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a Institute of Soil, Water and Environmental Sciences, the Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan 50250, Israel
b Faculty of Sciences, Univ. of A Coruna, A Zapateira s/n, 15071 A Coruna, Spain
* Corresponding author (meni{at}volcani.agri.gov.il).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: CEC, cation-exchange capacity • EC, electrical conductivity • ESP, exchangeable sodium percentage • OM, organic matter • SAR, sodium adsorption ratio
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Shainberg and Letey (1984) and Ben-Hur and Letey (1989) indicated that, in the absence of raindrop impact and with no deposition of kinetic energy on the soil surface, the soil permeability depends on the hydraulic conductivity (K) of the bulk soil. In contrast, when the soil surface is exposed to raindrop impact, the infiltration rate is affected by seal formation (Morin et al., 1981): the water drop impact energy causes the soil surface to break down and consequently a seal is formed. Under these conditions, the infiltration rate decreases, and the runoff and soil erosion increase. The present paper is focused on Ks in the absence of raindrop impact and its overall objective is to study the effects of the interaction between OM content and soil aggregate size on the processes and mechanisms that lead to the changes in the soil structure during wetting and leaching.
In the absence of raindrop impact, clay swelling and dispersion are two major mechanisms that have been hypothesized to cause the Ks reduction in soils when they are leached with deionized water (Shainberg and Letey, 1984). Frenkel et al. (1978) found a significant reduction in the Ks of montmorillonitic soils with exchangeable sodium percentage (ESP) values ranging from 10 to 30, when exposed to a 10-mmol L–1 electrolyte solution. They suggested that plugging of pores with dispersed clay particles was the main cause of Ks reduction. In contrast, Pupisky and Shainberg (1979) suggested that at electrolyte concentrations above 10 mmol L–1, clay swelling is the main process that causes the Ks reduction, whereas at solution concentrations below the flocculation value, clay dispersion and migration of the dispersed particles into conducting pores are the dominant processes. Alperovitch et al. (1985) also concluded that clay dispersion was the main cause of Ks reduction in montmorillonite-sand mixtures leached with deionized water, but that swelling was the main factor in Ks reduction when the mixtures were leached with 10-mmol L–1 electrolyte solution.
Aggregate disintegration caused by a slaking process during wetting is well documented (Le Bissonnais and Arrouays, 1997; Abu-Sharar et al., 1987; Loch, 1994). Slaking occurs when the aggregate is not strong enough to withstand the stresses produced by differential swelling, entrapped air, rapid release of heat during wetting, and the mechanical action of moving water (Emerson, 1977; Collis-George and Green, 1979; Kay and Angers, 1999). In spite of the importance of the slaking process in aggregate disintegration, only a few studies have addressed the effects of this process on the Ks. Shainberg et al. (2001) studied the effects of prewetting rate on Ks of various soils with differing textures and ESPs, and found that the effect of prewetting rate on Ks intensified as the clay content in the soil increased and the ESP decreased. They attributed the decrease in Ks with increasing prewetting rate to slaking of the aggregates in these soils.
The OM content has been found to be one of the main factors controlling the aggregate stability of soils with low ESP (Emerson, 1977; Elliot, 1986; Golchin et al., 1995). Soil OM compounds bind the primary particles in the aggregate, physically and chemically, and this, in turn, increases the stability of the aggregates and limits their breakdown during wetting. Chaney and Swift (1984) used wet sieving to measure the aggregate stability of 26 agricultural soils with differing properties, and they found a high positive correlation between aggregate stability and OM content, suggesting that OM is an important controlling factor. Benito and Diaz-Fierros (1992) studied the effects of various cropping systems on the structural stability of soils containing various OM contents. They found that a decrease of OM content in the soil led to a decrease in soil structure stability.
Emerson (1977) suggested that OM stabilized the aggregates mainly by forming and strengthening bonds between the particles within them. Tisdall and Oades (1982) classified these organic binding agents into: (i) transient—mainly polysaccharides; (ii) temporary—roots and fungal hyphae; and (iii) persistent—resistant aromatic components associated with polyvalent metal cations, and strongly sorbed polymers. Tisdall and Oades (1982) concluded that macroaggregates (>0.25 µm in diameter) are weaker than microaggregates (<0.25 µm in diameter) because the former are stabilized by transient binding, such as roots, hyphae, and polysaccharides, whereas the latter are bound by persistent aromatic humic material associated with amorphous Fe and Al compounds. Oades and Waters (1991) also indicated that there is an aggregate hierarchy in which microaggregates are associated by the binding action of hydrous oxides of Fe and Al and OM, to form macroaggregates.
Although many studies have determined the effects of OM content on aggregate stability and on the mechanical properties of soil (e.g., Ekwe, 1991; Guerra, 1994; Zhang and Hartge, 1995; Le Bissonnais and Arrouays, 1997; Paz, 2000; Castro Filho et al., 2002), few have investigated the effects of OM content on soil Ks. Likewise, there is little information about the physical mechanisms that cause the Ks reduction during soil wetting and leaching (Lebron et al., 2002). Black and Abdul-Hakim (1985) found that 2 yr of wheat growing, with the consequent decrease of soil OM content, decreased the soil permeability significantly more than pasture growing in the same type of soil. Mbagwu and Auerswald (1999) measured the percolation stability (the amount of water percolating during 10 min through soil samples packed in tubes) of soils under various crops, and found that in soils with OM content ranging from 0.12 to 5.0%, the percolation stability of the soils was positively correlated with OM content. Auerswald (1995) showed that percolation stability increased as OM increased, and attributed this to the effect of OM in preventing aggregate disintegration caused by the pressure of air entrapped during the wetting process.
The aggregate-size distribution in the soil determines the soil bulk density and the volume and shape of the pores, which in turn, affect the soil Ks. However, because there are interactions between aggregate size, OM content, and aggregate stability, it is hypothesized that these interactions could affect the soil Ks. Thus, the specific objective of the present study was to investigate the effects of soil OM content, aggregate size, and their interaction on the mechanisms that degrade the structure of soils with low ESP, and on their Ks under wetting and leaching with water of different qualities and without raindrop impact.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Samples of the various aggregate size groups for each soil were subjected to the three tests described below.
Hydraulic Conductivity Experiment
The Ks experiments were performed in 8-cm long plastic cylindrical columns with an internal diameter of 5 cm. The column was filled with the soil sample to a thickness of 2.0 cm, on top of a 5-cm layer of acid-washed coarse quartz-sand. The soil sample for each column was divided into two equal portions, each of which was packed gently in the column to achieve a quite uniform bulk density. The bulk densities in the columns of the low and high-OM soils for each aggregate size were similar: 1.32, 1.06, and 0.98 Mg m–3 for the columns with the aggregate-size groups of <2, 2 to 4, and 4 to 6 mm, respectively. The thickness of the soil and the sand layers in the column were similar to those used in a laboratory rainfall simulator (Ben-Hur et al., 1985). The Ks values measured in the columns could therefore be compared with the infiltration rate measured under simulated rainfall. In a previous study, no differences were found between the Ks values in columns with soil layer thicknesses of 5 and 2 cm. This indicates that there was no significant edge effect in the Ks measurements with a 2-cm layer. A filter paper (Whatman 42, Whatman Ltd., Maidstone, UK) was placed on the top of the soil layer in the column to prevent mixing and turbulence when water was added. The Ks of the coarse sand alone was 1041 mm h–1, with standard deviation of ±62.
Each column was initially wetted from the bottom with tap water with electrical conductivity (EC) of 0.9 dS m–1 and sodium absorption ration (SAR) of 2.5 (mmol L–1)0.5 at an average rate of 10 mm h–1. After saturation was reached, the direction of the flow was reversed so that the water entered from the top of the column, and about four pore volumes of tap water, followed by a certain volume of deionized water were percolated through each column under saturation conditions. The leaching with deionized water was continued until steady state was achieved with respect to both the EC of the leachate solution and the Ks of the soil.
The Ks was determined by means of a constant-head device (a Marriot bottle). The outflow solution was collected with a fraction collector and its volume and EC were measured. Three replicates (three columns) of each treatment were performed. An extra column for each soil and each aggregate size was left for 7 d at room temperature with 20°C to drain and dry after the initial wetting from the bottom with tap water. Then, the plastic cylinder was cut gently, and the soil profile in the column was photographed.
Swelling and Slaking Tests
These tests were conducted for both soils in the 2- to 4 and 4- to 6-mm aggregate sizes. The <2-mm aggregates were not tested because they were too small to yield reliable results. In each test, 20 air-dry aggregates were placed in a Petri plate (Fig. 1)
, and then the image area of each aggregate was scanned and measured using the UTHSCSA imagetool program (University of Texas Health Science Center, San Antonio, TX). The aggregates were considered as spheres, and their volumes were calculated from the radii of the scanned area. After scanning, the aggregates were wetted for 2 min with fog-type rain, to avoid water-drop impact, with an intensity of 35 mm h–1. The excess water between the aggregates was carefully removed with absorbent paper. After 5 min, during which the aggregates remained at rest, the aggregate slaking and swelling were determined. A slaked aggregate was defined as one that had been broken down to microaggregates, and a swollen aggregate was defined as one whose volume had increased after the wetting, but that had not broken down (Fig. 1). Three replicates (three Petri plates) for each treatment were performed in this test.
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Dispersion Test
The soil dispersivity was determined as proposed by Gupta et al. (1984). Two grams of each soil, of each aggregate size were suspended in 0.02 L of liquid in a 0.04-L centrifuge tube. The liquids were deionized water and two Ca and NaCl solutions with EC of 0.2 and 0.5 dS m–1 and SAR of 0.63. The centrifuge tubes containing the suspension were shaken in a reciprocal shaker for 30 min at 20 rpm, and centrifuged immediately after at relative centrifugal force of 960 for 5 min. The amount of dispersed clay in the turbid supernatant was determined by measuring the absorbance at 420 nm with a spectrophotometer. The concentration of the dispersed clay was calculated by using calibration curve of absorbance vs. clay concentration in the suspension prepared for each soil type. The dispersion value for each soil sample was determined as the percentage of clay content in the soil samples that was suspended. Each treatment was tested in three replicates (three centrifuge tubes).
Statistical Analysis
All the studies were conducted in three replicates, and the differences of the means and the interaction between studied parameters were tested using analysis of variance (ANOVA) as a complete randomized design. Separation of means was subjected to Tukey's honestly significant difference test (Steel and Torrie, 1981). All tests were performed at the 0.05 significance level.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The EC values of the leachates of the soils with the two different OM contents are presented in Fig. 3 as functions of the leachate volume, for two different water qualities and for aggregate sizes of <2 and 2 to 4 mm. No significant differences were observed between the patterns of EC values in the leachates from the various combinations of soils and aggregate sizes (Fig. 3). During the leaching with tap water, the EC in the leachate was fairly constant approximately 1.3 dS m–1. When the tap water was replaced with deionized water, the EC value of the leachate decreased sharply to an average, steady-state value of 0.05 dS m–1 (Fig. 3).
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During the leaching of the soils with tap water, the EC of the leachate was approximately 1.3 dS m–1 (Fig. 3), which is above the flocculation value of the soils (van Olphen, 1977; Shainberg and Letey, 1984). Therefore, it can be concluded that the occurrence of lower Ks values in the low-OM soil than in the high-OM soil during the wetting and leaching with tap water (Fig. 2) could not have been due to clay dispersion, but could have resulted from the swelling and/or the slaking process.
The slaking and swelling values of the two soils, with tap and deionized water, and aggregates sizes of 2 to 4 and 4 to 6 mm are presented in Table 2. For both water qualities and both aggregate sizes, the slaking values of the low-OM soil were significantly higher than those of the high-OM soil: >93 and <6.7%, respectively (Table 2). However, no significant differences were found between the slaking values in each soil, with the different aggregate sizes and water qualities. These differences in the slaking values between the high-OM soils and low-OM soils most likely resulted from the differences in the OM contents in these two soils. In the former soil, the high OM content acted as a cement, holding the particles in the aggregate together against the slaking forces, whereas in the low-OM soil, the OM content was too low to prevent the slaking. These results are consistent with the studies of Chenu et al. (2000) and Rachman et al. (2003), who observed that greater aggregate stability occurred in soils with higher OM.
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The visual effects of the slaking process on the soil structure after the initial wetting of the soils columns from the bottom with tap water can be seen in Fig. 4 , which shows the significantly higher breakdown of the aggregates in the low-OM soil than in the high-OM soil for the three aggregate sizes. Most of the aggregates in the high-OM soil were not broken, whereas most of those in the low-OM soil were slaked. With the 4- to 6-mm aggregate size in the low-OM soil, however, the slaking occurred mostly in the aggregates in the lower part of the soil column.
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Replacing the percolating tap water with deionized water did not significantly change the Ks of the high-OM soil with 2- to 4-mm aggregates (Fig. 2). In contrast, in all the other soil/aggregate-size combinations, leaching with deionized water decreased the Ks values gradually. In the low-OM soil with both aggregate sizes, the average steady-state Ks values at the end of the leaching with deionized water were significantly lower than those measured during the leaching with tap water (Fig. 2). This decrease in Ks could have been caused by an increase in the soil swelling and/or clay dispersion that resulted from the diminution of the electrolyte concentration in the soil solution.
Clay dispersion values, as determined by the dispersion test, of the two soils with the various aggregate sizes are presented in Fig. 5
as functions of the suspension EC. For both soils and most EC values, no significant differences in the dispersion values were observed between the aggregate-size groups in each soil, except for the high-OM soil and EC 
0 (Fig. 5). In this case, the dispersion value for the <2-mm aggregates was significantly higher than that for the other aggregate sizes. For both soils, a decrease of the EC from 0.5 to 0.2 dS m–1 did not change the dispersion values significantly. In contrast, a further decrease of the EC to approximately 0 significantly increased the dispersion value of both soils, with all the various aggregate sizes; however, the dispersion value at EC 0 was significantly higher in the low-OM than in the high-OM soil (Fig. 5). These results indicate that in spite of the very low ESP of the studied soils (Table 1), clay dispersion occurred when the electrolyte concentration in the suspension was low, and increasing the OM content in the soil decreased the clay dispersion.
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The interaction between the aggregate size and the OM content, in their effects on the Ks (Fig. 2 and Table 1), could be explained as follows. Because of the high structural stability of the high-OM soil (low slaking and dispersion values, Table 2 and Fig. 5), the wetting and leaching of this soil had little effect on its structure (Fig. 4). In this case, the difference in porosity between the soils with 2- to 4- and <2-mm aggregates remained high after the soil wetting and leaching, and this, in turn, led to a large difference in Ks values between the soils with the respective aggregate sizes (Fig. 2). In contrast, in the low-OM soil, which had a low structural stability, the high aggregate slaking (Table 2 and Fig. 4) and the high clay dispersion (Fig. 5) that occurred during the wetting and leaching, led to extensive structural degradation and to a sharp decrease in the Ks. However, in this soil with the <2-mm aggregates, the porosity and the Ks were already relatively low before the wetting and leaching, therefore the structural degradation in this case and its effect on the Ks were small. In contrast, in the low-OM soil with 2- to 4-mm aggregates, the porosity before the wetting and leaching was relatively high, therefore, the structural degradation during the wetting and leaching processes decreased the Ks sharply and significantly. Consequently, the difference in the Ks between soils with aggregate sizes of <2 and of 2 to 4 mm after the wetting and leaching was smaller in the low-OM than in the high-OM soil. For the same reason, the difference in Ks between the low-OM and high-OM soils was greater in those with 2- to 4-mm aggregates than in those with <2-mm aggregates. There is a practical aspect of this interaction effect: cultivation of soil with high OM will be effective because in this soil, the large aggregates will remain stable for a long time during wetting and leaching of the soil. In contrast, in soils with low OM, the cultivation will have only a short-lived effect, because in this soil, most of the large aggregates will be broken down and dispersed at the beginning of the wetting and leaching process.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication January 21, 2003.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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