stabilization of dissolved organic matter by sorption to the mineral soil

Stabilization of dissolved organic matter by sorption to the mineral soil

Karsten Kalbitza,*, David Schwesiga,1, Janet Rethemeyerb, Egbert Matznera

aDepartment of Soil Ecology, Bayreuth Institute for Terrestrial Ecosystem Research (BITOK),

University of Bayreuth, D-95440 Bayreuth, GermanybLeibniz-Labor for Radiometric Dating and Isotope Research, Christian-Albrechts-University Kiel, Max-Eyth-Str. 11-13, D-24118 Kiel, Germany

Received 22 September 2003; received in revised form 23 April 2004; accepted 16 November 2004

Abstract

The main process by which dissolved organic matter (DOM) is retained in forest soils is likely to be sorption in the mineral horizons that

adds to stabilized organic matter (OM) pools. The objectives of this study were to determine the extent of degradation of sorbed OM and to

investigate changes in its composition during degradation. DOM of different origins was sorbed to a subsoil and incubated for 1 year. We

quantified mineralized C by frequent CO2 measurements in the headspace of the incubation vessels and calculated mean residence times by a

double exponential model. Mineralization of C of the corresponding DOM in solution was used as a control to estimate the extent of DOM

stabilization by sorption. Changes in the composition of sorbed OM during the incubation were studied by spectroscopic (UV, fluorescence)

and isotope (13C, 14C) measurements after hot-water extraction of OM.

The fraction of sorbed organic C mineralized during the incubation was only one-third to one-sixth of that mineralized in solution. The

mean residence time of the most stable OM sample was estimated to increase from 28 years in solution to 91 years after sorption. For highly

degradable DOM samples, the portion of stable C calculated by a double exponential model nearly doubled upon sorption. With less

degradable DOM the stability increased by only 20% after sorption. Therefore, the increase in stability due to sorption is large for labile

DOM high in carbohydrates and relatively small for stable DOM high in aromatic and complex molecules. Nevertheless, in terms of stability

the rank order of OM types after sorption was the same as in solution. Furthermore, the extent of sorption of recalcitrant compounds was

much larger than sorption of labile compounds. Thus, sorptive stabilization of this stable DOM sample was four times larger than for the

labile ones. We conclude that stabilization of OM by sorption depends on the intrinsic stability of organic compounds sorbed. We propose

that the main stabilization processes are selective sorption of intrinsically stable compounds and strong chemical bonds to the mineral soil

and/or a physical inaccessibility of OM to microorganisms. The UV, fluorescence and 13C measurements indicated that aromatic and

complex compounds, probably derived from lignin, were preferentially stabilized by sorption of DOM. The 13C and 14C data showed that

degradation of the indigenous OM in the mineral soil decreased after sorption of DOM. We estimated DOM sorption stabilizes about

24 Mg C haK1 highlighting the importance of sorption for accumulation and preservation of OM in soil.

q 2005 Elsevier Ltd. All rights reserved.

Keywords: 13C; 14C; Dissolved organic matter; Fluorescence spectroscopy; Hot-water extraction; Mineralization; Sorption; Stabilization; UV/VIS

spectroscopy

1. Introduction

At a time of growing concern about global climate

change soils deserve greater attention as a sink for

0038-0717/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.soilbio.2004.11.028

* Corresponding author. Tel.: C49 921 555624; fax: C49 921 555799.

E-mail address: [email protected] (K. Kalbitz).1 Present address: IWW Rhenish-Westfalian Institute for Water

Research, Moritzstr. 26, D-45476 Mulheim an der Ruhr, Germany.

atmospheric carbon dioxide (CO2). The release of dissolved

organic matter (DOM) from vegetation and organic soil

horizons constitutes an important flux of C into mineral

soils. In forest ecosystems, the flux of dissolved organic

carbon (DOC) from the forest floor into the mineral subsoil

has been estimated at 115–500 kg C haK1 yearK1 (Guggen-

berger and Zech, 1993; Currie et al., 1996; Michalzik and

Matzner, 1999; Solinger et al., 2001; Kaiser et al., 2001).

Concentrations of DOC in deep soil horizons and its export

Soil Biology & Biochemistry 37 (2005) 1319–1331

www.elsevier.com/locate/soilbio

http://www.elsevier.com/locate/soilbio

K. Kalbitz et al. / Soil Biology & Biochemistry 37 (2005) 1319–13311320

from mineral subsoil are usually small (Guggenberger and

Zech, 1993; Michalzik and Matzner, 1999; Solinger et al.,

2001). Typically, 40–370 kg DOC haK1 yearK1 are

retained in the mineral subsoil (Currie et al., 1996;

Guggenberger and Kaiser, 2003). Sorptive stabilization of

DOC is likely the main process for this retention

(summarized by Kalbitz et al., 2000; Kaiser and Guggen-

berger, 2000). Precipitation of DOM may also contribute to

the formation of stable organic matter (OM) but this process

has not been investigated so far.

The extent of DOM degradation after sorption to the

mineral soil is almost unknown. Jones and Edwards (1998),

Strom et al. (2001) and van Hees et al. (2003) found

decreased degradation of simple organic compounds such as

glucose, citrate, oxalate and malate after sorption to soil

minerals and mineral soils. Desorption of OM from soils

and sediments increased the mineralization rates for C up to

five orders of magnitude (Keil et al., 1994; Nelson et al.,

1994). If desorption of OM results in increased mineraliz-

ation, it seems reasonable to hypothesize that sorption

decreases the mineralization of C to a similar extent.

Recent budget calculations of DOC fluxes and available

sorption capacity in forest mineral soils challenged the

paradigm of sorptive OM stabilization in the mineral soil.

Guggenberger and Kaiser (2003) hypothesized that sorptive

stabilization is restricted to juvenile mineral surfaces,

whereas the degradation of DOM sorbed onto surfaces

already occupied with OM should be large. This hypothesis

implies that the contribution of DOM to the formation of

stable OM in the mineral soil is related to the availability of

mineral sorption sites. In turn, available sorption sites will

be replenished through the decomposition of sorbed OM.

Furthermore, the sorptive stabilization of OM also likely

depends on the intrinsic stability of sorbed organic

compounds (Henrichs, 1995; Kaiser and Guggenberger,

2000). None of these effects has been quantified so far.

Therefore, the extent of mineralization of organic matter

after sorption of DOM needs to be quantified.

Decomposition of sorbed OM should result in changes in

the composition of the remaining OM. In recent studies,

dissolved aromatic and complex compounds that possibly

derived from lignin were relatively stable and degraded

slowly (Zsolnay and Steindl, 1991; Hongve et al., 2000;

Moran et al., 2000; Parlanti et al., 2000; Pinney et al., 2000;

Hertkorn et al., 2002; Kalbitz et al., 2003b). Similar changes

Table 1

Parameters of the sorption procedure

Solution Soil/solution ratio Initial DOC

(mg lK1)

DOC after so

(mg lK1)

Spruce Oi 1:42 185 140

Spruce Oa 1:100 43.3 24.3

Maize 1:13 964 722

Water 1:42 0.3 3.9

a Total organic carbon content; initial content of the used soil: 8.4 mg C gK1 s

in composition were found in studies assessing the stability

of sedimentary and soil OM (Baldock et al., 1992; Hedges

and Keil, 1995; Hedges et al., 1999). Therefore, it seems

reasonable to hypothesize analogous changes in the

composition of sorbed OM due to degradation. However,

this has not been investigated so far.

The objectives of this study were to (i) determine the

degradation of sorbed OM in order to estimate the sorptive

stabilization and whether this was dependent on its stability in

solution and (ii) investigate changes in the composition of

sorbed OM as affected by degradation. Our approach was to

sorb DOM of three different origins to a mineral subsoil and to

quantify its stability by measuring CO2. We also analyzed the

composition of hot-water extractable OM to track changes in

the composition of OM affected by sorption and incubation.

Changes in OM composition were detected by UV and

fluorescence spectroscopy and determination of 13C and 14C.

These isotopic techniques were also used to determine the

degradation of the sorbed OM in comparison to the CO2

measurements. This enabled us to detect priming effects of

DOM sorption on the indigenous OM in the mineral soil.

2. Materials and methods

2.1. Sample preparation and incubation

Mineral soil material was taken from the Bw horizon of a

Haplic Podzol (FAO, 1998) under a Norway spruce (Picea

abies [L.] Karst.) stand in the Fichtelgebirge mountains,

Germany. The soil was homogenized, sieved (!2 mm) and

air-dried. In a batch procedure, aliquots of the soil were

equilibrated with filtered (0.2 mm cellulose acetate; OE 66,

Schleicher and Schuell) DOM solutions that had been

extracted from the Oi and Oa horizon of the same site, and

from fresh maize straw. A detailed description of the

extraction of DOM samples is given in Kalbitz et al.

(2003a). Sorption was carried out by end-over-end shaking

at 5 8C in the dark for 18 h. For the sorption procedure,

different soil:solution ratios were used to achieve an

increase in C of the mineral soil from the initial

8.4 mg C gK1 soil (mineral soil before sorption) to approxi-

mately 10 mg C gK1 soil (Table 1). Soil suspensions were

shaken in 1-l amber borosilicate bottles using soil aliquots

of 7.5 g (OM solution from Oa horizon), 15 g (OM solution

rption Portion of sorbed

DOC (%)

Sorbed DOC

(mg C gK1 soil)

TOC content of the

soil after sorptiona

(mg C gK1 soil)

24 1.89 10.3

44 1.90 10.3

25 3.15 11.6

K0.15 8.3

oil.

K. Kalbitz et al. / Soil Biology & Biochemistry 37 (2005) 1319–1331 1321

from Oi horizon) and 50 g (OM solution from maize straw)

and water volumes between 630 and 750 ml. After the batch

sorption procedure, soil aliquots enriched with the same

type of OM were combined, resulting in 160–200 g of

‘enriched’ soil for each type of OM. The amount of OM

sorbed to the mineral soil was calculated from concen-

trations of DOC in the solutions before and after the batch

procedure. Control samples of the mineral soil were

prepared using ultra-pure water instead of DOM solution.

After the sorption procedure, the soil samples were adjusted

to a water content of 60% of the water holding capacity

using a ceramic suction plate. Subsequently, aliquots equal

to 10 g dry weight of the soil material were transferred into

120 ml serum flasks (glass), sealed and incubated at 20 8C in

the dark. Five sets of three replicates were prepared for each

DOM sample. One set per sample was harvested and freeze-

dried immediately after sorption. Another set was incubated

for a period of 1 year and harvested afterwards. In this set,

CO2 concentrations were measured during the entire

incubation. The remaining three sets were successively

harvested at three times during the 1-year incubation period

in order to track changes in OM composition. We adapted

times of harvesting to the different biodegradability of OM

in solution: a shorter period of time preceded the first

harvest for OM with large biodegradability in solution

(maize, Oi) compared to that for OM with small

biodegradability (Oa) or for the control (soil without

additional OM sorbed). A slight pressure of approximately

15 kPa above ambient atmospheric pressure was applied to

the incubation flasks to enable an appropriate sampling of

the gas phase. To avoid anoxic conditions, samples were

flushed with ambient air each time the concentration of CO2

in the headspace exceeded 5% (v/v). We did not inoculate

the soils because we wanted to use the indigenous microbial

community of the mineral soil. In the control treatment we

measured a mineralization of about 1% of soil organic C

during the 1-year incubation.

For comparison, three replicates (600 ml) of each OM

solution used for the sorption procedure were incubated in

1-l incubation flasks. Before incubation, OM solutions were

diluted to 20 mg C lK1 to avoid extensive growth of

microorganisms. A mixed inoculum was added (1% v/v)

with no additional nutrient amendment. The incubation

flasks were sealed and incubated in the dark at 20 8C for

1 year. CO2 evolution was measured to estimate C

mineralization. Details on this incubation experiment

including inoculation were given in Kalbitz et al. (2003a).

From all soil aliquots sampled before, during and after the

incubation experiment, 5 g of soil were extracted with ultra-

pure water using an accelerated solvent extraction device

(ASE; Dionex). Cold-water extraction yielded insufficient

amounts of OM. The extraction conditions were: cell size,

22 ml; temperature, 150 8C; pressure, 10 MPa; extraction

time, 10 min; flush volume, 60%; purge time, 90 s (for

details see Schwesig et al., 1999). The volume of the

aqueous extract was measured (about 35 ml) to quantify the

amount of hot-water extractable organic carbon.

2.2. Analytical methods

Mineralization of C was determined by measuring CO2

concentrations in the headspace of each incubation flask

(gas chromatograph HP 6890, Hewlett Packard, thermal

conductivity detector). The coefficient of variation of

replicate analyses was less than 4%. Measurements were

performed daily at the beginning of the experiment, but at

longer intervals (up to monthly) towards the end.

The hot-water extracts were analyzed for DOC using an

aliquot of 2 ml which was diluted with ultra-pure water by a

factor of 25 (High TOC, Elementar). A further aliquot of

about 0.5 ml was diluted to 10 mg C l–1 for UV and

fluorescence measurements. We measured the UV absor-

bance at 280 nm (UVIKON 930, BIO-TEK Instruments) to

estimate the aromaticity of organic matter (Chin et al., 1994;

McKnight et al., 1997) and recorded emission fluorescence

spectra (SFM 25, BIO-TEK Instruments) followed by

calculations of humification indices (HIXem; Zsolnay et al.,

1999) as a measure of the complexity of the organic

molecules. The remaining portions of the hot-water extracts

(32 ml) were freeze-dried and analyzed for d13C ratios

(Deltaplus, ThermoQuest–Finnigan MAT, connected to an

elemental analyzer, NA 2500, ThermoQuest–Fisons). The

isotope ratios were compared with that of reference CO2

(Linde AG, Unterschleibheim, Germany) and calibrated

against NBS19-limestone (National Institute of Standards

and Technology, Gaithersburg, MD) and sucrose ANU

(International Atomic Energy Agency, Vienna). The d13C

value expresses the enrichment of 13C in a sample relative to

the 13C of CO2 prepared from a calcareous belemnite of the

cretaceous Peedee formation, South Carolina. The analytical

precision of the measurements was 0.1‰.

The DOM solutions used for the sorption experiment were

analyzed for DOC and spectroscopic parameters before and

after sorption as described above. Furthermore, we deter-

mined the d13C ratios and the 14C content in freeze-dried

aliquots of DOM samples before sorption. 14C measurements

were carried out by accelerator mass spectrometry (AMS).

DOM samples were combusted at 900 8C for 4 h in

evacuated, flame-sealed quartz tubes together with CuO

wire and silver wool. The resulting CO2 was collected in a

cold trap with liquid nitrogen and subsequently reduced to

graphite with an iron catalyst and with a twofold excess of

hydrogen at 600 8C (Nadeau et al., 1997, 1998). The 14C

concentration is given in pmC (% modern C) and referred to

94.9% of the concentration of the NBS oxalic acid standard in

1950. The precision of the AMS measurements is in the range

of 0.3 pmC for modern samples (Nadeau et al., 1998). The

results were corrected for natural and sputtering fractionation

to a base of d13CZK25‰ (Stuiver and Polach, 1977).

Aliquots of all soil samples taken before, during and after

the incubation were also analyzed for d13C ratios. We


analyzed the 14C content of composite soil samples of

the three replicates of each treatment (including the control)

before and after incubation as described above.

2.3. Calculations

The amount of CO2 in the gas phase was calculated using

the general gas equation. For DOM solutions, CO2 in the

liquid phase was calculated by using temperature, pH and

tabulated solubility constants. To calculate CO2 in the liquid

phase for each measurement, a linear change of the HC

concentration during the incubation from the initial to the

final pH in the solutions was assumed.

To calculate the mineralized fraction of freshly sorbed

OM, the C mineralization of the control soil sample

(extracted with water) was subtracted from the samples

with sorbed OM, assuming that priming effects of the

freshly sorbed OM on the mineralization of the indigenous

C in the Bw soil material were negligible. We used two

methods to test this assumption. First, we utilized the 13C

signatures of the maize-derived DOM sample and of the

soils before and after sorption of maize DOM, as well as

their changes during the incubation to calculate the

mineralization of C for the sorbed maize OM. The fraction

of sorbed maize OM (fOM sorbed) was calculated using a

mixing model for each sampling time (at the start of the

incubation, three times during incubation, and after

incubation; Balesdent et al., 1988)

fOM sorbed Z ðdsoil sorbed OM Kdsoil controlÞ=ðdDOM maize

Kdsoil controlÞ

where dsoil sorbed OM is the 13C value of the soil sample with

maize OM sorbed, dsoil control is the 13C value of the control

treatment and dDOM maize is the 13C value of the maize-

derived DOM. The decline in the fraction of sorbed maize-

derived OM during the incubation was used to calculate the

degradation of the OM sorbed.

Secondly, we used the 14C content of all DOM samples

and the 14C contents of the soils before and after DOM

sorption and at the end of the incubation experiment to

calculate the degradation of the freshly sorbed OM in way

similar to that described above for the 13C ratios of the

maize DOM sample.

A double exponential model with two distinct C pools

with different mineralization rate constants was fitted to the

mineralization data to describe the process of mineralization

of organic C both in solution and sorbed to mineral soil

(Kalbitz et al., 2003a), using a least square optimization

method (Quasi-Newton):

mineralized C ð% of total CÞ

Z að1 KeKk1tÞC ð100 KaÞð1 KeKk2tÞ;

where

t
time (days)
a
the part of C that is rapidly mineralizedZlabile C
(%)

100Ka the part of C that is slowly mineralizedZstable C

(%)

k1 mineralization rate constant of labile C (day–1)

k2
mineralization rate constant of stable C (dayK1)
The mineralization rate constants were converted from

20 8C to a temperature of 5 8C, the average annual

temperature of the site from which the Oi and Oa samples

and the mineral soil came. We assumed a Q10 value of 2.4

for this correction, reflecting the global Q10 median of soil

respiration (Raich and Schlesinger, 1992). We are aware

that Q10 values are temperature-dependent and not constant

over a temperature range of 15 8C. Therefore, our correction

should be considered as a first attempt to relate values

measured in the laboratory to the situation in the field. We

applied the following equation (Tjoelker et al., 2001):

kx
Zkx0=Qðtx0KtxÞ=10 8C10
tx Z5 8C (specific temperature: aim of conversion)

tx 0 Z20 8C (reference temperature: incubation)

kx Zmineralization rate constant at 5 8C

kx 0 Zmineralization rate constant at 20 8C

We calculated the mean residence times of the labile and

stable DOC pools as the reciprocal value of the respective

mineralization rate constant.

2.4. Statistics

Differences in the degradation of OM in solution and after

sorption, including the parameters obtained by the double

exponential model, were proven to be significant for each

type of OM using paired t-tests (p!0.001). Differences

between initial and final contents of hot-water extractable

OM, spectroscopic properties and of d13C ratios were also

tested to be significant with paired t-tests (p!0.05). The t-

test for independent samples (two-tailed; p!0.05) was used

to test whether the sorptive enrichment with the three types of

OM resulted in significant differences in properties of the

mineral soil. Furthermore, we calculated linear regressions

between the analyzed properties of hot-water extractable OM

and the mineralized portions of sorbed OM at a significance

level of p!0.05. The slopes of these regressions were tested

to be significantly different between the samples using the

procedure described in Zar (1996).

3. Results

3.1. Degradation of sorbed organic matter in comparison

to organic matter in solution

After 1 year of incubating the DOM solutions, 28–91%

of the DOC was mineralized. DOM from Oi material and

Fig. 1. Carbon mineralization of three different OM samples in solution and

after sorption during the course of 1 year. Symbols and error bars represent

mean and standard deviation of three replicates. Lines represent results

from the fitting of the double exponential model.


maize straw was mineralized to a greater extent than DOM

from Oa material. These results were in agreement with

findings of Kalbitz et al. (2003a) which showed that

mineralization of DOC decreased with increasing degree

of decomposition of the parent solid material.

Sorption of DOM to mineral soil reduced the mineral-

ization of C significantly by 64% for DOM from maize

straw (from 91 to 27%) and the Oi horizon (from 77 to

13%), and by 23% for DOM from the Oa horizons (from 28

to 5%) compared to mineralization in solutions (Fig. 1).

Despite this stabilization, the order of degradability in

solution and after sorption was the same (maize straw OMOOi horizon OMOOa horizon OM).

In all samples, mineralization was initially high and then

decreased quickly. These changes can be adequately

described using the double exponential model (Table 2).

The parameters calculated from this model illustrate the

stabilization of OM by sorption. The greatest effect was

found for OM characterized by large mineralization in

Table 2

Quantitative measures of the C mineralization of three different OM samples in s

mineralized C, sizes of the labile and stable C pools, mineralization rate constan

OM sample Mineralized C

(% of (dissolved)

organic C)

Labile Ca

(% of (dissolved)

organic C)

Stable Cb

(% of (dissolved)

organic C)

Maize: solution 90.9 51.0 49.0

Maize: sorbed 27.4 20.2 79.8

Oi: solution 77.3 51.1 48.9

Oi: sorbed 13.4 6.8 93.2

Oa: solution 27.8 18.6 81.4

Oa: sorbed 4.7 0.8 99.2

Samples represent the mean of three replicates. r2: coefficient of determination oa rapidly mineralizable C; calculated using a double exponential model.b slowly mineralizable C; calculated using a double exponential model.c mineralization rate constant of the labile C pool (double exponential model; cd mineralization rate constant of the stable C pool (double exponential model;e mean residence time of the labile C pool (1/k1).f mean residence time of the stable C pool (1/k2).

solution (i.e. maize straw, Oi horizon). The portion of stable

C nearly doubled upon sorption for OM extracted from

maize and the Oi horizon and increased by one-fifth for OM

from the Oa horizon (Table 2). However, the percentage of

stable C after sorption was significantly larger for OM from

the Oa horizon (99%) than for the other two treatments

(80–93%). By sorption, the mineralization rate constants of

the stable C pool decreased significantly by a factor between

3 and 25, with the greatest increase for OM with large

mineralization in solution (maize straw, Oi horizon). The

estimated mean residence time of the stable pool was

longest for sorbed OM from the Oa horizon (91 years) in

comparison to 37–43 years of those from maize straw and

the Oi horizon. The mineralization rate constants of the

labile C pool were less affected by sorption. They decreased

significantly by a factor of 1.6–1.9 for OM from maize straw

and the Oi horizon and remained almost unchanged for OM

from the Oa horizon.

In summary, there is a distinct decrease in C mineral-

ization upon sorption, with the greatest absolute effect for

OM samples with large mineralization in solution before

sorption. However, the stabilization seems to be more

pronounced for that portion of OM that inherently degrades

slowly.

3.2. Changes of properties of sorbed organic matter

during degradation

The proportion of total soil C that was hot-water

extractable (Chw) was strongly affected by sorption of

DOM. Sorption of DOM from the Oi horizon and from

maize straw increased the Chw contents of the mineral soil

significantly as compared to that of the control (Table 3).

This increase in Chw was equivalent to the increase in total C

of the mineral soil. Therefore, the Chw portions of total C

were similar for the soils treated with DOM from the Oi

horizon and from maize straw as compared to the control

olution and after sorption as obtained by 1-year incubation: percentage of

ts and mean residence times of the labile (k1) and the stable (k2) C pools

k1c (dayK1) k2d (dayK1) MRT-1e

(days)

MRT-2f

(years)

r2

0.0421 0.001796 24 1.5 0.98

0.0215 0.000074 46 36.9 0.98

0.0478 0.000855 21 3.2 0.95

0.0294 0.000064 34 43.0 0.97

0.0220 0.000099 46 27.8 0.98

0.0252 0.000030 40 91.0 0.98

f the double exponential model.

onverted to a temperature of 5 8C).

converted to a temperature of 5 8C).

Table 3

Content of hot-water extractable C (Chw) and its relation to total organic C before, during and after incubation of sorbed organic matter

Treatment Sampling 1a Sampling 2a Sampling 3a Sampling 4a Sampling 5a

Content of Chw (mg gK1 soil)

Control 2.08G0.06 1.96G0.09 1.78G0.26 1.74G0.04 1.88G0.04

Sorbed maize-OM 2.62G0.19 2.06G0.25 2.15G0.12 2.00G0.40 2.20G0.03

Sorbed Oi-OM 2.42G0.20 2.06G0.20 1.76G0.08 1.76G0.14 1.78G0.06

Sorbed Oa-OM 2.09G0.07 2.11G0.37 2.03G0.12 1.89G0.06 2.11G0.17

Relation of Chw to total C (in %)

Control 25.2G0.7 23.7G1.1 21.6G3.1 21.1G0.5 22.8G0.5

Sorbed maize-OM 22.8G1.6 17.9G2.2 18.7G1.1 17.4G3.5 19.1G0.3

Sorbed Oi-OM 23.5G1.9 20.0G1.9 17.0G0.7 17.1G1.3 17.3G0.6

Sorbed Oa-OM 20.3G0.6 20.5G3.6 19.8G1.2 18.4G0.6 20.4G1.7

Mean and standard deviation of three replicates.a Sampling time during the incubation of sorbed OM (sampling 1, after sorption and before incubation; sampling 4, after 270 days of incubation; sampling 5,

after 375 days of incubation; samplings 2 and 3 differed between the treatments because of different C evolution, see Fig. 3 for these sampling times).

Fig. 2. Portions of hot-water extractable organic C (Chw) on total C in

relation to the C mineralization after sorption of DOM obtained from maize

straw and from the Oi horizon of a Norway spruce forest.


(Table 3). Sorption of DOM from the Oa horizon did not

change the Chw content of the mineral soil despite of the

increased C content. Therefore, sorption of that DOM

significantly decreased the portion of total C being hot-

water extractable compared with the control (and to DOM

from the Oi horizon) (Table 3).

The content of Chw and its relation to total C decreased

significantly during the incubation except for the mineral

soil with sorbed OM from the Oa horizon (Table 3). This

decrease was most evident during the first phase of the

experiment and corresponded to changes in mineralization.

Therefore, we found significant correlations between the

mineralization of C during the incubation and the portion of

total C being hot-water extractable (control: rZK0.56; Oi:

rZK0.86, maize: rZK0.55) (Fig. 2)

At the beginning of the incubation, the specific

absorbance at 280 nm (A 280) and the humification indices

deduced from emission fluorescence spectra (HIXem) of the

hot-water extractable OM depended on the source of OM

sorbed to the mineral soil (Figs. 3 and 4). The values were

largest for soil samples with sorbed OM derived from the Oa

horizon and smallest for OM from maize straw, reflecting

the differences in the aromaticity and complexity of the

molecules of OM from these sources. The HIXem of the

control were statistically equal to those of the treatment with

OM from the Oa horizon, whereas the specific UV

absorbance of the control was relatively small.

For almost all samples (except A 280 of the control), the

values of the spectroscopic properties increased signifi-

cantly during incubation, however, to a different extent. The

temporal course of the humification indices and the specific

UV absorbances were similar to changes in C mineraliz-

ation: a large increase at the beginning of the experiment

and only small changes afterwards (Figs. 3 and 4). This was

especially pronounced for the sample with the largest

biodegradability—DOM from maize straw sorbed to the

mineral soil. Furthermore, HIXem values seem to reflect

the dynamics better than the specific UV absorbance. The

dynamics of OM properties during incubation are reflected

in significant correlations between the mineralization of

sorbed C and the spectroscopic properties of hot-water

extractable OM (Figs. 3 and 4). That means the greater

the mineralization of sorbed OM the larger is the aromatic

and complex character of the remaining hot-water extrac-

table OM within each sample. These correlations indicate

the preferential ‘survival’ of aromatic and complex

structures of sorbed OM during the incubation. In most

cases, the slopes of the regressions between the extent of

mineralization and the spectroscopic properties were

statistically equal for the different types of OM samples.

Only the slope of OM from the Oa horizon was significantly

larger than that of maize straw using the specific UV

absorbance (Fig. 4). The differences in the spectroscopic

properties between the different OM sorbed to the mineral

soil were equalized during incubation. In case of the specific

UV absorbance, this trend was not as strong.

The d13C ratios of the soil solid phase treated with

different DOM solutions (before incubation) reflected the

differences in the d13C ratios of the DOM samples (Fig. 5).

Largest d13C ratios were observed after sorption of maize

DOM (d13C of maize DOM: K12.9‰). Sorption of DOM

from the Oi and the Oa horizon decreased the d13C ratios

slightly (significantly for DOM from the Oa horizon)

because the d13C values of the DOM (Oi: K26.3‰, Oa:

K26.9‰) were smaller than that of the original organic

Fig. 3. Humification indices (deduced from fluorescence emission spectra) of hot-water extractable OM during the 1-year incubation of sorbed OM to a mineral

soil; mean and standard deviation of three replicates (a); humification index of the remaining hot-water extractable OM after incubation in relation to the

mineralized amount of sorbed organic matter (b).

Fig. 4. Specific UV absorbance at 280 nm of hot-water extractable OM during the 1-year incubation of sorbed OM to a mineral soil; mean and standard

deviation of three replicates (a); specific UV absorbance of the remaining hot-water extractable OM after incubation in relation to the mineralized amount of

sorbed organic matter (b).


matter in the mineral soil (K25.4‰). We found no

significant differences between the d13C ratios of the soil

treated with different DOM and the corresponding hot-water

extractable OM except of significantly larger values for the

maize treatment in the extracts (data not shown).

During incubation, the d13C ratios of the maize treatment

decreased strongly (Fig. 5). However, for the other

treatments, we found a significant isotopic shift of about

K0.2‰ during the 1-year incubation indicating changes

in the composition of organic matter during incubation

(Fig. 5).

Fig. 5. d13C ratios of the soils during the 1-year incubation of OM sorbed to a miner

and standard deviation of three replicates.

3.3. Estimation of mineralization of sorbed C using changes

in d13C ratios and 14C of the soil samples

We found a similar mineralization dynamics of sorbed

maize-derived C based on CO2 and 13C measurements

(Fig. 6), but a slightly larger mineralization of 8% based on

the 13C values of the soils (Table 4). These results indicate

preferential mineralization of compounds rich in 13C.

However, the differences between 13C and CO2 data were

equivalent to 0.2‰, which is very small. The error of

estimation of the mineralization using the 13C approach was

al soil (a) with enlarged scale for the control, Oi and Oa treatment (b); mean

Fig. 6. Comparison of the dynamic of C mineralization of maize straw derived

OM sample after sorption to a mineral soil based on CO2 measurements and13C analysis; mean and standard deviation of three replicates.


G3.9% because of the analytical error of 13C measurements

(G0.1‰).

We had a strong 14C label to trace the mineralization of

sorbed C by comparing the 14C values of the mineral soil

(79.8 pmC) and of the DOM (O109 pmC, Table 4). The

measured 14C contents after sorption also represented the

percentage of sorbed OM as illustrated in Table 1. After 1-

year incubation, taking into account also the changes in the

control sample, we found a decrease in 14C. The decrease in14C was used to calculate the mineralization of sorbed C,

which was much larger than mineralization measured by

analysis of the CO2 evolution. However, the error of

estimation of the mineralization of sorbed C using 14C is

large. For example, an analytical error of 0.3 pmC is

equivalent to 5% mineralization of sorbed C and five 14C

measurements are necessary to calculate the mineralization

of one sample (DOM sample before sorption, control and

treatment before and after incubation). Despite of the large

error of estimation there is a preferential degradation of

compounds with high 14C contents, particularly in the soil

sorbed with DOM from the Oi horizon and from maize straw.

4. Discussion

4.1. Stabilization of DOM by sorption

According to recent knowledge, sorption of DOM on

mineral surfaces rather than mineralization to CO2 is

Table 414C content of the used DOM samples and of the mineral soil; estimation of C mi

maize derived DOM and estimation of C mineralization using changes in the 14C

Treatment 14C content (pmC)

DOM solution Soil after sorption Soil after sor

and incubatio

Control 79.8 81.8

Sorbed maize-OM 109.0 86.5 84.7

Sorbed Oi-OM 112.1 84.5 83.9

Sorbed Oa-OM 110.2 86.4 87.1

responsible for maintaining low DOC concentrations in

the mineral subsoil (summarized by Kalbitz et al., 2000).

Furthermore, sorption of DOM is considered to be an

important stabilization process of organic matter in mineral

subsoils (Kaiser and Guggenberger, 2000; Guggenberger

and Kaiser, 2003). Two factors that could account for the

decrease in degradation of sorbed OM observed in our

experiment are: (i) selective sorption of inherently more

stable components and (ii) stabilization of organic com-

pounds due to sorption to the mineral phase.

Sorption of DOM to the mineral soil is considered to be

selective for hydrophobic and aromatic compounds (Kaiser

et al., 1996; McKnight et al., 1997). Therefore, it should also

be selective with respect to the biodegradability of sorbed

OM, because aromatic, complex and hydrophobic com-

pounds are the most stable DOM components (Kalbitz et al.,

2003a,b). Indeed, we observed a significant decrease in the

specific UV absorbance (A 280) by 33% and in the

humification indices deduced from the fluorescence spectra

(HIXem) by 21% for the DOM solution from the Oa horizon

after sorption to the mineral soil (Fig. 7). These changes

indicated a preferential sorption of DOM fractions with

large aromaticity and complexity. The sorbed fraction

amounted to 44% of the total DOC in DOM from the Oa

horizon (Table 1). The other two samples with less sorption

to the mineral soil (portion of DOC sorbed: 24–25%;

Table 1) did not show such pronounced changes in

spectroscopic properties after sorption. The A 280

decreased by about 10%, whereas the HIXem remained

unchanged (Fig. 7). Therefore, selective sorption of stable

compounds should be of minor importance for decreasing

degradation of sorbed OM from maize straw and the Oi

horizon.

Kalbitz et al. (2003a) suggested that biodegradability of

DOM can be estimated using the values of specific UV

absorbance and HIXem in the solution before incubation

because of a close and inverse relationship between DOC

mineralization and these spectroscopic properties. We used

the changes in these spectroscopic properties of the OM

solutions after sorption to estimate the contribution of

selective sorption of aromatic and complex compounds to

sorptive OM stabilization (Table 5). The estimated

percentages of mineralized C were in the range of the

measured mineralization for OM solutions before sorption

(compare Tables 2 and 5). The good agreement indicates

neralization using changes in the d13C ratios of the mineral soil sorbed with

content during 1-year incubation

Mineralization of sorbed organic C (% of initial values)

ption

n

Measured by CO2 Estimated by 13C Estimated by 14C

27 35 53

13 52

5 14

Fig. 7. Humification indices deduced from fluorescence emission spectra (HIXem) (a) and specific UV absorbance at 280 nm (b) of three DOM solutions before

and after sorption to the mineral soil (mean and standard deviation of six replicates). Solution after sorption was obtained by centrifugation and filtration.

Table 5

Estimated C mineralization of three DOM solutions before and after

sorption to a mineral soil using regressions between spectroscopic

properties and the percentage of mineralizable DOC published by Kalbitz

et al. (2003a)

Estimated mineralization

using A 280a (% of initial

DOC)

Estimated mineralization

using HIXemb (% of initial

DOC)

Before

sorption

After

sorptionc

Before

sorption

After

sorptionc

Maize-DOM 100 106 102 87

Oi-DOM 77 82 70 69

Oa-DOM 16 34 15 21

a Specific absorbance at 280 nm: mineralized DOC (% of initial

DOC)ZK33.23475K106.4627!log(A 280).b Humification index deduced from fluorescence emission spectra:

mineralized DOC (% of initial DOC)Z89.1459K58.3983!log (HIXem).c Of remaining DOC after sorption.


the potential of spectroscopic properties to predict the

percentage of mineralizable DOC (Kalbitz et al., 2003a).

Based on this approach, we estimated the percentage of

completely mineralizable C remaining in solution after

sorption to increase by 12% for DOM from the Oa horizon

(mean of the values deduced from A 280 and HIXem;

Table 5). This means that the sorbed C should be

correspondingly less mineralizable. Thus, half of the

measured decrease of 23% in mineralization after sorption

of this DOM sample (Table 2) can be explained by selective

sorption of intrinsically stable compounds.

This selective sorption of stable compounds can be

disregarded for the other two DOM samples because of the

small differences in the calculated percentage of miner-

alized DOC before and after sorption (Table 5). The DOM

solutions from maize straw and the Oi horizon were

characterized by strong mineralization and little sorption.

On the other hand, the relative increase in stability after

sorption was even larger for these fairly labile DOM

samples than for DOM from the Oa horizon, indicating a

large stabilizing effect of sorption to the mineral soil.

A strong stabilization of intrinsically labile but strongly

sorbing organic compounds was shown for low-molecular-

weight organic acids (citrate) with a decrease in mineral-

ization from about 60% of added citrate to less than 5%

when comparing soils with small and large sorption capacity

(van Hees et al., 2003). The authors observed that the larger

the sorption, the smaller the mineralization of citrate. In

another study, sorption significantly reduced the mineraliz-

ation of citrate from 76 to only 1% after sorption to Fe(OH)3

(Jones and Edwards, 1998). In an alternative approach, Keil

et al. (1994) and Nelson et al. (1994) desorbed OM from

sediments and soils and found mineralization to increase

strongly after desorption. Between 70 and 95% of desorbed

organic C could be mineralized within 7 days (Keil et al.,

1994). These values are similar to the mineralization of

DOC from maize straw and the Oi horizon supporting the

idea of stabilization of intrinsically labile DOM by sorption.

Despite the relatively large increase in stability by

sorption of DOM from the Oi horizon and from maize straw

compared that of DOM from the Oa horizon, the differences

in the mineralization between the samples remained similar

after sorption. This means that the mineralization of C after

sorption of DOM from less decomposed organic material

(i.e. maize, Oi) was faster and larger than for DOM from

highly decomposed organic matter (i.e. Oa). These results

confirm that the composition of DOM is important for its

degradation even after sorption. DOM from the Oa horizon

had a much greater aromaticity and complexity of the

molecules (larger A 280 and HIXem) and smaller contents of

carbohydrates than DOM derived from the two other

materials (Fig. 7; Kalbitz et al., 2003a). Furthermore,

sorption of DOM from the Oa horizon to the mineral soil

was stronger and selective for stable compounds. In turn,

sorption of DOM from less-decomposed organic materials

was weaker and non-selective, likely because of the large

percentage of carbohydrates, which bind weakly to soils

(Kaiser and Guggenberger, 2000). The weak sorption of

DOM from less-decomposed materials resulted in larger

contents of hot-water extractable C in comparison to that


from the Oa horizon (Table 3). Thus, weak sorption of

intrinsically labile compounds resulted in greater mineral-

ization of C than did strong sorption of intrinsically more

stable OM.

The formation of strong chemical bonds between the

mineral phase and the organic molecules resulting in

changed conformation and electron distribution (Kaiser

et al., 1997; Khanna et al., 1998) is likely responsible for the

reduced mineralization of C after sorption. Furthermore,

physical separation of OM from microorganisms and/or

hydrolytic enzymes by sorption into small pores (Mayer,

1994) can contribute to decreasing C mineralization.

Sorption of DOM at the mouths of micropores favoring

multiple ligand attachment of organic molecules to the

sorbing surface can explain how the soil nanoporosity may

restrict desorption and accessibility of OM for microorgan-

isms (Kaiser and Guggenberger, 2003).

4.2. Usefulness of hot-water extraction to trace the fate

of sorbed organic matter

Hot-water extracts were used to track changes in the

composition of water-soluble OM after sorption of DOM to

the mineral soil and during the subsequent incubation. Cold-

water extraction yielded insufficient OM for further

analyses. Extraction at 150 8C and 10 MPa yields OM

with a similar composition to that from cold-water extracts,

at least with respect to properties detectable by UV and 1H-

and 13C-NMR spectroscopy (Schwesig et al., 1999).

The strong sorption of DOM from the Oa horizon was

reflected by the contents of hot-water extractable organic

carbon (Chw), which did not change after sorption (Table 3).

The large stability of sorbed OM from the Oa horizon was

also reflected by little changes in Chw during the 1-year

incubation. In contrast, the weak sorption of DOM from the

Oi horizon and from maize straw resulted in an equivalent

increase in Chw prior to the incubation. During the

incubation, Chw decreased considerably for the weakly

sorbing OM. The inverse relationship between the Chw

portion of total C and the percentage of mineralized C

illustrated the usefulness of hot water to extract weakly

sorbed and largely degradable OM. Also, hot-water extracts

reflected the order of aromaticity and complexity of sorbed

OM (as expected from DOM properties prior to sorption)

using A 280 and HIXem values (Fig. 7): maize straw!Oi

horizon!Oa horizon (Figs. 3 and 4). Thus, hot-water

extraction can be recommended to track changes in OM

composition after DOM sorption.

4.3. Selective stabilization of aromatic and complex

compounds

In the hot-water extracts, increasing values of UV

absorbance and HIXem indicated relative enrichment of

aromatic and complex compounds during the incubation of

soils with sorbed OM. Linear correlations between

the spectroscopic properties of remaining OM and the

extent of mineralized C confirmed the hypothesis that

aromatic and complex compounds are relatively stable

against degradation (Kalbitz et al., 2003a,b). This hypoth-

esis is in agreement with the results of previous studies

dealing with degradation of OM in solutions and solid

samples (Zsolnay and Steindl, 1991; Baldock et al., 1992;

Haider, 1992; Almendros and Dorado, 1999; Hongve et al.,

2000; Moran et al., 2000; Parlanti et al., 2000; Pinney et al.,

2000; Dai et al., 2001; Yanagi et al., 2002). Therefore, it

seems reasonable to conclude that aromatic and complex

structures are enriched during the degradation of OM

regardless of the sampled stratum. The slightly decreasing

d13C ratios of soil samples and hot-water extracts indicated

that these compounds might originate from lignin since

lignin-derived compounds are depleted in 13C (Benner et al.,

1987; Schulten and Gleixner, 1999; Lichtfouse, 2000). This

relative enrichment was greatest for the soil enriched with

OM from the Oa horizon. This may also explain

the extremely high proportion of stable organic C and the

longest mean residence time (Table 2) observed among

the samples. Our results also indicated that differences in the

composition of extractable OM diminished during the 1-

year incubation. This does not mean that all stable OM is

the same. At the end of the incubation, considerable

differences in the composition of extractable OM were

detectable by the relatively unspecific spectroscopic

methods applied.

4.4. Priming effects after sorption of DOM

to the mineral soil

Priming effects are strong short-term changes in the

turnover of soil OM caused by comparatively moderate

treatments of the soil (Kuzyakov et al., 2000). Such

treatments might be the input of OM by the sorption of

DOM. The addition of OM to the soil might cause not only

an acceleration of mineralization (positive priming effect)

but also its reduction or an immobilization of the added C

(negative priming effect; Kuzyakov et al., 2000).

The mineralization of sorbed C from maize straw as

estimated from 13C measurements was slightly larger than

that calculated from the CO2 measurements. This result

indicated a preferential mineralization of sorbed C in

comparison to the indigenous OM of the mineral soil. The14C values also suggest a preferential mineralization of

sorbed C with large 14C contents, indicating again that the

mineralization of the indigenous soil C decreased after

sorption of DOM. However, this negative priming effect

cannot by itself explain the large decline in the 14C content

after incubation. If we assume that all CO2 evolved from

mineralization of sorbed C, and neglecting the CO2

evolution from the original soil (which was subtracted

from the treatments), the percentage of mineralized C would

only increase from 4.7, 13.4 and 27.3 to 8.5, 17.3 and 29.7%

for sorbed OM from the Oa horizon, Oi horizon and maize


straw, respectively. Therefore, additional mineralization of

young organic compounds with large 14C contents of the

indigenous soil OM pool must have occurred during the

incubation. The C mineralization of the control treatment

resulted in an increasing 14C content of the soil also

indicating a selective degradation of very young compounds

with recent 14C values and an enrichment of components

containing bomb 14C from the 1950s and 1960s. The

observed 13C isotopic shift during the incubation in the

control of K0.2‰ indicates that this C may be derived from

fresh biomass and/or carbohydrates (Kracht and Gleixner,

2000; Lichtfouse, 2000). The differences between mineral-

ization of sorbed C from maize straw, as estimated from 13C

values on the one hand, and mineralization calculated from

CO2 measurements on the other hand, may relate to an

isotopic shift in addition to the 0.2‰ observed for the

control and the other OM samples. This isotopic shift might

be caused by larger degradation of carbohydrates enriched

in 13C (Lichtfouse, 2000). Larger degradation of carbo-

hydrates from sorbed OM that derived from maize straw as

compared with the other treatment seems reasonable,

because the largest C mineralization occurred for the

maize-derived OM, and close relations were reported

between the extent of C mineralization and the content of

carbohydrates (Kalbitz et al., 2003a; Kawahigashi et al.,

2004).

In summary, we expect a negative priming effect with a

preferential mineralization of the sorbed C in comparison to

the indigenous soil C after sorption of DOM. Therefore, the

CO2 measurements may slightly overestimate the stability

of sorbed OM. A positive priming effect may result in

preferential degradation of young compounds from the

indigenous soil organic pool, probably fresh biomass.

5. Conclusions and implications for OM stabilizationin soils

Sorption of DOM to the mineral soil results in its

stabilization. We propose that the main mechanisms for

this stabilization are the selective sorption of intrinsically

stable compounds and strong chemical bonds to the mineral

soil and/or a physical inaccessibility of OM to microorgan-

isms. The importance of selective sorption of intrinsically

stable DOM components increases with increasing extent of

DOM sorption.

The stabilizing effect of sorption is relatively large for

labile DOM samples and relatively small for stable ones high

in aromaticity and complexity of molecules. However, the

extent of sorption and the mean residence time after sorption

were estimated to be twice as large for stable DOM samples

compared to labile ones resulting in a four times larger

sorptive stabilization. We conclude that the stronger the

sorption, the greater the amount and stability of sorbed OM.

The extent of stabilization and the stability of sorbed OM

depend on DOM composition and soil properties. Therefore,

properties of DOM and sorbents need to be considered in

order to accurately estimate the potential of DOM to form

stable OM. Often, at large water fluxes after heavy rainfall

events DOM contains more carbohydrates, is less aromatic

and more hydrophilic than at small water fluxes (Kaiser and

Guggenberger, 2005), resulting in larger mineralization and

less sorption. Thus, these large DOM fluxes should

contribute much less to sorptive stabilization than continu-

ous DOM fluxes under base flow conditions.

Kaiser and Guggenberger (2003) suggested that the point

of time when DOM sorption occurs to mineral surfaces is

crucial because the stability of OM is largest for molecules

that sorb first to the juvenile mineral surface. In our

experiment, we used a mineral subsoil containing

8.4 mg C gK1 soil. This suggests that mineral surfaces

were no longer juvenile. Therefore, the measured OM

stability in the present study should be smaller than the

stability of OM sorbed to juvenile mineral surfaces.

The calculated mean residence time of sorbed OM in the

mineral soil increased up to 90 years. We estimated sorptive

DOM stabilization of about 24 Mg C ha–1 in the mineral soil

profile from which the DOM sample (Oa horizon) and the

mineral soil came. For this calculation we assumed a uniform

sorption behavior of the mineral soil, neglected preferential

flow and divided the annual DOC retention of about

260 kg C haK1 yearK1 (Kalbitz et al., 2004) by the measured

decomposition rate of the sorbed OM (Olson, 1963).

Currently, 110 Mg C haK1 are stored in the mineral soil at

this site with an important part likely derived from DOM.

We are aware that the estimated 24 Mg stable C ha–1

derived from DOM has some uncertainties. We do not know

whether the calculated amount of DOM-derived stable C

can be stabilized yet or has been already stabilized. A larger

mineralization than we measured is possible in the field if

sorption occurred at places with a high microbial activity

(Guggenberger and Kaiser, 2003). In contrast, we can

assume that the ‘sorption front’ is moving downwards in the

soil profile as a part of pedogenesis towards a region with a

larger number of available sorption sites (Guggenberger and

Kaiser, 2003; Ussiri and Johnson, 2004). This will result in

an even larger percentage of stable OM deriving from DOM

than we estimated. In any case, our results emphasize the

importance of the mineral soil for sorptive stabilization of

DOM.

Acknowledgements

This study was funded by the Deutsche Forschungsge-

meinschaft (DFG) as part of the priority programme SPP

1090 ‘Soils as source and sink of CO2’, and by the German

Ministry of Education and Research (BMBF) under grant

No. PT BEO 51—0339476. We thank the members of the

Central Analytical Department of BITOK for support,

P. Grootes, K. Kaiser and E. Gregorich for helpful

comments and B. Glaser for the 13C measurements.


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