Relationship of Soil Physical & Chemical Properties with Aggregate Stability in Rice-Wheat Soil
Soil Wettability, Aggregate Stability, And The
Transcript of 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
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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
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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.
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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.
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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).
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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.
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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.
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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)
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.
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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)
Grassland (2-4 mm aggregates)
Grassland (homogenised)
R2
= 0.38P
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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.
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