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SOIL PHYSICS

Water Repellency of Aggregate Size Fractions of a Volcanic Ash Soil

^a Graduate School of Science and Engineering, Saitama Univ., 255 Shimo-okubo, Sakura-ku, Saitama, 338-8570, Japan
^b Environmental Engineering Section, Dep. of Biotechnology Chemistry, and Environmental Engineering, Aalborg Univ., Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
^c Dep. of Agroecology, Univ. of Aarhus, P.O. Box 50, DK-8830 Tjele, Denmark

^* Corresponding author (kawamoto{at}post.saitama-u.ac.jp).

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Water repellency (WR) of soils can induce hydrological problems such as reduced water infiltration, enhanced surface runoff and erosion, and the forming of preferential flow patterns in soil. Although soil organic matter (SOM) may cause both soil aggregation and a hydrophobic-material-coating of aggregates, little is known about WR in aggregated soils. We investigated the degree of WR as functions of volumetric water content () and pF [= log (-; soil-water potential)] for sieved fractions of a volcanic ash soil samples from different depths with varying soil organic carbon (SOC) contents of between 1.1 and 12.3%. Water repellency was determined by the molarity of ethanol droplet (MED) test. Water repellency was observed in the samples with SOC 4.6%, and the effects of sample pretreatments (pressure chamber desorption, air-drying at 20°C, and oven-drying at 60°C) on the degree of WR were small. The degree of WR varied greatly with both SOC content, , and pF. Peaks of WR were observed when the water retained in intra-aggregate pores was drained to a moderate extent with the corresponding pF values located in a relatively narrow range from 3.2 to 3.6. This indicates that the hydrophobicity of high-SOC aggregate surfaces might be enhanced the most at a specific soil-water potential. Examining relations between water repellency parameters, the integrated areas below the WR- and WR-pF curves were useful indexes for characterizing WR, and linear relationships between the integrated areas and both SOC and water contents at maximum repellency were found.

Abbreviations: MED, molarity of ethanol droplet • SOC, soil organic carbon • SOM, soil organic matter • WR, water repellency • VG, van Genuchten

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Water repellency of soils has been observed in various soil types such as sandy, loamy, clayey, peat, and volcanic ash soils (Wallis and Horne, 1992; Dekker and Ritsema, 1994). Water repellency can have a range of soil hydrological consequences such as reduction of infiltration rates (DeBano, 1971; Wallis et al., 1990a), acceleration of surface runoff and erosion (Osborn et al., 1964), and occurrence of fingering flow (Ritsema et al., 1993). Water repellency is also closely related to suppression of germination rate and plant growth (Wallis et al., 1990b). Thus proper management of WR is important to optimize sustainability and productivity of farming systems in water-repellent soils (Blackwell, 2000).

Water repellency can be caused by coverage of particles by hydrophobic surface films produced by hydrophobic organic matter and waxes from plant leaves, or the growth of microorganisms (Roberts and Carbon, 1972; Wallis and Horne, 1992; Bisdom et al., 1993). Some studies have found the degree of WR to be positively correlated with SOM/soil organic carbon (SOC) (Chenu et al., 2000; Hallett et al., 2001; Mataix-Solera and Doerr, 2004). In contrast, for example, Ellerbrock et al. (2005) found that the increase in SOC imparted both increase and decrease in the degree of WR for forest soils. Based on measurements of hydrophobic and hydrophilic functional groups of SOM for extraction fractions, they suggested that spatial orientation of the SOM such as hydrophobic molecular layers on the outer surface controlled the soil wettability and assumed that hydrophobic character of the SOM could be more effective under relatively low and high SOC conditions than at medium SOC content. The hydrophobic organic matter is also important in soil aggregation and aggregate stability. Piccolo and Mbagwu (1999) found that the hydrophobic components of organic matter were more effective to bind aggregates than those of hydrophilic compounds. Goebel et al. (2005) showed that the increase of WR of soils decreased the initial aggregate breakdown (slaking).

The degree of WR of a soil and its hydrophobic character of SOM are greatly influenced by water content of soils, ambient relative humidity, and corresponding soil-water potential (Dekker and Ritsema, 1994; Roy and McGill, 2000; Doerr and Thomas, 2000; Doerr et al., 2002). In general WR varies nonlinearly with the soil water content. King (1981), for example, showed that the WR increased with increasing water content from air-dry to wilting point [ = pF 4.2; where pF = log (-; soil-water potential in cm H₂O)], reached the maximum WR (peak) around the wilting point, and then decreased rapidly to zero with increasing water content to field capacity. De Jonge et al. (1999) examined effects of water contents on the degree of WR using the MED test for sandy soils, and showed that there were two types of curves that describe the WR as a function of water content: a single-peak curve where the maximum WR was recorded at intermediate water content (the soil became wettable at water contents near zero and at higher water contents); and a double-peak curve where the two maximum WR were recorded at water contents near zero and at intermediate water content (the soil became wettable at higher water contents). Goebel et al. (2004) assessed the soil wettability by the advancing contact angle from the capillary rise method, and showed that the degree of WR for aggregated samples (most of aggregates were larger than 1 mm) was enhanced the most at an intermediate soil-water potential (pF 2.5).

Several different kinds of mechanisms that may explain the increase/decrease in WR depending on water content relative humidity, and the corresponding soil-water potential have been proposed. Ma'shum and Farmer (1985) and Wallis et al. (1990a) speculated that the nonlinear behavior in WR with increasing water content may have been facilitated by changes in the molecular conformation of the SOM responsible for WR. Goebel et al. (2004) assumed that changes in the surface free energy of soil particles caused by the formation of thin water film as indicated by Derjaguin and Churaev (1986) had occurred in the experiments at various water potentials. Jex et al. (1985) also reported the increase in WR with increasing relative humidity due to an enhanced microbial activity especially in case of long-term equilibration periods at relative humidity of 100%.

It is well known that the organic-rich surface layers of volcanic ash soils (Andisols) have a highly aggregated soil structure that retains water in both inter-aggregate and intra-aggregate pores (Maeda and Soma, 1986; Iwata et al., 1995). Due to the high degree of soil structure, water-repellent aggregated soils would exhibit a WR curve as a function of water content/water potential different from normal water-repellent sandy soils. For example, the WR of the poorly structured sandy soils will be caused by the coverage of granules by hydrophobic films, while the WR of the highly structured aggregates will be influenced not only by the hydrophobic coverage of aggregates (i.e., hydrophobicity in inter-aggregate pores) but by the degree of hydrophobicity in intra-aggregates pores. Furthermore, as reported by Capriel et al. (1995), the hydrophobic character of soil organic compounds will be different between sandy soils and aggregated soils. However, only limited information about WR of aggregated soils are available, and the relationship between the degree of WR and soil structure is not fully understood. Recently a detailed parameter study on characterizing WR in volcanic ash soils from La Gomera Island (Canary Islands, Spain) was presented by Regalado and Ritter (2005). They investigated the water dependent WR using the MED test and discussed correlations among several WR parameters. They found that the integrated area below the WR curve (WR as a function of water content; see Fig. 1 in their study) introduced by de Jonge et al. (1999) was strongly correlated with not only the SOM content but also the water content at minimum WR. They concluded that the integrated area below the WR curve is a key index for characterizing WR.

View larger version (10K): [in this window] [in a new window]   Fig. 1. Soil water characteristic curves for four soil samples. Fitted curves with the bimodal van Genuchten soil-water retention model (Durner, 1994) by Eq. [1] are given. The s values are plotted at pF = 0.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Sampling Site and Soil Materials
Soil samples from different depths of a single soil profile down to 0.3-m depth were used in this study (Table 1 ). The sampling site was a forested hill-site at Fukushima in northeastern Japan (37°08' N, 140°09' E). Red pine trees (Pinus densiflora), bamboo grass, and various understory species coexisted at the site. The soil was a volcanic ash soil (Andisol) originating from the Nasu-volcanic zone. The soil profile down to 0.3-m depth consisted of two layers: the A horizon from 0 to 0.12 m was a black surface soil and the structure was highly aggregated with numerous macropores. The B horizon below 0.12 m was a light-brown subsoil, and the structure was blocky with some macropores as a result of root decay. The field dry bulk density and total porosity of undisturbed soils determined by 100-cm³ cores were 0.57 and 0.77 Mg m^–3 at 0.05-m depth, and 0.54 and 0.80 Mg m^–3 at 0.2-m depth. The field water contents in the soil profile down to 0.3-m depth were around 0.45 m³ m^–3. Soil sampling was performed in April 2005 after a period with no rainfall of at least 2 wk. The sampled materials were stored in refrigerator at 4°C until used for laboratory tests.

View larger version (15K): [in this window] [in a new window]   Fig. 2. Chart of pretreatment procedures for the soil samples before the molarity of ethanol droplet (MED) tests. "Soil samples at pF 3.3 and 4.1" are soil samples with volumetric water contents corresponding to pF 3.3 and 4.1 in the soil-water retention curve. The water repellency (WR) curves (see Fig. 3) are depicted by using data for oven-dried samples at 60°C (subchart given by the gray boxes).

Figure 1 implies that all soil samples were aggregated containing both intra-aggregate and inter-aggregate pores (according to the bimodal form of the curves) and the soil water was drained to a large extent around potentials at pF = 1.5 and 3.2. The measured values were fitted with the bimodal van Genuchten (1980) (VG) soil-water retention model (Durner, 1994),

[1]

where is volumetric water content (m³ m^–3), _r is residual water content (m³ m^–3), _s is volumetric water content at saturation, is soil-water potential (cm)_1,2 , n_1,2 , m_1,2(=1–n_1,2) are fitting parameters, and w_1,2 (0 < w_1,2 < 1; w_1,2 = 1) is weighting parameter. The first and second brackets of the right-hand side of Eq. [1] represent the volumetric water content of intra-aggregate pores, _intra, and the volumetric water content of the inter-aggregate pores, _inter, respectively.

Fitted parameters for the bimodal van Genuchten (VG) soil-water retention model are shown in Table 2 . The model fitted the measured values well for each soil depth (soil sample). The weighting parameters,w₁ and w₂, were around 0.5, indicating that the soil pores were composed equally of the intra-aggregate and inter-aggregate pores. The fitting curves of the bimodal VG soil-water retention model were used for converting to pF [= log (–)] to describe the WR as a function of soil-water potential.

Pure water and different concentration of ethanol solutions of 0.0 to 9.0 molarity in 0.2 molarity steps were used in the measurements. The liquid surface tension of pure water was taken as 0.072 N m^–1. Conversion from the molarity of ethanol solution (>0.2 molarity) to the WR (surface tension) was done by using an equation proposed by Roy and McGill (2000): y = [61.05 – 14.75ln(x + 0.5)/1000], where x is the molarity of ethanol solution and y is the surface tension (N m^–1).

In most WR tests, the soil samples are dried at 60°C followed by 48 h of equilibration at room temperature (Dekker and Ritsema, 1994; de Jonge et al., 1999). The degree of WR, however, is dependent on drying temperature and the pretreatment procedures for the soil samples before the MED test and needs careful consideration (e.g., John, 1978; Dekker and Ritsema, 1994; Franco et al., 1995; Ritsema and Dekker, 1996; Ritsema et al., 1997; Dekker et al., 1998; Ziogas et al., 2005). Overall, the previous studies did not find consistent evidence of the effect of drying temperature on the degree of WR. Franco et al. (1995) reported differences in WR of samples dried at 25, 70, and 105°C and found that the WR of samples were highest when dried at 105°C, while Ziogas et al. (2005) assessed the differences in WR of samples dried at 30, 60, and 105°C and found that drying at higher temperatures led to a decrease and disappearance of the WR. Ritsema et al. (1997) found no differences in the degree of WR for dune sands dried at 25, 45, and 65°C.

Pretreatment procedures for the soil samples in this study are shown in Fig. 2. Soil samples with different volumetric water contents () below the field water content were obtained by drying at 20, 60, or 105°C for different amounts of time followed by 48-h equilibration at room temperature. Drying at 20°C and equilibration was achieved in a constant temperature room at 20°C, and drying at 60 and 105°C were conducted in an oven. About 50 g of soil at field water content was placed on a ceramic plate and put in an oven (60 or 105°C) for the time necessary to reach a given water content. For drying at 60 and 105°C, it took about 0.1 to 3 h to obtain a given . Air-dried samples heated at 60 and 105°C were prepared by drying for 24 h in an oven and equilibration at room temperature of 20°C. For air-drying at 20°C, it took >2 wk to reach water contents at pF 3.3 and pF 4.1 and about 1 mo to obtain air-dry conditions. To examine the detailed effects of soil-water content on the degree of WR, a series of volumetric water contents () from 0.02 m³ m^–3 (air dry) to 0.45 m³ m^–3 (close to the field water content) with steps of 0.02 m³ m^–3 were prepared using soil samples dried at 60°C. For comparison of effect of drying methods on the degree of WR, soil samples at pF 3.3 and pF 4.1 were also prepared by desorption (after water saturation) in a pressure chamber at 20°C (Fig. 2).

After pretreatment, soil samples were packed in an acrylic cylindrical ring (5 cm i.d. and 2 cm height) with the same dry bulk density of 0.56 Mg m^–3 as in the soil-water retention tests. Here, the weight of wet soil packed in the ring was calculated by ring volume, dry bulk density, and gravimetric water content of each soil sample (determined before packing). Special care was taken to minimize the forces exerted on the aggregates in the cylindrical ring at repacking. After repacking, all MED tests were conducted in a constant temperature room at 20°C. In the MED tests, at least three droplets of ethanol solution were placed at different positions on each sample and the median result was used for WR (surface tension) of the sample.

Additionally to the measurements on different soil-water content samples, measurements were conducted on sieved fractions of air-dried samples at 60°C (Fig. 2). The air-dried samples were sieved into six fractions (0.85–2, 0.425–0.85, 0.25–0.425, 0.106–0.25, 0.075–0.106, and <0.075 mm), and WR (N m–1), dry mass fraction (kg kg–1, %), and SOC (kg kg–1, %) for each fraction were determined.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
In this study soil hydrophobicity is defined by the MED test. The test allows the degree of WR to be easily and rapidly evaluated by measuring the surface tension of an aqueous ethanol droplet that can infiltrate into the soil sample within 5 s. However, infiltration of such a droplet is not only governed by the intrinsic wetting properties of the surface but may also be affected by grain size distribution and surface roughness, the hydraulic properties of the soil, and the mixing processes of the water-ethanol solution and the soil solution. Here, we will use soil media with relatively similar mineral-grain size distributions and hydraulic properties (water retention and hydraulic conductivity) but the possible limitations of the MED methods when interpreting MED-test derived WR data are noted.

Effects of Pretreatment Drying Temperature and Desorption on Soil Water Repellency
Water repellency was observed only for soil samples from the upper four depth intervals, 0.00 to 0.05, 0.03 to 0.06, 0.05 to 0.10, and 0.10 to 0.15 m with SOC from 4.6 to 12.3%, among samples collected from a total of seven depth intervals (Table 1). For the soil samples from the upper four depth intervals, the WR at three moisture conditions, air-dry and corresponding to soil-water potentials of pF 3.3 and 4.1 in the soil-water retention curve, are shown in Table 3 .

View this table: [in this window] [in a new window]   Table 3. Water repellency, WR, of three moisture conditions of soil samples, air-dry and samples with volumetric water contents corresponding to two different soil-water potentials, pF = 3.3 and 4.1, in the soil-water retention curve. The soil samples were prepared by air-drying at 20°C and oven-drying at 60 and 105°C followed by 48 h of equilibration at 20°C. The soil samples at pF = 3.3 and 4.1 were also prepared by water desorption method with a pressure chamber at 20°C.

View larger version (10K): [in this window] [in a new window]   Fig. 3. Water repellency (WR) curve as a function of (a) volumetric soil water content and (b) pF for the four water repellent sieved soil samples (<2 mm) at each soil depth. The pF values were obtained by using regression curves in Fig. 1. The WR-max, non-WR, pFWR-max, and pFnon-WR for the soil of 0.10–0.15 m depth are given by arrows.

Soil Water Repellency for Sieved Fractions of Air-Dried Samples
The degree of WR for six fractions of air-dried aggregates (pretreated by oven-dried at 60°C followed by 48-h equilibration at 20°C) is given in Table 4 along with mass fraction and SOC for each fraction. Water repellency was observed for soil samples from three depth intervals, 0.00 to 0.05, 0.03 to 0.06, and 0.05 to 0.10 m, and the WR could not be detected by the MED tests (contact angles of <90°) for air-dried fractions from 0.10- to 0.15-m depth interval. The SOC and WR for the three air-dried soil samples (all fractions) are also shown in the table. The mass fraction of coarser aggregates, 0.85 to 2 and 0.425 to 0.85 mm, accounted for about 60%, and the mass fraction of the finest aggregate (<0.075 mm) was <2%.

Soil Water Repellency as a Function of Water Content/Soil-Water Potential
Water repellency for soil samples with various volumetric water contents () prepared by oven-drying at 60°C for different amounts of time and 48-h equilibration was measured, and, the WR curve (WR as a function of ) is shown in Fig. 3a. The degree of WR varied greatly with depending on sampled depth intervals with different SOC contents from 4.6 to 12.3%. The WR increased with increasing water content and reached a peak or a plateau of maximum repellency (WR_max). For the samples of 0.00- to 0.05- and 0.03- to 0.06-m depths with high SOC, the plateau of maximum WR maintained until around = 0.38 m³ m^–3, then decreased rapidly to zero (<90° contact angle) with a slight increase of water content. For the sample of 0.10- to 0.15-m depth with low SOC, the WR decreased rapidly with a slight increase of water content just after the peak. For the sample of 0.05- to 0.10-m depth, the WR curve was located in between those for 0.10- to 0.15-m depth and 0.00- to 0.05-m depth. The soil-water contents at maximum repellency, _WR-max, and the soil-water contents at which repellency is subcritical (<90° contact angle), _non-WR, are tabulated together with the maximum water repellency, WR_max in Table 5 . Both the _WR-max, and the _non-WR decreased with decreasing SOC.

View this table: [in this window] [in a new window]   Table 5. Water repellency parameters: soil-water contents at maximum and minimum repellency, WR-max and non-WR; pF values at maximum and minimum repellency, pFWR-max and pFnon-WR; integrated areas below WR curves, SWR() and SWR(pF). The soil organic carbon, SOC, and maximum water repellency, WRmax, for each soil depth are also given.

As shown in Table 3, the degree of WR for soil samples in this study varied depending on pretreatment methods; the oven-drying at 105°C gave the highest WR and the desorption treatment by a pressure chamber tended to give lower degree of WR compared with air-drying at 20°C and oven-drying at 60°C. However, the differences in WR among desorption and air- and oven-drying (except for oven-drying at 105°C) were relatively small (Table 3). Based on these results, we assume that the hydrophobicity measured for soil samples used here after oven-drying at 60°C to a specific water content approximately equals that at the same specific water content followed water desorption. With this assumption, the WR was plotted as a function of soil-water potentials (pF) estimated from the soil-water retention curve (Fig. 3b). Here, the volumetric water content was converted to pF [= log (-)] using fitting curves of the bimodal VG soil-water retention model in Fig. 1. The pF value at maximum repellency, pF_WR-max, and the pF value at which repellency is negligible (non-repellency), pF_non-WR, are also tabulated in Table 5.

The WR increased suddenly with increasing pF and reached the peak values. After the peaks, the WR decreased with increasing pF. For the samples of 0.05- to 0.10- and 0.10- to 0.15-m depths, the WR diminished to be zero (<90° contact angle) with increasing of pF. It is noted that both pF_WR-max and pF_non-WR were within a narrow range irrespective of depths for the soil samples, the former ranged from pF 3.2 to pF 3.6 and the latter ranged from pF 2.8 to pF 3.2. Figure 4 shows a comparison of the WR curve (WR as a function of pF) and the soil-water retention curve for soil samples. For the soil-water retention curve, the boundary between the inter-aggregate and intra-aggregate pores is indicated. It is interesting that the peak of WR, that is, pF_WR-max, occurred where the water retained in intra-aggregate pores was drained to a moderate extent for all soil samples.

View larger version (16K): [in this window] [in a new window]   Fig. 4. Comparison of volumetric water content and water repellency (WR) as a function of pF. Soil samples are from depths of (a) 0.00–0.05 m, (b) 0.03–0.06 m, (c) 0.05–0.10 m, and (d) 0.10–0.15 m.

Relationships Among Water Repellency Parameters from the Water Repellency Curves for Repellency Characterization
The trapezoidal integrated area below the WR curves in Fig. 3, S_WR() and S_WR(pF), are given in Table 5. The data imply linear relationships among some WR parameters (S_WR(), S_WR(pF), _WR-max, _non-WR, and SOC), and the relationships are shown in Fig. 5 . A good correlation between S_WR() and SOC was also reported by de Jonge et al. (1999), and similar correlations between S_WR() and _WR-max (or _non-WR) were also reported by Regalado and Ritter (2005).

View larger version (9K): [in this window] [in a new window]   Fig. 5. Relationships among WR parameters. (a) integrated areas below WR curves, SWR() and SWR(pF), as a function of soil organic carbon, SOC, (b) soil-water content at maximum/minimum repellency, WR-max/non-WR, and (c) SWR() as a function of WR-max/non-WR. Regression lines and intercept values are given.

WR()

WR()

Both _WR-max and _non-WR increased with increasing SOC (Fig. 5b), and the S_WR() increased with increasing _WR-max (or _non-WR) (Fig. 5c). From the regression lines, the values of _WR-max (or _non-WR) at S_WR() = 0 and at SOC = 0 were estimated as 0.21 and 0.25, respectively. Here, the _WR-max = 0.21 can be seen as the initial water content at which the WR occurs in a soil sample with the minimum SOC content necessary for introducing WR (can be estimated by S_WR() = 0 in Fig. 5a). Unfortunately, in the present study we cannot estimate directly the peak value of WR, WR_max, based on correlations among WR parameters and SOC. However, the combination of WR parameters and SOC would be a useful tool for characterizing the soil-water content/soil-water potential dependent repellency in aggregated soils.

The WR parameters obtained from the WR curves are likely related to soil texture and porous media structure since the WR curves depend greatly on soil type and soil management (de Jonge et al., 1999). In the present study we examined correlations among WR parameters and SOC for soil samples from a single soil profile. Thus, WR parameter correlations for additional, differently textured soils and soil profiles will be needed for further analysis and discussion. In addition, the WR curves also vary depending on the composition and spatial structure of SOM (Bisdom et al., 1993; Ellerbrock et al., 2005). Because the composition of SOM varies depending on the vegetation that covers the soil surface (Wallis and Horne, 1992) and is controlled by the microbial activities (Roberts and Carbon, 1972; Piccolo and Mbagwu. 1999), correlations between the WR parameters and characteristics of SOM should be further investigated for a better understanding of water-repellent soils.

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Water repellency of sieved fractions from an aggregated volcanic ash soil with different SOC contents was quantified using MED tests. The degree of WR varied nonlinearly with both SOC content and (and corresponding pF value). Irrespective of soil-depths, the peak values of WR were located in a relatively narrow range of pF values from 3.2 to 3.6 at which the water retained in intra-aggregate pores was drained to a moderate extent in the soil-water retention curve. The results in this study suggested that the hydrophobicity of the aggregate surface changed with the and the corresponding pF value and that the hydrophobicity might be enhanced the most at a specific soil-water potential depending on SOC of soil samples. Correlations between WR parameters from WR curves (WR- and WR-pF) were examined. Linear relationships were found among several WR parameters including integrated areas below WR curves and water contents at maximum/minimum repellency, and SOC. This implies that the combination of WR parameters and SOC would be a useful tool for characterizing the soil-water content/soil-water potential dependent water repellency in unsaturated soils.

	ACKNOWLEDGMENTS

 
This work was partially supported by a grant from the Innovative Research Organization, Saitama University. Part of this work was also supported by the grant-in-aid for Young Scientists (A) (No. 18686039) from the Japanese Ministry of Education, Science, Sports, and Culture (Monbukagakusyo). Three anonymous reviewers helped greatly in improving the paper. We especially acknowledge the careful and dedicated laboratory work by former students, Hisato Nakamura and Yasutaka Yoshikawa, Saitama University.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
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Received for publication August 17, 2006.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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