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Soil physical characteristics of peat soil
Article in Journal of Plant Nutrition and Soil Science · August 2002
DOI: 10.1002/1522-2624(200208)165:4<479::AID-JPLN479>3.0.CO;2-8
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479
Soil physical characteristics of peat soils
Kai Schwärzel1*, Manfred Renger1, Robert Sauerbrey2, and Gerd Wessolek1
1
Institute of Ecology, Department of Soil Sciences and Soil protection, Technical University of Berlin, Salzufer 12,
D-10587 Berlin, Germany
2
Institute of Crop Sciences, Department of Ecology and Use of Resources, Humboldt University of Berlin,
Invalidenstr. 14, D-10115 Berlin, Germany
Accepted 31 May 2002
Summary ± Zusammenfassung
Drainage and intensive use of fens lead to alterations in the
physical characteristics of peat soils. This was demonstrated using
parameters of water balance (available water capacity) and the
evaluated unsaturated hydraulic conductivity. Deriving the distribution of the pore size from the water retention curve was flawed
because of shrinkage due to drainage, especially at high soil water
potentials. These errors became greater as the peat was less
influenced by soil-genetic processes. The water retention curves
(desorption) evaluated in the field and the laboratory satisfactorily
corresponded. However, the wetting- and drainage-curves obtained
in the field differed up to 30 vol.-% water content at same soil
water potentials. These differences were largely due to a wetting
inhibition.
Key words: peat soils / soil water characteristics / hydraulic
conductivity / shrinkage / water repellency / hysteresis / fen
pedogenesis
1 Introduction
The key to moorland conversation is a well-balanced water
management. To achieve this knowledge is needed on the
demand for water by these soils in regard to different
groundwater levels. Wessolek et al. (2002) used a model to
predict the soil water components and CO2 release for
different peat soils, various climate conditions and groundwater levels. Soil hydraulic properties were of high
importance as input parameters (Weiss et al. 1998; Letts et
al., 2000).
Much data on lab measurements of water retention of peat
soils have been reported (e.g. Zeitz, 1992; Okruszko, 1993;
Schäfer, 1996; Weiss et al., 1998; Silins and Rothwell, 1998).
Despite promising results, the influence of shrinkage on the
water retention curve is not well understood (Kellner and
Halldin, 2002). Morever, for a maximum accuracy in the
model prediction, in situ measurements are preferred since
laboratory and field measurements can differ significantly as
shown by Royer and Vachaud (1975).
Aside from water retention, it is necessary to understand
the relation between the hydraulic conductivity and the
water tension; i.e. soil moisture for a physically wellfounded model of the water regime. In contrast to the water
retention, the knowledge about the unsaturated hydraulic
* Correspondence: Dr. K. Schwärzel; E-mail: Kai.Schwaerzel@TU-Berlin.de
J. Plant Nutr. Soil Sci. (2002), 165, 479±486 (2002)
Bodenphysikalische Eigenschaften von
Niedermoortorfen
Die Entwässerung und intensive Nutzung der Niedermoore führt
zu Veränderungen der bodenphysikalischen Eigenschaften der
Torfe. Anhand der Kennwerte des Wasserhaushaltes und auch am
Beispiel der ermittelten ungesättigten hydraulischen Leitfähigkeit
wird dies gezeigt. Die Ableitung der Porengröûenverteilung aus der
Wasserretentionskurve ist auf Grund der entwässerungsbedingten
Schrumpfung der Torfe vor allem im hohen Wasserspannungsbereich mit Fehlern behaftet. Diese Fehler sind um so gröûer, je
weniger der Torf von bodengenetischen Prozessen geprägt wurde.
Die Übereinstimmung zwischen den im Labor und im Feld
ermittelten Wasserretentionskurven (Desorption) ist zufriedenstellend. Unterschiede von bis zu 30 Vol.-% im Wassergehalt wurden
aber bei gleicher Wasserspannung zwischen den im Feld erhobenen
Be- und Entwässerungskurven festgestellt. Diese Unterschiede
beruhen vor allem auf Benetzungshemmung.
conductivity, especially in highly degraded peat layers, is
unsatisfactory. The aim of this study was to determine the
hydraulic function (water retention and hydraulic conductivity) for different peat soils. These results were a basis for
the modeling of CO2 release (Wessolek et al., 2002) and for
practical recommendations for the preservation of fens
(Renger et al., 2002).
2 Material and methods
2.1 Study area
The study area of Rhinluch is located ca. 60 km northwest of Berlin (Fig.
1). It is part of the Havelland basin, a fen area of ca. 87000 ha.
The peat formation in the Rhinluch was dominated by bogginess. The
filling-in processes were of minor importance in the peat sediment. The
average thickness of the peat was 120 cm. Underlying are glacifluvial sands
(mostly fine sand) and limnic sediments such as detritus- or calcerous mud
(Zeitz, 1993). The upper peat layers are strongly decomposed and
pedogenically altered. Earthified and strongly earthified peat soils can be
found mostly at the surface. The deeper layers are dominated by sedge(carex) and reed-peats (phragmites), often showing a mixture of both. The
average annual temperature is 8.1 oC and the average annual precipitation
is 526 mm. This makes the Rhinluch one of the regions with the lowest
precipitation in Germany. The climatic water balance in the summer is
negative for the Rhinluch area. Between 1993 and 1998, the average was
about minus 250 mm.
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2002
1436-8730/02/0408-479 $17.50+.50/0
480
Schwärzel, Renger, Sauerbrey, and Wessolek
2.2 Research methods
The characteristics determined in the laboratory and the methods used are
listed in Tab. 1.
2.3 Statistical evaluation
In order to summarize the distribution of the characteristics the
arithmetical average to mark the central tendency and the standard
deviation to mark the variation of the data were used. Variance analyses
were carried out in order to verify hypotheses on the differences. The
necessary prerequisites (standard distribution, variance homogeneity, and
the necessary extent of samples) were examined as follows. The standard
distribution was examined using the Komolgorov-Smirnov-test (Sachs,
1997). The variance homogeneity was tested with the Levene-test (Sachs,
1997). If ANOVA led to a significant F-value, the mean values that
differed significantly from the other mean values were examined using the
ScheffØ-test (Sachs, 1997). The stochastic context was calculated using the
product-moment-correlation coefficient according to Pearson (Sachs,
1997).
3 Results and discussion
3.1 Influence of fen formation and pedogenesis on soilphysical characteristics
The groundwater regulation of fen peat soils in the
northeast of Germany led to the formation of characteristic
soil horizons, depending on the intensity of drainage and use.
The soil horizons differ mainly in the development of their
soil structure. In order to standardize these developments of
soil structure, Schmidt and Illner (1976) developed a
classification system based on the differentiation of various
soil structures for fens in East Germany (Tab. 2).
Schmidt (1986) suggested the water index according to
Ohde (1951) as an objective, easily determinable parameter
for an exact classification of pedogenically altered fen peat
substrates. The standardized water index (W1) corresponds
to the water content of the soil after consolidation with a load
of 100 kPa (W1 = Mass of water relative to the mass of dry
soil). The W1, according to Ohde (1951) is suitable for
describing degrees of soil development of peat soils with
Figure 1: Location of the study area
Abbildung 1: Lage des Untersuchungsgebietes
Table 1: Laboratory methods
Tabelle 1: Labormethoden
Characteristics
Method
Table 2: Classification of fen peat soil horizons (AG Boden, 1994; from
Schäfer, 1996), W1 = Water index according Schmidt (1986)
Tabelle 2: Klassifikation der Niedermoorhorizonte (AG Boden, 1994; aus
Schäfer 1996), W1 = Einheitswasserzahl (Schmidt, 1986)
Symbol
Description
nHm
Strongly
earthified
horizon
Topsoil horizon of intensive drained
peatlands and with an intensive tillage
action, strongly earthified; high degree
of decompsotion, when dry: very fine
granular structure (dusty), high water
repellency (W1 < 1.8)
nHv
Earthified
horizon
Topsoil horizon of drained mires;
poorly to moderately earthified by
aerobe mineralization and humification, crumb or fine subangular structure
(2.2 < W1 < 1.8)
nHa
Aggregate
horizon
Subsoil-horizon, coarse to fine angular
blocky structure caused by shrinkage
and swelling processes
nHt
Shrinkage
horizon
Subsoil-horizon, vertical cracks and
coarse prismatic structure caused by
shrinkage
nHr
Peat horizon
Permanently below the ground or
perched water-table and preserved in a
reduced state
Dry Bulk Density dB [g cm±3] Thermogravimetrical dessication at
105 oC (DIN 19683, 1998)
Mean Particle Density dF
[g cm±3]
Heliumpyknometer (Quanta Chrome)
(DIN 19683)
Ignition Loss X [M.-%]
Four-hour incineration at 550 oC
Total pore volume P
Mathematically out of dB and dF
(DIN 19683, 1998)
Water retention curve
Up to pF 2.0 hanging water column, above
pF 2.2 overpressure in a pressure pot,
fourfold repetition, 100 cm3 short core
samples
Unsaturated hydraulic
conductivity
Stationary and non-stationary according to
Plagge (1991) with threefold repetition on
10 cm high short core samplers (237 i.e.
550 cm3)
Shrinkage
Measuring with a caliper rule after each
pF-level
Characteristics
481
Soil physical characteristics of peat soils
Table 3: Soil physical parameters of substrate-horizon-groups of drained and agriculturally used fen soils (standard deviation in brackets).
X = ignition loss, P = porosity, AC = air capacity, AWC = available water capacity (pF 1.8 to pF 4.2), GPII = é <50 ± 10 mm, MPI = é <10 ± 3 mm
Tabelle 3: Bodenphysikalische Kennwerte von Substrat-Horizont-Gruppen entwässerter und landwirtschaftlich genutzter Niedermoore (in Klammern
Standardabweichungen).
X = Glühverlust, P = Porosität, AC = Luftkapazität, AWC = nutzbare Feldkapazität (pF 1.8 bis pF 4.2), GPII = é <50 ± 10 mm, MPI = é <10 ± 3 mm
Horizon
Kind of peat
Source
n
X
M.-%
dB
g cm±3
P
V.-%
AC
V.-%
AWC
V.-%
GPII
V.-%
MP
V.-%
nHm
Strongly
earthified
own
10
76a
(2)
0.36a
(0.04)
77a
(4)
13ab
(4)
29a
(5)
9ab
(4)
20a
(6)
Peat,
X < 70 M.-%
Zeitz
(1992)
79
±
±
79
(3)
24
(8)
27
(6)
6
(9)
21
(7)
Earthified
Peat,
Own
24
80b
(3)
0.31a
(0.06)
80a
(3)
11a
(6)
31a
(9)
6ab
(4)
24a
(7)
X < 70 M.-%
Zeitz
(1992)
79
±
±
82
(5)
14
(6)
38
(13)
7
(6)
39
(12)
ReedSedge-
own
13
86c
(2)
0.20b
(0.03)
87b
(2)
13ab
(4)
43b
(8)
4a
(4)
38b
(6)
Mixed-Peat,
X < 80 M.-%
Zeitz
(1992)
49
±
±
88
(3)
16
(7)
42
(11)
7
(6)
35
(9)
ReedSedge-
own
16
86c
(2)
0.18bc
(0.03)
88bc
(2)
12ab
(3)
49b
(6)
8ab
(4)
39b
(4)
Mixed-Peat,
X < 80 M.-%
Zeitz
(1992)
30
±
±
88
(3)
15
(7)
44
(10)
9
(7)
35
(11)
ReedSedge-
own
13
85c
(2)
0.14c
(0.02)
91c
(1)
18b
(5)
50b
(8)
9b
(5)
30b
(10)
Mixed-Peat,
X < 80 M.-%
Zeitz
(1992)
29
±
±
91
(2)
14
(6)
58
(4)
17
(10)
42
(7)
nHv
nHa
nHt
nHr
Identical letters refer to statistically not confirmable differences between the characteristic values of peat groups from own evaluations (p ³ 0.05). Unequal
letters refer to statistically secured differences between the respective characteristic values of peat groups from the own survey (p £ 0.05).
ignition residues of < 30 mass-%. This is especially
appropriate for differentiating between earthified and
strongly earthified soils. The W1 is normally given as a
decimal without dimension. Slightly earthified peat soils
show a W1 greater than 2.2, earthified peat soils 2.2 to 1.8,
and strongly earthified peat soils have values lower than 1.8.
Based on the classification system by Schmidt and Illner
(1976), Zeitz (1992) divided the fen peat soils into substratehorizon-groups (SHG) in order to evaluate locations. Zeitz
(1992) assigned soil-physical parameters to these SHG. In
order to examine the influence of soil developing processes
on the soil-physical parameters, we organized the samples
that we analyzed in SHG as well. Mean values and standard
deviations of the soil-physical parameters were determined
for each SHG. The methods described above were used for
establishing which soil-physical parameters differed between the SHG.
Tab. 3 lists the mean values and standard deviations of the
soil physical parameter of individual SHG for the samples
that we analyzed (see also Fig 2).
Tab. 3 also shows the results published by Zeitz (1992) in
order to verify the plausibility of our own results. The former
are based on a set of data consisting of more then 800
samples which were collected and analyzed from various fen
regions of Northern Germany (among others from the
Figure 2: Influence of soil development on the soil-physical characteristics of peat soils.
SV = volume of solids, AC = air capacity, AWC = available water capacity
(pF 1.8 to pF 4.2), PWP = permanent wilting point (soil moisture content at
pF 4.2) (v. also Tab. 2 and Tab. 3)
Abbildung 2: Einfluss der Bodenentwicklung auf die bodenphysikalischen Kennwerte von Niedermoortorfen.
SV = Substanzvolumen, AC = Luftkapazität, AWC = nutzbare Feldkapazität (pF 1.8 bis pF 4.2), PWP = Permanenter Welkepunkt (Bodenfeuchtegehalt bei pF 4.2) (s. auch Tab. 2 und Tab. 3)
482
Rhinluch) between 1975 and 1985. The derivation of the
distribution of the pore size from the water retention curve,
as well as plant-available water, serves only to distinguish
between peat substrates. Especially high water tensions can
cause a misinterpretation of the pore size distribution due to
shrinking processes and, consequently, amount of plantavailable water (see Fig. 5).
The characteristic values determined by us deviated only
slightly from the corresponding values of the investigations
by Zeitz (1992). The high scattering of our own values,
which is at times almost as high as the values from Zeitz
(1992), is conspicuous. This is remarkable because the
values of Zeitz (1992) were based on a much greater data set
and the samples were taken from diverse regions of Northern
Germany.
Fig. 3 shows the high scattering of the individual values on
the basis of the characteristic values for humus content (loss
of weight on ignition), bulk density, and available water
capacity. Furthermore, it became clear that the characteristic
losses on ignition and bulk densities were excellent
indicators for pedogenetic changes in peat soils.
The letters behind each mean value in Tab. 3 indicate
whether statistically secured differences existed between the
mean values of the individual SHG. Unequal letters show
differences in the mean values between the respective
characteristic values. For example, the characteristic value
of available water capacity (AWC) shows no differences
between the mean values of strongly earthified and
earthified peat soils (identical letters) but reveals differences
between the earthified peat soils and the reed-sedge peat
soils of the nHa horizon (unequal letters).
The facts presented in Tab. 3 and Fig. 2 and 3 allow for the
following conclusions regarding the influence of soil
development on the soil-physical characteristics:
(1) Progressive soil development increases the bulk density
due to subsiding, shrinkage or mineralization. On the
other hand, the content of organic substance and the
porosity decrease significantly (see also McLay et al.,
1992; Brandyk et al., 1995; Schäfer, 1996; Silins and
Rothwell, 1998).
(2) The higher bulk density of pedogenically altered peat
soils modifies the special structure of the pores (see also
Zeitz, 1992; Schäfer, 1996; Silins and Rothwell, 1998).
For example, the share of medium pores in a peat of the
Schwärzel, Renger, Sauerbrey, and Wessolek
topsoil is decreased by 15 to 20 percent compared to
peat soils of deeper layers.
(3) The characteristic value of available water capacity
illustrates the influence of pedogenic peat alteration due
to use (Zeitz, 1992; Okruszko, 1993; Schäfer, 1996). In
contrast to peat soils of the nHr horizon, the available
water capacity of strongly earthified peat soils was
decreased by roughly 40 %; from 50 to < 30 vol-%.
(4) There are no statistically secure differences regarding
the soil physical characteristic values of earthified and
strongly earthified peat topsoils.
(5) There are also no statistically secure differences
between the soil physical characteristic values of reedsedge-peat soils from the subsoil horizons nHa and nHt.
3.2 Shrinkage
An important characteristic of organic soils is the shrinkage
that accompanies the drainage and the resulting decrease of
the base volume. One has to differentiate between the
irreversible and the reversible shrinkage. The latter can be
observed as the so called mire-breathing. Moreover, in
strongly drained fen peat soils, high evaporation rates can
lead to reversible shrinkage cracks and clefts (Schmidt et al.,
1981; Schothorst, 1982).
The shrinkage of the soil material, especially the initial
shrinkage (lowering of groundwater), leads to strong
alterations of the special pore structure (Sauerbrey et al.,
1988). In the course of establishing a water retention curve
in the laboratory, the peat shrinkage after each pressure level
was quantified for some horizons by measurement with a
calliper rule.
The base volume of the peat was decreased due to the
shrinkage, as proven for the cases illustrated in Fig. 4. The
shrinking behavior of peat soils during drainage was
dependent on the peat condition i.e. the type of horizon.
Peat soils of a strongly earthified horizon started to
noticeably shrink at pF 3.5 (5 % loss of volume). Peat soils
of an aggregated horizon (nHa) showed shrinking behavior
at pF 3.0 (loss of volume at pF 3.5: 7 vol.-%). Peat soils from
deeper layers (nHr) showed shrinkage as early as pF 1.8 (loss
of volume at pF 3.5: 37 % vol.). Our results support the
conclusions of Hennings (1996) which state that the
shrinking behavior of peats is dependent on the degree of
secondary decompositon and the intensity of drainage.
Figure 3: Relations between humus content (ignition loss)
and bulk density (left side) and available water capacity and
bulk density for peat soils (right side) from the Rhinluch.
Ham = strongly earthified peat (n = 10); Hav = earthified peat
(n = 24); Hnp / Hnr = reed-sedge peat (n = 42)
Abbildung 3: Beziehungen zwischen Humus-Gehalt (Glühverlust), Lagerungsdichte und nutzbarer Feldkapazität für
Torfe aus dem Rhinluch.
Ham = vermulmter Torf (n = 10); Hav = vererdeter Torf (n =
24); Hnp/ Hnr = Schilf-Seggen-Torf (n = 42)
483
Soil physical characteristics of peat soils
3.3 Water retention ± comparison between field- and
laboratory measurements
Figure 4: Loss of volume (%) of peat soils from drained fen locations.
nHr: depth 95 to 100 cm, moss peat (Bryidae) H2-3, nHt: 55 to 60 cm,
wood peat (Alnus glutinosa) H6-7, nHa: 35 to 40 cm, strongly decomposed
peat, nHv: 10 to 15 cm, earthified peat, nHm: 10 to 15 cm, strongly
earthified peat
Abbildung 4: Prozentuale Verringerung des Ausgangsvolumens von
Torfen entwässerter Niedermoorstandorte während der Erstellung von
Labor-Wasserretentionskurven.
nHr: Tiefe 95 bis 100 cm, Laubmoostorf (Bryidae) H2-3, nHt: 55 bis 60
cm, Erlenbruchtorf (Alnus glutinosa) H6-7, nHa: 35 bis 40 cm, stark
zersetzter Torf, nHv: 10 bis 15 cm, vererdeter Torf, nHm: 10 bis 15 cm,
vermulmter Torf
The benchmark figures of the laboratory water retention
curves for various peat soils have already been presented in
Tab. 3 and Fig. 2. Since the relation between the water
content and the water tension is needed as an input value for
modeling the water regime and the CO2 release (Wessolek et
al., 2002), it has to be determined if the curves established in
laboratory conditions can be transferred to field conditions.
We ran two stations in the Rhinluch for the recording of
components of the water regime. Among others, water
retention curves were established under field conditions for
various peat horizons with the help of the TDR- and
tensiometer-technique. Fig. 6 shows the results of these
experiments for two horizons from four measurement sites.
It also shows the first course of desorption during spring.
In Fig. 6, the water retention curves established in the
laboratory are compared with those obtained from field
values. The cases listed in Fig. 6 show only minor deviations
between laboratory and field retention curves, an average of
less than 4 vol.-%. The field values followed the laboratory
curves to a satisfactory extent. However, it is remarkable
that the field values from different measurement sites,
especially of the earthified peat, vary up to 9 % vol. at
constant water tension. This is equivalent to a standard
deviation of close to 5 % vol. The individual measurement
sites were only a few meters apart. Thus, the facts depicted
in Fig. 6 reflect a high spatial variability of the water
retention in one location. The variability is, therefore,
exactly as high as recorded by Zeitz (1992).
3.4 Hysteresis
Figure 5: Water retention (laboratory, desorption) of peat soils with and
without consideration of shrinkage.
Moss peat = depth: 95 bis 105 cm, nHr; Reed-Seedge-Peat = 35 bis 45 cm,
nHa
Abbildung 5: Wasserretention (Labor, Desorption) von Torfen mit und
ohne Berücksichtigung der Schrumpfung.
Braunmoostorf = Tiefe: 95 bis 105 cm, nHr; Schilf-Seggen-Torf = 35 bis
45 cm, nHa
What influence does peat shrinkage caused by drainage
have on the course of the water retention? Fig. 5 compares
the water retention curves of two slightly decomposed peat
soils, with and without shrinkage consideration.
The water retention curves that considered shrinkageinduced decreased base volume showed a steeper ascent in
high water tension than the curves that were drawn up
without acknowledging the shrinkage. Taking the shrinkage
into consideration, peat soils showed a substantially higher
volumetric water content at ranges of high water tension and,
consequently, higher shares of fine pores than without
shrinkage consideration. Therefore, deriving the distribution
of the pore size from the water retention is prone to errors
when knowledge of the shrinkage behavior of peat soils at
high water tensions is not known. These errors were greater
the less the peat was subjected to pedogenic processes. Silins
and Rothwell (1998) also observed an underestimation of
water content owing to shrinkage.
It is well known that the relation between soil humidity and
water tension is subject to the hysteresis. The causes of this
phenomenon can be the inclusion of air, the formation of
water-repelling films (hydophobia), effects of the pore
geometry (ink-bottle-effect), as well as alterations of the
spatial structure of pores due to shrinkage.
During the field experiments, hysteresis loops of the water
retention were recorded for different peat horizons. One can
infer from Fig. 7 that the differences between the curves for
Figure 6: Water retention (desorption, without consideration of the
shrinkage) from the laboratory and the field for two peat horizons
(Rhinluch).
^ Laboratory values, & * ~ ´: Field values of various measurement sites
Abbildung 6: Wasserretention (Desorption, ohne Berücksichtigung der
Schrumpfung) aus Labor und Feld für zwei Torfhorizonte (Rhinluch).
^ Laborwerte, & * ~ ´: Feldwerte unterschiedlicher Messplätze
484
Schwärzel, Renger, Sauerbrey, and Wessolek
drainage and wetting of the strongly earthified surface layer
(15 to 25 cm depth, strongly earthified horizon) are more
pronounced than that for the strongly decomposed peat
(aggregation horizon, 35 to 45 cm depth). Whereas the soil
moisture content of the peat layer from a depth of 35 to 45
cm varied by a maximum of 8 vol.-% for the same water
tension, differences of almost 30 vol.-% were shown for the
peat close to the surface during the first humidification
period at the end of the summer drought (August). After the
second humidification period, there was still a value 20 %
vol.
The large variations between drainage and wetting curves
observed during August were mainly based on wetting
inhibitory surfaces that were formed in the course of the
desiccation period. The high wetting resistance of the
strongly earthified peat layers inhibited water uptake; the
water seeping in due to precipitation moved to greater depths
and the groundwater level rose very quickly. The unusually
high precipitation frequency during the months of October
and November are required to achieve an intensive
continuous humidification of the rooting zone which has
dried out during the summer (Schwärzel, 2000).
Kellner and Halldin (2002) reported that the variable peat
volume was a major reason for the highly hysteretic
relationship between the water content and groundwater
level in a mire. However in our case, a hysteresis of the water
retention due to shrinkage is unlikely. Water withdrawal
caused the peat matrix to shrink. As a result, the proportional
volume share of medium and fine pores increased, as seen in
Fig. 5 for mildly decomposed peat soils, i.e. the ascent of the
water retention curve is steeper. Thus, in cases of high water
tension, more water remains in the peat and the amount of
water available to the plants decreases. However, a field
experiment revealed that plants exhausted the volumetric
water content evaluated in the laboratory up to pF 4.2
(without consideration of the shrinkage) (Schwärzel, 2000).
The second wetting curve registered for the strongly
earthified peat in October shows a considerably less steep
increase when compared to the wetting curve recorded in
August (Fig. 7). At this time, the identifiable differences
between the drainage and wetting curves are more an effect
of the pore geometry than of a wetting inhibition.
Figure 7: Hysteresis of water retention (field experiment)
Strongly earthified Peat = 15 to 25 cm, strongly decomposed peat = 35 to
45 cm
Abbildung 7: Hysterese der Wasserretention (Feldversuch)
Vermulmter Torf = 15 bis 25 cm, stark zersetzter Torf = 35 bis 45 cm
3.5 Unsaturated hydraulic conductivity
Aside from the water retention, it is necessary to understand the relation between the hydraulic conductivity and the
water tension i.e. soil humidity for a physically well-founded
model of the water regime. The conductivity values
registered for various peat soils in the course of this study
were grouped according to pedogenic alterations. Peat soils
(i) with recognizable plant substance were distinguished
from earthified peat soils (ii) and strongly earthified peat
soils (iii). The result of this classification is summarized in
Tab. 4. However, statistically secure differences between the
mean values of the unsaturated hydraulic conductivity at the
respective water tensions could not be established.
The observed differences in the unsaturated conductivities
of the individual peat classes corresponded well to the shifts
in the distribution of pore size resulting from soil
development as discussed above. The proportions of wide
macropores (é >100 mm) were diminished by sinking,
shrinkage, and mineralization, and the proportions of narrow
macropores were increased. Looking at the peat groups, this
caused the observed differences in the water conductivity at
water tensions between 30 and 60 hPa. The highest hydraulic
conductivities were seen in strongly earthified peat soils and
the lowest in peat soils with noticeable plant species. The
Table 4: Unsaturated hydraulic conductivity of the investigated peat soils in dependence on pedogenic development.
mean values, standard deviation in brackets, n = number of horizons, a = 2 horizons, Hn = peat of low to medium decomposition, Hav = earthified peat,
Ham = strongly earthified peat
Tabelle 4: Ungesättigte hydraulische Leitfähigkeit der untersuchten Torfe in Abhängigkeit der pedogenen Entwicklung.
Mittelwerte, in Klammern Standardabweichungen, n = Anzahl der Horizonte, a = 2 Horizonte, Hn = Torf geringer bis mittlerer Zersetzung, Hav =
vererdeter Torf, Ham = vermulmter Torf
Kind of
peat
n
Ham
Hydraulic conductivity [mm d±1] at
60 hPa
100 hPa
150 hPa
P
[Vol.-%]
X
[M.-%]
dB
[g cm±3]
30 hPa
6
75
(2)
75
(5)
0.39
(0.05)
10.4
(8.9)
4.9
(3.8)
1.6
(1.0)
0.5
(0.4)
0.05
(0.03)
Hav
6
78
(3)
77
(2)
0.34
(0.05)
7.3
(3.2)
3.6
(2.9)
0.9
(0.6)
0.4
(0.2)
0.08
(0.06)
Hn
8
87
(3)
83
(3)
0.20
(0.04)
7.4
(6.0)
2.2
(1.6)
0.9
(0.9)
0.4
(0.3)
0.12
(0.09)
300 hPa
485
Soil physical characteristics of peat soils
reasons for the good water conductivity of strongly
earthified peat soils can also be explained by earthifying.
This process is marked by increasing of a fine granular
structure in the topsoil and, consequently, an increasing
share of continuous macropores.
At first it seems contradictory that peat soils with clearly
recognizable plant tissue have a lower hydraulic conductivity at water tensions of 30 to 100 hPa than strongly
earthified peat soils, regardless of a comparable air capacity.
However, one has to keep in mind the sponge-like structure
of peat soils with recognizable plant tissue. The high air
capacity of these peat soils is ensured mainly by the high
amount of macropores. Silins and Rothwell (1998) reported
that greater peat bulk density after drainage and subsidence
was associated with a loss of macropores (>600 mm é) with
a concurrent increase in micropores (3±30 mm é). If these
macropores are drained, the hydraulic conductivity is
considerably reduced (Baird, 1997). In the water tension
range of 100 to 200 hPa, there are almost no differences
between the conductivity of the individual groups. The
courses of the conductivity of the individual peat groups do
not diverge again until water tensions of 300 hPa and above.
Peat soils with recognizable plant tissue show higher
conductivity values than earthified or strongly earthified
peat soils. This fact can be explained by the continuously
decreasing share of medium pores (é 10 to 2 mm, see Tab. 3)
in the course of the secondary soil formation.
Our findings do not support the conclusions of Hennings
(1996) and Sauerbrey and Zeitz (1999) which state that
progressive soil development decreases the unsaturated
hydraulic conductivity. Silins and Rothwell (1998) found
that the mean unsaturated conductivity of drained peat was
roughly five times greater than undrained peat in the water
tension range of 25 to 1000 cm.
Fig. 8 describes graphically the relations between water
tension and hydraulic conductivity for strongly pedogenetically altered peat soils. Noteworthy is the comparably low
conductivity of the strongly decomposed and segregated
peat. Results for the strongly decomposed peat of the
aggregated horizon were established for one horizon only.
However, Hennings (1996) also found conductivity values
for a peat soil of the aggregation horizon that were ten times
lower than that for the earthified horizon at the same water
tension. This peat, showing a polyhedral fabric with more or
less large fragments and sharp edges (see Tab. 2), had a
hydraulic conductivity comparable to that of earthified and
strongly earthified peat soils at water tensions of ca. 300 hPa.
Due to soil genetic factors, this peat had a significantly lower
hydraulic conductivity in the region close to saturation.
Such layers with clearly reduced hydraulic conductivity
can often be found at strongly drained and degraded peat
locations. At present it is not clear whether these layers
result from peat degradation or whether, in turn, the low
water conductivity was the reason for degradation. However,
Schmidt et al. (1981) proved that the soil development due to
drainage and use of fens does not necessarily result in
strongly degraded fen peat soils.
4 Conclusions
In general, the predicted soil physical parameters showed
good agreement with the results of Zeitz (1992). Nevertheless, we proved that peat shrinks during draining cycles,
especially at higher water tensions. Taking this shrinkage
into consideration, peat soils showed a substantially higher
volumetric water content at high water tensions. Therefore,
deriving the pore size distribution only from water retention
data leads to an underestimation of water content for high
water tensions. These errors decreased with the degree of
soil pedogenic processes. Results show the effect of
hydrophobicity on soil wetting at the end of the summer
drought. The high wetting resistance of peat soils inhibits the
soil water uptake by plants. As a result of water repellency
processes, the water infiltration moves faster to greater
depths and the groundwater level rises very quickly. In this
case, a preferential transport of water and solutes to the
groundwater might be possible.
In contrast to the water retention, the unsaturated hydraulic
conductivity functions, especially that of high degraded peat
layers, are underrepresented in the modeling literature. More
research is needed to understand the relation between peat
soil development and unsaturated hydraulic conductivity.
Acknowledgment
We would like to thank the German Research Association (Deutsche
Forschungsgemeinschaft DFG) for the financial support of this work. This
study was developed within the scope of the DFG research group.
References
Figure 8: Unsaturated hydraulic conductivity (Ku) of pedogenically
altered fen peat soils of the Rhinluch
laboratory results; strongly earthified peat: n = 6; earthified peat: n = 6;
strongly decomposed peat: n = 1
Abbildung 8: Ungesättigte hydraulische Leitfähigkeit (Ku) pedogen
veränderter Niedermoortorfe des Rhinluchs
Laborwerte; vermulmter Torf: n = 6; vererdeter Torf: n = 6; stark zersetzter
Torf: n = 1
AG Boden (1994): Bodenkundliche Kartieranleitung. 4th edn. Hannover,
E. Schweitzbart©sche Verlagsbuchhandlung
Baird, A. J. (1997): Field estimation of macropore functioning and surface
hydraulic conductivity in a fen peat. Hydrol. Process. 11, 287±295.
Brandyk, T., J. Szuniewicz, K. Skapski, and J. Szatylowicz (1995): The soil
moisture regime study of fen peat soils in the Middle Biebrza Basin as a
Basis for soil protection.. Z. Kulturtechnik Landentw. 36 (2), 78±83.
486
DIN 19683 (1998): Physikalische Untersuchungen von Böden.
Hennings, H. H. (1996): Zur Wiedervernässbarkeit von Niedermooren.
PhD, Georg-August-Universität Göttingen, Germany.
Kellner, E., and S. Halldin (2002): Water budget and surface-layer water
storage in Sphagnum bog in central Sweden. Hydrol. Process. 16, 87±
103.
Letts, M. G., N. T. Roulet, N. T. Comer, M. R. Skarupa, and D. L. Verseghy
(2000): Parametrization of peatland hydraulic properties for the
Canadian Land Surface Scheme. Atmosph. Ocean. 38 (1), 141±160.
McLay, C. D. A., F. R. Allbrook, and K. Thompson (1992): Effect of
development and cultivation on physical properties of peat soils in New
Zealand. Geoderma 54, 23±37.
Ohde, J. (1951): Neue Erdstoff-Kennwerte. Die Bautechnik 27, 345±351.
Okruszko, H. (1993): Transformation of fen-peat soil under the impact of
draining. Pols. Akad, zesz. 406, 3-75.
Plagge, R. (1991): Bestimmung der ungesättigten hydraulischen Leitfähigkeit. PhD, TU Berlin, FG Bodenkunde, Germany.
Renger, M., G. Wessolek, K. Schwärzel, R. Sauerbrey, and C. Siewert
(2002): Aspects of peat conservation and water management. J. Plant
Nutr. Soil Sci. 165, 487±493.
Royer, J. M., and G. Vachaud (1975): Field Determination of Hysteresis in
Soil-Water Characteristics. Soil Sci. Soc. Am. J. 39, 221±223.
Sachs, L. (1997): Angewandte Statistik. Springer Verlag, Berlin.
Sauerbrey, R., E. Gebhardt, and H. Raasch (1988): Methodische
Untersuchungen der pF-Bestimmungen an Niedermoortorfen und daraus
ableitbare Schluûfolgerungen. Tag.-Ber., Akad. Landwirtsch.-Wiss.
DDR, Berlin 269, 581 ± 584.
Sauerbrey, R., and J. Zeitz (1999): Handbuch der Bodenkunde. Kap. 3.3.3.7
6. Erg. Lfg. 7/99 S. 1±24.
Schäfer, W. (1996): Changes in physical properties of organic soils induced
by land use. Proc. 10th International Peat Congress, Vol. 4, 77±83,
Bremen.
View publication stats
Schwärzel, Renger, Sauerbrey, and Wessolek
Schmidt, W., and K. Illner (1976): Die Bodenformen landwirtschaftlich
genutzter Niedermoore. Z. Melioration. Landwirtschaftsbau 10, 166±
168.
Schmidt, W. (1986): Zur Bestimmung der Einheitswasserzahl von Torfen.
Archiv Acker- Pflanzenbau Bodenkd. 30, 251±257.
Schmidt, W., G. Mundel, A., W. Scholz, and W. v. d. Waydbrink (1981):
Kennzeichnung und Beurteilung der Bodenentwicklung auf Niedermoor
unter besonderer Berücksichtigung der Degradierung. Forsch.-Bericht
Inst. Futterproduktion Paulinenaue AdL der DDR.
Schothorst, C. J. (1982): Drainage and Behaviour of Peat Soils. Proc.
Symp. On Peatlands below the Sea Level, Wageningen.
Schwärzel, K. (2000): Dynamik des Wasserhaushaltes von Niedermooren.
PhD, TU Berlin, FG Standortkunde/Bodenschutz, Germany.
Silinis, U., and R. L. Rothwell (1998): Forest Peatland drainage and
subsidence affect soil water retention and transport properties in an
Alberta peatland. Soil Sci. Soc. Am. J. 62, 1048±1056.
Weiss, R., J. Alm, R. Laiho, and J. Laine (1998): Modeling moisture
retention in peat soils. Soil Sci. Soc. Am. J. 62, 305±313.
Wessolek, G., K. Schwärzel, M. Renger, R. Sauerbrey, and C. Siewert
(2002): Soil hydrology and CO2 release of peat soils. J. Plant Nutr. Soil
Sci. 165, 494±500.
Zeitz, J. (1992): Bodenphysikalische Eigenschaften von Substrat-HorizontGruppen in landwirtschaftlich genutzten Niedermooren. Z. Kulturtechnik Landentw. 33, 301±307.
Zeitz, J. (1993): Zustandserfassung und Kartierung der Moorböden im
Niedermoorgebiet Oberes Rhinluch als Grundlage für die Planung von
standortangepassten, umweltschonenden Nutzungsformen. MUNR Forschungsbericht, Germany.
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