SOIL DEVELOPMENT IN A CHRONOSEQUENCE OF FLUVIAL TERRACES OF THE ALMAR RIVER (SALAMANCA, CENTRAL WESTERN SPAIN)
C. DORRONSORO and P. ALONSO
Dpto. de Edafología. Facultad de Farmacia. Univeridad de Salamanca.
SOIL DEVELOPMENT INDICES VERSUS HORIZON DEPTH WITH AGE
It is generally accepted that most soil properties are time-dependent variables (Jenny, 1941). Therefore it is likely that soils of different ages will display their properties in different ways, especially under conditions in which the actions of other soil formation factors remain constant. Thus, the study of how the properties of soils vary with age is of great interest and may offer useful information in regard to their genesis.
Studies of soil chronosequences have focused on widely differing materials such
as fluvial materials, eolian, lacustrine material, marine materials, dunes,
glaciar morraines, volcanic ashes and even on anthropogenic deposits such as
the slag heaps of abandoned mines. Of all these types of chronosequence, river
terraces have undoubtedly been the focus of most attention since that they are
very good examples of natural soil sequences whose evolution has been shaped
by time. Although river terraces do cover an extended period of time, the changes
occurring in their soils should not be attributed exclusively to the action
of this parameter; rather, to climatic changes and those due to the development
of biological organisms should also be taken into account, insofar that these
will also have affected soil development.
The aims of the present work are several: i) to evaluate how the properties
and development of soils change with the progression of time; ii) to define
soil development age trends; iii) to analyze whether the development of the
soils under study becomes fixed at a steady state or not.
The soils examined in this study were from a sequence of river terraces formed
by the Almar River in the province of Salamanca (Central Western Spain).
Over geological time this river has left a typical scaled relief, with abundant
horizontal surfaces placed between sharp scarps. According to Vreeken (1975),
these are post-incisive sequences. The river terraces analyzed are located in
the neighborhood of Macotera village (UTM coordinates: 466694 and 471698 of
National Grid Map 479).
A representative profile was chosen from each of the seven geomorphological
surfaces distinguished, although in some cases, due to lateral changes, two
profiles were sampled (Table 1). The
sequence comprised all soils from the riverbank to a terrace 64 m above the
riverbed.
The fluvial deposits have thicknesses ranging between 1.5 and 5 m and are composed
mainly of gravels and sands that originated from the erosion of granitic rocks,
slates, quartzites and siliceous sediments.
The present day climate can be classified as subhumid (mean annual rainfall
412 mm), mesic, (mean annual T°, 11°C) of the continental Mediterranean
type).
The climax vegetation is composed of oaks from the genisto- histricis-Quercetum
rotundifoliae sigmetum series. In certain areas this vegetation has been altered
by human activity for crop cultivation and in others it is characterized by
more or less open holm-oak populations with abundant undergrowth.
The soil surfaces have been dated by Santonja et al (1976; 1982, 1984) and the
IGME (1982), mainly by archaeological and stratigraphic methods. The calculated
ages for these Spanish soils have been contrasted (figure
1) with the values obtained by other authors (Meixner and Singer, 1981;
Harden, 1982; Busacca, 1987) for Californian soils that were formed under similar
conditions to ours: fluvial terraces formed by materials originated from the
erosion of granitic rocks, Mediterranean climate, with average annual precipitation
of 310 to 640 mm. We illustrate the results for the rate of soil formation in
figure 1. We have simply calculated
these values on the basis of the proportion between the soil depth in millimeters
(A hor. + B hor. + 1/2AC hor. + 1/2 BC hor. excluding CA, CB, or C horizons)
and the soil age in years (soils with anomalous performances have not been represented).
A strong agreement was found between the values respective to our Spanish soils
in relation to the Californian soils.
The formation rate drastically decreases with age. The soils deepen at a rate
of 0.2 to 0.6 mm/year for those that are less than 1000 years old in these soil
conditions. The soil that is 10,000 years old has formed at 0.1 mm/year. The
soil that is 100,000 years old has formed at 0.01 mm/year. The rate decreased
to 0.005 mm/year at 500,000 years old. Finally, rate decreased to 0.001 mm/year
at about one million years.
The descriptions of the morphological properties and the physical and chemical
analyses of the soils were conducted according to traditional methods (Soil
Survey Staffs, 1951 and 1984). The mineralogy of the sand and silt fractions
was determined by X-ray diffraction.
In order to analyze the soil development trend with age, representative samples
were chosen:
1) The properties of Ap horizon always refer to those of the horizon closest
to the surface.
2) When two or more Bt horizons within one soil are referred to, the following
have been selected:
-The maximum value of all the Bt horizons for the clay content, water retention
at 0.03 and 1.5 MPa, available water and cation exchange capacity.
-For the contents in sands and silts, those corresponding to the horizon with
the maximum enrichment in clays were selected
-In the case of clay, we also calculated the accumulation indices at preset
depths in the 15, 50, 100, 150, 200 and 250 cm profile (cumulative values of
the percentage of clay particles by horizon thickness in meters)
-The mineralogy of the sand and silt fractions was always calculated by taking
the values corresponding to the second-most superficial horizon of each profile.
-Solum thickness was calculated as the sum of the thicknesses corresponding
to Ap and Bt horizons (measured in centimeters) plus half the thickness of the
BC and CB transition horizons .
-The thickness of the Bt horizon was also considered in this way, logically
without taking the Ap horizons into account.
Several authors have stressed the considerable difficulty in evaluating the
degree of development shown by soils due to the enormous amount of data about
different soil properties that such studies usually generate. To overcome this
problem, quantitative indices have been developed; using a single value these
quantitative indices evaluate the degree of evolution among different soils.
They can also be applied to the different horizons of a single soil (Buntley
and Westin, 1965; Walker and Green, 1976; Bilzi and Ciolkosz, 1977; Harden.
1982; Birkeland, 1984 a and b). These indices were calculated by determining
the intensity of the change occurring between the properties of the horizons
and those of the original material.
MORPHOLOGICAL INDICES (MI)
To calculate the MI we followed the guidelines of Harden (1982), using seven
properties chosen by this author; namely: i) structure (type and degree of development);
ii) total texture (textural class + type of stickiness and plasticity of the
wet consistence); iii) dry consistence (class;); iv) moist consistence (class);
v) clay films (abundance, thickness and location); vi) melanization (color value);
vii) rubification (color hue and chroma).
A detailed morphological description of the soil profile was the starting point
for the calculations. Quantification of the field properties was modelled after
Bilzi and Ciolkosz (1977). These authors assign points on the basis of the the
difference between the properties observed in the soil horizons and the original
material. The B-C index yields very good results for comparing soils developed
from different original materials, although in the particular case of chronosequences
developed on river terraces it is not completely satisfactory, as demonstrated
by Meixner and Singer (1981). For comparative purposes, better results are obtained
using the indices of Harden (1982) in large part because of the introduction
of thickness and to the ability to combine several properties.
To calculate the MI, the following steps were observed (Harden, 1982; Busacca,
1987). 1) a description of the soil profile; 2) assessment of parent materials
(measurement of fresh deposits or deep C horizons); 3) quantification of each
field property for each horizon (assessing ten points to step increases); 4)
normalization of quantified properties (division by maximum quantified property)
to obtain the MI for each property and horizon. 5) multiplication of the value
obtained in the previous step by the thickness (in cm) of the horizon and adding
all of the values corresponding to all of the horizons of a given soil yields
the MI for a single property for each profile; 6) if all the normalized values,
calculated in step 4, are added up and are then divided by the number of properties
considered, one obtains the morphological index (general properties) per horizon;
7) by multiplying the latter values by the thickness corresponding to each horizon
and then adding these products, one obtains the general MI for each profile.
So that the thickness of the soil will not be overvalued in these indices, Harden
and Taylor (1983), Birkeland (1984 a and b), and Busacca (1987) proposed that
the values could be divided by the true thickness of each soil. In this same
sense, Birkeland (1984b) suggested the use of a constant thickness for each
of the soils, choosing the thickness of the deepest soil in the chronosequence
as a homogenization factor, artificially prolonging the thinnest soils until
the thickness of the deepest soil is reached. We obtained good results by dividing
the value of the index by the true thickness of each soil (the values are not
reproduced here) and also for a standard thickness of 2 m.
For the results of the physical and chemical analyses, we calculated the indices
of Birkeland (1984). These indices are a modification of the index of profile
anisotropy (IPA) of Walker and Green (1976) and are calculated thus:
mIPA= D/PM
where D represents the numerical difference between the value of the horizon
property considered and its value in the original material, and PM refers precisely
to this latter value.
To calculate the AI we followed steps through 7 described in the previous section
corresponding to the MI (step 4, AI for each property and horizon; step 5, AI
for each property and profile; step 6, general AI for all properties and horizon;
step 7, general AI for all properties and profile). As was done for the MI,
calculations were made for normalized thicknesses (AI divided by the thickness
of each soil and AI for standard thicknesses of 2 m).
In this study, we calculated the AI for a single property and profile for 9
properties: the % of sand, the % of clay, water retention at 0.03 and 1.5 MPa,
available water, % organic matter, % of calcium carbonate, pH and cation exchange
capacity. We also calculated the general AI (all properties) for horizon and
the general AI for profile.
Likewise we calculated MineI as a function of the contents of quartz and feldspars
of the sand and silt fractions of the soil.
To calculate these MineI we followed the same steps as for the AI, calculating
the general MineI as a function of the values of the quartz/feldspars ratio.
A selection of the results of the morphological, physical, chemical and mineralogical
analyses of these soils is offered in the tables
2 and 3.
In this chronosequence, it is possible to observe a progressive and pronounced
development of the soil with age. In the current flood plain there are Xerorthents.
The soils of the Upper Pleistocene surfaces have become Haploxeralfs. Finally,
in the Middle Pleistocene there are Palexeralfs.
SOIL DEVELOPMENT INDICES VERSUS HORIZON DEPTH
WITH AGE
Figures 2, 3
and 4 summarize the values of the three
general indices: morphological, analytical and mineralogical (MI, AI and MineI).
It can be seen how the distributions of these indices follow a progressive evolution
with age, and form two general developmental trends. One corresponds to the
global values of the indices and the other refers to the degree of differentiation
among the horizons of each soil.
A marked increase with age is observed in regard to the global values of these
indices. The flood plain soils are outstanding due to their very low values.
In the soils of Upper Pleistocene age, the indices corresponding to the mineral
alterations continue to be spectacularly low, while those corresponding to MI
and AI have moderate values. In the soils of the Middle Pleistocene age, all
the indices increase as the age of the soils advances.
In regards to the degree of differentiation of the horizons of each soil, the
following trend is observed: the soils of the present flood plains show very
underdeveloped profiles. The soils of the Upper Pleistocene surfaces have a
pronounced horizoning; these soils show a horizon of maximum differentiation
situated at about 100 cm depth. In the soils of the Middle Pleistocene age the
most developed horizons increase in their absolute values and become broader
towards horizons above and below. Very similar distributions have been reported
for the MI of Californian soils by Harden (1982, fig.
7) and Busacca (1987, fig. 3).
The MineI disclose the very low degree of alteration shown by soils of the Holocene
and Upper Pleistocene ages (AM7, AM14, AM18) whereas for the other indices --MI
and AI-- these soils are already clearly developed. The main process responsible
for the development of these soils is clay illuviation; this process begins
in the initial phase of soil development, long before mineral alteration is
developed. We believe that this is due to the special characteristics of the
fluvial deposits over which the soils have been formed.
First, the deposits of fluvial sands display a certain mineral stability since,
on one hand, due to their size the mineral fragments are fairly well protected
from weathering and, on the other, these fluvial deposits contain reasonably
stable minerals, since the unstable ones would have decomposed during previous
phases of erosion and river transport.
Secondly, the fluvial deposits are mostly formed of loose grains of sand and
are therefore very porous, thereby enormously facilitating the infiltration
of rainwater and hence the vertical transport of clay suspensions. Since they
are present in small amounts, the clays are weakly retained in the soils and
migrate with ease. Indeed, micromorphological study clearly shows that at the
present time clay illuviation has already developed on the flood plain. Rapid
clay translocation at on early stage seems to explains the absence of Inceptisols
in this developmental sequence.
Particular types of behavior of different soil properties can be grouped in
three broad categories, according to whether the value of the property tends
to increase with age; decrease, or whether there is no relationship to age at
all. For the first two groups, a series of subgroups can be established as a
function of the intensity and constancy of the change.
1.- Properties that increase with age
Many properties show a direct dependence with age. According to the evolution
of the property with age, several groups can be differentiated thus:
1.1. Constant and regular increase throughout the chronosequence depending strongly
on age: the available water and COLE (coefficient of linear extensibility) of
the Bt horizon (figure 5).
1.2 Strong increase only during the first phases. Another group of properties
shows strong increases with age for the Holocene and Upper Pleistocene soils,
thereafter undergoing a change in their development in the final stages, corresponding
to the Middle Pleistocene. This change may follow different trends:
i) Some properties continue to increase with age but more moderately (figure
6): water retention at 0.03 and 1.5MPa and cation exchange capacity of the
Ap horizon (also in Dickson and Crocker, 1953; Torrent, 1976; Ahmad et al.,
1977; Jongmans et al., 1991), Fed of the Bt horizon (Hendershot et al.,1979;
Torrent et al., 1980; Alexander and Holowaychuck, 1983; Peña and Torrent,
1984; Arduino et al., 1984 and 1986; and Aniku and Singer,1990); solum thickness
(Ahmad et al., 1977; Meixner and Singer, 1981; Little and Ward, 1981; Harden,
1982; Muhs, 1982; Alexander and Holowaychuck, 1983; Arduino et al., 1984; Chittleborough
el al., 1984; Arduino et al., 1986; Busacca, 1987; Ajmone et al., 1988; Reheis
et al., 1989), clay accumulations at 15, 50, 100, 150, 200 and 250 cm depth,
quartz content and quartz/feldspar ratio (also in Ruhe, 1956; Barshad, 1955;
Muhs, 1982).
ii) Other properties cease to increase for the soils from the Middle Pleistocene.
In this situation are the silt and available water of the Ap horizon and the
% of clay, available water at 0.03 and 1.5MPa and cation exchange capacity of
the Bt horizons (figure 7).
A highly representative property of this type of study is the concentration
of clay accumulated in the B horizon of the soils. Logically, as age increases
a progressive increase in the amount of clay present would also be expected
to occur. This kind of behavior has been observed both in our soils and in many
others studied (Ruhe, 1956; Franzmeier and Whiteside, 1963; Janda and Croft
1967; Torrent, 1976; Ahmad et al., 1977; Pastor and Bockheim, 1980; Meixner
and Singer, 1981; Dorronsoro et al. 1988; Fine et al., 1989; Reheis et al.,1989;
Aniku and Singer, 1990). All of these chronofunctions can be adapted to power
and logarithmic models without a value of maximum enrichment being reached after
which the values remain constant, although this does occur in some cases (Peña
and Torrent, 1984) and also in this investigation for the total enrichments
in clay at defined depths (% clay x thickness) of 50, 100, 150 and 200 cm.
1.3 Strong increase only during the last phases. Another group of properties
shows strong increases with age for the Middle Pleistocene soils. In this situation
are the % of clay and nitrogen of the Ap horizon, the thickness of the Bt horizons
and carbonate accumulations (figure 8).
Some of these properties are not strongly age dependent. So, these trends could
go to next group 3 of not age related.
2.Properties that decrease with age
These show the opposite behavior to that of the previous group; that is, the
property becomes increasingly less degree of expression as the soil ages.
The properties belonging to this group are minor. All of these properties (figure
9) show strong decreases up to the Upper Pleistocene soils, thereafter decreasing
very moderately (feldspar contents), or cease to increase for the soils from
the Middle Pleistocene (percentages in sand of the Ap and Bt horizons) or show
increase during the last phases (bulk density of the Bt horizon).
3. Properties that are not age-related.
Finally, for one group of properties it is not possible to establish any logical
relationships between their degree of manifestation and the age of the soil
(pH and base saturation of Ap horizon, and the silt of Bt horizon). Figure
10.
The percentages of organic matter are expected to increase with age (Dickson
and Crocker, 1953; Ruhe, 1956; Viereck, 1966; Leisman, 1957; Olson, 1958; Syers
et al. 1970; Mahaney, 1974), although in our soils, an increase is observed
only for the very young soils; after the Upper Pleistocene the soils display
a chaotic behavior, possibly due to the intense anthropogenic activity to which
they have been subjected.
The absence of a dependence between the base saturation and age could be the
result of land practices.
In other chronosequences the pH of the soils clearly reflects the progressive
acidification that atmospheric precipitation tends to produce on the surface
horizon (Dickson and Crocker, 1954; Crocker and Dickson, 1957; Wilson, 1960;
Cowie, 1968; Ugolini, 1968; Viereck, 1970; Bockheim, 1979; Alexander and Holowaychuck,
1983; Arduino et al., 1984; Chittleborough et al., 1984; Peña and Torrent,
1984; Aniku and Singer, 1990).
Morphological indices
As shown in figure 11, all of the MI
show strong increases during the initial phases of soil formation; as from the
Upper Pleistocene soils, these increases are reduced to the minimum (MI of texture,
dry consistency, and the general one for all the properties together) or may
even remain constant (moist consistency and rubification MI). A similar kind
of behavior, with continuous increases, and mostly in the early phases of the
development of the soils has been reported for these MI by Harden (1982), Birkeland
(1984b), Busacca (1987) and Reheis et al. (1989).
Analytical indices
As occurs with the MI, the AI show a very dominant type of behavior. The values
of most of the AI increase throughout the chronosequence but only do so very
intensively in the case of the soils of the Holocene and Upper Pleistocene age
(figure 12). Similar increases to those
of these AI have been described by Birkeland (1984b) for soils of the Holocene
age in New Zealand. Some of these AI are not strongly age dependent, like organic
carbon and carbonate.
Mineralogical indices.
The MineI are also well related with age. These MineI point to a constant and
intense increase with age (figure 13).
Five models of regression equations were assayed for the chronofunctions of
these soils: linear (Y=a + bX), second degree polynomial (Y= a + bX + cX2),
power (Y=aXb), logarithmic (Y= a + blogX) and exponential (Y= abX). Of all these
models, the power, second degree polynomial and logarithmic equations proved
to be the ones best represented, almost all the regressions defined for the
MI and AI and also for many of the properties corresponded to these equations
(tables 4 and 5).
The linear model was the best fit in very few cases; the exponential equations
had no representativity at all for these soils. In previous works (Birkeland,
1974 and 1984; Yaalon, 1975; Bockheim, 1980 and 1990; Little and Ward, 1981;
Harden, 1982; Muhs, 1982) the equations most frequently found have been the
logarithmic and power models.
In general, the correlation coefficients obtained had high values. The regression
equations with the best fits for each chronofunction are shown in table
5.
Finally, it is interesting to note that very few properties of the soils studied
here seem to reach steady state during their development. All of them, with
the only exception of the percentages of sands, continue to undergo changes
even in the oldest soils of the chronosequence. However, for a large number
of properties, development manifests only during the first period, corresponding
to the Holocene and Upper Pleistocene soils (up to about 100,000 years), with
the rate of development slowing thereafter.
The paucity of results confirming the steady-state theory seems to be quite
patent in most chronosequences investigated by different authors. Bockheim (1980)
indicates that "the trends shown in this study cast some doubt as to whether
soils reach a steady-state (dynamic equilibrium) with their environment. Most
of the properties investigated continue to change despite the passage of as
many as 106 years". Similar conclusions were reached by Muhs (1982) for
the soils of marine terraces in California of up to 106 years old. Also, Busacca
(1987), studying a chronosequence of river terraces of the Sacramento Valley
(California) of up to 106 years concluded "Apparently none of the soils
in the sequence has reached a steady state of development".
In light of the foregoing, it seems clear that most soil properties continue
to develop over time and that no final stage of development is reached. Despite
this, within this possible kind of development it would be necessary to take
into account that during the development of a chronosequence not only are there
changes in time but also modifications in climatic factors and the subordinate
factor of organisms participate; these may have led to important variations
in the final behavior of the soils of the chronosequence (in fact in most studies
investigators should be using the term climobiochronosequences).
The following conclusions can be drawn from the results:
1.- The soil properties show several types of behavior.
1.1- As age progresses, same properties increase regularly throughout the chronosequence.
This is strongly dependent on the age, COLE and the available water of the Bt
horizon.
1.2- Some properties continue to increase with age but more moderately during
the last phases: for example, water retention at 0.03 and 1.5MPa and cation
exchange capacity of the Ap horizon, Fed of the Bt horizon; solum thickness,
clay accumulations at 15, 50, 100, 150, 200 and 250 cm depth, quartz content
and quartz/feldspar ratio.
1.3- Other properties cease to change in the soils from the Middle Pleistocene.
In this situation are the silt and available water of the Ap horizon and the
% of clay, water retention at 0.03 and 1.5MPa and cation exchange capacity of
the Bt horizons.
1.4- Still other properties show a strong increase only during the last phases
(Middle Pleistocene and older soils). In this situation are the % of clay and
nitrogen of the Ap horizon, the thickness of the Bt horizons and carbonate accumulations.
1.5-Other properties decrease with age: with strong decreases up to the Upper
Pleistocene soils, thereafter decreasing very moderately (feldspar contents),
or cease to increase for the soils from the Middle Pleistocene (percentages
in sand of the Ap and Bt horizons) or show an increase during the last phases
(bulk density of the Bt horizon).
1.6- Other properties are not age-related. Within this group there are two subgroups.
One refers to those properties that remain constant (base saturation of the
Ap horizon), and another to those that vary chaotically: organic matter and
pH of the Ap horizon and the silt of the Bt horizon.
2.- The horizon development indices show marked increases in their values with
the advancing age of the soils and also show a progressive differentiation over
time for the horizons of the soil profile.
3.- The soil development indices show very good correlations with age. In the
vast majority of cases the tendency to increase falls off strongly for the oldest
soils of the Middle Pleistocene age.
4.- In most cases the properties and development indices continue to evolve
constantly throughout the chronosequence without steady-state being reached;
however, the soils of the Middle Pleistocene age do reach a state of evident
maturity from which development processes very slowly.
ACKNOWLEDGEMENTS
The authors are grateful to the Spanish DGICYT for the financial support received
for this study (PB88-0378)
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