The role of DOM sorption to mineral surfaces in the
preservation of organic matter in soils
Klaus Kaiser *, Georg Guggenberger
Institute of Soil Science and Soil Geography, University of Bayreuth, 95440 Bayreuth, Germany
Abstract
Sorption of dissolved organic matter (DOM) is considered to be a major process in the preservation of organic matter (OM) in marine sediments. Evidence for this hypothesis includes the close relationship between sediment surface area (SA) and organic carbon (OC) concentrations and the strongly reduced biological degradability after DOM has sorbed to mineral surfaces. The aim of this study was to discuss the possibility of a similar process in the soil envir-onment. We accomplished this by gathering information from the literature, and by an evaluation of our own studies on DOM sorption and accumulation of OM in soil. We found that in soil a close association of OM with the mineral matrix exists. Both the concentration of soil OM associated with the mineral matrix, and the sorption of DOM are related to reactive mineral phases such as Al and Fe oxyhydroxides. Sorption of DOM derived from the oxidative decomposition of lignocellulose to Al and Fe oxyhydroxides involves strong complexation bondings between surface metals and acidic organic ligands, particularly with those associated with aromatic structures. The strength of the sorption relates to the surface area but more importantly to the surface properties of the sorbing mineral phase. The sorption of a large part of DOM is hardly reversible under conditions similar to those during sorption (hysteresis). Because sorption of the more labile polysacchariderived DOM on mineral surfaces is weaker, adsorptive and de-sorptive processes strongly favour the accumulation of the more recalcitrant lignin-derived DOM. In addition, we found the soil OM in an alluvial B horizon and in the clay fraction of a topsoil strongly resembling lignin-derived DOM from the overlying forest ¯oors. Hence, it seems likely that sorption of DOM contributes considerably to the accumulation and preservation of OM in soil. However, this does not result in a signi®cant relationship between OC concentration and SA. Reasons for that ®nding may be the ''masking'' of mineral surfaces by adsorbed OM, the clustering of OM patches at highly reactive sites of metal hydroxides, and/or the absence of a relationship between SA and the concentration of surface-active Fe and Al oxyhydroxides in some soil types. Overall, we conclude that sorptive preservation of OM in soil is aected by the chemical structure of the sorbing DOM and the surface properties of the mineral matrix. Localisation and conformation of sorbed OM remains unclear and therefore should be subject of fur-ther research.#2000 Elsevier Science Ltd. All rights reserved.
Keywords:Dissolved organic matter; Sorption of DOM; Mineral surfaces; Preservation of OM
1. Introduction
In soils and in various wetland ecosystems, vascular plants are the most important source of organic material. During bacterial and fungal degradation of detrital lig-nocellulose, the polymer is partially hydrolysed and
solubilized through the activity of exoenzymes (Hoppe, 1983; Haider et al., 1985). Oxidatively altered water-soluble intermediates of lignocellulose decomposition are released into the wetland environment (Moran and Hodson, 1989) and into soil solution (Guggenberger et al., 1994a) where they represent a major proportion of the dissolved organic matter (DOM). Besides, by inducing partial solubilization of plant residues, micro-organisms themselves add to DOM by release of exopolysaccharides and by cell lysis (Guggenberger et al., 1994a).
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* Corresponding author. Tel.: 921-55-2318; fax: +49-921-55-2246.
The organic forest ¯oor layer is the major source of DOM in many forest soils as re¯ected by large concentra-tions of dissolved organic carbon (DOC) in seepage waters beneath the forest ¯oor (up to 8 mmol lÿ1; Cronan
and Aiken, 1985; Guggenberger and Zech, 1993). On contact with mineral soil horizons rich in Al and Fe oxides and hydroxides, DOC concentrations decrease sharply due to sorption in most soils (McDowell and Likens, 1988; Kaiser et al., 1996). Export of DOC to riverine systems is higher from soils with organic layers as major hydrologic pathway compartments (Cronan, 1990), from soils with aquic moisture regime and reducing conditions (McLaughlin et al., 1994; Hagedorn et al., 2000), and from slightly developed shallow soils (Kaiser et al., 1996) due to the lack of eective sorbents for DOM.
In the streams, DOM can be rapidly removed from solution by chemical adsorption onto sediment parti-cles, either in the streambed (McDowell, 1985; McKnight and Bencala, 1990) or suspended in the water (Ertel et al., 1986). In soils as well as in riverine sedi-ments, the capacity to adsorb DOM relates to the pre-sence of Al and Fe oxides and hydroxides (McKnight et al., 1992; Kaiser et al., 1996).
Hedges et al. (1997) provided evidence that terrestrial OM adsorbed to ®ne suspended riverine sediment material can be accumulated in coastal marine sediments, whereas DOM discharged by rivers into the open sea is subjected to rapid oxic biodegradation and/or photolysis (Mopper et al., 1991; Opsahl and Benner, 1997). Using the stable carbon isotope approach, Keil et al. (1997) estimated that usually more than half of the total OM bound to the surfaces of river-derived mineral particles in the Amazon delta is of terrestrial origin. However, the proportion of marine, phytoplankton-derived OM adsorbed to mineral grains, is much higher in sediments accumulating outside deltas along continental shelves and upper slopes (Showers and Angle, 1986).
Along continental shelves and upper slopes, investi-gations on diagenetically consolidated sediments have shown that more than 90% of organic carbon (OC) cannot be physically separated from its mineral matrix, and that this strongly mineral-associated OC shows a direct correlation to the surface area (SA) of the sediments, giving calculated surface loadings of 0.6±1.5 mg OC mÿ2(Keil et al., 1994a; Bergamaschi et al., 1997). These loadings were considered to represent the ''monolayer equivalent'' (ME) range for OM associated with mineral particles (Mayer, 1994a). Hedges and Keil (1995) assumed that this ®nding is indicative of DOM sorption to the mineral grains. As OM desorbed from sediments was mineralised by the microbial community present in the seawater at a rate of ®ve orders of magnitude faster than the sorbed OM, Keil et al. (1994a) concluded that association of OM with minerals provides protection against rapid microbial decay (=sorptive preservation)
and that for marine sediments sorption of OM is the largest single factor controlling OM preservation.
Mayer (1994b) extended his work on the relationship between the OC concentration and the N2-BET SA
from marine sediments to a range of North and Central American topsoils. After removal of low-density parti-culate OM (d< 1.9 g cmÿ3), he found a close relation-ship between the remaining OC and the N2-BET SA
with a good ®t into the ME range for about half of the soils. Soils with high carbonate content, low pH, or poor drainage showed OC concentrations above the ME level, whereas OC concentrations below the ME level were found for arid soils. The latter ®nding was attrib-uted to low primary production (Mayer 1994b).
Haider (1992) reported for soils that OM must be desorbed from the mineral matrix to render the organic substances susceptible to microbial decomposition. This was con®rmed by Jones and Edwards (1998) who showed that sorption to clay minerals and ferric hydroxide reduces the microbial utilisation of labile organic molecules such as glucose and citrate. In turn, biological decomposition of OM desorbed from soil is rapid (Nelson et al., 1994). As Hedges and Oades (1997) emphasised the generally comparative organic geochemistries of soils and sedi-ments, the objective of this paper is to discuss our results on the role and the mechanisms of DOM sorption to soils in light of recent knowledge on sorption as an OM-conserving process in marine sedimentary systems.
2. Materials and methods
2.1. Dissolved organic matter
Dissolved organic matter for the sorption experiments was obtained from the Oa horizon of the mor layer of an Entic Haplorthod by adding 2 l distilled H2O to 200
g of organic material. After 15 min of stirring, the sus-pension was allowed to stand for 18 h and then ®ltered through 0.45-mm polysulfone membrane ®lters. The DOC concentration of the stock solution was 8.4 mmol lÿ1. The extraction was carried out just before the sorp-tion experiments. A proporsorp-tion of the solusorp-tion was separated into a hydrophilic and a hydrophobic fraction according to Aiken and Leenheer (1993) using XAD-8 resin (Rohm and Haas Comp., Philadelphia, PA), pro-tonated and ®nally freeze dried. These samples were used for wet-chemical analyses and 13C-NMR
spectro-scopy. Detailed information on the composition of the DOM is given in Kaiser and Zech (1998, 1999) and Kaiser et al. (1997).
2.2. Soils and mineral phases
Germany, Sweden and The Netherlands. All sampling sites were forested. The dominant tree species were Norway spruce (Picea abies (L.) Karst.), European beech (Fagus sylvatica L.), Scots pine (Pinus sylvestris
L.), and European larch (Larix europaea Mill.). The sampled soil pro®les represented the main soil orders of temperate climatic zones: Spodosols, Vertisols, Mollisols, Al®sols, Inceptisols, and Entisols (Soil Survey Sta, 1994). The clay mineralogy of the studied soils comprised kaolinitic (2 pro®les), illitic (12 pro®les), vermiculitic (3 pro®les), smectitic (3 pro®les) and mixed systems (14 pro®les). The study was restricted to topsoils and illuvial subsoil horizons as we considered these soil compartments to receive DOM permanently from the percolation water. Further informations on the soils used are presented by Kaiser et al. (1996).
The bulk samples were air dried and passed through a 2-mm sieve. Discrete particulate OM not associated with minerals, mainly plant debris (Amelung et al., 1998), was removed from subsamples by heavy liquid ¯otation using sodium polytungstate (Sometu, Berlin, Germany) at a density of 1.6 g cmÿ3. Soil samples (5 g)
were slurried in 25 ml of sodium polytungstate solution, shaken for 24 h, and centrifuged at 10 000gfor 30 min. The supernatants were removed and the settled soil materials washed intensively with deionized H2O until
the electrical conductivity of the solution was < 50mS cmÿ1. Thereafter, the samples were air dried.
Total C content was measured on ground subsamples of the bulk soil and the density fraction > 1.6 g cmÿ3with a CHNS analyser (Vario EL, Elementar GmbH, Hanau, Germany). In carbonate-free samples, the total C value represented the total concentration of OC. For samples containing carbonate, OC was determined by a second measurement after destruction of carbonates with 10% HCl. Carbonate C (CO3±C) was calculated from the
dif-ference between total C and OC. Aluminium and Fe in amorphous oxides were extracted from bulk soil samples with 0.2 M NH4-oxalate (pH 3) according to Schwertmann
(1964). Iron in amorphous and crystalline oxides was esti-mated by means of the Na-dithionite-citrate-bicarbonate method (Mehra and Jackson, 1960). Aluminium and Fe in the extracts were measured with atomic absorption spectrometry (AA-400, Varian Inc., Palo Alto, CA).
Amorphous Al(OH)3 gel was precipitated from a
solution of 1 M Al(NO3)3by slow addition of NaOH
until pH of 7 was reached (Huang et al., 1977). The precipitate was washed with deionized water, dialysed against deionized water for 7 days, then freeze dried and ®nally sieved to particles <0.63 mm. The product was X-ray amorphous and had a N2-BET SA (see below) of
285 m2 gÿ1. Goethite (-FeOOH) was prepared by
adjusting the pH of a 0.5 M FeCl3solution to 12 with
NaOH (Atkinson et al., 1967). The precipitate was aged at 333 K for 48 h, then washed with deionized water and dialysed against deionized water for 14 days. Finally, it
was freeze dried and sieved to particles <0.63 mm. X-ray diraction showed that the sample was pure and highly crystalline. It had a N2-BET SA of 47 m2gÿ1.
The organic matter for covering mineral surfaces was extracted with 0.05 M NaOH from the Oa horizon of a mor layer. Thereafter, the hydrophobic fraction was separated using XAD-8 resin and freeze dried. The pre-paration procedure of OM for the coverage of mineral surfaces is given in detail in Kaiser and Zech (1998).
Covers of oxyhydroxides on soil surfaces were pro-duced by suspending 200 g of soil material from the Bs horizon of an Entic Haplorthod in 1 l of a suspension containing the desired amount of a synthetic oxyhydroxide. The suspension was stirred for 10 min, allowed to stand for 24 h, and then centrifuged at 2000 rpm. The super-natant was removed and the soil material air dried. The added amounts of the oxides were 20, 40 and 80 g kgÿ1 for goethite and 20, 40, 80 and 120 g kgÿ1for amorphous
Al(OH)3. For all treatments, there was a complete
retention of the added oxides by the soil material. For preparing OM coatings, the desired amount of freeze-dried OM (20, 35 and 80 g kgÿ1) was dissolved in 1 l
H2O and 200 g soil material was added. The suspension
was shaken for 24 h, then centrifuged at 2500g. After removal of the supernatant the soil material was air dried. Further details on the covering of mineral surfaces are given in Kaiser and Zech (1998).
The SA of the mineral phases, of the high density frac-tion of soil samples, and of samples with and without OM or oxyhydroxide coverage was determined by multiple-point BET (Brunauer-Emmett-Teller) adsorption isotherms (Gregg and Singh, 1982) using automatic surface analysers (Quantasorb Surface Analyzer, Quantachrome, Inc., Syosset, CA, or Gemini 2370, Micromeritics Instrument Corp., Norcross, GA) and N2as the adsorbent. Prior to
the SA analyses, the samples were degassed for 24 h at 25C under continuous stream of He. This procedure
was chosen to avoid thermal degradation of OM and conversion of less ordered mineral phases into more crystalline ones which may take place during high-temperature degassing (> 50C). For example, already
at 40C, amorphous Al(OH)
3is converted into gibbsite
(Kyle et al., 1975). The conversion of poorly ordered into crystalline mineral phases is associated with a strong reduction in SA (e.g. Schwertmann and Cornell, 1991). This is of great importance for many acid soils as their SA is mainly due to Al and Fe oxides (Borggaard, 1982; Feller et al., 1992).
2.3. Sorption to and desorption from soils and mineral phases of dissolved organic matter
Al(OH)3, goethite). Sorption on soil materials was tested
by adding 40 ml of a solution containing 0±6.0 mmol DOC lÿ1to 8 g of soil material. In the case of the pure mineral phases, 200 ml of solution were added to 2.5 g of the synthetic oxyhydroxides. All added solutions had a pH value of 3.95 and an ionic strength 0.002 mol lÿ1. The suspensions were shaken (60 rpm) at 293 K for 24 h and then ®ltered through 0.45-mm polysulfone mem-brane ®lters. In the ®ltrate, the concentration of total DOC and the distribution among hydrophilic and hydrophobic DOC (Aiken and Leenheer, 1993) were determined. Due to the release of indigenous OM from the soils, the results of the sorption experiments could not be analysed by Freundlich or Langmuir isotherms. For systems in which native adsorbed substances need to be considered, the initial mass approach (Nodvin et al., 1986) is useful. In this approach, the concentration of a substance adsorbed or released (normalised to soil mass) is plotted against the initial concentration of the substance (normalised to soil mass). In addition to sorption experiments, desorption of sorbed OM was tested for the pure mineral phases and one subsoil, a 3 Bw of an Oxyaquic Haplumbrept. Detailed descriptions of the DOM sorption and desorption experiments with soils and mineral phases are given by Kaiser et al. (1996) and Kaiser and Zech (1998, 1999).
The analytical and spectroscopic methods (13C-NMR,
DRIFT) used to study the nature of the DOM binding to soil and Al and Fe oxides/hydroxides are presented in Kaiser et al. (1997).
2.4. Comparison of forest ¯oor-derived dissolved and mineral soil organic matter
Soil OM from the illuvial Bs horizon of a forested (Picea abies (L.) Karst.) Entic Haplorthod in NE Bavaria, Germany, was extracted by 0.5 M NaOH at a soil-to-solution ratio of 1:6 (w/v). The Bs horizon derived from the same soil pro®le where the mor forest ¯oor layer for the extraction of DOM was taken from. After ultrasonic dispersion (100 J mlÿ1) and 24 h of
agitation, the suspensions were centrifuged (2500 g). The supernatants were passed through glass ®bre ®lters (GF/F, Whatman, Inc., Spring®eld Mill, UK), diluted by addition of deionized water, then pumped through a column ®lled with a strongly acidic cation exchange resin (AG MP-50, BioRad Laboratories, Richmond, CA) in order to remove cations from the solution. The column euent was acidi®ed to pH=2 by addition of HCl and thereafter pumped through a column ®lled with a macroporous resin (Amberlite XAD-8, Rohm and Haas Comp., Philadelphia, PA). Organic matter adsorbed to the resin was recovered by back-¯ush elu-tion of the column with 0.1 M NaOH. The desorbed OM was protonated by pumping the solution through a column ®lled with AG MP-50 and ®nally freeze-dried.
Liquid-state 13C-NMR spectra of the
alkaline-extractable soil OM and forest ¯oor-derived DOM samples (see above) were recorded on an AM 500 spectro-meter (Bruker Analytik GmbH, Karlsruhe, Germany) at a 13C-NMR resonance frequency of 125.77 MHz.
Sample quantities of 120±180 mg were dissolved in 3 ml of 0.5 M NaOD in 10 mm tubes. At a pulse angle of 45,
2.0-s pulse delay and inverse-gated decoupling, about 15000 scans were accumulated for each sample (Preston, 1996). The signal-to-noise ratio was improved by using a line-broadening of 100 Hz. Resonance areas were calcu-lated by electronic integration. In general, the13C-NMR
spectra of natural OM consisted of four regions (Wil-son, 1987): (a) the alkyl region (0±50 ppm) mainly representing C atoms bonded to other C atoms (methyl, methylene, and methine groups), (b) the O-alkyl region (50±110 ppm) mainly representing C bonded to O or N (carbohydrates, alcohols, ethers, N-substituted carbon), (c) the aromatic region (110±160 ppm) representing C in aromatic systems and ole®ns, and (d) the carbonyl region (160±210 ppm) including carboxyl and amide C (160±190 ppm). Because of the small content of N in the samples (<1.6%) we assumed that the contribution of compounds containing N to the spectral signals was small. Furthermore, the abundance of signals between 160 and 190 ppm ®tted the amount of carboxyl groups as assessed by titration or IR spectroscopy. This accords with previous studies (e.g. Celi et al., 1997). We, therefore, con-sider the signals within the carbonyl region to be mainly due to carboxyl C.
In addition, forest ¯oor-derived DOM and OM in the clay fraction from the A horizon beneath the forest ¯oor layer of a forested (Picea abies(L.) Karst.) Typic Dystro-chrept were characterised by wet-chemical methods (see below). The DOM was collected using zero-tension lysimeters beneath the forest ¯oor layer. Details on the site and the particle-size fractionation are given by Guggenberger et al. (1994b, 1995).
Clay fraction-associated and dissolved OM were ana-lysed for carbohydrates (gas chromatographic analysis of monomers after hydrolysis with 4 M tri¯uoroacetic acid; Amelung et al., 1996) and lignin-derived con-stituents (gas chromatographic analysis of monomers released by alkaline CuO oxidation; Hedges and Ertel, 1985).
3. Results and discussion
3.1. Coverage of soil mineral surfaces by organic matter
Sweden, and The Netherlands. Like in the studies on American topsoils (Mayer, 1994b) and on marine sedi-ments (Mayer, 1994a,b; Keil et al., 1994b; Bergamaschi et al., 1997), the major proportion of OC (76±99%) in the subsoils and most of the topsoils was within the soil fraction > 1.6 g cmÿ3. Exceptions were a few topsoils which contained up to 77% of their OC in particulate matter. In contrast to the results reported by Mayer (1994a,b), Keil et al. (1994b), and Bergamaschi et al. (1997) ) there was only a weak correlation between OC concentration and SA of the soil fraction >1.6 g cmÿ3
(Fig. 1a). The slopes of the relationship between OC
concentration and SA of topsoils and subsoils were sig-ni®cantly dierent (P< 0.05). In addition, most soils were lying high above the ME range, and OC/SA ratios of up to 88 mg OC mÿ2were observed. On the other hand, we
found highly signi®cant correlations of OC concentra-tions with indicators for Al and Fe oxides and hydro-xides, e.g., oxalate-extractable Al or dithionite-citrate-bicarbonate extractable Fe (FeDCB; Fig. 1b). The slopes
of the relationship between OC and FeDCB
concentra-tions of topsoils and subsoils were not signi®cantly dif-ferent (P> 0.995).
The FeDCB-OC relationship was corroborated by
sorption experiments carried out on soil material of a spodic Bs horizon with dierent extents of coatings of goethite (-FeOOH) and amorphous Al(OH)3. Due to
the large SA of the hydrous oxides used (goethite: 47 m2gÿ1, amorphous Al(OH)
3: 285 m2gÿ1), the SA of the
soil increased strongly with increasing coatings (Fig. 2a). This exempli®es the large contribution of Al and Fe oxides/hydroxides to the SA of soils (see Borggaard, 1982; Feller et al., 1992). Dissolved OM sorption was strongly enhanced by hydrous oxides coatings (Fig. 2b, 2c), in particular by amorphous Al(OH)3 (Fig. 2b).
These results ®t well to laboratory studies on DOM sorption to minerals. Tipping (1981) and Davis (1982) identi®ed Fe and Al oxides and hydroxides as highly eective potential adsorbers for soluble OM.
However, not only Fe and Al oxide/hydroxide coatings but also sorbed OM may in¯uence the SA of a soil. Due to the small N2-BET SA of OM (< 1 m2gÿ1; Chiou et
al., 1990; Chiou et al., 1993), sorption of OM to oxide/ hydroxide surfaces in soil can reduce the SA of the sorbing material. This is illustrated in Fig. 3a, where the coating of a spodic Bs horizon with the XAD-8-adsorbable (hydrophobic) fraction (Aiken and Leenheer, 1993) of forest ¯oor-derived OM progressively decreases the N2-BET SA. Thus sorbed OM seems to ``mask''
mineral surfaces (Burford et al., 1964; Feller et al., 1992; Pennell et al., 1995) by reducing the surface roughness of the minerals. Additionally, organic macro-molecules may link small mineral particles to microaggregates. According to SuÈsser and Schwertmann (1983), the interior of these microaggregates may not be accessible for N2. Contrary eects of Al and Fe oxides and OM on
the SA of soil were also found by multiple regression analysis with SA as the dependent variable and OC and dithionite-citrate-bicarbonate extractable Fe as the independent variables (Kaiser et al., 1996).
The reduction in the surface roughness and/or the blockage of active sites for chemical bondings results in a decrease of further DOM sorption to the spodic Bs horizon covered with OM (Fig. 3b). This means that the N2-BET SA of minerals (i.e. the external surface) may
govern the potential amount of OM that can be adsor-bed, but once the OM is adsorbed the true SA of the mineral surface is not accessible any more. Hence,
Fig. 1. Organic carbon (OC) concentration vs. surface area (SA) in the soil fraction > 1.6 g cmÿ3(a) and
dithionite-citrate-bicarbonate extractable Fe (FeDCB) (b) for topsoils (n=41) and
masking of mineral surfaces by sorbed OM may be the reason for the non-conformity of the OC±SA relation-ship in the soils.
These results contrast with those of Mayer (1994a,b) who found no increase of SA of soils and marine sedi-ments after removal of OM by chemical oxidation. One possible reason for the dierent ®ndings may be the dierent treatments of the samples prior to the SA measurement. In our work, as in the studies of Feller et al. (1992) and Pennell et al. (1995), the samples were dried under a slow stream of N2 or He and at a
tem-perature of 4 105C. Mayer (1994a,b), Keil et al.
(1994b), and Bergamaschi et al. (1997) used higher dry-ing temperatures (150±350C). According to Miltner
and Zech (1997) such high temperatures may cause loss (up to 40% of total OC in form of CO2) and strong
alteration of OM. This may reduce the dierences between samples with and without chemical removal of organic matter.
3.2. Sorption of DOM to soils and minerals
While in the previous section quantitative aspects of DOM sorption to soils have been elucidated, this section
Fig. 2. In¯uence of coatings of amorphous Al(OH)3 and of
goethite on the surface area (SA) of a subsoil horizon (Bs) of an Entic Haplorthod (a) and dissolved organic carbon (DOC) sorption on this soil coated with dierent amounts of amor-phous Al(OH)3(b) or goethite (c). Error bars in (a) indicate the
standard error of the mean. Error bars in (b) and (c) represent least signi®cant dierences (0:01).
will focus on the type of bonding between DOM and soil minerals. As pointed out by Henrichs (1995), pre-servation of intrinsically labile organic compounds by sorption requires strong bonding between adsorbate and adsorber. In case the sorption is highly reversible, the compounds could be desorbed easily and decomposed thereafter.
The strong dependence of the DOM sorption on pH, the competition of DOM with speci®cally binding inorganic anions such as sulphate and phosphate for sorption sites, and the release of OHÿduring the sorption suggest that surface complexation of functional groups via ligand exchange is the most important process in the sorption of OM on mineral phases (Tipping, 1981; Mazet et al., 1990; Gu et al., 1994; 1995; Edwards et al., 1996; Weigand and Totsche, 1998). Especially the for-mation of bidentate complexes between two organic ligands inorthoposition of an aromatic ring and a metal at the surface of oxides and hydroxides causes a strong chemi-sorptive binding (Par®tt et al., 1977; Jekel, 1986; Gu et al., 1994, 1995). Such favourable chemical struc-tures are preferentially found in the lignocellulose-derived hydrophobic DOM fraction (Dai et al., 1996; Guggenberger et al., 1998). This fraction is also less biodegradable than the polysaccharide-derived hydro-philic fraction (Qualls and Haines, 1992b; Jandl and Sollins, 1997).
Kaiser et al. (1997) examined the organic compounds involved in the sorption of total forest ¯oor-derived DOM and of its hydrophobic fraction on intact soil cores by solution13C-NMR. They comparatively analysed the
DOC composition in the initial solution and in the euent (i.e. the portion of DOC not adsorbed). Likewise, they compared the chemical structures of DOC before and
after a batch sorption experiment with goethite and amorphous Al(OH)3using13C-NMR spectroscopy.
According to 13C-NMR spectroscopy, the
hydro-phobic DOM of the euents from the soil cores were depleted in carbonyl and aromatic C as compared with the hydrophobic fraction of the initial solution (Table 1). The same trends were observed after sorption of the hydrophobic fraction on goethite and Al(OH)3. In
agreement with results of McKnight et al. (1992) on sorption of riverine DOM to streambed Al and Fe hydrous oxides, this suggested that carboxyl groups bonded to aromatic structures are preferentially sorbed to hydrous Al and Fe components of soils. In contrast, alkyl C accumulates in the solution. The comparatively weak binding of aliphatic structures being low in carboxyl groups indicates that hydrophobic interactions are neg-ligible for DOM sorption.
Another useful approach to investigate the chemical structures involved in the sorption of DOM to mineral surfaces uses DRIFT spectroscopy. In this approach, spectra of a mineral phase with or without adsorbed OM are recorded. Thereafter, a dierence spectrum between the two spectra is calculated. The resulting spectrum, i.e., the spectrum of the OM sorbed on the mineral surface, is compared with that of the initial OM (Gu et al., 1994, 1995; Kaiser et al., 1997).
The DRIFT spectrum of total DOM prior to sorption (Fig. 4a) is dominated by the bands of carboxyl groups, including those of the CO stretching of protonated
carboxyl groups at 1725 cmÿ1and of the carboxylate at 1625 cmÿ1. The band at 1400 cmÿ1might result from complexed carboxylate. The spectrum of the hydro-phobic fraction generally resembles that of the total DOM. Dierences are a lower intensity of the shoulder
Table 1
Distribution of C moieties of the hydrophobic acidic fraction of DOM from the mor forest ¯oor layer of an Entic Haplorthod in the solution prior to the sorption experiment, in the euent of the sorption experiments with mineral soil cores, and in the ®ltrate of batch experiments with oxyhydroxide phases according to liquid-state13C-NMR spectroscopya
Sample C moieties (%) Carbonyl Cb
160±210 ppm
Aromatic C 110±160 ppm
O-alkyl C 50±110 ppm
Alkyl C 0±50 ppm
Solution before the sorption experiment
Original 21a 31a 28a 20a
Soil core euents
2Bw 18b 27b 29a 26b
Bw 19a 28b 28a 25b
Batch experiment ®ltrates
Al(OH)3 16c 24c 29a 31c
Goethite 17c 25c 28a 30c
a Repeated measurements on the same sample showed variations in the distribution of C moieties of42%; data from Kaiser et al.
(1997).
at 1465 cmÿ1, indicating less alkyl structures in the hydrophobic fraction, and a stronger intensity of the band at 1275 cmÿ1, suggesting a larger content of phenolic structures.
Sorption of total DOM to goethite (Fig. 4b) resulted in a sharp decrease of the band at 1715 cmÿ1. This was accompanied by a strong increase of the carboxylate band together with a shift from 1625 to 1600±1605 cmÿ1 and increasing absorption at 1400 cmÿ1. According to
Par®tt et al. (1977) and Gu et al. (1994, 1995) such observations are due to complexation of carboxyl groups with metals on the mineral surface resulting from ligand exchange reactions. The fact that the major part of the carboxyl groups seems to be involved in complexation reactions at the goethite surface suggests
that each organic macromolecule is sorbed by many bonds. Increased absorption occurred also at 1270 cmÿ1, which agrees with the suggestion of Jekel (1986) and Gu et al. (1994, 1995) that phenolic groups are also involved in the sorption of DOM on hydrous oxide sur-faces. Such a sorption with formation of multiple bonds per molecule (octopus eect of organic oligomers; Podoll et al., 1987) might reduce the desorption of OM from the surfaces of Al and Fe oxides and hydroxides.
Comparison of the smaller surface coverage by organic matter (0.39 mg C mÿ2; Fig. 4b) with the larger
surface coverage by organic matter (0.72 mg C mÿ2; Fig. 4c) showed that the band intensity at 1715 cmÿ1 was
more intensive for the latter. The larger surface coverage seems to enhance the interference between the organic polyelectrolytes at the mineral surface resulting in fewer ligands involved in the binding (Podoll et al., 1987). Hence, the number of bindings between OM and mineral phases, i.e. the strength of the bonding, depends on the degree of the surface coverage.
Kaiser et al. (1997) reported similar results for sorp-tion of total DOM on goethite, ferrihydrite and Al(OH)3. However, they found that the number of
ligands per organic molecule involved in binding reactions was larger on the goethite and Al(OH)3surfaces than on
the ferrihydrite surface. Though the surface coverage on the goethite exceeded that of ferrihydrite by factor of 4, the number of complexed carboxyl groups at the goethite surface was above that at the ferrihydrite surface. A possible reason for the dierent binding of DOM to goethite and ferrihydrite could result from the favour-able hydroxyl con®guration at the goethite surface. All faces of the goethite provide pairs of contiguous singly coordinated OH groups which are involved in the for-mation of bidentate surface complexes (BarroÂn and Torrent, 1996). None of those OH groups occur on any of the crystal faces of hematite (BarroÂn and Torrent, 1996) to which ferrihydrite resembles structurally (Towe and Bradley, 1967). This means that the strength of the binding and the extent of the sorptive preservation was not related solely to the SA but also to the surface properties of the sorbing mineral.
3.3. Desorption of DOM from soils and minerals
Up to now, we provided evidence that hydrous metal oxides are the most important sorbents for DOM in the soil environment and that the major bonding type is ligand exchange reactions. But only knowledge on the reversibility of the sorption process gives an estimation of the importance of DOM sorption to mineral surfaces in the preservation of OM in soils.
To study the hysteresis of DOM sorption, Kaiser and Zech (1999) carried out sorption-desorption experiments on a subsoil (3Bw) horizon and on hydrous oxides. Fig. 5 shows the linear relationship between the added and
Fig. 4. DRIFT spectrum of dissolved organic matter (DOM) deriving from the mor forest ¯oor of an Entic Haplorthod prior to sorption (a) and the spectra of DOM sorbed on goethite at small surface coverage (0.39 mg C mÿ2) (b) and at larger
the sorbed amount of DOC. The linear relationship indicated that the sorption capacity of the sorbing materials was not exhausted by the added amounts of DOM. Twenty-four hours after sorption, less than 3% of the sorbed OC was released from goethite and Al(OH)3 under solution conditions similar to those
during the sorption step. Also for the 3Bw horizon the desorption was low though it was not possible to quantify the exact amount as the soil material still released indi-genous OC. Comparable results have been reported for hematite (-Fe2O3) and the AB horizon of a Hapludult
(Gu et al., 1994; Qualls and Haines, 1992a). The rever-sibility of the sorption of OM even decreased with increasing residence time on the adsorbers (Kaiser and Zech, 1999).
Extraction with high concentrations of inorganic oxyanions which are known to compete with DOM for binding sites, such as SO42ÿand H2PO4ÿ(Tipping, 1981;
Gu et al., 1994), resulted in a considerable release of OC from goethite (Fig. 6a and 6b). In particular H2PO4ÿ, an
anion which forms strong bondings on Al and Fe oxide surfaces via complexation-ligand exchange (e.g. Barrow and Shaw, 1975), released high amounts of sorbed OM. However, Fig. 6a, 6b also show that desorption was always considerably stronger for the hydrophilic OC than for the hydrophobic OC. As stated above hydro-phobic DOM is rich in carboxyl and aromatic (phenolic) C. Because these structures form strong complexes with Al and Fe oxides, low desorption of the hydrophobic fraction is to be expected. The hydrophilic DOM fraction, in contrast, completely lacks aromatic C (Table 2), and, therefore, cannot be sorbed to the hydrous metal oxides by the strong ligand exchange binding. The fact that no preferential removal of any structural unit from solution occurred in the case of the hydrophilic DOM (Table 2) suggests sorption by weaker, non-speci®c bondings. Weak outer-sphere bondings may also explain the almost complete removal of hydrophilic OM by the speci®cally sorbing H2PO4ÿand the considerable
deso-rption of hydrophilic OM by SO42ÿ.
The desorption of hydrophilic OC by H2PO4ÿ was
complete for all investigated surface loadings. In con-trast, the desorption of hydrophobic OC decreased with increasing loading of the sorbent. At ®rst, this appears contradictory to the reduced sorption due to the smaller number of ligands involved per organic molecule at high OC surface loadings as shown above. However, we assume that at high surface coverage, the binding organic ligands are eectively shielded against exchange with competing anions by other negatively charged parts of the macromolecule. Non-binding carboxylate groups occur in particular at high surface loading with OM (see above). If they are oriented towards the solution they repulse other anions and thus hinder them to approach the binding sites. This assumption was con-®rmed by the increase of negative surface charge of Al and Fe hydrous oxides along with increasing of sorbed OM (Tipping, 1981; Kaiser and Zech, 1999).
Because the desorption of the hydrophilic OM with H2PO4ÿwas complete, the composition of the desorbed
OC did not dier from that of the sorbed material (Table 2). In contrast, desorption of hydrophobic OM again led to a fractionation among structural elements.
13C-NMR spectra of desorbed hydrophobic OM
showed that alkyl and O-alkyl C was released to a higher extent than carboxylic and aromatic C. It seems that molecules containing aromatic acid structures
Fig. 5. Sorption of dissolved organic carbon (DOC) to amor-phous Al(OH)3(a), goethite (b), and a subsoil horizon (3Bw) of
an Oxyaquic Dystrochrept (c) and the subsequent desorption by a solution of the same inorganic composition as the sorption solutions but without DOC. Sorption is given as the relation-ship between added and sorbed OC; the coecient of determi-nationr2for the linear regression between added and sorbed
forming strong complexes on the goethite surface are more dicult to remove than molecules rich in O-alkyl and alkyl C.
These results indicated that desorption of OM from hydrous oxides and soils is controlled by the same fac-tors that are governing DOM sorption, and they agree well with the ®nding that sorption of DOM to hydrous oxides involves the formation of strong chemisorptive
bondings by ligand exchange between acidic organic ligands and OH groups at the surface of the adsorber. The strong hysteresis of DOM sorption on soils as well as on hydrous metal oxides suggests that sorption to these mineral phases is an important factor for storage and stabilisation of OM in soils (see also Blaser et al., 1997). If this is the case, then the controlling factors for DOM sorption and OC concentration in soils as well as
Fig. 6. Desorption of the hydrophilic (a) and hydrophobic (b) dissolved organic carbon (DOC) fraction from goethite by dierent solutions. ``Soil solution'' was a DOC-free solution of similar inorganic composition as the DOC solutions used for the sorption; modi®ed from Kaiser and Zech (1999). Sorption is given as the relationship between added and sorbed OC; the coecient of deter-minationr2for the linear regression between added and sorbed OC is given. Error bars indicating the standard error of the mean for
the sorption data are given where larger than symbols. Error bars for the desorption data represent least signi®cant dierences (0:05).
Table 2
Distribution of C moieties in initial, sorbed, and desorbed hydrophilic and hydrophobic acidic DOM fractions according to liquid-phase13C-NMR spectroscopya
Sample C moieties (%) Carbonyl Cb
160±210 ppm
Aromatic C 110±160 ppm
O-alkyl C 50±110 ppm
Alkyl C 0±50 ppm
Hydrophilic DOM
Initial 15a 0a 69a 16a
Sorbed 16a 0a 70a 14a
Desorbed 15a 0a 70a 15a
Hydrophobic acid DOM
Initial 21a 31a 28a 20a
Sorbed 25b 37b 28a 10b
Desorbed 19a 34c 32c 15c
a The sorbing mineral phase was goethite and the desorbing solution was 0.1 M NaH
2PO4. Desorption was carried out 24 h after
the sorption experiment. The species distribution of the sorbed DOM was calculated by dierence from the weighted species dis-tribution of DOM remaining dissolved during the sorption experiment. Repeated measurements on the same sample showed varia-tions in the distribution of C moieties of42%; data from Kaiser and Zech (1999).
the chemical compositions of the DOM and parts of the soil OM should be similar.
3.4. Analogies between sorbed DOM and SOM
As presented above, Fe and Al oxides and hydroxides control DOM sorption in the soils used in this study. In particular, amorphous Al(OH)3shows a large capacity
to sorb DOM by chemisorptive bondings. Likewise, we found close correlation between the contents of hydrous oxides with the OC concentration in a range of genetically dierent soils (Fig. 1b). The latter ®nding is con®rmed by Torn et al. (1997) who investigated the relationship between soil mineralogy, OC storage and turnover in a soil chronosequence at Hawaii. Their conclusions are that in particular the non-crystalline minerals such as amorphous Al(OH)3control OC storage as well as OC
turnover in soil, and that it is the passive OC pool which depends on the soil mineralogy. This strongly suggests a sorptive control of OM preservation in soil.
If soil OM is due to sorbed DOM at a considerable proportion, then a resemblance of soil OM and DOM is to be expected. Fig. 7 shows the liquid-state13C-NMR
spectra of the XAD-8-adsorbable fraction of forest ¯oor-derived DOM (i.e. the DOM entering the mineral
soil) and the XAD-8-adsorbable fraction of OM extracted with 0.5 M NaOH from the Bs horizon of a spodosol (i.e. the soil horizon where DOM is retained). The XAD-8-adsorbable acidic DOM fraction represents DOM components which are strongly sorbed on soils and hydrous metal oxides (Jardine et al., 1989; Kaiser and Zech, 1998). The extractability of OC from the Bs horizon by NaOH was about 45%.
The spectra show close similarity. The major dier-ences between the two spectra are higher abundances of O-alkyl C and of O±CH3, and lower abundances of
aro-matic and ole®nic C in the DOM spectrum compared to the spectrum of the extracted soil OM (Table 3).
Spodosols are soils where it appears obvious that soil OM in illuvial horizons is strongly related to DOM. Therefore, we compared the composition of DOM in the mineral soil input (collected by zero-tension lysimeters beneath the forest ¯oor) with that of OM from an A horizon of a Typic Dystrochrept. For the comparison, we chose OM associated with clay-sized separates because soil OM in larger particle size classes consists primarily of particulate OM representing weakly decomposed debris (Christensen, 1992; Guggenberger et al., 1995; Amelung et al., 1998). The fact that the clay-sized separates of soils usually show a pro-nounced enrichment of OM (Christensen, 1992) which is closely associated with poorly crystalline Fe oxides (Shang and Tiessen, 1998) corresponds well with the hypothesis that DOM sorption is an important mechanism in the preservation of OM in soil. It seems therefore reasonable to compare the chemical composition of DOM and OM associated with clay-sized separates. Table 4 summarises the lignin and carbohydrate sig-natures of total DOM, the hydrophobic acidic fraction of DOM, and of OM localised in the clay fraction from the A horizon of a Typic Dystrochrept. Interestingly, the carbohydrate and lignin signatures of DOM resemble much those of the clay-sized separates, suggesting that DOM may be a considerable source of SOM also in this type of soil.
4. General discussion
As shown above, there is evidence that sorption of DOM by formation of strong chemisorptive bonds to metal oxides and hydroxides in soils can be an impor-tant mechanism in the preservation of soil OM. Also for marine sediments, Hedges and Keil (1995) stated that the adsorbed organic substances were at some time dis-solved. However, there is discussion on the nature of the organic matter sorbed to marine sediments. Keil et al. (1994a) suggested that intrinsically labile organic com-pounds are preserved by reversible adsorption. In contrast, Henrichs (1995) demonstrated that easily reversible sorp-tion cannot preserve such labile OM.
Fig. 7. Liquid-state 13C-NMR spectra of the
Our soil data suggest that, during percolation of DOM through the solum, the soil acts as a chromato-graphic system, and the sorption process of DOM leads to fractionation of compounds according to sorption intensity (Guggenberger and Zech, 1993; Kaiser et al., 1998). The hydrophobic DOM sorbs strongly whereas the hydrophilic fraction sorbs weakly and is discriminated. Beside the weaker binding, the hydrophilic fraction consists of more labile structures (Qualls and Haines, 1992b; Jandl and Sollins, 1997), e.g. free polysaccharides (Guggenberger et al., 1994a). The strongly sorbing, pri-marily lignocellulose-derived hydrophobic DOM is more recalcitrant (Qualls and Haines, 1992b; Jandl and Sollins, 1997). As the intensity of sorption required for preservation is inversely proportional to the rate at which DOM is decomposed (Henrichs, 1995), strong
binding of a refractory fraction of DOM is best pre-requisite for a sorptive preservation of OM.
Dissolved OM sorption experiments on soils and minerals, and comparative analyses of soil mineralogy and soil OC contents suggest that the soil mineralogy is the primary controlling factor in this sorptive preservation of SOM. In many soils, the OC/SA ratio exceeds by far the ME range. This can be partly due to the masking of mineral surfaces by OM (Feller et al., 1992; Pennell et al., 1995). At high coverage of the active mineral surface, a considerable part of the acidic functional groups of the organic macromolecules are not involved in the chemisorptive bonding, and due to their polarity it is reasonable to assume that they are tailing into the solu-tion. Hence, not the whole organic macromolecule must be in direct contact with the mineral surface. This may be another reason why the ME range is exceeded.
In a recent study, Mayer (1999) used the adsorption energetics of N2at the BET experiment to estimate the
fraction of mineral surface coated with OM. He found that all sediment samples within the ME level had less than 22% of their mineral surfaces covered with OC, and recommended that the term monolayer equivalent should be no more used. Mayer (1999) concluded that OM patches thicker than a monolayer must be localised on mineral surfaces. These patches of sorbed OM may occur at the edge sites of clay minerals (see also Schul-thess and Huang, 1991; Kubicki et al., 1997) or may be associated with distinct compositional phases, such as highly sorptive Fe oxyhydroxides (see also Shang and Tiessen, 1998).
In the soils we studied, the OC concentration was closely related to the indicators for Fe oxides and hydroxides. These reactive Fe and Al oxyhydroxide phases may be covered with thick packages of OM. In the soil mineral matrix the oxyhydroxides are ''diluted'' with coarse mineral phases such as sand-sized quartz grains which are low in SA and sorb little to no OM (Liu and Amy, 1993; Weigand and Totsche, 1998). E.g. a sandy soil relatively rich in oxyhydroxide phases therefore may have a small overall SA and contain large amounts of mineral-associated OC. This refers to all soils where Al and/or Fe oxyhydroxides are selectively
Table 3
Distribution of C moieties in the XAD-8-adsorbable (hydrophobic acid) fraction of DOM from the Oa horizon of an Entic Hap-lorthod and the XAD-8-adsorbable fraction of OM extracted from the Bs horizon of an Entic HapHap-lorthod using 0.05 M NaOHa
Sample C moieties (%) Carbonyl C 160±210 ppm
Aromatic C 110±160 ppm
O-alkyl C 50±110 ppm
Alkyl C 0±50 ppm
Hydrophobic acid DOM 21 31 28 20
XAD-8-adsorbable alkaline-extractable OM 22 34 23 21
a Repeated measurements on the same sample showed variations in the distribution of C moieties of42%.
Table 4
Lignin and carbohydrate signatures of total DOM, XAD-8-adsorbable (hydrophobic) acidic DOM, and of OM in the clay fraction from an A horizon of an Inceptisola
Clay OM
Total DOM
Hydrophobic acidic DOM
Lignin
V+S+C [mg gÿ1C] 14.9 9.2 8.4
(ac/al)v 1.0 1.2 1.0 Carbohydrates
(man+gal)/(ara+xyl) 1.6 1.7 1.6 (rha+fuc)/(ara+xyl) 0.8 1.0 0.7
a The yield of lignin-derived CuO oxidation products is
expressed as the sum of vanillyl (= V), syringyl (= S) and cinnamyl (= C) units (V+S+C). The degree of lignin oxida-tion is given by the acid-to-aldehyde ratio of vanillyl units ([ac/ al]v). The carbohydrate content was calculated as the sum of
translocated and accumulated in distinct horizons due to podzolisation or hydromorphy, or where Al and Fe oxyhydroxides are residual products of intensive weath-ering. Besides a possible masking of SA by sorbed OM, this oers another plausible explanation why we failed to establish a signi®cant relationship between soil OC and SA. Hence, the SA of the strongly sorbing mineral soil constituents, such as Fe and Al oxides and hydroxides, probably would be a better measure than the SA of the bulk soil.
This hypothesis ®ts well to the ®nding of Ransom et al. (1998). They compared the OM preservation on continental slopes with dierent clay mineralogy and found that OM appears to be preferentially sequestered in sediments being rich in smectite but also in metal oxyhydroxides. Using TEM images of in-situ OM of continental margin sediments together with theoretical considerations, Ransom et al. (1997, 1998) showed that the vast bulk of OM in marine sediments is not in direct contact with a mineral surface. Rather, it is only a small proportion of any given organic globule that shares this interface.
Hence, in both environments, soils and marine sedi-mentary systems, the surface chemistry of the minerals has to be taken into account for a better understanding of sorptive preservation of OC. Likewise, it appears for both systems that the organic substances sorbed on minerals represent a continuum with respect to their strength of bonding. The chemical structure of DOM is important not only for the intrinsic recalcitrance but also for the strength of bondings onto mineral surfaces. While there seems to be no doubt that strong sorption of DOM to surface active minerals is an important process in the preservation of OM, it is not clear what mechanisms cause the inaccessibility to microbial and enzymatic degradation. The rapid improvement of micro- and nanoscale microscopic and spectrometric techniques is a key factor to get a better insight into the localisation of sorbed OM at the mineral surface and into conformational changes of the OM associated with the sorption. Such investigations may give important informations how the sorbed OM is protected from enzymatic attack in soil and sedimentary environments (e.g. mesopore theory, surface-induced condensation theory).
Acknowledgements
This research was in parts ®nanced by the Deutsche Forschungsgemeinschaft (joint research program ''ROSIG''). We are grateful to U. Roth and H. Ciglasch for laboratory assistance and in particular to L. Haumaier for recording the NMR spectra and for valuable dis-cussions. We are indebted to W. Zech for his constant support during all phases of this study.
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