguilera et al.(2009)Sewage sludge DOC was extracted in a soil:water ratio of1:10(w/v)after1h agitation.Wet combustion withchromate followed by backtitrationGasco´and Lobo(2007)River water Natural water from riverfiltered by0.22m mfilter DOC by wet oxidation TOCanalyzerKrachler et al.(2005)Peat water Peat waterfiltered through0.45m mmembranefilters DOC was analyzed using ahigh-temperature catalyticoxidation method(Dohrman DC-190analyzer)Rixen et al.(2008)River water Filtered through0.7m m glassfiberfilter In situ optical technologyusingfluorescenceSpencer et al.(2007)(continued)Table2(continued)Samples Extraction of DOM Measurement of DOM ReferenceSea water Filtered through0.45m m polysulforemembrane High-temperaturecombustion instrument tomeasure isotopecomposition of DOCLang et al.(2007)Freshwater Filtered through0.7m m glassfiberfilter Acid-peroxydisulfatedigestion and high-temperature catalyticoxidation(HTCO)withUV detectionTue-Ngeun et al.(2005) Effluent water–In situ UV spectrophotometer Rieger et al.(2004)Groundwater,lake water, and effluent –High-performance liquidchromatography-sizeexclusion chromatography-UVAfluorescence systemHer et al.(2003)Sea water and effluent Filtered through0.7m m glassfiberfilter Measurement of carbonatomic emission intensity ininductively coupled plasmaatomic emissionspectrometry(ICP-OES)Maestre et al.(2003)Lake water Water samplesfiltered using precombustedGF/Ffilters TOC analyzer(TOC5000;Shimadzu)Ishikawa et al.(2006)Soil solution and stream water from forested catchments Samples werefiltered through0.45m mfiltersDOC by Shimadzu TOC5050A analyzerVestin et al.(2008)Dissolved Organic Matter11 Sanderman et al.,2008).In forest ecosystems,which are the most intensively studied with regard to C cycling and its associated DOM dynamics,the canopy and forest floor layers are the primary sources of DOM(Kaiser et al., 1996;Kalbitz et al.,2007;Park and Matzner,2003).However,it is still unclear whether DOM originates primarily from recently deposited litter or from relatively stable organic matter in the deeper part of the organic horizon(Kalbitz et al.,2007).In a temperate,deciduous forest,the source of DOM leaching from the forest floor(O layer)is generally a water-soluble material from freshly fallen leaf litter and throughfall(Kalbitz et al.,2007;Qualls et al.,1991).Appar-ently all of the DOM and dissolved organic N(DON)could have origi-nated from the Oi(freshly fallen litter)and Oe(partially decomposed litter) horizons.They further observed that,while about27%of the freshly shed litter C was soluble,only18.4%of the C input in litterfall was leached in solutions from the bottom of the forest floor.Virtually all the DOM leached from the forest floor appeared to have originated from the upper forest floor,with none coming from the lower forest floor—an indication of the role of this litter layer as a sink.The role of freshly deposited litter as DOM source was further corroborated by laboratory studies(Magill and Aber, 2000;Moore and Dalva,2001;Muller et al.,2009;Sanderman et al.,2008). Michalzik and Matzner(1999)found high fluxes of DOM from the Oi layer than from the Oe and Oa layers and indicated that the bottom organic layers acted instead as a sink rather than as a source of DOM.Logically,however, because of the more advanced state of decomposition,the bottom litter layers could produce more DOM than the surface layer.Indeed,Solinger et al.(2001)measured greater DOM fluxes out of the Oa than out of the Oi layer.Recently,Froberg et al.(2003)and Uselman et al.(2007)confirmed with14C data that the Oi layer is not a major source of DOM leached from the Oe layer.In a comprehensive synthesis of42case studies in temperate forests, Michalzik et al.(2001)observed that,although concentrations and fluxes differed widely among sites,the greatest concentrations of DOM(and DON)were generally observed in forest floor leachates from the A horizon and were heavily influenced by annual precipitation.However,somewhat surprisingly,there were no meaningful differences in DOM concentrations and fluxes in forest floor leachates between coniferous and hardwood sites. The flux of soluble organic compounds from throughfall and the litter layer could amount to1–19%of the total litterfall C flux and1–5%of the net primary productivity(Froberg et al.,2007;McDowell and Likens,1988; Qualls et al.,1991).Nearly one-third of the DOM leaving the bottom of the forest floor originated from throughfall and stemflow(Qualls et al.,1991; Uselman et al.,2007).Values for the potential solubility of litter in the field and in laboratory studies are in the5–25%range of the litter dry mass and 5–15%of the litter C content(Hagedorn and Machwitz,2007;McDowell12Nanthi S.Bolan et al. and Likens,1988;Muller et al.,2009;Sanderman et al.,2008;Zsolnay and Steindl,1991).In typical soils,DOM concentrations may decrease by50–90%from the surface organic layers to mineral subsoils(Cronan and Aiken,1985;Dosskey and Bertsch,1997;Worrall and Burt,2007).Similarly,fluxes of DOM in surface soil range from10to85g C mÀ2yrÀ1,decreasing to2–40g C mÀ2 yrÀ1in the subsoils(Neff and Asner,2001).In cultivated and pastoral soils,plant residues provide the major source of DOM,while in forest soils,litter and throughfall serve as the major source (Ghani et al.,2007;Laik et al.,2009).In forest soils,DOM represents a significant proportion of the total C budget.For example,Liu et al.(2002) calculated the total C budgets of Ontario’s forest ecosystems(excluding peat lands)to be12.65Pg(1015g),including1.70Pg in living biomass and10.95 Pg in DOM in soils.Koprivnjak and Moore(1992)determined DOM concentrations and fluxes in a small subarctic catchment,which is composed of an upland component with forest over mineral soils and peat land in the lower section.DOM concentrations were low(1–2mg LÀ1)in precipita-tion and increased in tree and shrub throughfall(17–150mg LÀ1),the leachate of the surface lichens and mosses(30mg LÀ1),and the soil A horizon(40mg LÀ1).Concentrations decreased in the B horizon(17mg LÀ1)and there was evidence of strong DOM adsorption by the subsoils.Khomutova et al.(2000)examined the production of organic matter in undisturbed soil monoliths of a deciduous forest,a pine plantation,and a pasture under constant temperature(20 C)and moisture.After20weeks of leaching with synthetic rain water at pH5,the cumulative values of DOM production followed:coniferous forest>deciduous forest>pasture,the difference being attributed to the nature of carbon compounds in the original residues.The residues from the coniferous forest were found to contain more labile organic components.Among ecosystems types,Zsolnay(1996)indicated that DOM tends to be greater in forest than agricultural soils:5–440mg LÀ1from the forest floor compared with0–70mg LÀ1from arable soils.Other studies have also indicated greater concentrations of DOM and concentrations in grasslands than in arable soils(Ghani et al.,2007;Gregorich et al.,2000;Haynes, 2000).In general,DOM concentration decreases in the order:forest floor> grassland A horizon>arable A horizon(Chantigny,2003).The rhizosphere is commonly associated with large C flux due to root decay and exudation(Muller et al.,2009;Uselman et al.,2007;Vogt et al., 1983).Microbial activity in the rhizosphere is enhanced by readily available organic substances that serve as an energy source for these organisms (Paterson et al.,2007;Phillips et al.,2008).Because of their turnover,soil microbial biomass is also considered as an important source of DOM in soils (Ghani et al.,2007;Steenwerth and Belina,2008;Williams and Edwards, 1993).Thus,microbial metabolites may represent a substantial proportionDissolved Organic Matter13 of the soil’s DOM.It may well be that the rate of DOM production and extent of DOM dynamics in soil is regulated by the rate of litter/residue incorporation in soils,kinetics of their decomposition,and various biotic and abiotic factors(Ghani et al.,2007;Kalbitz et al.,2000;Michalzik and Matzner,1999;Zech et al.,1996).In summary,the various C pools in an ecosystem represent the sources of DOM in soils.Due to their abundance,recently deposited litter and humus are considered the two most important sources of DOM in forest soils. Similarly,recently deposited crop residues and application of organic amendment such as biosolids and manures are the most important sources of DOM in arable soils.However,the role of root decay and/or exudates and microbial metabolites cannot be downplayed in both forested and arable ecosystems.3.Properties and Chemical Composition ofDissolved Organic Matter in Soils3.1.Structural componentsBecause DOM is a heterogeneous composite of soluble organic compounds arising from the decomposition of various carbonaceous materials of plant origin,including soluble microbial metabolites from the organic layers in the case of forest ecosystem,DOM constituents can be grouped into “labile”DOM and“recalcitrant”DOM(Marschner and Kalbitz,2003). Labile DOM consists mainly of simple carbohydrate compounds(i.e., glucose and fructose),low molecular weight(LMW)organic acids,amino sugars,and LMW proteins(Guggenberger et al.,1994b;Kaiser et al.,2001; Qualls and Haines,1992).Recalcitrant DOM consists of polysaccharides (i.e.,breakdown products of cellulose and hemicellulose)and other plant compounds,and/or microbially derived degradation products(Marschner and Kalbitz,2003)(Table3).Soil solution DOM consists of LMW carbox-ylic acids,amino acids,carbohydrates,and fulvic acids—the first comprising less than10%of total DOM in most soil solutions and the last(i.e.,fulvic acid)being typically the most abundant fractions of DOM(Strobel et al., 1999,2001;Thurman,1985;van Hees et al.,1996).Dissolved organic matter is separated into fractions based on solubility, molecular weight,and sorption chromatography.Fractionation of DOM by molecular size and sorption chromatography separate DOM according to properties(hydrophobic and hydrophilic)which regulate its interaction with organic contaminants and soil surfaces.The most common technique for the fractionation of aquatic DOM is based on its sorption to non-ionic and ion-exchange resins(Leenheer,1981).。
