Morphology and Micromorphology of Paddy Soils under Different Soil Moisture Regime and Ground Water Table in Mazandaran Province, Northern Iran, Amol
The morphology and micromorphology of paddy soils under
different soil moisture regime and groundwater table of Northern Iran
were investigated. Aquic conditions in soils are often associated with
redoximorphic features. The depths at which the features occur are often
used as an indicator of the location of the seasonal water table. Morphological
and micromorphological characteristics could be used to identify the hydromorphism
degree of soils and their properties. In this study, morphological and
micromorphological characteristics of paddy soil samples with different
water table depth and drainage were investigated. Thirty nine undisturbed
blocks of soil were taken. Thin sections were observed with a polarized
microscope and described according to thin section interpretation. This
study demonstrated the extensive pedogenic changes and micromorphological
evidences. Studied soil samples are exposed to alternative fluctuations
with water saturation in different time. The various type of iron and
manganese oxides show the difference of environment and their movement
and formation are the most important factor to form hydragric anthrosol.
This study demonstrated that moisture regimes in soils with anthraquic
saturation can be characterized in the same way as in soils with either
episaturation or endosaturation. And also the effect of irrigation water
on pedogenesis with respect to saturation, reduction and redoximorphic
features was greater than that of the shallow ground water.
to cite this article:
H. Hassannezhad, A. Pashaee, F. Khormali and M. Mohammadian, 2008. Morphology and Micromorphology of Paddy Soils under Different Soil Moisture Regime and Ground Water Table in Mazandaran Province, Northern Iran, Amol. International Journal of Soil Science, 3: 149-156.
Paddy soils are a kind of artificial hydromorphism or hydragric Anthrosols;
soils with prominent characteristics result from human activities, virtually
any soil material, modified through cultivation or by addition of material
(Wilding and Ahrens, 2005; Aimrun et al., 2003).
Hsue and Chen (2001) confirmed that the formation of redoximorphic features
occurred as Fe and Mn concretion and depletion associated with the fluctuations
of seasonal water table. Zhang and Gong (2003) found deposition of sediment
in area with low groundwater table has changed the soil hydrology. The
objectives of this research were to determine: (i) the impact of anthraquic
conditions on morphology, the different forms of redoximorphic features
and distributions and (ii) the processes responsible for the formation
or transformation of soil in general, or of specific features.
MATERIALS AND METHODS
The study area (at the east of Amol city) is located in Mazandaran
Province, northern Iran, in the southern coast of the Caspian Sea (Fig.
1), with different water table depth (Fig. 2). Studied
area has general slope from south to east. Its physiographic unit is alluvium
||The location of Mazandaran province and Amol area
||The location of pedons according to water table depth
A humid climate with moderate temperature characterizes the area. The
mean annual temperature is approximately 16.0°C. This area has an
average annual rainfall of 848.4 mm (1989-1999). The soil moisture and
temperature regimes are Wet Ustic and Thermic, respectively. The parent
materials are derived from sediments weathered from primary rocks content
Calcite, Dolomite and loess moved to this plain.
This study was accomplished from September 2005 to September 2006.
Seven pedons (35 paddy soil samples) and 1 pedon (4 soil samples) as a
blank (soil under cultivation of citrus fruit) were selected and sampled
according to genetic horizons when the fields were drained after rice
harvest. Soil Samples described and classified, according to Keys to soil
taxonomy (Soil Survey Staff, 2006).
Thirty nine undisturbed blocks of soil were taken with Kubiena boxes
in order to have representative sampling of the different horizons of
pedons. In order to avoid structural modifications, all of the blocks
were impregnated with a polyester resin after replacing water by acetone.
After air drying, vertical and horizontal oriented thin sections with
a thickness of 30 micrometer were prepared. Finally, thin sections were
observed with a polarized microscope (euromex type) and described according
to guidelines of Stoops (2003).
Air-dried soil samples were crushed and passed through a 2 mm sieve.
Samples were dispersed using sodium hexametaphosphate for determination
of sand, silt and clay fraction by the hydrometric method (Bouyoucos,
1962). Organic matter was measured by wet oxidation (Nelson and Sommers,
1982). Alkaline-earth carbonate (lime) was measured by acid neutralization
(Ali Ehyaee and Behbahanizade, 1993). Soil pH was determined with a glass
electrode in saturated paste (Ritvo et al., 2003). Electrical conductivity
(total soluble salt) was measured in the saturation extract (Ali Ehyaee
and Behbahanizade, 1993). Cation Exchange Capacity (CEC) was determined
using sodium acetate (NaOAc) at a pH of 2/8 (Bower and Hatchea, 1966).
RESULTS AND DISCUSSION
Soil profile description and some physical and chemical properties are
reported in Table 1. In studied area pedon 5 is located
northern and lowland part of this region. The water table was at (or close
to) the surface through out the year. Only some years in harvesting time
and 2-3 weeks after harvesting is not flooded, but the soil surface is
mostly wet because of arrival deposition from top land. Soil development
of this pedon is the last, which has undergone little soil formation.
The other pedons were not saturated all of the year and the water table
fluctuation was influenced by cropping and flooding of the rice-growing
and rainfall. Pedon 7 and 8 were at southern of studied area that have
the height water table depth and locate in top land. Pedons 1, 2, 3 and
4 were located between pedons 5 and 7. Water ponding as in paddy soils,
usually leads to development of aquic characteristics in the surface and
subsurface horizons where under natural conditions such characteristics
would be absent or only weakly developed. In paddy soil samples with high
water table, completely reduced layer is located below the 100 cm depth
and saturation condition from the surface and oxidized zone have been
seen in Apg, Ag and Bg1 below the plow
layer and between these two layer respectively. In pedons with low water
level and poorly drainage (pedon 5), water level is stayed at 70 cm and
soil texture in this pedon is sandy and the thickness between saturated
layer in surface and depth is the least. Because of arrival deposition
of sediment from top land, soil evolution of this pedon is low. Soil color
patterns and redoximorphic features are the most significant soil characteristics
affected by water table depth and its fluctuation in studied soils that
include low chroma and gray color. They could be used to predict where
seasonal saturation occurs in soils and reflect the environment conditions
according to He et al. (2003) and Jien et al. (2004). The
redoximorphic features are the main sign of aquic condition in saturated
soils with low water level and the distribution patterns of pedogenic
oxides are depending on intensity of reduced condition. Stolt et al.
(2001) and Fiedler and Sommer (2004) also reported this observation. Alternating
cycles of reduction and oxidation in soils over prolonged periods and
the consequent mobility and accumulation or depletion of Fe and Mn, result
in the formation of redoximorphic features associated with water table
depth that is according to Costantini et al. (2006), Hsue and Chen
(1996) and Khan and Fenton (1996). One of the most important phenomena
occurs in plow layer of studied rice soils is also reduced Fe and Mn leaching
and their different kind of features. In reduced condition, ferrous iron
is far more soluble than ferric iron, thus creating an increase in Fe
mobility. The Fe transported within soils and landscapes via soil solution.
Areas with frequent reducing conditions lose Fe+2 with out
flowing water. This redistribution is visible in the field by the change
from brown to gray colored soil horizons. In some of the samples, a large
part of reduced Fe became oxide by drainage and dryness. Brown spots and
rusty mottles formed that show oxidized condition.
When reoxidized condition existed in the soil, Fe and Mn were accumulated
as coating in the voids, root channels and macropores, particularly in
the root channels and other biopores caused by rice cultivation. After
alternative redox processes, irregular soft masses were concentrated close
to the pedosurface; Fe and Mn nodules were further formed as well. In
thin section observations, redoximorphic features have been seen in all
studied samples as a different type of Fe and Mn coating, hypocoating
(Fig. 3b) and nodules (Fig. 3a). This
study confirmed that although Fe-Mn nodules have been seen in all studied
samples, the maximums amount and size occur in the optimum reducing and
saturation condition. Reverse effects in the development of Fe-Mn nodules
are excessive submergence and less saturation and reduction.
||Fe-Mn oxide orthic nodule (a) coating and hypocoating around void
(b) 10X magnification
The fact that there are much more void type like root channels and other
biopores near the plow layer (caused by human activities for rice production)
accounts for why the patterns of Fe-Mn nodules tend to have different
shapes such as angular, sub angular, slice and imperfectly round shapes.
Hydromorphism in the soils led to nodule formation; the intensity of which
appears to have a maximum in the wetter, but not the wettest soils. The
least amounts of Fe-Mn nodule through the profile were found in pedon
5, which contained the poorest drainage and was closest to the see level.
Their coating and hypocoating were also exist in well-drained samples
(pedon 1, 2, 6 and 8) to moderate drainage system (pedon 3, 4) that have
sharp boundaries relate to matrix and can be seen distinctly. But in poorly-drained
paddy soil (pedon 5), because of low chroma of matrix, these pedofeatures
have diffused boundaries. In this soil sample shiny iron cutan have been
observed that moved with water table fluctuation through the pedon and
precipitated in situ after the water table comes down. The coating
materials is believed to move from the top soil under hydraulic pressure
when the soil are flooded; coating is most prominent in deeply flooded
soils and least developed in soils that are only shallowly flooded for
short periods. The size of coating and hypocoating are estimated approximately
200 to 1000 μ. Nodules are kind of typic (with poorly to moderately
saturation), orthic (those formed in situ) and anorthic or lithomorphic
(nodular bodies inherited from the parent material), they are about 100
to 1000 μ and the biggest nodules are exist in samples with moderately
Microstructure studies of soil samples show in Apg horizons,
there is no special microstructure as a result of soil compaction. In
this case, soils have massive microstructure and most pores are vesicles
and vughs (Fig. 4a). Existence of angular and sub angular
microstructure in B horizons reflect the effect of accumulation of organic
matter, as a result of low ground water table (Fig. 4b).
This micromorphological evidence confirmed morphologic field observations.
Micromorphological evidence of poor clay coating in thin sections (Fig.
5a,b ) show the clay movement downward the profiles by irrigation
and flooded water. Truly, clays have been seen in samples are the particles
that formed by cultivation and irrigation water leaching. Additionally,
there is no micromorphological sign of increased illuviation such as clay
skin in studied wet-cultivated rice lands. The lower clay content near
the some surface of wet-cultivated soils is probably due mainly to their
weathering associated with alternative flooding and drainage. Another
process that can contribute to loss of clay from the surface soil is its
removal to the surface water during puddling when muddy water outflows
from the higher to the lower fields. Although there is no reason to believe
that wet rice cultivation enhances clay illuviation, it probably promotes
downward movement of clay, silt and organic matter through cracks and
pores that corresponded to Liu et al. (2001), Mori et al.
(1999) and Tuong et al. (1996). In pedon 5, clay movement downwards
the profile is less than the others, because of low leaching and low water
|Fig. 4: (a)
||Massive and Angular blocky microstructure (b) 4X magnification
||Slice clay coating in groundmass (a, b) 10 X magnifications
Growing rice in soils on alluvial plains is very popular in northern
Iran. Surface water irrigation and shallow ground water influence the
soil morphology of these soils simultaneously. The main pedogenic processes
forming rice soils have been discussed by many investigators. The most
important processes have been influenced by redox condition, addition
and removal of chemical components and soil particles and changes in physical,
chemical and microbiological properties through irrigation or drainage,
or both. In other words, glayzation and eluviations, mottle forming (oxidized
illuviation and segregation of iron and manganese and separation of manganese
from iron), grayzation, plow-sole forming, illimerization or degradation,
cutan forming, redistribution of exchangeable bases, accumulation (or
decomposition) and alteration of organic matter and other processes lead
to the profile differentiation of rice soils. Results indicated that the
effect of irrigation water on pedogenesis with respect to saturation,
reduction and redoximorphic features was greater than that of the shallow
ground water. On the contrary, shallow ground water was the main contributor
to saturation at the depth of 50 cm in the pedon 5. And also this study
demonstrated that moisture regimes in soils with anthraquic saturation
can be characterized in the same way as in soils with either episaturation
On alluvial plains, the soil conditions range from continuously reducing
controlled by high groundwater to alternating reduction and oxidation
because of artificial submergence and groundwater fluctuations. Artificially
induced leaching losses, previously prevented by a high groundwater table,
are partly compensated during paddy cultivation by the rejuvenation process
of alluvial deposition. Soil samples with no longer receives river-sediment,
are more strongly developed. In the polder area, apparent lowering of
groundwater table by deposition of sediment is less developed classified
in Entisols order. Micromorphology provides a means to identify the presence
of hydromorphic features that may otherwise be missed in the field if
only the naked eye is used. The observation of hydromorphic features in
thin sections suggests that careful observations in the field, using a
hand lens, may increase the likelihood of identification of horizons in
problem soils with aquic conditions. Fe nodules may be one of the easiest
diagnostic features to find because they have sharp boundaries. However,
these small features appear very similar to Fe-coated coarse fragments,
and, in addition, nodules with sharp boundaries are often interpreted
as relict features. Therefore, the use of nodules with sharp boundaries
may result in misinterpretations.
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