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a Inst. of Soil, Water, and Environmental Sci., The Volcani Center, Agricultural Research Organization, P.O. Box 6, Bet Dagan, 50250, Israel
b Faculty of Sci., Univ. of La Coruna, A Zapateira s/n, 15071 La 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 DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CEC, cation exchange capacity • DI, dispersion index • EC, electrical conductivity • IR, infiltration rate • MWD, mean weight diameter • OM, organic matter • SAR, sodium adsorption ratio • SEM, scanning electron microscope
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Lado et al. (2004) found that for aggregate sizes of <2 and 2 to 4 mm under absence of raindrop impact, the saturated hydraulic conductivity of the soil with high OM content (3.5%) was higher than that of the low-OM (2.3%) soil. These differences in the hydraulic conductivity between the two soils resulted from structural degradation that occurred to a greater extent in the low-OM soil than in the high-OM one. During the leaching of the soils with tap water [electrical conductivity (EC) = 0.9 dS m–1 and sodium adsorption ratio (SAR) = 2.5 (mmol L–1)0.5], aggregate slaking was the main mechanism that degraded the soil structure. Leaching the soils with deionized water led to further reductions of the saturated hydraulic conductivities of the low-OM soil with aggregate sizes of <2 and 2 to 4 mm, and of the high-OM soil with <2-mm aggregates. These decreases resulted from clay dispersion that was larger in the low- than in the high-OM soil.
When the soil surface is exposed to raindrop impact, the permeability of the soil is affected by seal formation (Morin et al., 1981; Ben-Hur and Letey, 1989). The seal formation reduces the infiltration rate (IR), thus increasing runoff (Morin et al., 1981) and may increase the soil loss (Ben-Hur et al., 1992). In general, the lower the aggregate stability at the soil surface, the higher the susceptibility of the soil to seal formation (Le Bissonnais, 1996) and to soil loss (Singer et al., 1982). Thus, under raindrop impact, the mechanisms that affect the soil hydraulic properties could be different from those in the absence of raindrop impact.
McIntyre (1958) found that a seal consists of two distinct parts: an upper skin seal attributed to compaction by raindrop impact, and a washed-in zone of decreased porosity, attributed to the accumulation of small particles. Agassi et al. (1981) suggested that the seal formation is a result of two complementary mechanisms: (i) a physical disintegration of surface soil aggregates caused mainly by the impact energy of the raindrops that leads to the formation of the upper skin layer; and (ii) the physicochemical dispersion of soil clays, which migrate into the soil with the infiltrating water and clog immediately beneath the surface to form the washed-in zone.
Organic matter content could decrease the susceptibility of the soil to seal formation. Le Bissonnais and Arrouays (1997) found that reduction of the organic carbon content of a loamy soil below 1.5 to 2.0% decreased the aggregate stability and the soil IR under rainfall simulator conditions. Ekwe (1991) used a rainfall simulator to measure the soil detachment from five different soils with OM contents ranging from 1.2 to 5.6%, and found that the soil detachment decreased significantly as the OM content increased. Likewise, Fullen (1991) indicated that loamy sand with OM contents <2% were very prone to erosion. Guerra (1994) used a rainfall simulator to measure IR, runoff, and soil loss in sandy loam soils with various OM contents, and found that the soil OM content played an important role in aggregate stability and soil erodibility. Guerra (1994) found that an OM content of 3% was a threshold value, below which the aggregates were unstable and the soil erodibility was high. However, in spite of the dominant effect of the OM content on seal formation, there is little information on the effects of the OM on the two mechanisms, the physical disintegration of surface soil aggregates and the physicochemical dispersion of soil clays (Agassi et al., 1981), that form the seal.
The initial size of aggregates in the soil is also an important factor in seal formation, IR, runoff, and soil loss (e.g., Moldenhauer and Kemper, 1969; Ekwe, 1991; Freebairn et al., 1991; Shainberg et al., 1997). Moldenhauer and Kemper (1969) and Freebairn et al. (1991) indicated that as the initial aggregate size of the soil increased, the IR during a rainstorm dropped more gradually because of a delay in seal formation. However, the final IR of the studied soils was independent of the initial aggregate size. Shainberg et al. (1997) studied the effects of aggregate size of two soils containing 465 and 190 g kg–1 clay and 2.07 and 1.49% OM, respectively. In both soils, the aggregate stability increased and the seal formation rate decreased when the initial aggregate size increased from <4 to between 9.5 and 12 mm.
Many studies have been focused on the effects of OM content and aggregate size on seal formation and soil loss. We hypothesized that there are interactions between aggregate size, OM content, and aggregate stability, and that these interactions affect seal formation, IR, and soil loss. However, little is known about these interactions and their effects on the above parameters. Thus, the objectives of the present study were (i) to investigate the effect of soil OM content and aggregate size on the mechanisms that form the seal; and (ii) to study the OM content and aggregate size interactions in seal formation and morphology, IR, and soil loss under raindrop impact conditions.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Fast-Wetting Aggregate Stability Test
The soil aggregate stability was determined by the fast-wetting method proposed by Le Bissonnais (1996). This method was chosen because in the rainfall simulator study, which is described below, the soils were under fast-wetting conditions (Levy et al., 1997). For each soil, aggregates of <2 or 2 to 4 or 4 to 6 mm were put in the oven at 40°C for 24 h, and 5 g of the oven-dry aggregates were gently immersed in a beaker containing 50 cm3 of deionized water for 10 min. The water was then sucked off with a pipette. The soil material was transferred to a 50-µm sieve that had previously been immersed in ethanol and gently moved up and down in ethanol five times to separate the <50-µm fragments from the >50-µm ones. The >50-µm fraction was oven dried and then gently dry sieved by hand on a column of sieves of mesh sizes 4.00, 2.00, 1.00, 0.50, 0.25, 0.10, and 0.05 mm. The weight of each fraction was then calculated; that of the <50-µm fraction was the difference between the initial weight and the sum of the weights of the other seven fractions. The aggregate stability for each soil sample was expressed by calculating the mean weight diameter (MWD) of the eight classes:
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i.
Rainfall Simulator Study
The IR and the soil loss were determined by means of a laboratory rainfall simulator (Morin et al., 1967). The typical mechanical parameters of the simulated rainfall were: rainfall intensity, 42 mm h–1; raindrop mean diameter, 1.9 mm; median drop velocity, 6.2 m s–1; and kinetic energy, 18.1 J m–2.
The soil samples were placed in perforated trays measuring 0.30 by 0.50 m and 0.02 m deep. For all the aggregate sizes tested, a 1-cm-thick layer of soil with aggregate size of <2 mm was packed on the bottom of the tray. Above this layer was packed a 1-cm-thick layer of soil with the tested aggregate size. The bulk densities of the upper layer for the <2-, 2- to 4-, and 4- to 6-mm aggregate sizes were 1.32, 1.06, and 0.98 Mg m–3, respectively. The packed soil tray was placed on an 8-cm-thick layer of coarse sand, in a box positioned under the rainfall simulator at a slope of 9%. The experimental treatments comprised the various initial aggregate sizes and the soil OM contents.
The soils in the rainfall simulator were wetted almost to saturation with tap water [EC = 0.9 dS m–1 and SAR = 2.5 (mmol L–1)0.5] introduced from below, and were then exposed to a simulated rainstorm of 80 mm of deionized water. Water percolating through the soil was collected and measured at various times during the rainstorm, to determine the IR. Likewise, the runoff from the entire rainstorm was collected and measured. The soil that was eroded from each soil tray was measured by drying the runoff samples and weighing the dry material. Because the length of the tray in the rainfall simulator was short (0.5 m), the measured soil loss could be considered because of interrill erosion (Meyer and Harmon, 1984). Nine grams of the eroded material from each soil tray were analyzed by the hydrometer method (Day, 1956) to determine the clay content in the sediment.
Scanning Electron Microscope Observations
After the rainstorms, the trays with the sealed soils were air-dried for a week. A piece of crust from the <2 and 4- to 6-mm aggregate size samples of each soil was carefully broken, along its natural planes of weakness, into representative fragments from the 0- to 15-mm increment. The top of the crust was mechanically stabilized by coating it with a gold layer. The crust sample was attached to the scanning electron microscope (SEM) stub with conductive carbon glue, and the fracture was covered by sputtering with a thin layer of gold, approximately 40 nm in thickness. The samples were then placed in the SEM and micrographs were taken.
Statistical Analysis
All the studies were conducted in three replicates, and the differences of the means and the interaction between studied parameters were tested with ANOVA as a complete randomized design. Separation of means was subjected to Tukey's honestly significant difference test (Steel and Torrie, 1960). 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 DISCUSSIONCONCLUSIONSREFERENCES |
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The seal formation process comprises two main stages (Mcintyre, 1958; Agassi et al., 1981; Morin et al., 1981): (i) breakdown of the aggregates and dispersion of clay at the soil surface; and (ii) rearrangement of these detached particles into a seal layer. The small, detached particles at the impacted soil surface can form a compacted sealing skin with low hydraulic conductivity, whereas the dispersed clay can move from the soil surface with the infiltrating water to form the washed-in zone (Mcintyre, 1958; Gal et al., 1984; Wakindiki and Ben-Hur, 2002). Ben-Hur and Letey (1989) found that clay dispersion at the soil surface significantly controls the final IR and the rate of seal formation. Clay dispersion at the soil surface can occur when rainwater (distilled water) reduces the electrolyte concentration at the soil surface below the flocculation value (Agassi et al., 1981).
Soil dispersibility under rainfall conditions was expressed as a dispersion index (DI), which was calculated by dividing the percentage of clay in the runoff sediments by that in the original soil (Stern et al., 1991). A DI value of 1 indicates that no clay dispersion occurred at the soil surface during the rainstorm. In contrast, when the clay fraction at the soil surface is dispersed, the clay percentage in the sediments should be higher than that in the original soil because the clay particles are more easily transported by the overland flow than the bigger silt and sand particles. In this case, the DI > 1: the higher the DI, the more dispersive the soil.
Dispersion indices for the various aggregate sizes of the two soils are presented in Fig. 3
. The DI values of the high-OM soil for all three aggregate sizes were 
1, which indicates that in this soil, clay dispersion at the surface during the rainstorm was insignificant; it is likely that the cementing effect of the 3.5% OM in this soil limited the clay dispersion at the soil surface. In contrast, the DI values of the low-OM soil for all three aggregate sizes were >2.1, significantly higher than that of the high-OM soil (Fig. 3), indicating that high clay dispersion had occurred at the low-OM soil surface during seal formation.
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1.1 mm, and that this crust was comprised of three parts (Fig. 4): (i) the uppermost layer, from the 0- to 0.1-mm depth, containing particles 0.05 mm in size with no fine materials between them; (ii) a transition layer, from the 0.1- to 0.45-mm depth, containing a mixture of 0.05-mm particles and fine materials; and (iii) the innermost layer of the crust, from 0.45 to 1.1 mm, which was visually very dense, probably because of the accumulation of dispersed clay that was washed in from the upper layers. This last layer is the washed-in zone. In the high-OM soil with <2-mm aggregates (Fig. 4), the thickness of the crust was 0.8 mm, and it comprised some relatively large particles, 0.1 mm in size, mixed with fine materials. In this crust, no distinct washed-in zone was observed.
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0.7 mm, and it was mainly comprised of large particles, 0.05 to 0.20 mm in size, with some fine materials but less than in the crust on the low-OM soil. No distinct washed-in zone was observed in these two crusts. Observations of the soil surfaces before and after the rainstorm (Fig. 2) indicated that the rainstorm caused more extensive aggregate breakdown in the low- than in the high-OM soil for both aggregate sizes. In the low-OM soil with both the <2-mm and 4- to 6-mm aggregates, and in the high-OM soil with the <2-mm aggregates, most of the aggregates at the soil surface were broken and a smooth crust was developed. However, more unbroken aggregates were observed at the surfaces of the low-OM soil with 4- to 6-mm aggregates and of the high-OM soil with <2-mm aggregates than at the surface of the low-OM soil with <2-mm aggregates. In contrast, in the high-OM soil with 4- to 6-mm aggregates, many fewer aggregates were broken down, and a less continuous crust was developed than on the other soils.
The increase in the OM content in the soil increased the aggregate stability (Table 2) and decreased the soil dispersivity (Fig. 3). Therefore, it can be suggested from the photographs of the soil surface after the rainstorm (Fig. 2) and the micromorphologies of the crusts (Fig. 4 and 5) that the final IR values were lower in the low- than in the high-OM soil because (i) there was more extensive breakdown and dispersion of the aggregates at the surface of the low-OM soil than at that of the high-OM soil, so that a more continuous crust was formed on the former soil; (ii) the rearrangements of the detached and dispersed particles in the crust differed between the two soils, so that a thicker, higher-density crust (in some cases, with washed-in zone) formed on the low- than on the high-OM soil.
The final IRs and the rates of seal formation in the two soils were also affected by the initial aggregate sizes (Fig. 1 and Table 3). In general, the larger the aggregate size, the greater the final IR and the lower the rate of the seal formation. The effect of the initial aggregate size on the seal formation rate was more pronounced than its effect on the final IR. The results presented in Table 3 also indicate that there was a significant interaction between soil OM content and aggregate size, in their effects on the final IR and seal formation rate. The effects of the aggregate size on the seal characteristics (seal formation rate and final IR) were more pronounced in the high- than in the low-OM soil (Fig. 1 and Table 3).
This interaction between soil OM content and aggregate size could be explained as follows. Because of the low aggregate stability (Table 2) and the high dispersivity (Fig. 3) of the low-OM soil, the breakdown and dispersion of the aggregates at the surface of this soil, under raindrop impact, were extensive even for the large aggregate size (4–6 mm). Consequently, a well-developed seal was formed on the low-OM soil for all aggregate sizes (Fig. 2, 4, and 5), and the effect of the initial aggregate size on the IR was negligible (Fig. 1). In contrast, the higher aggregate stability (Table 2) and the lower dispersivity (Fig. 3) of the high-OM soil limited the breakdown and dispersion of the aggregates at its surface. Therefore, the difference in IR caused by the presence of large (4- to 6-mm) or small (<2-mm) aggregates under raindrop impact was relatively great (Fig. 1).
Freebairn et al. (1991) and Shainberg et al. (1997) found that for various smectitic soils with <3.1% OM content, initial aggregate size had significant effects on the rate of seal formation, but not on the final IR values. These results are different from the results obtained in the present study (Fig. 1 and Table 3), particularly, for the high-OM soil. This was probably because the differences in the mineralogy of these soils. The dominant clays in the soils of the present study were vermiculite and kaolinite, but in the soils of Freebairn et al. (1991) and Shainberg et al. (1997), smectite was dominant. This smectite probably decreased the aggregate stability (Stern et al., 1991; Wakindiki and Ben-Hur, 2002) of the soils of Freebairn et al. (1991) and Shainberg et al. (1997) and, therefore, the aggregate breakdown by the raindrop impact in these soils was more extensive even for the soils with high OM content, and a well-developed seal with low final IR was formed on all the soils with all aggregate sizes.
The interrill soil losses of the two soils, with the various aggregate sizes, are presented in Fig. 6 . For each aggregate size, the soil loss was significantly larger in the low- than in the high-OM soil (Fig. 6). The soil losses ranged from 4.5 x 10–3 to 6.6 x 10–3 kg m–2 mm–1 for the low-OM soil and from 1.2 x 10–3 to 3.8 x 10–3 kg m–2 mm–1 for the high-OM soil. In both soils, the soil loss was significantly greater for the <2-mm aggregates than for the other two aggregate sizes, but the soil losses for the two larger sizes did not differ significantly from one another. Unlike the final IR, no significant interaction was found between soil OM content and aggregate size in their effects on soil loss.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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There was an interaction between OM content and aggregate size in seal formation and IR. Because of the low aggregate stability and the high dispersivity of the low-OM soil, the breakdown and dispersion of the aggregates at the surface of this soil, under raindrop impact, were extensive even for the large aggregate size. Consequently, a well-developed seal was formed on the low-OM soil for all aggregate sizes, and the effect of the initial aggregate size on the IR was negligible. In contrast, the higher aggregate stability and the lower dispersivity of the high-OM soil limited the breakdown and dispersion of the aggregates at its surface. Therefore, the differences in IR under raindrop impact caused by the presence of large or small initial aggregates were relatively great in the high-OM soil.
There is a practical aspect of this interaction between OM content and aggregate size. Cultivation of soil with high OM will be effective because in this soil, the large aggregates will remain stable during the rainy season and will maintain relatively high IR. In contrast, in soil 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 rainy season.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Received for publication July 17, 2003.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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= 0.05) among the treatments.
