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SOIL PHYSICS

Pore Shrinkage Dependency of Inorganic and Organic Soils on Wetting and Drying Cycles

^a Institute of Plant Nutrition and Soil Science, Christian-Albrechts Univ., Olshausenstrasse 40, 24118, Kiel, Germany
^b Dep. of Crop and Soil Science, Michigan State Univ., East Lansing, MI 48824

^* Corresponding author (xh.peng{at}soils.uni-kiel.de).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Cycles of wetting and drying (WD) occur naturally in soils and affect the pore structure through altered hydraulic stresses. Two organic-rich soils, a Eutric Histosol and a Histic Gleysol, and two inorganic soils, a Calcic Gleysol and a Dystric Gleysol, ranging in texture and microstructure, were investigated. Undisturbed soil samples were predried to either –100 kPa water potential by using a ceramic plate or to 30°C by using an oven and then resaturated for one or three WD cycles. In addition, different combinations defined by the intensity, frequency, and sequence of WD cycles were analyzed. Soil structure was altered significantly if the intensity of drying was severe at 30°C, while drying to –100 kPa had only a small effect. The frequency and sequence of WD cycles did not alter the structure and shrinkage behavior significantly. Compared with the initial pore volume, intense WD cycles decreased it by 23.6 to 60.1% in the two organic-rich soils, whereas it increased by 1.5 to 4.8% in the silty Calcic Gleysol and by 3.6 to 15.1% in the clayey Dystric Gleysol. Both organic-rich soils showed more shrinkage but less swelling than did the two inorganic soils. Intense WD cycles altered the water potential vs. void ratio curves of the two organic-rich soils more gradually, while steeper patterns were observed for the two inorganic soils. This study shows that the changes in soil structure and pore shrinkage depend mostly on the maximum intensity of previous WD cycles.

Abbreviations: COLE, coefficient of linear extensibility • SEM, scanning electron micrographs • WD, wetting and drying • pore shrinkage index

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Changes in soil volume can occur by external stresses (compaction, load) or by internal stresses (capillary, hydraulic) (Horn and Baumgartl, 2000). Soil shrinkage and swelling under in situ conditions are, therefore, controlled by the level of the actual, in comparison to the previous, internal stresses, as defined by the intensity and number of WD cycles. Nonrigid soils undergo an increase in volume during water uptake and shrinkage after water removal. To understand the dynamics of soil structure, the effects of drying intensity, frequency, and their combination in WD cycles must be known, because they influence the paths of soil swelling and shrinkage.

Cycles of WD alter the capillary stress in pores. The stress conditions during WD cycles are complex and involve stress and shear. Basma et al. (1996) and Tripathy et al. (2002) reported that modifications of homogeneous soil structure were greatest following the first cycle of WD, and then decreased with subsequent WD cycling. Few changes were observed after three or five cycles. Multiple investigations (Alonso et al., 2005; Boivin et al., 2004; Tripathy et al., 2002) reported volume changes of artificial soils containing expansive clay when subjected to repeated WD cycling, even when external stresses were applied. Because these studies used the same WD intensity during each cycle, it was impossible to compare results with diverse natural environments. Since natural environments generate multiple numbers of WD cycles, each with a different intensity, we hypothesized that soil structure will establish a new equilibrium when the intensity of WD cycles is altered, which also influences the development of the stresses in the following cycles. Thus, aggregate formation hysteresis is altered by each WD cycle (Ghezzehei and Or, 2000, 2001).

Repeated WD cycles result in a hysteresis of soil swelling and shrinkage. Peng and Horn (2007) found a more pronounced hysteresis in organic soils than in inorganic soils. Organic soils show more shrinkage and less swelling than do the inorganic soils. Clay chemistry, including clay type (Boivin et al., 2004) and specific ion concentrations in soil pore solutions (Di Maio et al., 2004; Peng et al., 2005), control shrink–swell capacities of soils. Macropores are generally more sensitive to soil deformation than micropores, because shrinkage and swelling varies with pore size and pore rigidity when external stresses are imposed (Kutílek et al., 2006). Braudeau et al. (2004) suggested that the shrinkage capacity of macropores was less than that of micropores. The small capillary stresses in large pores cannot pull soil particles and aggregates together even if the pore rigidity is weak (Peng and Horn, 2007). One of the most important dynamic, nonrigid soil properties during multiple WD cycles is the opening and closure of cracks. Preexisting cracks rapidly redistribute water contents, altering the soil volume and pore size distribution.

Stress-modified soil volume can be described by comparing void ratio responses to soil water potentials (Baumgartl and Köck, 2004). This graphic relationship is composed primarily of two components: (i) reshrinkage under small stress; and (ii) new and initial shrinkage under higher stress exceeding the most negative ones that ever occurred in the past. Stress corresponding to the inflection point between them is defined as a critical preshrinkage stress according to Casagrande's method (Baumgartl and Horn, 1999; Baumgartl and Köck, 2004). If the present stress is higher than the preshrinkage stress, soil volume decreases considerably and linearly on a semilogarithmic stress scale. Consequently, particles and aggregates on the microstructure scale are reoriented (Mitchell and Soga, 2005).

The soil shrinkage graph, comparing void ratio responses to ratios of soil water contents, demonstrates a sigmoidal response composed of four characteristic zones including the structural, proportional, residual, and zero shrinkage phases from the wet to the dry range of soil water contents (Peng and Horn, 2005). The structural phase is consistent with the range of macropores, while the residual or zero shrinkage phase is induced by fine pores within the clayey platelets (Braudeau et al., 2004), and these phases demonstrate how soil volumes are altered by four characterized phases during WD cycles.

Our first hypothesis in this study was that the water potential vs. void ratio relationship and soil shrinkage curves depend on inherent soil properties, especially active clay and soil organic content, and on the stress history of WD cycles. A second hypothesis was that soil pore structures establish new equilibrium states when the intensity of WD cycles, defined as negative pore water pressure or matric potential, exceeds the previous maximum stress. Thus, the objectives of this study were (i) to quantify the changes in soil structure as a function of WD cycles, (ii) to quantify alteration of pore shrinkage capacity after WD cycles, and (iii) to identify alteration after WD cycles of the soil shrinkage curve, which relates soil water potentials to concomitant void ratios.

	MATERIALS AND METHODS

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The Soils
Four soils under pasture, a Histosol and three Gleysols having a wide range of texture and soil organic matter, were sampled in the southern part of Schleswig-Holstein, Germany, in June 2005. The Gleysols were derived from glacial sediments, with illite as the main clay mineral. Some of their chemical and physical properties are listed in Table 1. The Eutric Histosol contains mainly sands while the Dystric Gleysol contains mainly clay. The Eutric Histosol and Histic Gleysol contain >16% of organic C, whereas the other two Gleysols have <1.5%.

View larger version (157K): [in this window] [in a new window]   Fig. 1. Scanning electron micrograph features of the four investigated soils.

Modeling and Quantification of Shrinkage
A three-parameter model (Peng and Horn, 2005), which takes into account the actual ranges in void ratio (e) and moisture ratio (), was used to fit the measured data:

[1]

swhere , p, and q are dimensionless fitting parameters, e_s and e_r are the saturated and residual void ratios, respectively, which can be obtained either by measurement or by fitting—in this study we used the measured data, and _s is the saturated moisture ratio. The transition points of the four characteristic shrinkage phases were quantified numerically as reported earlier by Peng and Horn (2005).

The magnitude of shrinkage can be quantified by the coefficient of linear extensibility (COLE), which defines the one-dimensional variation of soils between wet and dry states (Grossman et al., 1968). Grossman et al. (1968) further defined the -33 kPa water potential as the wet state and oven drying at 105°C as the dry state. Since soil volumes change immediately on drying, we took complete water saturation as the wet-state boundary. The magnitude of shrinkage can be ranked into three classes: low, COLE < 0.03; moderate, 0.03 < COLE < 0.06; and high, COLE > 0.06. To isolate the shrinkage magnitude of different pore sizes, we separated large pores (>50 mm), medium pores (0.6–50 mm), and fine pores (<0.6 mm) according to the theory of cylindrical capillary (Kutílek and Nielsen, 1994), with equivalent water potential ranges of greater than -6, -6 to -500, and less than -500 kPa, respectively. The relative classification is limited to saturated soil structure due to pore shrinkage.

[2]

where L₀, L_-6, L_-500, and L_105°C are the length of the sample at water potentials of 0, -6 kPa, -500 kPa, and after oven drying at 105°C, respectively. The three components on the right side of Eq. [2] define the shrinkage magnitude of large, medium, and fine pores, respectively. The shrinkage capacity can be defined by the pore shrinkage index (PSI) as the shrinkage volume of soil (V_t) per dehydrating water-filled pore volume (V_p) as follows:

[3]

where i is the large, medium, or fine pores or all of them. If a soil shrinkage curve is available, Eq. [3] can be rewritten

[4]

Equation [4] is the slope or the first differentiation of the soil shrinkage curve.

Soil Analysis
Disturbed samples of all soils were ground to pass a 2-mm mesh. Soils <2 mm were collected for routine measurement. The following measurements were made (Klute, 1986): particle size distribution by the pipette method after removing organic matter; soil organic C by dry combustion at 1000°C; cation exchange capacity by the NH4OAc method; and particle density by the pycnometer method. Soil microstructure was observed by SEM (CamScan 44, Obducat CamScan Ltd., Waterbeach, UK) after oven drying at 40°C.

Statistical Analysis
Analyses of variance tested the effects of WD cycling treatment and pore size on shrinkage capacity with multiple comparisons (SPSS, 2001). Differences of means between WD cycling treatments and pore sizes were assessed by LSD tests.

	RESULTS

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Changes in Void Ratio and Moisture Ratio during Wetting and Drying Cycles
Changes in void ratio on predrying at -100 kPa and at 30°C are illustrated in Fig. 2. The void ratio decreased by 1.3 to 22.2% after the first drying to -100 kPa in all soils, while intense drying at 30°C decreased the void ratio by 51.7 to 68.3% for the Eutric Histosol, 42.0 to 48.9% for the Histic Gleysol, 23.0 to 30.2% for the Calcic Gleysol, and 56.3 to 60.6% for the Dystric Gleysol. Soil swelling also depends on the intensity of WD. The intense WD cycles increased the initial void ratio by 0.1 to 4.8% for the Calcic Gleysol and by 3.6 to 15.1% for the Dystric Gleysol, whereas the two organic-rich soils lost by 23.6 to 60.1%. With increasing slight WD cycles of the -100kPa(III) treatment, the soil volume varied by only a minor extent (P > 0.05). The three continuously intense WD cycles of the 30C(III) treatment, however, decreased the void ratio for the two organic-rich soils but resulted in small increases for the two inorganic soils. After one intense WD cycle followed by two slight WD cycles, or vice versa, i.e., treatments of 30C(I)-100kPa(II) and -100kPa(II)30C(I), soils showed no significantly different final void ratio values (P > 0.05). Furthermore, volumes of the two organic-rich soils subjected to the treatment of 30C(I)100kPa(II) always remained similar to those of the first intense WD cycle, even if the following two cycles were already shifted to slight ones.

View larger version (28K): [in this window] [in a new window]   Fig. 2. Changes in void ratio of the four soils on intensity, frequency, and sequence of wetting and drying cycles. Bars are the standard deviations. See Table 2 for treatment definitions.

View larger version (32K): [in this window] [in a new window]   Fig. 3. Changes in moisture ratio of the four soils on intensity, frequency, and sequence of wetting and drying cycles. Bars are the standard deviations. See Table 2 for treatment definitions. See Table 2 for treatment definitions.

View larger version (42K): [in this window] [in a new window]   Fig. 4. Distribution of large (>50 µm), medium (0.6–50 µm), and fine (<0.6 µm) pores of the four soils at saturation after wetting and drying cycles. Bars are the standard deviations. Different lowercase letters on the bars indicate significant difference of total pores between treatments at P < 0.05. See Table 2 for treatment definitions.

View larger version (40K): [in this window] [in a new window]   Fig. 5. Coefficient of linear extensibility (COLE) of large (>50 µm), medium (0.6–50 µm), and fine (<0.6 µm) pores of the four soils after wetting and drying cycles. Bars are the standard deviations. Different lowercase letters on the bars indicate significant difference of total COLE between treatments at P < 0.05. See Table 2 for treatment definitions.

View larger version (39K): [in this window] [in a new window]   Fig. 6. Shrinkage index of total large (>50 µm), medium (0.6–50 µm), and fine (<0.6 µm) pores of the four soils after wetting and drying cycles. Bars are the standard deviations. Different uppercase letters on the bars indicate significant difference between pore sizes at P < 0.05. Different lowercase letters on the bars indicate significant difference between treatments at P < 0.05. See Table 2 for treatment definitions.

View larger version (35K): [in this window] [in a new window]   Fig. 7. The changes in the water potential vs. void ratio relationship of the four soils as a function of the intensity, frequency, and sequence of wetting and drying cycles. The whole dryness is composed of 0 to –1000 kPa water potential, air dryness for 2, 4, and 7 d, and oven dryness at 30, 60, and 105°C. Bars are the standard deviations. See Table 2 for treatment definitions.

View larger version (29K): [in this window] [in a new window]   Fig. 8. Changes in the soil shrinkage curves of the four soils as a function of the intensity, frequency, and sequence of wetting and drying cycles. The dots are the lines fitted by Eq. [1]. See Table 2 for treatment definitions.

	DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

First, changes in soil structure of organic and inorganic soils differed in their responses to the intensity of WD cycles. Slight WD cycles (i.e., repeated drying at -100 kPa and following saturation) did not modify soil structure significantly (P > 0.05). Intense WD cycles (i.e., repeated drying at 30°C and following saturation), which exceeded the maximum predrying intensity, however, decreased initial pore volume considerably (23.0–68.3%) during the drying stage. Such great changes in soil structure were due to the stress of drying at 30°C beyond the threshold value of soil shrinkage as defined by preshrinkage stress (Baumgartl and Köck, 2004), which further involved the irreversible plastic component of soil deformation (Mitchell and Soga, 2005). Thus, after an intense WD cycle, soil structure did not return to the initial conditions. A new equilibrium state of the water phase within the pore system could be established.

Second, if soils were subjected to intense WD cycles, frequency dependency on soil structure faded, because the arrangement as mentioned above was so intense that the new quasiequilibrium state could not be alleviated during the following reswelling. This was established after just one cycle and was verified by the similarity of final pore size distribution and shrinkage magnitude between the two treatments of 30C(I) and 30C(III). Furthermore, because there were no differences in pore volume and shrinkage behavior between the -100kPa(II)30C(I) and 30C(I)-100kPa(II) treatments, modifications of soil structure appear to depend primarily on the maximal WD intensity but not on the sequence of slight and intense WD cycles. This will simplify evaluation of soil structure dynamics under in situ conditions if the maximal predrying WD intensity is known.

Third, the changes in soil structure are furthermore enhanced by soil textural effects, as can be detected by the direct interaction between particle size and possible rearrangement of particles through shrinkage. Especially the clayey Dystric Gleysol was deformed more easily than the silty and dense Calcic Gleysol. Interactions between organic compounds and the WD cycles, however, also need to be considered. The organic-rich soils resulted in a pronounced hysteretic shrinkage, but in a minor swelling. The latter is in agreement with Schwärzel et al. (2002), who observed an intense shrinkage behavior in peat soils, but a minor, or even a negligibly small, reswelling in them.

As a result, the changes in soil structure during WD cycles modified the pore size distribution as well as the shrinkage magnitude. Intense WD cycles enhanced the volume of large pores markedly in all soils except the silty Calcic Gleysol, although the volume of total pores was reduced in the two organic-rich soils and increased in both inorganic soils. The increase of large pores in organic-rich soils can be ascribed to their great shrinkage and small swelling. This was also shown by the SEM. The formation of new large pores in the Dystric Gleysol due to crack formation developed generally under heterogeneous internal stresses during WD cycles. The reduced total pore volume in the two organic-rich soils resulted in a decrease in shrinkage magnitude.

The shrinkage capacity of pores depends both on the pore rigidity and on the internal stresses including capillary stress. Thus, to investigate the shrinkage capacity of different pore sizes, we introduced the pore shrinkage index (see Eq. [3]), which shows the apparent soil deformation because partial shrinkage can be overlapped by the irregular pores and by the heterogeneous orientation of particles and aggregates. Our results confirm that the pore shrinkage index depends on soil type and on the intensity of WD cycles. The angular, blocky structure of the Calcic Gleysol is much stronger (defined by the angle of internal friction and the cohesion) than the platy-structured Dystric Gleysol due to its high cohesion (Horn and Baumgartl, 2000). This, in turn, induces a smaller shrinkage for the former soil. In the two inorganic soils, the shrinkage index of fine pores was smaller than that of large and medium pores, although they dominated soil shrinkage (55.7–96.0%). This is partly because the total internal stress is much smaller than the shear strength so that particles and aggregates are not able to be rearranged, and partly because the shrinkage of fine pores is counteracted by the larger pores but not vice versa.

The shrinkage index of fine pores in the Eutric Histosol was higher than 1.0 during the intense WD cycles. The extra volume loss may come from the collapse of pores where air is expelled during drying, as mentioned by Groenevelt and Grant (2001). The intense WD cycles decreased the shrinkage index of large pores in the two organic-rich soils, but increased it in inorganic soils, although the volumes of large pores all increased. The discrepancy indicates that the large pores in organic soils became more rigid while the new cracks in inorganic soils were still unstable.

The shrinkage index of differently sized pores can also be determined from the soil shrinkage curve. A small volume change during the structural shrinkage phase indicates a low shrinkage capacity of large pores, which agrees with the results of Braudeau et al. (2004). A large volume decrease in the proportional shrinkage phase resulted from the high shrinkage index of the medium and fine pores for the two organic-rich soils and from the greater shrinkage capacity of the medium pores for the two inorganic soils. A low shrinkage in the residual phase and nearly no shrinkage in the zero phase in the two inorganic soils were due to a very small shrinkage index or the rigidity of fine pores. Thus, WD cycles do not always result in an identical structure formation. The pore water pressure history, however, in combination with the available structure, determines to a great extent the processes of particle rearrangement, pore formation, and the corresponding soil strength.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Soil structure and pore shrinkage were influenced by inherent soil properties and by WD cycles. Organic-rich soils showed similar shrinkage behavior, with more shrinkage and less swelling during WD cycles than inorganic soils. Inorganic soils, with a higher clay content, were more sensitive to deformation. The pore size distribution and pore shrinkage capacity were modified significantly, if the maximal predrying stresses were exceeded. Intense WD cycles increased the volume of large pores markedly in all soils except the silty Calcic Gleysol, although they decreased the total pore volume in the organic-rich soils and increased it in the inorganic soils. The greater volume of large pores increased the water flux and aeration in the structural shrinkage phase. After intense WD cycles, the rigidity of large pores in organic-rich soils was improved, but the newly formed cracks in the inorganic soils were still unstable and could only reach a more rigid state after several WD cycles. Consequently, organic-rich soils are characterized by more gradual water potential vs. void ratio curves, while inorganic soils with a given natural structure show much steeper curve patterns compared with soils subjected to slight or no WD cycles.

	ACKNOWLEDGMENTS

 
X. Peng gratefully thanks the German Research Foundation (DFG PE1404/1), who provided the postdoctoral fellowship at Christian-Albrechts Univ., Kiel, Germany. Thanks are also given to Prof. M.B. Kirkham for the improvement of our English.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 
Current Address: Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, People's Republic of China

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Received for publication April 13, 2006.

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TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Peng, X. Articles by Smucker, A. Search for Related Content PubMed Articles by Peng, X. Articles by Smucker, A. GeoRef GeoRef Citation Agricola Articles by Peng, X. Articles by Smucker, A. Related Collections Structure and Properties Soil Physics