Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue7-8.2000:
Organic Geochemistry 31 (2000) 711±725
www.elsevier.nl/locate/orggeochem
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 environment. 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 polysaccharide-derived DOM on mineral surfaces is weaker, adsorptive and desorptive 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 further 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 lignocellulose, the polymer is partially hydrolysed and
* Corresponding author. Tel.: +49-921-55-2318; fax: +49921-55-2246.
E-mail address: [email protected] (K. Kaiser).
solubilized through the activity of exoenzymes (Hoppe,
1983; Haider et al., 1985). Oxidatively altered watersoluble 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).
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00046-2
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K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
The organic forest ¯oor layer is the major source of
DOM in many forest soils as re¯ected by large concentrations 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 particles, 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 sediments, the capacity to adsorb DOM relates to the presence 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, investigations 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 particulate OM (d < 1.9 g cmÿ3), he found a close relationship 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 attributed 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 sediments, 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 OMconserving 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 suspension 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 sorption experiments. A proportion of the solution 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), protonated and ®nally freeze dried. These samples were
used for wet-chemical analyses and 13C-NMR spectroscopy. 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
Samples of mineral topsoil and illuvial subsoil horizons were collected from 34 soil pro®les in Belgium,
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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 10 000 g for 30 min.
The supernatants were removed and the settled soil
materials washed intensively with deionized H2O until
the electrical conductivity of the solution was < 50 mS
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 cmÿ3 with
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 difference 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 estimated 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)3 by 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 1.6 g cmÿ3 (a) and dithionite-citratebicarbonate extractable Fe (FeDCB) (b) for topsoils (n=41) and
spodosol illuvial subsoils (n=18) of forest soils (*, **=signi®cant at P < 0.05 and P < 0.01, respectively).
715
concentration and SA of topsoils and subsoils were signi®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 mÿ2 were observed. On the other hand, we
found highly signi®cant correlations of OC concentrations with indicators for Al and Fe oxides and hydroxides, e.g., oxalate-extractable Al or dithionite-citratebicarbonate extractable Fe (FeDCB; Fig. 1b). The slopes
of the relationship between OC and FeDCB concentrations of topsoils and subsoils were not signi®cantly different (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 m2 gÿ1; 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-8adsorbable (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 adsorbed, but once the OM is adsorbed the true SA of the
mineral surface is not accessible any more. Hence,
716
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
masking of mineral surfaces by sorbed OM may be the
reason for the non-conformity of the OC±SA relationship in the soils.
These results contrast with those of Mayer (1994a,b)
who found no increase of SA of soils and marine sediments 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 temperature of 4 105 C. Mayer (1994a,b), Keil et al.
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 amorphous 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).
(1994b), and Bergamaschi et al. (1997) used higher drying temperatures (150±350 C). 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. 3. In¯uence of coatings of hydrophobic organic matter 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 OC (b). Error bars
in (a) indicate the standard error of the mean. Error bars in (b)
represent least signi®cant dierences ( 0:01).
717
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
will focus on the type of bonding between DOM and
soil minerals. As pointed out by Henrichs (1995), preservation 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 OHÿ during 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 formation of bidentate complexes between two organic
ligands in ortho position 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 structures are preferentially found in the lignocellulosederived hydrophobic DOM fraction (Dai et al., 1996;
Guggenberger et al., 1998). This fraction is also less
biodegradable than the polysaccharide-derived hydrophilic 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 solution 13C-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)3 using 13C-NMR spectroscopy.
According to 13C-NMR spectroscopy, the hydrophobic 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 negligible 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 cmÿ1 and of the carboxylate at
1625 cmÿ1. The band at 1400 cmÿ1 might result from
complexed carboxylate. The spectrum of the hydrophobic 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-state 13C-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
Bw
18b
19a
27b
28b
29a
28a
26b
25b
Batch experiment ®ltrates
Al(OH)3
16c
Goethite
17c
24c
25c
29a
28a
31c
30c
a
Repeated measurements on the same sample showed variations in the distribution of C moieties of 4 2%; data from Kaiser et al.
(1997).
b
Dierent letters within a column indicate that the values were signi®cantly dierent from the value of the original sample at the
P < 0.05 level (one-way ANOVA).
718
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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 surface coverage (0.72 mg C mÿ2) (c); 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 surfaces. 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 sorption 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)3 surfaces 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 favourable 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 formation 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
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
Fig. 5. Sorption of dissolved organic carbon (DOC) to amorphous 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 relationship between added and sorbed OC; the coecient of determination r2 for the linear regression between added and sorbed
OC is given. Error bars indicating the standard error of the
mean are given where larger than symbols.
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 indigenous OC. Comparable results have been reported for
hematite ( -Fe2O3) and the AB horizon of a Hapludult
719
(Gu et al., 1994; Qualls and Haines, 1992a). The reversibility 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
ÿ
binding sites, such as SO2ÿ
4 and H2PO4 (Tipping, 1981;
Gu et al., 1994), resulted in a considerable release of OC
from goethite (Fig. 6a and 6b). In particular H2POÿ
4 , 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 hydrophobic 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 H2POÿ
4 and the considerable desorption of hydrophilic OM by SO2ÿ
4 .
The desorption of hydrophilic OC by H2POÿ
4 was
complete for all investigated surface loadings. In contrast, 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
H2POÿ
4 was 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.
13
C-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
720
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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 factors 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 determination r2 for 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 liquidphase 13C-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
Sorbed
Desorbed
15a
16a
15a
0a
0a
0a
69a
70a
70a
16a
14a
15a
Hydrophobic acid DOM
Initial
Sorbed
Desorbed
21a
25b
19a
31a
37b
34c
28a
28a
32c
20a
10b
15c
a
The sorbing mineral phase was goethite and the desorbing solution was 0.1 M NaH2PO4. 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 distribution of DOM remaining dissolved during the sorption experiment. Repeated measurements on the same sample showed variations in the distribution of C moieties of 4 2%; data from Kaiser and Zech (1999).
b
Dierent letters within a column and for a DOM fraction indicate that the values were signi®cantly dierent from another at the
P < 0.05 level (one-way ANOVA).
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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)3 shows 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)3 control 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-state 13C-NMR
spectra of the XAD-8-adsorbable fraction of forest
¯oor-derived DOM (i.e. the DOM entering the mineral
721
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 dierences between the two spectra are higher abundances of
O-alkyl C and of O±CH3, and lower abundances of aromatic 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 pronounced 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 signatures 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
Fig. 7. Liquid-state 13C-NMR spectra of the XAD-8-adsorbable acidic dissolved organic carbon fraction (a) and XAD-8adsorbable acidic organic carbon fraction extracted by 0.5 M
NaOH from the Bs horizon of an Entic Haplorthod (b).
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 important 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 dissolved. However, there is discussion on the nature of the
organic matter sorbed to marine sediments. Keil et al.
(1994a) suggested that intrinsically labile organic compounds are preserved by reversible adsorption. In contrast,
Henrichs (1995) demonstrated that easily reversible sorption cannot preserve such labile OM.
722
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
Table 3
Distribution of C moieties in the XAD-8-adsorbable (hydrophobic acid) fraction of DOM from the Oa horizon of an Entic Haplorthod and the XAD-8-adsorbable fraction of OM extracted from the Bs horizon of an Entic Haplorthod using 0.05 M NaOHa
Sample
C moieties (%)
Hydrophobic acid DOM
XAD-8-adsorbable alkaline-extractable OM
a
Carbonyl C
160±210 ppm
Aromatic C
110±160 ppm
O-alkyl C
50±110 ppm
Alkyl C
0±50 ppm
21
22
31
34
28
23
20
21
Repeated measurements on the same sample showed variations in the distribution of C moieties of 4 2%.
Our soil data suggest that, during percolation of
DOM through the solum, the soil acts as a chromatographic 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, primarily 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
Table 4
Lignin and carbohydrate signatures of total DOM, XAD-8adsorbable (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 gÿ1 C]
(ac/al)v
14.9
1.0
9.2
1.2
8.4
1.0
Carbohydrates
(man+gal)/(ara+xyl)
(rha+fuc)/(ara+xyl)
1.6
0.8
1.7
1.0
1.6
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 oxidation is given by the acid-to-aldehyde ratio of vanillyl units ([ac/
al]v). The carbohydrate content was calculated as the sum of
monosaccharides released by TFA hydrolysis. Ratios of mannose plus galactose to arabinose plus xylose ([man+gal]/
[ara+xyl]) and rhamnose plus fucose to arabinose plus xylose
([rha+fuc]/[ara+xyl]) provide information about the origin of
the carbohydrates. According to Oades (1984), the ratios indicate a predominantly microbial source; data from Guggenberger and Zech (1993) and Guggenberger et al. (1994a,b).
binding of a refractory fraction of DOM is best prerequisite 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 solution. Hence, not the whole organic macromolecule must
be in direct contact with the mineral surface.
www.elsevier.nl/locate/orggeochem
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 environment. 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 polysaccharide-derived DOM on mineral surfaces is weaker, adsorptive and desorptive 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 further 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 lignocellulose, the polymer is partially hydrolysed and
* Corresponding author. Tel.: +49-921-55-2318; fax: +49921-55-2246.
E-mail address: [email protected] (K. Kaiser).
solubilized through the activity of exoenzymes (Hoppe,
1983; Haider et al., 1985). Oxidatively altered watersoluble 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).
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00046-2
712
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
The organic forest ¯oor layer is the major source of
DOM in many forest soils as re¯ected by large concentrations 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 particles, 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 sediments, the capacity to adsorb DOM relates to the presence 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, investigations 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 particulate OM (d < 1.9 g cmÿ3), he found a close relationship 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 attributed 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 sediments, 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 OMconserving 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 suspension 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 sorption experiments. A proportion of the solution 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), protonated and ®nally freeze dried. These samples were
used for wet-chemical analyses and 13C-NMR spectroscopy. 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
Samples of mineral topsoil and illuvial subsoil horizons were collected from 34 soil pro®les in Belgium,
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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 10 000 g for 30 min.
The supernatants were removed and the settled soil
materials washed intensively with deionized H2O until
the electrical conductivity of the solution was < 50 mS
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 cmÿ3 with
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 difference 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 estimated 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)3 by 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 1.6 g cmÿ3 (a) and dithionite-citratebicarbonate extractable Fe (FeDCB) (b) for topsoils (n=41) and
spodosol illuvial subsoils (n=18) of forest soils (*, **=signi®cant at P < 0.05 and P < 0.01, respectively).
715
concentration and SA of topsoils and subsoils were signi®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 mÿ2 were observed. On the other hand, we
found highly signi®cant correlations of OC concentrations with indicators for Al and Fe oxides and hydroxides, e.g., oxalate-extractable Al or dithionite-citratebicarbonate extractable Fe (FeDCB; Fig. 1b). The slopes
of the relationship between OC and FeDCB concentrations of topsoils and subsoils were not signi®cantly different (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 m2 gÿ1; 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-8adsorbable (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 adsorbed, but once the OM is adsorbed the true SA of the
mineral surface is not accessible any more. Hence,
716
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
masking of mineral surfaces by sorbed OM may be the
reason for the non-conformity of the OC±SA relationship in the soils.
These results contrast with those of Mayer (1994a,b)
who found no increase of SA of soils and marine sediments 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 temperature of 4 105 C. Mayer (1994a,b), Keil et al.
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 amorphous 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).
(1994b), and Bergamaschi et al. (1997) used higher drying temperatures (150±350 C). 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. 3. In¯uence of coatings of hydrophobic organic matter 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 OC (b). Error bars
in (a) indicate the standard error of the mean. Error bars in (b)
represent least signi®cant dierences ( 0:01).
717
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
will focus on the type of bonding between DOM and
soil minerals. As pointed out by Henrichs (1995), preservation 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 OHÿ during 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 formation of bidentate complexes between two organic
ligands in ortho position 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 structures are preferentially found in the lignocellulosederived hydrophobic DOM fraction (Dai et al., 1996;
Guggenberger et al., 1998). This fraction is also less
biodegradable than the polysaccharide-derived hydrophilic 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 solution 13C-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)3 using 13C-NMR spectroscopy.
According to 13C-NMR spectroscopy, the hydrophobic 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 negligible 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 cmÿ1 and of the carboxylate at
1625 cmÿ1. The band at 1400 cmÿ1 might result from
complexed carboxylate. The spectrum of the hydrophobic 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-state 13C-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
Bw
18b
19a
27b
28b
29a
28a
26b
25b
Batch experiment ®ltrates
Al(OH)3
16c
Goethite
17c
24c
25c
29a
28a
31c
30c
a
Repeated measurements on the same sample showed variations in the distribution of C moieties of 4 2%; data from Kaiser et al.
(1997).
b
Dierent letters within a column indicate that the values were signi®cantly dierent from the value of the original sample at the
P < 0.05 level (one-way ANOVA).
718
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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 surface coverage (0.72 mg C mÿ2) (c); 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 surfaces. 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 sorption 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)3 surfaces 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 favourable 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 formation 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
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
Fig. 5. Sorption of dissolved organic carbon (DOC) to amorphous 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 relationship between added and sorbed OC; the coecient of determination r2 for the linear regression between added and sorbed
OC is given. Error bars indicating the standard error of the
mean are given where larger than symbols.
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 indigenous OC. Comparable results have been reported for
hematite ( -Fe2O3) and the AB horizon of a Hapludult
719
(Gu et al., 1994; Qualls and Haines, 1992a). The reversibility 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
ÿ
binding sites, such as SO2ÿ
4 and H2PO4 (Tipping, 1981;
Gu et al., 1994), resulted in a considerable release of OC
from goethite (Fig. 6a and 6b). In particular H2POÿ
4 , 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 hydrophobic 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 H2POÿ
4 and the considerable desorption of hydrophilic OM by SO2ÿ
4 .
The desorption of hydrophilic OC by H2POÿ
4 was
complete for all investigated surface loadings. In contrast, 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
H2POÿ
4 was 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.
13
C-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
720
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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 factors 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 determination r2 for 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 liquidphase 13C-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
Sorbed
Desorbed
15a
16a
15a
0a
0a
0a
69a
70a
70a
16a
14a
15a
Hydrophobic acid DOM
Initial
Sorbed
Desorbed
21a
25b
19a
31a
37b
34c
28a
28a
32c
20a
10b
15c
a
The sorbing mineral phase was goethite and the desorbing solution was 0.1 M NaH2PO4. 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 distribution of DOM remaining dissolved during the sorption experiment. Repeated measurements on the same sample showed variations in the distribution of C moieties of 4 2%; data from Kaiser and Zech (1999).
b
Dierent letters within a column and for a DOM fraction indicate that the values were signi®cantly dierent from another at the
P < 0.05 level (one-way ANOVA).
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
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)3 shows 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)3 control 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-state 13C-NMR
spectra of the XAD-8-adsorbable fraction of forest
¯oor-derived DOM (i.e. the DOM entering the mineral
721
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 dierences between the two spectra are higher abundances of
O-alkyl C and of O±CH3, and lower abundances of aromatic 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 pronounced 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 signatures 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
Fig. 7. Liquid-state 13C-NMR spectra of the XAD-8-adsorbable acidic dissolved organic carbon fraction (a) and XAD-8adsorbable acidic organic carbon fraction extracted by 0.5 M
NaOH from the Bs horizon of an Entic Haplorthod (b).
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 important 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 dissolved. However, there is discussion on the nature of the
organic matter sorbed to marine sediments. Keil et al.
(1994a) suggested that intrinsically labile organic compounds are preserved by reversible adsorption. In contrast,
Henrichs (1995) demonstrated that easily reversible sorption cannot preserve such labile OM.
722
K. Kaiser, G. Guggenberger / Organic Geochemistry 31 (2000) 711±725
Table 3
Distribution of C moieties in the XAD-8-adsorbable (hydrophobic acid) fraction of DOM from the Oa horizon of an Entic Haplorthod and the XAD-8-adsorbable fraction of OM extracted from the Bs horizon of an Entic Haplorthod using 0.05 M NaOHa
Sample
C moieties (%)
Hydrophobic acid DOM
XAD-8-adsorbable alkaline-extractable OM
a
Carbonyl C
160±210 ppm
Aromatic C
110±160 ppm
O-alkyl C
50±110 ppm
Alkyl C
0±50 ppm
21
22
31
34
28
23
20
21
Repeated measurements on the same sample showed variations in the distribution of C moieties of 4 2%.
Our soil data suggest that, during percolation of
DOM through the solum, the soil acts as a chromatographic 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, primarily 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
Table 4
Lignin and carbohydrate signatures of total DOM, XAD-8adsorbable (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 gÿ1 C]
(ac/al)v
14.9
1.0
9.2
1.2
8.4
1.0
Carbohydrates
(man+gal)/(ara+xyl)
(rha+fuc)/(ara+xyl)
1.6
0.8
1.7
1.0
1.6
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 oxidation is given by the acid-to-aldehyde ratio of vanillyl units ([ac/
al]v). The carbohydrate content was calculated as the sum of
monosaccharides released by TFA hydrolysis. Ratios of mannose plus galactose to arabinose plus xylose ([man+gal]/
[ara+xyl]) and rhamnose plus fucose to arabinose plus xylose
([rha+fuc]/[ara+xyl]) provide information about the origin of
the carbohydrates. According to Oades (1984), the ratios indicate a predominantly microbial source; data from Guggenberger and Zech (1993) and Guggenberger et al. (1994a,b).
binding of a refractory fraction of DOM is best prerequisite 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 solution. Hence, not the whole organic macromolecule must
be in direct contact with the mineral surface.