soil wettability, aggregate stability, and the

7/30/2019 Soil Wettability, Aggregate Stability, And The

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Soil wettability, aggregate stability, and the

decomposition of soil organic matter

Marc-O. Goebel*, Joerg Bachmann, Susanne K. Woche, Walter R. Fischer

Institute of Soil Science, University of Hannover, Herrenhaeuser Str. 2, D-30419 Hannover, Germany

Available online 12 January 2005

Abstract

In well-structured topsoils, part of the soil organic matter (SOM) is located in the interior of the soil aggregates. Because of

its location, this part of the SOM is little accessible to micro-organisms, and consequently not readily mineralised. Additionally,

the physico-chemical conditions on the aggregate surfaces, being the main habitat of the organisms, control the accessibility,

and hence, the rate of mineralisation. We hypothesise that hydrophobic conditions on aggregate surfaces reduce the rate of

mineralisation of inside SOM, and simultaneously enhance the aggregate stability. The objectives of this study were therefore to

study the significance of soil wettability with respect to both SOM mineralisation and aggregate stability. We used soil material

from a loess-derived Gleyic Luvisol, either used as cropland or as grassland. Wettability was measured in terms of both the

advancing soilwater contact angle and the solid surface free energy. Aggregate stability was assessed by immersion ofaggregates in waterethanol mixtures of varying surface tension. The impact of aggregation on SOM mineralisation was

determined by respiration experiments that measured the CO2-release both from the aggregates and from the corresponding

homogenised soil material. It was found that the contact angle of the soil samples ranged from 17 8 to 798, and the solid surface

free energy from 34 to 68 mJ m2. Aggregates showed increasing stability with decreasing surface tension of the testing liquid.

With increasing contact angle, the initial aggregate breakdown was decreased, which we attribute to the wettability-dependence

of the liquid adsorption rates of the aggregates. Soil respiration measurements showed that microbial SOM decomposition was

affected by the aggregation status, i.e., the homogenised samples released significantly more CO2 than the aggregates. We

conclude that even subcritical soil water repellency (with contact angles b908) can have a significant impact on the protection of

SOM against microbial decomposition.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Soil organic matter; Soilwater contact angle; Solid surface free energy; Aggregate stability; Soil respiration

1. Introduction

Scenarios regarding the impact of future land

management on soil carbon content require an under-

standing of the physical mechanisms that affect organic

matter degradation. The rate at which soil organic

0016-7061/$ - see front matterD 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.geoderma.2004.12.016

* Corresponding author. Fax: +49 511 762 5559.

E-mail address: [email protected] (M.-O. Goebel).

Geoderma 128 (2005) 8093

www.elsevier.com/locate/geoderma


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carbon (SOC) is returned to the atmosphere, either as

CO2 or CH4, depends on its bioavailability. Both

processes are significantly linked with either the

moisture and nutrient status or with the aggregationstatus of the soil. Generally, many processes like

wetting, adsorption, flocculation, dispersion and solute

transfer depend on the interfacial interactions between

solid and liquid. These interactions are mainly con-

trolled by the interfacial energy between the two

phases, which controls the strength of interaction.

Characterisation of the surface properties and espe-

cially the surface free energy components of the solids

provides essential insight into the mechanisms of

surface based phenomena, especially for carbon

stabilising processes. Particular the organic moleculeswith amphiphilic properties like long chain C16 to C32fatty acids, fulvic and humic acids can determine the

wetting behaviour of the soil (MaShum et al., 1988;

Franco et al., 2003).

Regarding carbon sequestration, an important effect

of water repellency on the physical stabilisation of soil

organic matter (SOM) is its influence on the dynamics

of soil moisture. A contact angle of 08 indicates

wettable surfaces of the capillarywater system,

resulting in capillary uptake, whereas values around

908 indicate the transition to water repellency. Hydro-

phobic surfaces, indicated by contact angles N908,

feature capillary depression. For contact angles N908,

the moisture transfer is reduced to zero which results in

dry soil domains. These domains cause preferential

flow and are excluded from water and nutrient flow. It

was shown by Dekker and Ritsema (1996) that

irregular wetting patterns can persist for months. With

increasing rainfall, the wetting resistance of the soil is

reduced, when soil moisture content rises to a certain

value, called the critical water content hc.

Another effect of water repellent organic matter is

the impact on the stabilisation of soil aggregates.Hydrophobicity, caused by organic substances,

favours the formation and the protection of stable

aggregates (Sullivan, 1990) which, in turn, stabilise

the encapsulated organic substances against microbial

degradation and mineralisation (Tisdall and Oades,

1982; Tisdall, 1996). Recent results confirmed exper-

imentally that not only the amount of SOC, but also

the composition is of importance for the stabilisation

of soil aggregates (Hassink and Whitmore, 1997;

Piccolo et al., 1999; Chenu et al., 2000). Capriel

(1997) demonstrated that the hydrophobicity of SOM

changes as a function of soil management. These

authors found that the amount of aliphatic CH units

can be influenced by agricultural management. Adecrease of hydrophobicity, as determined by infrared

spectroscopy, was accompanied by a decrease of

aggregate stability. This was also confirmed by the

findings of Piccolo and Mbagwu (1999).

A t hi rd e f fect of physical stabilis ation was

described by Spaccini et al. (2002). These authors

studied the effect of humified organic matter on the

mineralisation of a representative labile organic

compound in soil. Their results suggest that labile

organic compounds may be effectively protected in

soil by humified organic matter that also reducesmicrobial mineralisation. Innovative soil management

practices, employing hydrophobic humic substances,

may increase the biological stability of SOM and thus

contribute to reduced CO2-emissions from agricultural

soils (Spaccini et al., 2002).

To date, no systematic study has been performed

on the wetting properties of typical (obviously non-

repellent) soils in Central Europe and to link

subcritical wetting properties both to aggregate

stability and to CO2-release as an important mecha-

nism of the physical protection provided by soil

structure. Therefore, our objective was to assess the

SOM mineralisation of a typical loess soil under

cropland and grassland use, and to relate it to its

wetting properties and its structural status (aggregates

vs. homogenised material).

2. Materials

The soil investigated in this study was a loess-

derived Gleyic Luvisol with silt loam texture from the

Rotthalmuenster test site in Bavaria, Germany. Thesite is used as a test plot for various manuring

managements with different crops. Our samples came

from the wheat plot with NPK manuring and from the

grassland plot. The wheat soil was sampled from 0

30, 3045, and 4565 cm, the grassland soil was

sampled from 010, 1020, 2030, 3045, and 4565

cm. Table 1 gives basic data of the soils. The particle

size distribution was determined for the maize plot

and is considered as representative of the whole site.

The data showed only small differences in particle

M.-O. Goebel et al. / Geoderma 128 (2005) 8093 81


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size distribution between the sampling depths. With

pH values below 6 both soils were slightly acidic. The

SOC content of the topsoil was around 1.3% for thewheat plot (030 cm) and slightly above 2.3% for the

grassland plot (010 cm) and decreased with depth for

both variants. Both soils were carbonate free.

3. Methods

3.1. Capillary rise method

To evaluate the wetting properties of the soil

material, we determined the advancing soilwater

contact angle with the capillary rise method (CRM). It

has been used for the measurement of powders like

silica flour and limestone (Siebold et al., 1997), for

peat material (Michel et al., 2001), and, recently, for

soil aggregates (Goebel et al., 2004).

The contact angle h (8) is calculated with the

Washburn (1921) equation from which Siebold et al.

(1997) derived an expression for the weight increase

of the soil column during the capillary rise process.

This expression can be written as:

w 2 c q2

clcoshg

t 1

where w is the weight of the soil column (including

the weight of the liquid), (kg), q is the liquid density

(Mg m3), cl is the liquid surface tension (J m2), g is

the viscosity of the liquid (Pa s), t is the time (s), and c

is a geometry factor (m5) that reflects the porosity and

tortuosity of the capillaries and depends on particle

size and packing density of the soil column. If a liquid

with a non-zero-degrees contact angle is used, two

unknown variables, c and h, have to be determined in

Eq. (1). Therefore, c has to be evaluated independ-

ently for each sample using a liquid (n-hexane) which

wets the soil particles completely (h=08

). Accordingto Eq. (1), the c-factor was determined from the slope

of the n-hexane adsorption rate in the linear range of

the w2(t)-function. The contact angle was then

calculated from the slope of the water adsorption rate

and the material specific c-factor. For details of the

method, see Siebold et al. (1997).

For the measurements, 2 g of soil material was

filled into a glass tube with a sintered glass at the

bottom which is covered by a filter paper. The

material was compacted by tapping the sample with

30 similar impacts to get nearly identical maximum

packing densities, i.e., no further compaction of the

samples in contact with the testing liquid. Thereafter,

the tube was attached to an electronic balance (DCAT

11, DataPhysics, Filderstadt, Germany) and was

brought into contact with the respective testing liquids

(water, n-hexane). The weight gain of the soil material

during its contact with the liquid was recorded with a

frequency of 30 measurements per second. The c-

factor was calculated on the basis of three independent

measurements with n-hexane. Contact angles for

water were calculated on the basis of five independent

measurements. For a detailed discussion, we refer toGoebel et al. (2004).

Additionally, the water drop penetration time test

(Dekker, 1998) was applied for all samples to provide

a commonly used measure for the determination of

soil water repellency.

3.2. Solid surface free energy

For a complete physical characterisation of the

wetting properties which is independent of the wetting

Table 1

Selected physical and chemical properties of the soils

Depth (cm) Particle size distribution (wt.%) SOC (%)a pHCaCl2

Gravel Sandb Siltb Clayb Wheat Grassland Wheat Grassland

010 2.5 10.2 72.6 17.2 1.2 2.3 5.5 5.9

1020 2.5 10.2 72.6 17.2 1.2 1.1 5.5 5.6

2030 2.5 10.2 72.6 17.2 1.2 0.7 5.5 5.7

3045 2.9 10.2 72.6 17.2 0.4 0.5 5.6 5.7

4565 0.1 11.4 72.3 16.3 0.2 0.3 5.7 5.6

a SOC is soil organic carbon.b Soil material b2 mm.

M.-O. Goebel et al. / Geoderma 128 (2005) 809382


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liquid, we calculated the dispersion and polar compo-

nents of the solid surface free energy. Knowledge of

the polar component of the solid surface free energy

provides a more detailed insight into the interactionsbetween solid surfaces and polar liquids like water.

For that purpose, each soil was tested additionally

against ethylene glycol and diiodomethane having

different amounts of dispersion and polar components

of the liquid surface free energy. Using an expression

originally derived by Owens and Wendt (1969), it was

possible to quantify the solid surface free energy

components from the contact angles for the different

testing liquids with known dispersion and polar

components of surface free energy,

1 cosh 2ffiffiffiffifficds

qffiffiffiffifficdl

q

cl

1A 2

ffiffiffiffiffic

ps

pffiffiffiffiffic

pl

q

cl

1A

0@

0@ 2

where cl is the liquid surface free energy (J m2), cl

d

and clp are the dispersion (Lifshitzvan der Waals) and

polar (acidbase) components of the liquid surface

free energy (J m2), and where csd and cs

p are the

corresponding dispersion and polar components of the

solid surface free energy (J m2). The contact angles

were determined in the same way as described for

water on the basis of three replicate measurements

with each testing liquid (for details, see Goebel et al.,

2004).

3.3. Aggregate stability

There are numerous different methods for testing

aggregate stability described in the literature. The

choice of a specific method depends mainly on the

breakdown mechanism to be investigated. Slaking,

dispersion, differential swelling and raindrop impact

are considered as the four main breakdown mecha-nisms (Le Bissonnais, 1996). To study the role of

wettability for aggregate stability, our investigation

was focussed mainly on the slaking process. Besides

wettability, slaking also depends on internal cohesion,

i.e., on clay and SOM content (Emerson and Green-

land, 1990).

Focussing on slaking, it was important to use a

method which did not involve any kind of mechanical

agitation. An appropriate procedure for measuring soil

aggregate stability is therefore the immersion of

aggregates in water as proposed by Emerson (1967).

Because preliminary tests showed, that most of the

aggregates were already completely disrupted after

short-term immersion in water we needed to enhancethe sensitivity of the measurements. Hence, we

extended the procedure and used waterethanol

mixtures to reduce the surface tension of the testing

liquid. This provided a sufficient differentiation

between the samples, and moreover, it allows for the

evaluation of the kinetics of aggregate breakdown.

Ethanol (96%) percentages (wt.%) were 100%,

80%, 60%, 40%, 20%, 10%, 5% and 0% (pure water).

The liquid surface tensions were determined by the

Du Noqy ring method (Adamson, 1990) with the

DCAT 11. The resolution of the measurements wasbetter than 0.01 mJ m2, and the accuracy was better

than 0.03 mJ m2. The relation between ethanol

percentages (molarity of ethanol) and the liquid

surface tension is given in Fig. 1.

Because slaking is most effective at low initial

water contents (Truman et al., 1990), air-dried

aggregates (46.3 mm) were used. Ambient relative

humidity was around 45% (at 20 8C) which corre-

sponds to initial water potentials of around 110 MPaand gravimetric water contents between 2% and 3%.

Ethanol percentage (weight-%)

0 20 40 60 80 100

Liquidsurfacetension(mJm

-2)

0

10

20

30

40

50

60

70

80

4.0 7.8 11.1 14.10.0 16.4

Molarity of ethanol (mol L-1)

Fig. 1. Relation between ethanol percentages (wt.%) and the surface

tension of the testing liquid. The corresponding molarity of ethanol

is given at the upper axis.



5/14

Ten aggregates of each soil sample were placed in

polyethylene containers and 4 mL liquid was added

simultaneously to cover the aggregates completely.

The aggregates were photographed with a digitalcamera after 1, 2, 5, 10, 20, 30, and 60 min of

immersion. At each time step, the number of intact

aggregates was counted by optical inspection on the

basis of the photographs. The occurrence of visible

alteration was used as a criterion for the distinction

between intact and disrupted aggregates.

Preliminary experiments showed that aggregate

breakdown is most intensive within the first 30 min.

Alteration after this time period generally was small.

To derive a parameter for the overall stability of the

aggregates we counted the number of intact aggre-gates Ai (number) in all containers (C=1,. . ., 8) after a

time period of 30 min and calculated the percentage of

intact aggregates related to the total of all aggregates

(n=80). This yields an index of aggregate stability

ASt % P8

C1 Ai

d

10080

, where larger values

indicate greater stability.

A second measure for aggregate stability was

defined by the surface tension ASSFT (mJ m2) at

which 100% of the aggregates are dispersed after a

time period of 30 min. This parameter indicates the

critical surface tension at which aggregates were

completely disrupted whereas larger values indicate

greater stability. The larger the surface tension

(smaller ethanol-percentages), the greater are the

capillary forces exerted during liquid adsorption in

case of wettable surfaces. Assuming non-wettable

material, disruptive processes would be absent. But

even for wettable material, there is a certain surface

tension below which the capillary forces are not

strong enough to disrupt the aggregates. This

surface tension is specific for an individual sample

and allows an assessment of the cohesive forces

between the solid particles which stabilise theaggregate.

A measure to quantify the kinetics of aggregate

breakdown can be obtained by graphing the number

of intact aggregates Ai vs. time t and fitting the

function with a double exponential model,

Ai ad ejdt bd eddt 3

where the parameterj (min1) describes the kinetics

of the rapid and the parameter d (min1) the kinetics

of the slow breakdown, t is the time (min). Hence,

parameter j is used for the description of the initial

breakdown which can mainly be attributed to slaking.

3.4. Soil respiration

For the soil respiration experiments, aggregates

(24 mm) and homogenised material from the

topsoil of both variants were equilibrated at a water

potential of 40 kPa. 20 g of each soil was filledinto a glass flask (300 mL) which was equipped with

a septum at the top. To minimize evaporation losses,

each flask was supplied with a water reservoir. The

samples were then incubated at 20 8C under

exclusion of light. Gas aliquots of 5 mL were taken

from the headspace of each incubation flask with amedical syringe. Measurements were performed in

intervals of 3 and 4 days, respectively. To prevent

inhibition effects due to CO2-accumulation, each

incubation flask was aired after the measurements.

The loss of water was controlled after each aeration

by weighing. CO2-release was quantified by gas

chromatography (Perkin Elmer, Auto System XL,

TCD, Ueberlingen, Germany). The molar amount of

CO2 was calculated using the ideal gas equation. The

percentage of the remaining SOC (SOCrem) was

calculated from the loss of carbon after each time

step and the initial SOC content at the beginning of

the incubation experiment. SOC contents (Table 1)

were measured by dry combustion and infrared

detection of CO2 (CNS analyser, LECO, CNS-

2000, Moenchengladbach, Germany).

A two-component first-order decay model with

two different mineralisation rates was fitted to the

measured data (after Qualls and Haines, 1992,

modified),

SOCrem % 100 b d ek1dt

bd ek2dt

4

where t is the time (day), (100b) and b are the initialpercentages of the rapidly and slowly decaying pools

(%), respectively, and k1 and k2 are the degradation

rate constants of the two pools (day1).

Preliminary experiments with four different soil

variants showed that the mean relative standard

deviation calculated on the basis of three independent

measurements including the instrument error was

generally below 8% indicating that a single measure-

ment for each soil variant would give sufficient

reliability.



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4. Results and discussion

4.1. Contact angles and solid surface free energies

The results of the contact angle measurements and

the solid surface free energies are given in Table 2.

Additionally, the water drop penetration times

(WDPT) are presented. As indicated by WDPT, all

samples seemed to be completely wettable. However,

the CRM contact angles covered a range from 178 to

around 798, indicating at least slight wetting resistan-

ces (subcritical water repellency). For both, wheat and

grassland plot, the largest contact angles were found

for the topsoil. The contact angles of the wheat soil

showed a tendency to decrease, from 66.58

in theupper 30 cm to 37.88 in 4565-cm depth. Conversely,

for the grassland soil, we found decreasing contact

angles within the upper 30 cm (from 79.18 to 17.18)

and below this depth increasing contact angles.

Material from the 2030-cm depth showed the small-

est contact angle (17.18) which also corresponded to

the largest solid surface free energy (68.0 mJ m2)

and the largest polar component of 56.0 mJ m2 of all

samples. Because interactions between water and the

solid surface mainly depend on the polar component

of solid surface free energy, the dispersion component

showed no clear relation to the contact angle.

Generally, the solid surface free energy as well as its

polar component showed inverse depth functions

compared to the corresponding contact angle, i.e.,

they increased with decreasing contact angle and vice

versa. All solid surface free energies were distinctly

lower than the surface tension of water (72.8 mJ m2

for 20 8C) which can be seen as a quantitative

parameter for water repellent behaviour, particularly

for the grassland topsoil with a solid surface free

energy of only 33.9 mJ m2.

Previous investigations confirmed the occurrenceof large topsoil contact angles compared to deeper

horizons (Woche et al., 2005). Because SOM is

considered as the main source of hydrophobicity, the

large topsoil contact angles can be explained by large

SOC contents. The decreasing contact angles with

depth for the wheat soil can be explained in the same

way by decreasing SOC contents with depth. How-

ever, the large contact angles found for the 3045-cm

depth of the grassland soil cannot be attributed to the

SOC content. This is in line with the findings of

Woche et al. (2005), who reported that large contactangles occurred even in soil with only small SOC

contents. These authors suggested that the composi-

tion of SOM seems to be more important for the

wetting properties than the content of SOC. This was

confirmed by Ellerbrock et al. (2005), who found that

the contact angle of bulk soil samples seemed to be

directly related to the ratio of CH to CMO groups of

SOM, independently of the SOC content.

4.2. Aggregate stability

The immersion experiments showed a clear rela-

tion between the surface tension of the testing liquid

and aggregate stability. For each soil variant, we

found decreasing aggregate stability with increasing

liquid surface tension. As a parameter for the

evaluation of the persistence of aggregate stability,

we used the percentage of intact aggregates after 30

min (ASt), and the surface tension at which 100% of

Table 2

Contact angles, water drop penetration times (WDPT), and solid surface free energies of the soils

Depth (cm) Contact angle (8)a WDPT (s) Solid surface free energy (mJ m2)b

cs csd cs

p cs csd cs

p

Wheat Grassland Wheat Grassland Wheat Grassland

010 66.5 (1.2) 79.1 (0.3) b5 b5 38.9 16.2 22.7 33.9 22.6 11.3

1020 66.5 (1.2) 46.6 (1.7) b5 b5 38.9 16.2 22.7 53.5 2.7 50.8

2030 66.5 (1.2) 17.1 (8.5) b5 b5 38.9 16.2 22.7 68.0 11.9 56.0

3045 43.6 (2.7) 48.0 (6.5) b5 b5 53.5 20.2 33.3 ND

4565 37.8 (2.2) ND b5 b5 58.9 10.3 48.6

a The values in parenthesis are the standard deviations of five replicates.b cs=solid surface free energy; cs

d=dispersion component of the solid surface free energy; csp=polar component of the solid surface free energy;

ND=not determined.



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the aggregates were disrupted after 30 min (ASSFT).

The results are given in Table 3.Comparing both variants, the ASt values suggested

that aggregates from the grassland soil were more

stable over the whole profile depth whereas in general

the topsoils showed the greatest stability. Fig. 2

summarizes the results and shows the percentage of

intact aggregates per container (ASc) as a function of

liquid surface tension and time for the topsoil of the

wheat (030 cm) and the grassland plot (010 cm).

Generally, ASc decreased with increasing surface

tension and time. For example, it can be seen that

immersion in water (cl=72.8 mJ m

2

) resulted in

complete aggregate breakdown for the wheat topsoil

already after 1 min (ASc=0%) whereas all aggregatesfrom the grassland topsoil were completely intact after

1 min (ASc=100%). This reflects the higher stability

of the aggregates from the grassland topsoil. Immer-

sion in a waterethanol mixture containing 20%

ethanol (cl=38.9 mJ m2) showed no breakdown of

the wheat aggregates after 1 min (ASc=100%), but

complete breakdown after 30 min (ASc=0%), (see Fig.

3). Fig. 2 further shows that the breakdown process is

most pronounced at the initial stage shortly after the

liquids were added. This is indicated by the nearly

vertical lines at the initial stage (b

5 min), particularly

Table 3

Aggregate stability parameters of the soils

Depth (cm) ASt (%)a ASSFT (mJ m

2)b Class ()c j (min1)d

Wheat Grassland Wheat Grassland Wheat Grassland Wheat Grassland

010 44 53 38.9 47.9 II I 0.84 0.18

1020 44 48 38.9 38.9 II II 0.84 0.92

2030 44 48 38.9 38.9 II II 0.84 1.89

3045 41 44 38.9 38.9 II II 1.10 1.38

4565 33 48 30.5 38.9 III II 1.75 1.22

a Percentage of intact aggregates after 30 min of immersion.b Liquid surface tension at which 100% of the aggregates are disrupted.c Class I: AStN50%; Class II: ASt=5040%; Class III: ASt b40%.d Rate constant describing the rapid aggregate breakdown.

Wheat (0-30 cm)

0

0

0 0 0

404040

40

40

20

20 20 20

100 100 100 1008080

80

80

6060

6060

60

Time (min)

Liquidsurfac

etension(mJ

m-2)

30

40

50

60

70

Grassland (0-10 cm)

20

20

20

80

80

80

80

80

80

60

60

60

60

60 40

40

40

40

100100

100100

Time (min)

5 10 15 20 25 305 10 15 20 25 30

30

40

50

60

70

Fig. 2. Contour plot of the percentage of intact aggregates per container (ASc) as a function of liquid surface tension and time for the topsoil of

the wheat (030 cm) and the grassland plot (010 cm).



8/14

for the wheat topsoil. For the wheat plot, the

maximum percentage of aggregate breakdown was

reached for most horizons already after approximately

10 min whereas aggregates of the grassland topsoil

showed progressing breakdown even after 30 min of

immersion. From Fig. 2, it becomes also clear that

aggregate breakdown did not occur at liquid surface

tensions below 30 mJ m2 for both topsoil horizons,

indicated by the horizontal lines.

T he A SSFT values confirmed the stability

expressed already by ASt. Aggregates from the

grassland topsoil were the most stable indicated by

the largest ASSFT value (47.9 mJ m2). Aggregates

from the lower grassland horizons and from the wheat

soil were already completely dispersed at lower

surface tensions (38.9 mJ m2) with the smallest

value for the lowermost horizon (4560 cm) of the

wheat soil (30.5 mJ m

2). Aggregates from thishorizon also showed the smallest percentage of stable

aggregates as indicated by ASt. Using the ASSFTvalues, we classified the tested soils into three groups

I, II, and III which correspond to ASt values ofN50,

5040, and b40%. With the exception of the grassland

topsoil (I) and the lowermost horizon from the wheat

soil (III), all soils fall within class II.

As a measure to assess the kinetics of aggregate

breakdown, we used the parameter j, obtained by

fitting Eq. (3) to the number of intact aggregates (Ai)

as a function of time, that represents the initial

breakdown process (Table 3). Large j values mean

rapid aggregate breakdown at the initial stage, small j

values indicate slow breakdown. For both variants, the

smallest j values were found for the topsoils

indicating the greatest initial resistance against break-

down. The wheat soil showed increasing j with

increasing depth whereas for the grassland soil j

increased from 0 to 30 cm and then decreased for the

two lowermost horizons.

Aggregate destructive forces can have various

causes. Most important in this context is the liquid

surface tension as the driving force for the liquid

adsorption process. In case of wettable soil aggregates

(solid surface free energy is larger than liquid surface

tension), a large liquid surface tension would enhance

the rate of liquid uptake into the aggregates that may

result in entrapped air compression and may causeaggregate breakdown by slaking (Loch, 1994). As the

particle size distribution was comparable for all soil

samples (Table 1), the differences in aggregate

breakdown can mainly be attributed to the wetting

properties of the aggregates resulting in different

liquid adsorption rates. The effect of entrapped air

compression is most effective when the liquid

adsorption rate is large (Loch, 1994). Therefore,

already a small decrease in wettability may have an

important effect on the stability of aggregates. Quirk

Fig. 3. Photographs of immersed aggregates from the wheat (030 cm) and grassland (010 cm) topsoil in water (72.8 mJ m 2) and a water

ethanol mixture (38.9 mJ m2) after 1 and 30 min of immersion.



9/14

and Panabokke (1962) showed that tilled aggregates

which had been wetted quickly to 0.2 kPa suctionslaked in water, whereas aggregates wetted slowly to

0.2 kPa did not. These authors further found that ahydrophobic soil conditioner applied to pores of 15

50 Am was able to decrease the rate of wetting. This is

because liquid adsorption rates are reduced and

breakdown due to the forces exerted by compressed

air entrapped during rewetting may become less

effective as the air has enough time to escape. Further,

liquid adsorption reduces the mechanical stability of

the aggregates because the stabilising cohesive forces

due to liquid menisci become ineffective. Addition-

ally, the effect of differential swelling could be of

some importance, particularly for soils with highercontents of swelling clay minerals (Emerson and

Greenland, 1990). The process of swelling can be

related to the dielectric constant which affects the

extent of the electrical double layer at the surface of

clay minerals (Adamson, 1990). The dielectric con-

stant is largest in pure water (80 for 20 8C) and

decreases with increasing ethanol percentage. All

mentioned breakdown mechanisms (entrapped air

compression, loss of stabilising liquid menisci,

swelling) would be the greatest for immersion in pure

water, however, since the investigated soils mainly

consisted of non-swelling clay minerals (illite and

kaolinite), swelling should be less important for

aggregate breakdown.

Linear correlations of the parameter j with the

contact angle (R2=0.84) and the solid surface freeenergy (R2=0.85) showed reasonable relations (Fig.

4), indicating that wettability and consequently the

liquid adsorption rate played an important role for the

initial stability of the aggregates. Particularly for the

grassland soil, the relation between j and the contact

angle was evident. The increasing j values within the

first 30 cm and decreasing j values at 3045-cm

depth were inversely related to the contact angle. The

resulting fragments of the initial aggregate breakdown

were mainly macroaggregates (N250 Am) thus indi-

cating that slaking was the predominant process. Afterthe initial stage, when the liquid adsorption rate was

lowered by a decreasing water potential gradient,

other mechanisms than slaking controlled the break-

down process. With progressing time, the macro-

aggregates were disrupted to microaggregates (b250

Am) and elementary particles, respectively, thus

confirming that dispersion was the main process of

aggregate breakdown during later stages (Le Bisson-

nais, 1996). Breakdown in liquids with smaller

surface tension was less pronounced resulting in

larger aggregate fragments compared to breakdown

in water.

Solid surface free energy (mJ m-2)

0.0

0.5

1.0

1.5

2.0

Contact angle ()

(min-1)

0.00 10 20 30 40 50 60 70 80 90 0 10 20 30 40 50 60 70 80 90

0.5

1.0

1.5

2.0

R2

= 0.84P< 0.01

R2

= 0.85P< 0.01

Fig. 4. Rate constantj describing the initial aggregate breakdown process as a function of the contact angle and the solid surface free energy.



10/14

Time (d)

0 50 100 150 200 250 300 350

SOC

re

m

(%)

96

97

98

99

100

Wheat (2-4 mm aggregates)

Wheat (homogenised)

Grassland (2-4 mm aggregates)

Grassland (homogenised)

0 50 100 150 200 250 300 350

CO2-release

pergram

soil(mgCO2gsoil-1)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Wheat (2-4 mm aggregates)

Wheat (homogenised)

0 50 100 150 200 250 300 350

C

O2-releaserate(mgCO2gsoil-1d

ay-1)

0.00

0.01

0.02

0.03Wheat (2-4 mm aggregates)

Wheat (homogenised)

0 50 100 150 200 250 300 350

0.00

0.01

0.02

0.03

0 50 100 150 200 250 300 350

0.0

0.5

1.0

1.5

2.0

2.5

3.0



Grassland (2-4 mm aggregates)Grassland (homogenised)

(a)

(b)

(c)

Fig. 5. CO2-release rates (a), cumulative CO2-releases (b), and remaining SOC (c) as functions of time for aggregates and corresponding

homogenised material from the topsoil of the wheat (030 cm) and grassland (010 cm) plot.



11/14

However, the contact angle and ASt as well as

ASSFT are poorly related to each other, indicated by

R2-values of 0.13 and 0.37, respectively. Linear

correlations of the stability parameters with SOCcontent give better results for ASt (R

2=0.66) and

ASSFT (R2=0.74) but smallerR2-values forj (0.72) in

comparison to correlation with contact angle and solid

surface free energy. This suggests that SOC content

was more important for the persistence of aggregate

stability but less important for the initial stability of

the aggregates.

4.3. Soil respiration

Incubation experiments were performed withaggregates and homogenised material from the topsoil

of the wheat (030 cm) and grassland (010 cm)

variant. For both variants, CO2-respiration rates were

largest at the beginning of the experiment (Fig. 5a).

After approximately 30 days of incubation, the rates

of all samples were reduced to 50% of the initial rates.

Overall, the homogenised material of both variants

had larger respiration rates than the aggregates. The

differences were most pronounced at the beginning

but remained apparent throughout the experiment

particularly for the grassland soil. The larger respira-

tion rates of the homogenised material may arise from

two reasons. One possible explanation can be the

restricted accessibility of water and nutrients as well

as oxygen resulting in a lower microbial activity in the

interior of the aggregates (Hartmann and Simmeth,

1990). A further reason may be the reduced microbial

accessibility of the SOM itself which resides in the

aggregates interior (Adu and Oades, 1978; Tisdall,

1996).

Both aggregates and homogenised material of the

grassland soil showed a larger cumulative CO2-release

per gram soil as indicated by Fig. 5b. This can beattributed to the larger SOC contents of the grassland

soil. To exclude this effect, we normalized the CO2-

release by the initial SOC contents of the samples and

calculated the remaining amount of SOC (Fig. 5c).

Considering the remaining SOC in the samples, both

the wheat aggregates as well as the homogenised

material showed a larger relative loss of carbon

compared to the grassland soil.

We propose three possible effects to explain the

smaller relative carbon loss of the grassland samples.

First of all, the grassland soil was not fertilized, so that

conditions for micro-organisms may be less optimal

resulting in smaller relative mineralisation rates. A

second effect may arise from the grassland SOM itselfwhich is more hydrophobic compared to SOM of the

wheat topsoil as indicated by larger contact angles,

because this may result in structural recalcitrance.

Furthermore, the lower wettability of the grassland

soil may also result in a reduced accessibility of water

and nutrients on the microscopic scale, which in turn

reduces the relative mineralisation rates. To prove the

hypothesis that reduced wettability may stabilise

SOM even in homogenised substrates, we additionally

measured contact angles of samples from various

agricultural soils whose respiration data were deter-mined by Springob and Kirchmann (2002).

Fig. 6 shows the relation between the sum of

respirated CO2 after 140 days and the contact angle.

Increasing cumulative respiration is correlated with a

decreasing contact angle. Our data fit well within the

data range of the regression based on soil data with a

great variation of contact angles, thus indicating that

Contact angle ()

0 20 40 60 80 100 120 140

CO2-releasepergram

SOC(mgg-1)(after1

40d)

0

50

100

150

200

250

Respiration data from Springob and Kirchmann (2002)

Wheat (2-4 mm aggregates)Wheat (homogenised)



R2

= 0.38P


12/14

the proposed effects of wettability can have a

significant effect on soil respiration.

Table 4 gives the constants derived from Eq. (4) on

the basis of the normalised respiration data. The

constants k1 and k2, which characterise the mineral-

isation rates of the labile and stable carbon pools,

respectively, were generally larger for the homoge-

nised soil, which indicated a retarded decomposition

of SOM encapsulated in aggregates. The rate of rapid

decay (k1) was larger for the grassland than for wheat

soil, whereas the rate of slow decay (k2) was larger for

the wheat than for the grassland soil. The constant b,which marks the transition point from rapid to slow

decay, was higher for the aggregated material in both

cases. It appears that encapsulation shifts parts of the

SOM from the rapidly degradable to the slowly

degradable fraction.

5. Conclusions

To assess the impact of soil wettability on the

stabilisation of SOM, we measured contact angles,solid surface free energies, aggregate stability, and soil

respiration rates of aggregates and homogenised

material of a cropland and a grassland soil. Three

different physical stabilisation mechanisms were pro-

posed. We conclude that wettability may have an

important effect on the stabilisation of SOM because

of the following reasons (Fig. 7): (1) Reduced

wettability enhances the stability of aggregates mainly

due to a reduction of liquid adsorption rates. The

results showed that small differences in wettability can

have an effect on aggregate stability; (2) Aggregation

dry

HYDROPHOBIC

moist: < c

HYDROPHOBIC

wet > c

HYDROPHILIC

moist

HYDROPHILIC

Water loss:

Increasinghydrophobicity Moisture statusof soil

Microbial activity high

Microbial activity low

Interface scale (m-mm) Aggregate scale (mm-cm) Profile scale (dm-m)

Encapsulated SOM is protected

inside aggregates by a reduced

microbial accessibility and a restricted

accessibility of water, nutrients and O2.

Liquid adsorption rate is reduced

by hydrophobic components

=> enhanced aggregate stability

Microbial activity is reduced by

permanent dry patterns in soil

(after Doerr et al. (2000), modified)

SOM is increasinglyprotected from minerali-

sation with increasing

hydrophobic character

of humic substances and

by a reduced accessibility

of water and nutrients.

Direct protection caused by

hydrophobic components

Fig. 7. Scale-dependent physical protection mechanisms on the interface, the aggregate and the profile scale. hc is the critical water content

(Doerr et al., 2000).

Table 4

Mineralisation rate constants and pool sizes of the rapidly and

slowly decaying components of the wheat (030 cm) and grassland

topsoil (010 cm)a

Soil Type of

sample

k1 (day1)b k2 (day

1)c 100b(%)d

b

(%)e

Wheat Aggregates

(24 mm)

4.31102 4.45105 0.38 99.62

Homogenised

material

4.78102 6.23105 0.57 99.43

Grassland Aggregates

(24 mm)

2.93102 6.94105 0.89 99.11

Homogenised

material

3.27102 8.18105 1.50 98.50

a Values based on normalised data (C-release per gram SOC).b Mineralisation rate constant of the labile pool.c

Mineralisation rate constant of the stable pool.d Portion of the labile pool.e Portion of the stable pool.



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stabilises encapsulated SOM by physical separation

and reduced accessibility for micro-organisms, as well

as restricted accessibility of water and nutrients; (3)

Hydrophobic SOM itself is more stable againstmicrobial decomposition.

All mentioned processes depend on soil wettability.

In real soils, the described processes act simultane-

ously, and can lead to a considerably greater

stabilisation effect than can be estimated from the

analysis of each single process. Another physical

stabilisation mechanism is likely to result because of

the exclusion of water from water repellent soil

domains (Dekker and Ritsema, 1996). The absence

of water thus could restrict the living conditions for

micro-organisms and therefore lower the rate ofmicrobial decomposition. We conclude that the wet-

ting properties of soils could be of central importance

for the understanding of the physical stabilisation

processes of SOM. Further work is needed to analyse

the simultaneous impact of all stabilisation effects

related to wettability.

Acknowledgements

We are grateful to R.R. van der Ploeg for his

critical reading and valuable suggestions to improve

the manuscript. We thank H. Flessa for providing the

soil samples and R. Jahn and G. Guggenberger for

providing the particle size distribution data. We thank

G. Springob and H. Kirchmann for providing the soil

samples from their respiration experiments. Financial

support provided by the bDeutsche Forschungsge-

meinschaft DFGQ (Priority program bSoils as source

and sink for CO2mechanisms and regulation of

organic matter stabilisation in soilsQ, SPP 1090, BA

1359/5-1) for this study is greatly appreciated.

References

Adamson, A.W., 1990. Physical chemistry of surfaces, 5th ed. John

Wiley and Sons, New York.

Adu, J.K., Oades, J.M., 1978. Physical factors influencing decom-

position of organic materials in soil aggregates. Soil Biol.

Biochem. 10, 109115.

Bachmann, J., Horton, R., van der Ploeg, R.R., Woche, S.K., 2000.

Modified sessile drop method for assessing initial soilwater

contact angle of sandy soil. Soil Sci. Soc. Am. J. 64, 564567.

Capriel, P., 1997. Hydrophobicity of organic matter in arable soils:

influence of management. Eur. J. Soil Sci. 48, 457462.

Chenu, C., Le Bissonnais, Y., Arrouays, D., 2000. Organic matter

influence on clay wettability and soil aggregate stability. Soil

Sci. Soc. Am. J. 64, 14791486.

Dekker, L.W., 1998. Moisture variability resulting from water

repellency in Dutch soils. PhD diss., Wageningen Agricultural

University, The Netherlands.

Dekker, L.W., Ritsema, C.J., 1996. Uneven moisture patterns in

water repellent soils. Geoderma 70, 8799.

Doerr, S.H., Shakesby, R.A., Walsh, R.P.D., 2000. Soil water

repellency: its causes, characteristics and hydro-geomorpholog-

ical significance. Earth-Sci. Rev. 51, 3365.

Ellerbrock, R.H., Gerke, H.H., Bachmann, J., Goebel, M.-O., 2005.

Composition of organic matter from forest soils in relation to

soil wettability. Soil Sci. Soc. Am. J. 69, 5766.

Emerson, W.W., 1967. A classification of soil aggregates based on

their coherence in water. Aust. J. Soil Res. 5, 4757.Emerson, W.W., Greenland, D.J., 1990. Soil aggregates-formation

and stability. In: De Boodt, M., Hayes, M., Herbillon, A. (Eds.),

Soil colloids and their associations in aggregates. Plenum Press,

New York, pp. 485 511.

Franco, C.M.M., Clarke, P.J., Tate, M.E., Oades, J.M., 2003.

Chemical characterisation of water repellent materials in

Australian sands. In: Ritsema, C.J., Dekker, L.W. (Eds.), Soil

water repellency. Elsevier Science B.V., Amsterdam, pp. 3748.

Goebel, M.-O., Bachmann, J., Woche, S.K., Fischer, W.R., Horton,

R., 2004. Water potential and aggregate size effects on contact

angle and surface energy. Soil Sci. Soc. Am. J. 68, 383393.

Hartmann, A., Simmeth, I., 1990. Einfluss des Bodenwasserpoten-

tials auf die Lokalisation mikrobieller Aktivit7t in Bodenag-

gregaten eines ariden Sandbodens (In German). Mitt. Dtsch.Bodenkdl. Ges. 62, 3942.

Hassink, J., Whitmore, A.P., 1997. A model of the physical

protection of organic matter in soils. Soil Sci. Soc. Am. J. 61,

131139.

Le Bissonnais, Y., 1996. Aggregate stability and assessment of soil

crustability and erodibility: I. theory and methodology. Eur. J.

Soil Sci. 47, 425 437.

Loch, R.J., 1994. A method for measuring aggregate water stability

with relevance to surface seal development. Aust. J. Soil Sci. 32,

687700.

MaShum, M., Tate, M.E., Jones, G.P., Oades, J.M., 1988.

Extraction and characterization of water-repellent materials

from Australian soils. J. Soil Sci. 39, 99110.

Michel, J.-C., Riviere, L.-M., Bellon-Fontaine, M.-N., 2001.

Measurement of the wettability of organic materials in relation

to water content by the capillary rise method. Eur. J. Soil Sci.

52, 459467.

Owens, D.K., Wendt, R.C., 1969. Estimation of the surface free

energy of polymers. Polym. Sci. 13, 17411747.

Piccolo, A., Mbagwu, J.S.C., 1999. Role of hydrophobic compo-

nents of soil organic matter in soil aggregate stability. Soil Sci.

Soc. Am. J. 63, 18011810.

Piccolo, A., Spaccini, R., Haberbauer, G., Gerzabek, M.H., 1999.

Increased sequestration of organic carbon by hydrophobic

protection. Naturwissenschaften 86, 496 499.



14/14

Qualls, R.G., Haines, B.L., 1992. Biodegradability of dissolved

organic matter in forest throughfall, soil solution, and stream

water. Soil Sci. Soc. Am. J. 56, 578586.

Quirk, J.P., Panabokke, C.R., 1962. Incipient failure of soil

aggregates. J. Soil Sci. 13, 6070.

Siebold, A., Walliser, A., Nardin, M., Oppliger, M., Schultz, J.,

1997. Capillary rise for thermodynamic characterization of solid

particle surface. J. Colloid Interface Sci. 186, 60 70.

Spaccini, R., Piccolo, A., Conte, P., Haberhauer, G., Gerzabek,

M.H., 2002. Increased soil organic carbon sequestration through

hydrophobic protection by humic substances. Soil Biol.

Biochem. 34, 18391851.

Springob, G., Kirchmann, H., 2002. C-rich sandy Ap horizons of

specific historical land-use contain large fractions of refractory

organic matter. Soil Biol. Biochem. 34, 15711581.

Sullivan, L.A., 1990. Soil organic matter, air encapsulation and

water-stable aggregation. J. Soil Sci. 41, 529534.

Tisdall, J.M., 1996. Formation of soil aggregates and accumulation

of soil organic matter. In: Carter, M.R., Stewart, B.A. (Eds.),

Structure and organic matter storage in agricultural soils, Adv.

Soil Sci., pp. 5796.

Tisdall, J.M., Oades, J.M., 1982. Organic matter and water-stable

aggregates in soils. J. Soil Sci. 33, 141163.

Truman, C.C., Bradford, J.M., Ferris, J.E., 1990. Antecedent water

content and rainfall energy influence on soil aggregate break-

down. Soil Sci. Soc. Am. J. 54, 13851392.

Washburn, E.W., 1921. The dynamics of capillary flow. Phys. Rev.

17, 273283.

Woche, S.K., Goebel, M.-O., Kirkham, M.B., Horton, R., van der

Ploeg, R.R., Bachmann, J., 2005. Contact angle of soils as

affected by depth, texture, and land management. Eur. J. Soil

Sci. (in press).


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