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a Environmental Engineering Section, Dep. of Life Sciences, Aalborg University, Sohngaardsholmsvej 57, DK-9000 Aalborg, Denmark
b Dep. of Agroecology, Danish Institute of Agricultural Sciences, P.O. Box 50, DK-8830 Tjele, Denmark
* Corresponding author (C.Kjaergaard{at}agrsci.dk).
Received 3 July 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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= –2.5, –100, and –15500 hPa), and wetting rate on WDC released in response to infiltration of low–ionic strength rainwater, using a low-energy input measurement of WDC (LE-WDC). These results were referenced by WDC obtained by a conventional, high-energy input measurement based on air-dried soil (HE-WDC). The energy input in the dispersion procedure significantly affected the release of WDC. The amount of HE-WDC increased with clay content, while the amount of LE-WDC decreased with increasing clay content. The decrease in LE-WDC was explained by an increase in cohesive strength, reflected by the increase in water-stable aggregates (
4 mm). A strong dependency of IMP on LE-WDC was observed, with maximum release of LE-WDC from soils that were at –2.5 hPa before measurement. Decreasing soil matric potential in the period before measurement reduced LE-WDC and also reduced the dependency of soil clay content, with soils incubated at –15500 hPa releasing a low amount of LE-WDC independent of clay content. The content of particulate organic C (POC) in the LE-WDC decreased with increasing clay content, and increased after drying to –15500 hPa. Colloid dispersibility changed as a function of time and moisture status, with the main changes occurring during or immediately after adjustment of the moisture content. Increasing the wetting rate resulted in a doubling of the amount of LE-WDC released from the initially dry soil (–15500 hPa), while no effect of wetting rate was observed at higher initial matric potentials.
Abbreviations: CEC, cation exchange capacity • DOC, dissolved organic C • EC, electrical conductivity • HE-WDC, high-energy water-dispersible colloids • IMP, initial matric potential • LE-WDC, low-energy water-dispersible colloids • POC, particulate organic C • SAR, sodium adsorption ratio • TOC, total organic C • WDC, water-dispersible colloids • WSA, water-stable aggregates
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In general, the main fraction of soil colloids is associated in aggregates as a result of the cohesive nature of colloids (Oades and Waters, 1991; Oades, 1993), and released following disintegration of aggregates. Le Bissonnais (1996) summarized four main mechanisms for disintegration of aggregates: (i) slaking, which is disintegration caused by the compression of entrapped air during wetting, (ii) differential swelling, (iii) mechanical breakdown by raindrop impact, and (iv) physicochemical dispersion. These processes differ in the intensity of disintegration and the size of the resulting fragments. Disintegration of aggregates by slaking and swelling results in the breakdown of larger aggregates into minor units, which consequently increases the surface area and exposes new surfaces, which may ultimately increase colloid dispersion. Dispersion is the ultimate state of breakdown that results in release of clay particles as a consequence of expanding electrical double layers and dominating repulsive forces as described by the DLVO theory (Derjaguin and Landau, 1948; Verwey and Overbeek, 1948). It is generally recognized that the fraction of clay that disperses in water has been found to have statistically significant relationships with soil erodibility (Miller and Baharuddin, 1986; Shainberg et al., 1992). The soil WDC fraction has been used as input parameter for predicting soil erosion (Brubaker et al., 1992), colloid leaching, and colloid-facilitated transport through the vadose zone (Jarvis et al., 1999; Villholth et al., 2000).
The predominant effect of clay mineralogy (e.g., Frenkel et al., 1978; Goldberg et al., 1988; Seta and Karathanasis, 1996), solution ionic strength and pH (e.g., Rengasamy, 1983; Rengasamy et al., 1984; Grolimund and Borkovec, 1999; Flury et al., 2002) on colloid dispersion has been documented from several studies. In natural field soils, the effect of mineralogy and solution ionic strength may, however, be influenced by the presence of surface adsorbed organic C, which may mask the direct importance of clay mineralogy or promote increased aggregate stability by increasing the bonding strength of aggregates or reducing the wettability and consequently the slaking potential of aggregates. From regression analysis comparing a range of soil factors, several studies have identified total clay content as one of the most important properties in determining the amount of WDC (Pojasok and Kay, 1990; Brubaker et al., 1992; Rasiah et al., 1992; Levy et al., 1993; Curtin et al., 1994). In addition, the matric potential of aggregates before measurement (e.g., Pojasok and Kay, 1990; Rasiah et al., 1992; Brubaker et al., 1992) and management in terms of crop sequence, application of organic manures, soil tillage, and field traffic (e.g., Watts et al., 1996a b; Watts and Dexter, 1997, Schjønning et al., 2002) have been shown to significantly affect dispersibility of clay in soil structural elements.
Different properties may influence WDC in different soil types, but comparison among data is complicated by the numerous methods that have been used to determine WDC. Studies have shown that values of aggregate stability and WDC depend on sample pretreatment, initial moisture content, length of time aggregates were allowed to wet up and equilibrate, and the length of the shaking period (e.g., Pojasok and Kay, 1990; Kay and Dexter, 1990). Generally, most studies have used air-dry soil for measurements of WDC. Air drying is meant to standardize initial conditions. However, air drying may introduce changes in chemical or physical characteristics that can alter stability (Alderfer, 1946; Reid and Goss, 1981). These changes are further augmented due to swelling or slaking during rewetting of the samples (Panabokke and Quirk, 1957). On the basis of these observations, Pojasok and Kay (1990) recommended that soil stability and dispersible clay should be measured using field-moist soils.
Water-dispersible colloids can be measured for different purposes, one of which is to yield an estimate of the fraction of potentially mobile colloids released during the infiltration of low–ionic strength rainwater through the soil. Clay may become mechanically dispersed at the soil surface layer by erosive raindrop impact. However, determinations of WDC with low energy inputs are probably more indicative of in situ colloid mobilization from the upper soil horizon, especially when the soil surface is protected from raindrop kinetic energy by vegetation cover. Although the most frequently applied dispersion methods involve sieved, air-dried soil and application of mechanical energy, we see a need for modified methodologies that better simulate the potential amount of WDC released from soil aggregates with different initial moisture conditions before a rain event. In this paper we explore the process of colloid dispersion with the following objectives: (i) evaluating the results from a low-energy input measurement of WDC against a conventional, high-energy input measurement based on air-dry soil, and (ii) using the low-energy input WDC measurement to determine the influence of soil clay content, initial soil matric potential, and wetting rate on colloid dispersion following infiltration of low–ionic strength rainwater. The soils used in these studies were collected from six locations along a naturally occurring clay gradient from an arable field (Lerbjerg, Denmark) with clay contents ranging from 12 to 43%. The use of soil with similar mineralogy and management history allows examination of the interacting effects of soil clay content and initial soil matric potential on colloid dispersion.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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2-µm fraction was separated by siphoning off the upper suspension in one-step gravity sedimentation. The amount of HE-WDC in the retrieved suspension was measured as mass dry weight. Measurements of HE-WDC were performed in triplicate.
The pretreatment procedures of the six soil types subjected to low-energy input dispersion measurements are illustrated in Fig. 2
. The pretreatment included (i) saturation by capillary wetting and drainage to three initial soil matric potentials = –2.5, –100, and –15500 hPa, which covered the moisture conditions from near saturation to the crop wilting point, and (ii) resaturation with either slow wetting with controlled tension for 7 d or fast wetting within 4 h. The experiments were performed using three replicates of each combination, giving a total of 108 samples. The pretreatment involved the following procedure. Field-moist soil, with known soil water content, was passed through an 8-mm sieve and packed in 100-cm3 steel cylinders at a bulk density of 1.1 Mg m–3. Care was taken not to press the soil through the sieve. Aggregates were separated by hand at the planes of weakness until they obtained a size that allowed them to pass the sieve. The packed samples were sealed at the bottom with a 0.45-µm nylon disc filter.
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After drainage all samples were weighed, sealed, and allowed to equilibrate for 14 d at 10°C. Water loss at –2.5 hPa was prevented by incubating the samples in containers with a water saturated atmosphere, and water gain at –15500 hPa was prevented by incubating the samples in containers with silica gel. After the 14-d incubation period the samples were reweighed. Water loss or gain was found to be insignificant. The packed soil samples were then placed on tension tables and resaturated with electrolyte solution. Samples incubated at –2.5 hPa were initially drained to –100 hPa and subsequently saturated. This procedure was done to have identical treatments with respect to changes in electrolyte composition for all samples. Half of the treatments were saturated by slow capillary wetting from an initial soil matric potential at –100 hPa with slowly increasing potential for 7 d, and the other half by fast capillary wetting within 4 h. The volume change after slow and fast wetting was measured with a specially constructed caliper.
Immediately after the resaturation, the samples were transferred quantitatively to 1-L polyethylene sedimentation bottles with electrolyte solution and further electrolyte solution was added to reach a final soil/water ratio of 1:8. The dispersion procedure for the LE-WDC measurement involved turning the soil suspension upside down manually 10 times. Also in this case, the colloid fraction (2-µm) was separated by one-step gravity sedimentation. The amount of LE-WDC after one-step gravity sedimentation was measured as mass dry weight. Stock suspensions from each soil were used for making turbidity calibration curves for determination of colloid concentration in later experiments. Fractions of LE-WDC released after slow rewetting (Fig. 2) were used for the following experimental investigations: (i) comparison of HE-WDC with LE-WDC, (ii) evaluating the effect of initial matric potential, (iii) investigating the time dependency of colloid dispersion, and finally (iv) investigating the effect of wetting rate by comparing with fractions of LE-WDC released after fast rewetting.
Characterization of Colloid Suspensions
The LE-WDC suspensions were characterized for total organic C (TOC), particulate organic C (POC), and dissolved organic C (DOC). Total organic C was measured by C combustion using a Total Organic Carbon Analyzer (TOC-5000A, Shimadzu Scientific, Kyoto, Japan) equipped with a suspended particle kit and coupled to an auto sampler with magnetic stirring. Dissolved organic C was determined on colloid suspensions after centrifuging for 1 h at 4180 g, yielding a size separation at approximately 50 nm, assuming spherical particles with 2.63 g cm–3 density. This separates the fraction of organic matter bound to the mineral fraction (mineral bound POC) from the DOC. Particulate organic C is given as POC = TOC – DOC. The results were used for calculating the POC/LE-WDC ratio.
Dispersion Kinetics
The time dependency of colloid dispersion was investigated using the same experimental setup as described for the low-energy input dispersion measurement. Low-energy water-dispersible colloids were measured at 11.5% clay with three initial soil matric potentials ( = –2.5, –100, and –15500 hPa) and five times (0, 12, 26, 72, and 145 h). Initially all samples were saturated with electrolyte solution by slow capillary infiltration on tension tables and allowed to equilibrate for 48 h. After initial saturation, one-third of the samples were drained to –2.5 hPa, and two-thirds were drained during 24 h to –100 hPa on tension tables. Half of the samples drained to –100 hPa were subsequently dried by passing them through dry air for 6 h until the samples reached a gravimetric water content corresponding to a soil matric potential of –15500 hPa. Immediately after reaching the desired soil matric potentials, LE-WDC was measured at time 0. The rest of the samples were capped and stored at 2°C, and samples were taken for LE-WDC measurements after 12, 26, 72, and 145 h.
Wet Aggregate Stability
Approximately 50 g of field-moist soil was taken from three intact soil cubes for determination of water-stable aggregates (WSA) by methods as described by Pojasok and Kay (1990). The soil was gently passed through an 8-mm sieve. A subsample of 10 g was used for determining water content, and 30 g were taken to a 4-mm sieve installed in a wet-sieving apparatus (Yoder, 1936). Thirty seconds of initial capillary wetting were followed by 2 min of vertical movement of the sieve (stroke length 32 mm, 38 strokes min–1). The water used for the wet-sieving procedure was identical to the electrolyte solution used for the HE-WDC and LE-WDC measurements. Stable aggregates remaining on the sieves (0.250 and 4 mm) were transferred quantitatively to a beaker. Water was evaporated at 80°C, and the soil was further dried at 105°C followed by weighing. Finally the soil was dispersed by end-over-end shaking for 24 h with a 0.002 M Na4P2O7 solution and poured through the 0.250- and 4-mm sieves. Primary particles 0.250 and 4 mm were weighed following drying at 105°C. Results are given as percentage of WSA.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The fact that all six soil types were sampled along a texture gradient within one single field is important in ruling out management-induced differences in clay dispersibility. Several investigations have shown that management may affect clay dispersibility (e.g., Watts et al., 1996ab; Watts and Dexter, 1997). As the six soils used for this study have experienced the same management in all respects, we consider them similar for the study. We cannot exclude some difference in the energy the soils have received during tillage, the soils holding more clay being cloddier and less friable than the less clayey soils. This would be expected also to influence clay dispersibility (Watts et al., 1996a; Schjønning et al., 2002). However, such effects would still be minor compared with soils sampled at different locations.
High- vs. Low-Energy Input Dispersible Colloids
Mechanically dispersible colloids obtained by the classical, HE-WDC procedure increased with increasing clay content from 1458 mg kg–1 at 11.5% clay to 5989 mg kg–1 at 28% clay, followed by an insignificant increase to 6283 mg kg–1 at 43% clay (Fig. 3a)
. The positive correlation between HE-WDC and total clay agrees with observations from other studies having different pretreatments but all involving mechanical shaking in the measuring procedure (Table 2). The application of mechanical energy breaks down the aggregates and thereby increases the surface area. As a consequence, more clay is dispersed as the soil content of clay increases. The maximum HE-WDC at 28% clay could be explained by the following:
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The employment of a low-energy input in the dispersion procedure displayed an opposite trend of a decreasing amount of LE-WDC with increasing clay content (Fig. 3b). These results also revealed a strong dependency of initial soil matric potential, and an interactive effect between clay content and initial soil matric potential on LE-WDC. For soils incubated at high water content (–2.5 hPa), LE-WDC decreased from 3174 mg kg–1 at 11.5% clay to 1247 mg kg–1 at 43% clay. The same pattern was observed for soils incubated at intermediate water content (–100 hPa), but the amount of LE-WDC was reduced, with LE-WDC decreasing from 1974 mg kg–1 at 11.5% clay to 1025 mg kg–1 at 43% clay. The L4 soil (28% clay) in both cases showed a considerably lower dispersibility than the other clay-rich soils. The reason for this lower dispersibility is not clear, but it could be due to differences among soils in the content of strongly bonding agents such as sesquioxides. Soils incubated at very low water content (–15500 hPa) released a small amount of LE-WDC, about 450 to 500 mg kg–1, which was independent of clay content.
The decrease in LE-WDC with increasing clay content can be explained by a combination of two interrelated properties: (i) increasing cohesive strength between clay particles due to the larger number of clay particles increasing the area of contact, and (ii) a higher ionic strength in the pore water of aggregates from the higher clay soils, as indicated by the increase in CEC with increasing clay content (Table 1). In addition, it is also evidenced from the ratio of POC to LE-WDC (Table 3) that the composition of the colloid fraction differs, with a decrease in the content of POC as the clay content increases. This is probably a consequence of the lower TOC/clay ratio with increasing clay content (Table 1) and may indicate that organic C associated with mineral colloids may increase the dispersibility of colloids. The influence of organic matter on aggregate stability and clay dispersion has been the subject of much discussion, since it has been found that organic matter may promote both aggregation and dispersion (e.g., Shanmuganathan and Oades, 1983; Durgin and Chaney, 1984; Barzegar et al., 1997). The stabilizing effect of soil organic matter has been attributed to the role that roots and hyphae have in stabilizing macroaggregates, thereby preventing clay dispersion. When these bonds are broken by aggregate disruption, the dispersive effects of organic matter may predominate (e.g., Goldberg et al., 1990; Nelson et al., 1998).
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0.250 and 4 mm) are correlated with the clay content. The fraction 0.250 mm represents the traditionally used size discrimination between micro- and macroaggregates (Tisdall and Oades, 1982), and the fraction 4 mm was chosen based on experiences from preliminary investigations. The fraction of WSA 0.250 mm showed a significant positive correlation (R2 = 0.827; P < 0.05) between the percentage of WSA and clay content, with the soil mass increasing from 51% at 11.5% clay to 74% at 43% clay. Using the fraction of WSA 4 mm significantly improved the correlation (R2 = 0.989; P < 0.001) between the percentage of WSA and clay content, with a soil mass ranging from 26% at 11.5% clay to 61% at 43% clay. This demonstrated that the increase in clay content from 11.5 to 43% resulted in a minor increase in the total percentage of stable macroaggregates 0.250 mm, while the increase in aggregate stability with increasing clay content is mainly explained by the fraction of very large WSA (4 mm). The positive correlation between clay content and WSA has been reported in several studies (e.g., Kemper et al., 1987; Pojasok and Kay, 1990). It may be speculated that estimates of WSA could be used to predict WDC. However, the results from this study showed that a poor correlation existed between LE-WDC and WSA (Fig. 5)
. At lower clay content (<25%), the amount of LE-WDC increased as WSA decreased, while at higher clay content (28%), no correlation existed between LE-WDC and WSA. Water-stable aggregates continued to increase with increasing clay content, while LE-WDC maintained a constant minimum value. Generally, attempts to use aggregate stability to predict soil susceptibility to erosion have had limited success due to conflicting results (Amezketa et al., 1996). The differences in HE-WDC and LE-WDC displayed from this study agree with the conclusions of Kay and Dexter (1990), who documented that the amount of mechanically dispersed clay increased with increasing aggregate size, while the amount of spontaneously dispersed clay increased with increasing surface area, which required initial aggregate breakdown. This highlights the importance in the choice of dispersion procedure, especially the input of energy. When using estimates of WDC as input parameter for modeling colloid mobilization and transport, one should be aware of the coherence between energy input and the result of the WDC measurement. The application of mechanical energy by the shaking procedure may resemble "worst case" scenarios, where storm flow events or soil tillage increases dispersion by breaking down aggregates, while the use of low-energy input dispersion measurements may resemble the in situ release of colloids from the upper horizons during average precipitation events.
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It is interesting that no effect of clay content on LE-WDC was evident after drying to –15500 hPa. Kemper and Rosenau (1984) found greater cohesion in 40% clay than in 15% clay after air drying. The reason that no effect of clay content is evident from this study might be that the cohesive forces created during the drying of low-clay soils were sufficient to withstand dispersion under the mild stresses applied by the low-energy input. It is important to note that the effect of initial soil matric potential declines with increasing clay content. This is probably a consequence of pores <30 µm constituting an increasing fraction of the pore volume with increasing clay content as documented for this clay gradient by Kjaergaard et al. (2004). These pores will retain water after drainage to –100 hPa, while only pores 30 µm will drain, and this consequently minimizes the effect of soil matric potential on colloid dispersion at higher clay contents.
Time Dependency of Colloid Dispersion
In the field, soil is subjected to drying and wetting events that continuously change the moisture status of the soil, and thus the dispersibility, as illustrated previously. This raises the important question of the time dependency of the dispersion process. This was investigated as a function of soil matric potential at 11.5% clay, since the effect of soil matric potential was most pronounced at this clay content. The results revealed that colloid dispersion changed as a function of time and that the rate of change depended on the moisture status of the soil (Fig. 6)
. When the moisture content was near saturation (–2.5 hPa), LE-WDC increased with time from 1636 mg kg–1 at time 0 to 3046 mg kg–1 after 26 h, when the rate of change leveled off, with only a minor increase to 3091 mg kg–1 after 145 h. Additionally, 24 h drainage of soils to –100 hPa immediately reduced LE-WDC to 1455 mg kg–1, followed by a continuing small decrease in LE-WDC during the incubation period to 1333 mg kg–1 after 166 h, with a rate of decreasing dispersibility of 0.859 mg kg–1 h–1. An additional 6 h of air drying to –15500 hPa resulted in an immediate reduction in LE-WDC to 409 mg kg–1, followed by a continuing small decrease in LE-WDC during the incubation to 303 mg kg–1 after 172 h, with a rate of decreasing dispersibility of 0.747 mg kg–1 h–1. At this clay content, the main change in dispersibility appeared to occur during or immediately after adjustment of the moisture status. The increase in dispersibility at –2.5 hPa occurred within the first 26 h, and the decrease in dispersibility at –100 and –15500 hPa occurred primarily during the change in moisture status. However, the dispersibility at –100 and –15500 hPa continued to decrease with time. This is consistent with the results of Kemper and Rosenau (1984), who found an increase in cohesive strength with duration of storage and suggested that the increase in cohesion under air-dry conditions indicates that migration of bonding components continue even when there is as little as one molecular layer of water on the mineral surfaces.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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This study also documented that there is no unique relation between WDC and clay content, as WDC significantly depended not only on the energy input but also on the initial conditions of the soils. This was reflected both by the pronounced effect of initial matric potential and by the interacting effect of initial matric potential and wetting rate on LE-WDC. On the basis of the sensitivity of the LE-WDC toward initial matric potential as well as wetting rate, we also anticipate that the use of the low-energy dispersion procedure would result in measurements of WDC that would be much more sensitive toward other important soil factors such as mineralogy, chemistry, management conditions, and the interactions among these properties, compared with the classical estimates of WDC. We therefore recommend that estimates of WDC to be used in predictive models should not be based on empirical relationships, but need to be measured using methods that resemble the conditions of interest.
Future studies intended to investigate the potential of colloid dispersion from aggregates should consider the importance of these different mechanisms of aggregate breakdown and colloid release. The results from this study additionally highlight the general problems of using air-dry or dried soils when investigating colloid mobilization and transport. Clearly the effect of initial matric potential should be taken into consideration when studying colloid mobilization. In addition, the time dynamics and reversibility of changing moisture conditions on colloid dispersibility should be explored further.
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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