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Soil Biology & Biochemistry 52 (2012) 29e32

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Soil Biology & Biochemistry

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Cycling downwards e dissolved organic matter in soils

Klaus Kaiser a, Karsten Kalbitz b,*

a Soil Sciences, Martin Luther University Halle-Wittenberg, von-Seckendorff-Platz 3, 06120 Halle (Saale), Germanyb Earth Surface Science, Institute for Biodiversity and Ecosystem Dynamics (IBED), University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

a r t i c l e i n f o

Article history:Received 29 August 2011Received in revised form29 March 2012Accepted 2 April 2012Available online 2 May 2012

Keywords:AdsorptionConceptual modelCo-precipitationDissolved organic matterMicrobial processingSoil organic matter

* Corresponding author. Tel.: þ31 (0)20 525 7457;E-mail addresses: klaus.kaiser@landw.uni-halle.de

(K. Kalbitz).

0038-0717/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.soilbio.2012.04.002

a b s t r a c t

Dissolved organic matter has been recognized as mobile, thus crucial to translocation of metals, pollutantsbut also of nutrients in soil. We present a conceptual model of the vertical movement of dissolved organicmatter with soil water, which deviates from the view of a chromatographic stripping along the flow path.It assumes temporal immobilization (sorptive or by co-precipitation), followed by microbial processing,and re-release (by desorption or dissolution) into soil water of altered compounds. The proposed schemeexplains well depth trends in age and composition of dissolved organic matter as well as of solid-phaseorganic matter in soil. It resolves the paradox of soil organic matter being oldest in the youngest part ofthe soil profile e the deep mineral subsoil.

� 2012 Elsevier Ltd. All rights reserved.

1. The concept

The movement of dissolved organic matter (DOM) is significantto the cycling and distribution of nutrients and carbon withinand between ecosystems and contributes to soil forming processes(e.g., Kalbitz et al., 2000). Most DOM produced in terrestrialecosystems becomes mineralized or retained in soils, with onlya part reaching aquatic systems. The rise in dissolved organic carbon(DOC) concentrations across Europe and North America during thelast decade has been attributed to increasing DOM leaching fromsoils (Roulet and Moore, 2006; Monteith et al., 2007). Also, DOM isa potential source of the stabilized carbon occurring in subsoils(Kalbitz and Kaiser, 2008; Schmidt et al., 2011) even though accu-mulation is not infinite (Guggenberger and Kaiser, 2003; Kalbitz andKaiser, 2008).

Rainwater, usually low in dissolved organic constituents,becomes increasingly enriched in DOMwhen passing the vegetationcover and the organic-rich upper soil compartments. In conse-quence, waters draining the organic layers and the upper mineralsoil horizons are rich in DOM, often causing intensive colour. Incontrast, water in subsoils has little DOM and is rather colourless(Kalbitz et al., 2000). In linewith the colour, DOMof surface-near soil

fax: þ31 (0)20 525 7832.(K. Kaiser), k.kalbitz@uva.nl

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compartments is rich in phenolic compounds, deriving from lignin,while that of subsoils is dominated by carbohydrates and nitrogen-rich compounds. Generally, DOM in the upper soil profile hasa vegetation-type signature, with lignin-derived phenols and plant-derived carbohydrates dominating, while most subsoil DOM is ofmicrobial origin (Guggenberger and Zech, 1994; Kaiser et al., 2004).

The initial idea explaining this phenomenon assumes produc-tion of soluble organic compounds during the decomposition oflitter and humus in surface-near, organic-rich soil compartments.These compounds then dissolve in the water percolating throughand are transported into underlying soil layers (McDowell andLikens, 1988). Here, most of the DOM is retained, either by sorp-tion to or by co-precipitation with reactive mineral phases, such asaluminium and iron hydrous oxides (McDowell and Likens, 1988).Those physico-chemical processes cause preferential removal fromsolution of phenolic acids while carbohydrates and nitrogen-richcompounds remain dissolved (Kaiser et al., 2004) and becomeeventually leached from soil and into the hydrosphere, where theyfuel biological cycles (Hopkins et al., 1998).

Consequences of that vieware that (I) soil acts towards DOMas achromatographic system, with the more sorptive (surface-reactive)compounds being retained and themoremobile ones being leached(Neff and Asner, 2001; Guggenberger and Kaiser, 2003); and(II) recalcitrant compounds remain in soil and become stabilizedagainst further biodegradation by sorptive interactions (Kaiserand Guggenberger, 2000; Kalbitz et al., 2005), while more labileand nutrient-rich matter is exported (Kaiser et al., 2004).

K. Kaiser, K. Kalbitz / Soil Biology & Biochemistry 52 (2012) 29e3230

However, there are clear indications that the transport of DOMthrough soil is not strictly following that scheme. Recent studiesshowed that DOM in surface-near compartments has a near-modern radiocarbon signature, thus comprises mainly recentphotosynthesis products (Sanderman et al., 2008; Fröberg et al.,2009). This is in good agreement with the assumption of DOMproduced during the decomposition of litter material. In contrast,subsoil DOM is often depleted in radiocarbon, which suggests thepresence of either aged compounds or compounds incorporatingolder carbon (Schiff et al., 1997; Sanderman et al., 2008; Fröberget al., 2009). In fact, little DOC derived from recent litter reachesthemineral subsoil (e.g., Fröberg et al., 2007; Hagedorn et al., 2012).Simple chromatographic stripping, as originally proposed, shouldresult in fairly similar radiocarbon contents of topsoil and subsoilDOM, with constituents deriving from fresh litter dominating. Also,subsoil DOM, despite being rich in nutrients (nitrogen, phosphorus)and comprising mainly carbohydrates, is hardly decomposable(Schwesig et al., 2003). This suggests that subsoil DOM, having thesignature of highly degraded organic material, seems to be ratherthe left over of decomposition than an energy and nutrient source.

Concepts explaining all the aspects of DOM within soil profilesneed to include strong retention of plant-derived compounds insurface-near compartments in exchange for older, partly microbialmetabolites, which then migrate into underlying soil layers. Partof the exchange could be due to physico-chemical displacementof previously sorbed plant-derived compounds by younger, morestrongly sorbing ones. In addition, DOM sorbed or co-precipitatedand forming into mineral-associated organic matter is turning overslow e but still undergoes microbial processing, resulting in partialdegradation (mineralization) but also transformation (Kalbitz et al.,

Fig. 1. Cycling of dissolved organic matter (DOM) in soils. The processes as illustrated in thentire profile. In consequence of continuous sorption and precipitation as well as of microbicompounds decrease with soil depth while those of microbial metabolites and aged/mic(SOM) changes accordingly. Depending on soil properties and hydrological conditions, part osoil without intimate contact with the soil matrix.

2005; Mikutta et al., 2007). The transformationedegradation prod-ucts may dissolve directly in soil water or are more easily desorbablethan the originally sorbed compounds (Fig. 1). Consequently, DOM inthe (deeper) mineral soil is not only result of physico-chemicalstripping during transport (fractional sorption and co-precipitation)but also of microbial processing and subsequent release of organicmatter previously sorbed in overlaying horizons.

The proposed concept explains both the compositional changes,i.e., from predominately plant-derived compounds, includingphenols, to predominatelymicrobial products, as well as the increasein radiocarbon age of DOM with depth. The driving factors are thesteady input of surface-reactive plant-derived compounds, whichforce less strongly binding compounds to move further down, andthe slowly proceeding degradation of bonded organic matter, lettingit grow older but at the same time rending it less strongly binding,thus prone to future translocation.

The proposed conceptual model also offers an immediateexplanation of the increase in radiocarbon age as well as of thecompositional change of solid-phase organic matter with depthobserved inmany soils (e.g., Rumpel et al., 2002;Mikutta et al., 2009;Schmidt et al., 2011). Note, the radiocarbon age of subsoil organicmatter contrasts sharply the assumption of roots being a majorsource of deep soil carbon. There is almost no awareness of the oldestorganicmatter being located in the youngest part of the soil profileethe deepmineral subsoil. So, the oldest organic matter resides at theweathering front. That means, aged organic matter has to be trans-ported down, with migration of DOM being a likely source of thesubsoil organic matter associated with mineral phases. Tipping et al.(2012) showed that DOM becoming stable mineral-associatedsoil organic matter had mean residence times of 100e200 years.

e enlarged outtake on the left hand side of the figure take place at throughout the soilal processing, desorption and dissolution the proportions of more recent plant-derivedrobially processed plant-derived compounds will increase. Solid soil organic matterf the DOM portion will be transported along preferential flow paths, thus pass through

K. Kaiser, K. Kalbitz / Soil Biology & Biochemistry 52 (2012) 29e32 31

By contrast, soil organic matter not associated with minerals andderiving from roots, had residence times of 20e30 years.

Overall, composition and age of DOM mirrors the compositionand age of solid-phase organic matter at a given soil depth (Fig. 1),being more similar to mineral-associated than to bulk soil organicmatter (Sanderman et al., 2008). Bulk organic matter includes rela-tive fresh litter material (debris), while the mineral-associatedmatter is made up by processed compounds, likely in equilibriumwith DOM. Therefore, DOM is probably not only indicative fortransformation and transport processes but is representative for thecycling of soil organic matter (SOM) in general.

The conceptual model applies tomineral soils where percolatingwater controls transport processes (e.g., Podzols, Cambisols,Luvisols, Andosols, Ferrasols). It does not apply to soils either withstagnating (Stagnosols) or ascending water (Calcisols, Solonetz,Solonchak, Gleysols) and to organic soils. Bioturbation (e.g., inChernozems and other soils featuring mollic topsoils), peloturba-tion (i.e., in Vertisols) and preferentialflow in largemacroporesmayfavour transport of particulate organic matter in subsoils. So, thesteep gradient in radiocarbon age and organic matter compositionas described above should be less pronounced and DOM of minorimportance to SOM formation. Fast water movement mightdecrease sorption/co-precipitation as well as microbial processing,resulting in litter-derived DOM transported deeper into the subsoil(e.g., Fröberg et al., 2007). Rapid water movement might bethe reason that modelling of DOC dynamics in such soils does notrequire consideration of microbial processes (Tipping et al., 2012).

The major advantage of the proposed concept is its simplicity;nevertheless, it is capable to explain complex relations as wellas seemingly contrasting observations (e.g., the poor degradabilityof subsoil DOM despite its content of microbial sugars and aminosugars), and it interlinks soil solid phase and solutionprocesses. Also,it is simpler than the conceptual model proposed by Sandermanet al. (2008). They introduced a “potentially exchangeable orreactive soil C pool” as an intermediate fraction between solid anddissolved organic matter, controlling amount and composition ofDOM at different depths of the soil profile.

Our concept consistently explains DOM in the soil profileas a result of continuous sorption and precipitation, combined withmicrobial processing and subsequent desorption and dissolution.That means, controls on these processes (e.g., reviewed by Kalbitzet al., 2000) will determine concentrations and fluxes of DOM andthe related soil organic matter accumulation. The conceptual modellinks the transport of DOMwithin the soil profile to composition andradiocarbon age of organic matter in subsoils. The timescale of ourconcept is that of SOM formation, ranging from very short periods(sorption/precipitation, turnover of very labile organic components)to millennia (turnover of SOM in the mineral subsoil). Tipping et al.(2012) demonstrated the importance to link SOM turnover to DOMtransport by applying the DyDOC model to an extensive experi-mental dataset. Implementation of our concept into process-orientedmodels is feasible and of great value for predictive modelling; also, itwill allow for quantitative validation of the concept. Overall, usingDOM as an indicator for processes controlling the turnover of soilorganic seems possible.

The potential of using DOM as an indicator for environmentalchanges and a tool for classifying ecosystems has been discovered inaquatic and marine sciences long ago and so an analogue use in soilscience seems desirable. This will require a much stronger focuson DOM composition, especially on molecular markers, instead ofthe simple records of concentrations that were prevailing the lastdecades. The composition of DOM can indicate processes governingthe accumulation and stabilization of soil organic matter. Forexample, preferential removal from solution is common for plant-derived compounds, attributed to their strong affinity for binding

sites. The parallel increase of microbial metabolites indicatesthat part of the binding is due to displacement of previously boundcompounds; near equivalent release of microbial matter wouldindicate a largely complete occupation of binding sites, thus limita-tions to sorptive accumulation of organicmatter. Also, determinationof temporal changes of DO14C at different soil depths may allow forconclusions on rates of organic matter accumulation and stabiliza-tion in different soil layers/horizons. That will facilitate quantitativetesting of the proposed concept as well. Considering given envi-ronmental conditions, such data can be used for deducing scenariosof future soil processes and related effects on climate (e.g., carbonsequestration, emission of greenhouse gases; Bengtson andBengtsson, 2007; McCarthy, 2005) and water quality (e.g., transferof terrestrial, mainly plant-derived organic matter into aquaticsystems; Roulet andMoore, 2006) in an efficient way, for DOMbeinga small but highly sensitive fraction of organic matter in soil.

Acknowledgements

We are grateful to Chiara Cerli and Marion Schrumpf for helpfuldiscussions and constructive comments on an earlier version of thismanuscript. We also appreciate the comments of Tim Moore andone anonymous reviewer.

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