SSSAJ Journal of Natural Resources and Life Sciences Education
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Zaher, H.
Right arrow Articles by Ouaki, B.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Zaher, H.
Right arrow Articles by Ouaki, B.
Agricola
Right arrow Articles by Zaher, H.
Right arrow Articles by Ouaki, B.
Related Collections
Right arrow Structure and Properties
Right arrow Soil Physics
Right arrow Pore-Scale Modeling
Right arrow Macroporous/Aggregated Media
Published in Soil Sci. Soc. Am. J. 69:1-12 (2005).
© 2005 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

Division S-1—Soil Physics

Modeling Aggregate Internal Pressure Evolution following Immersion to Quantify Mechanisms of Structural Stability

Hafida Zahera, Jean Caronb,* and Bennaceur Ouakic

a Division de l'Organisation, des Méthodes et de la Gestion Informatique Ministère de la Pêche Maritime, B.P. 476 Agdal, Rabat, Morocco
b Dép. des Sols et de Génie Agroalimentaire, Univ. of Laval, QC, Canada G1K 7P4
c Dép. Génie des Matériaux, Ecole Nationale de l‘Industrie Minérale, B.P. 753 Agdal, Rabat, Morocco

* Corresponding author (jean.caron{at}sga.ulaval.ca).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX A
 REFERENCES
 
Identification of the key components controlling aggregate stability is important in soil structure research. The deterioration of soil aggregates during rapid wetting has often been attributed to the swelling and internal pressure buildup resulting from the compression of entrapped air by the advancing wetting front. Organic matter is known to reduce the extent of slaking, but the different modes of action have not yet been quantified. The objective of the study was to use theoretical three-dimensional models to quantify the effect of paper sludge amendment on the key processes controlling internal pressure evolution. A clay loam and a silty-clay loam were incubated for a 2-wk period with different amounts and types of paper sludge. Aggregates were then selected, air dried, and then fixed to a hypodermic needle connected to a pressure transducer, and the whole system was immersed in distilled water while images and pressure evolution were recorded. For both soils, the maximum internal pressure was lower in the sludge-amended aggregates. From the models fitted to the observed data, it appears that the addition of paper sludge resulted in an increase of the potential at the wetting front and a decrease of the near saturated hydraulic conductivity. This result suggests that sludge addition reduces pressure buildup by reducing the rate of water entry, lowering the potential at the wetting front and reducing the hydraulic conductivity of the aggregate.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX A
 REFERENCES
 
ORGANIC MATTER is one of the essential elements that support soil microbial life and maintain soil structure and fertility. A marked reduction in the quantity and/or quality of organic matter often leads to a deterioration of the soil structure, and recent laboratory and field studies have shown the effectiveness of readily biodegradable organic matter in improving soil structure, even if the effect is temporary (Sidi and Pansu, 1990; Nemati et al., 2000). These studies highlighted the role of the microbial biomass (Metzger et al., 1987; Tisdall, 1994) and of transitional decomposition substances like carbohydrates and lipids (Metzger and Yaron, 1987; Dinel et al., 1990; Wright et al., 1999; Morse et al., 2000; Garrity et al., 1996) originating from microbial and plant activity in the improvement of soil structural stability. The organic agents involved in aggregate stabilization can generally be divided into three main groups according to the effect of the different carbonaceous fractions on structural stability, that is, transitional, temporary, or permanent (Tisdall and Oades, 1982).

As a dynamic soil property, structural stability can be altered by the action of degradation agents such as water (Yoder, 1936). Numerous laboratory tests have been performed in the past to study the action of water on soils in standard conditions in an effort to understand the effect of this stress on structure destabilization. Two factors appear to play an essential role in the process: swelling, which generates internal stresses and causes the dispersion of colloidal cements (therefore causing a loss of cohesion), and air pressure increase, which leads to the rupture of the aggregate (Grant and Dexter, 1990). The rate of pressure increase and the subsequent slaking depends chiefly on aggregate wettability and the possibility of trapped air to escape (Concaret, 1967). Indeed, when an aggregate is submerged, wettability of the aggregate influences the rate of water entry (Chenu et al., 2000), which then affects the rate of pressure increase. If the aggregate is wettable, water rapidly enters the capillary pores, and the liquid menisci only allow part of the air in the aggregate to escape through the few unobstructed (nonwetted) capillaries. Most of the air remains trapped in the aggregate and is compressed by the incoming water. The rupture of the aggregate occurs when the resulting internal pressure is great enough to overcome aggregate cohesion. Therefore, changes in cohesion may also affect stability. The cohesion in turn changes with the extent of swelling.

The predominant mechanisms among change in cohesion and pressure buildup for insuring stability is still subject to debate, and the conclusion may depend on the experimental setup used. According to Caron (1996), the major mechanism for decreased aggregate stability is the increased water entry into the aggregate. Changes in aggregate cohesion and swelling are less important factors. Dinel and Gregorich (1995) observed that the dispersal action of water had more effect on aggregates than the alteration caused by the air trapped during rapid wetting. Grant and Dexter (1990) concluded that both mechanisms alone are partially effective and that the two work synergistically.

It is generally understood that an aggregate from a given soil characterized by a certain number of properties can be subject to disintegration through a series of mechanisms induced by the shock of wetting. At this level, labile organic matter can effectively reduce slaking by modifying soil properties and also by acting on the two factors involved in the destruction of aggregate stability: cohesion and internal pressure.

Stroosnyder and Koorevaar (1972) presented an experimental device that could be used to follow the air pressure evolution in aggregates that could be coupled to sequential image analysis to investigate further the effect of organic matter on stability (Caron et al., 1998). The overall purpose of this study is to use such an approach to understand the mechanisms contributing to aggregate stability as influenced by paper sludge amendments so that the amendments can be used efficiently. The specific objectives of this study are to experimentally measure the change in internal pressure during aggregate immersion in water, and to estimate the key parameters involved in aggregate degradation and their relative contribution, as modified by sludge amendments.


   THEORY
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX A
 REFERENCES
 
On the basis of the formulations of Green and Ampt (1911) and the equations recently developed by Youngs et al. (1994), this study deals specifically with the rapid wetting of supposedly spherical aggregates in water, both with (Model 1) and without (Model 2) air escape. A list of the variables used is provided in the appendix.

Model 1: Water Infiltration with Air Escape
In Model 1, where it is assumed that the forward movement of the wetting front is not hindered by intraaggregate pressure, air will escape continuously without generating a pneumatic potential at the wetting front. In this case, the variation of the matric potential at the wetting front will be equal to h0-hf, where h0 and hf represent, respectively, the matric potential at the surface of the aggregate and at the wetting front. On the basis of Youngs et al. (1994), the water flux q that has penetrated the supposedly spherical aggregate of a radius a from its external surface after a given time t is:

[1]
where a and rf represent, respectively, the radius of the aggregate before immersion and the position of the wetting front at a given time t, and Kns is the near saturated hydraulic conductivity of the aggregate.

Since the water flux q is simply the variation in time of the cumulative infiltration (I), Youngs et al. (1994) demonstrated that this parameter could be calculated at each instant with the following equation of equilibrium:

[2]
where {theta}i and {theta}s are, respectively, the initial water content and the water content at saturation.

The flux q can then be reformulated as follows:

[3]
Combining Eq. [1] and [3] yields a differential equation representing the evolution of the wetting front:

[4]
According to the one-dimensional model of Caron et al. (1998), the near saturated hydraulic conductivity Kns is considered to decrease exponentially with the duration of wetting, as a result of slaking, pore clogging, or swelling. Hence, Kns can be expressed as:

[5]
where {alpha} is a constant characterizing the rate of loss of this conductivity during the wetting process.

Taking into account this last equation, the position of the wetting front rf as a function of time can be determined by the integration of Eq. [4] between the time t = 0 and a given time t. The following equation characterizes this evolution:

[6]

Model 2: Water Infiltration Without Air Escape
In model 2, it is assumed that infiltration occurs without air release from the aggregate. A pneumatic potential ha is generated at the wetting front in the aggregate due to trapped air, and this increase of the global matric potential is given by:

[7]
where hf is constant and ha is the variation of the potential generated at the wetting front in relation to the atmospheric pressure.

The pneumatic potential ha generated at the wetting front is no other than the variation of the pressure of the volume of air inside the part of the aggregate that has not been wetted (P) relative to the atmospheric pressure (P0). If V0 and V represent the initial volume of air in the dry aggregate and the volume of air after wetting at a time t, respectively, the pneumatic potential can be calculated as follows in accordance with Boyle's law:

[8]

If f represents the initial porosity of the aggregate (f = {theta}s – {theta}i), and the air volumes V0 and V are known, the pneumatic potential can be expressed in relation to the cumulative infiltration I. Therefore, taking into consideration Eq. [2], ha can be represented as follows:

[9]

Replacing hf by hg (hg = hf + ha) in Eq. [4] developed in model 1 and taking into account Eq. [2] and [5], we obtain the general equation controlling the infiltration of water into a sphere-shaped aggregate without air release and a changing hydraulic conductivity as the wetting front advances, in a form similar to that of Youngs et al. (1994):

[10]
where X is such that:

[11]
Solving this last equation for X using Maple V v. 4 (Mathsoft Corp., Maple Software, Waterloo, Cambridge, MA), gives:

[12]
with:

[13]

[14]

[15]
The constant (Ct) found in this equation is obtained from the initial conditions. The solution in X1 and in X is obtained using MathcadPlus (v. 6.0, professional ed., Mathsoft Corp.). The radius rf, is determined consequently from Eq. [13] and [2].

In both models (1 and 2), the solutions in rf consisted in two complex (imaginary) and one real roots as solutions to Eq. [6] and [12] respectively, in which we applied the pressure data collected (see below) of individual aggregates. Before comparing with experimental data, Eq. [6] or [12] were corrected to account for the fraction of air released from the aggregates and to the amount of swelling observed after the rapid immersion of the aggregates in water. Following Concaret (1967), if it is assumed that the volume of air Ve that could have escaped after each time t was under atmospheric pressure P0, the application of Boyle's law corrects the pressure P which can then be expressed as follows:

[16]
With regard to swelling, if rg represents the radius of the swollen aggregate at a given time t, the radius corresponding to the position of the wetting front determined by one of the two models described above (with and without air release) is corrected (rc) as follows:

[17]
Finally, taking into account the fraction of air released, aggregate swelling and the position of the wetting front determined by one of the two models formulated above, the following general equation can be used to determine the pressure prevailing inside the aggregate as the wetting front advances after rapid immersion in water:

[18]
Relative to the atmospheric pressure, the predicted pressure Pp corresponds to:

[19]
Then Pp(t) can be compared with measured pressure data. Since the pneumatic potential (air pressure) at the wetting front represents the variation in the measured internal pressure relative to the atmospheric pressure, it becomes, if pressures are expressed in terms of water heights,

[20]
Taking into account aggregate porosity, ha is expressed as

[21]


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX A
 REFERENCES
 
An experiment was designed to measure the following as a function of time during rapid water wetting of an aggregate: intraaggregate pressure, aggregate swelling, loss of aggregate matter, and volume of air released from aggregates treated with various organic soil amendments.

The two soils examined in this study were a clay loam soil from the Bedford series in Ste-Croix-de-Lotbinière (loamy, mixed, nonacid, frigid, Typic Humaquept) and a silty-clay loam from the Tilly series in St. Augustin (fine-silty, mixed, frigid, Aquic Haplorthod), both in the area of Quebec City, QC, Canada. The clay loam contained 27, 43, and 30 g of clay, silt, and sand, respectively, per 100 g soil, and 2.2 g organic C per 100 g soil. The silty-clay loam was composed of 39, 52, and 9 g of clay, loam, and sand, respectively, per 100 g soil, with 3.7 g organic C 100 g–1 soil. Six treatments were applied to each of the soils. The treatments were three application rates of deinking–secondary sludge mix (8, 16, and 24 Mg dry matter ha–1), one application rate of primary–secondary sludge mix (18 Mg oven-dry ha–1) containing the same quantity of C as the 24 Mg ha–1 rate of deinking–secondary mix. The treatments also included one application rate of composted deinking sludge (24 Mg ha–1) and a control treatment that received no amendment. The primary and the deinking sludges are composed primarily of wood fibers, [essentially cellulose (39%), hemicellulose (11%), and lignin (23%)], whereas the deinking sludge may also contain clay (aluminium silicates), ink residues, kaolinite, and charcoal black. Both primary and deinking sludges has initially a C/N ratio of about 300 (Brouillette et al., 1996). The secondary sludge (C/N {approx} 20) is a N-rich byproduct, rich in N and already inoculated with microbes decomposing wood fibers. Additional details regarding sludge composition, effects on the environment, and on soil structure can be found in Brouillette et al. (1996), Trépanier et al. (1998), Nemati et al. (2000), and Zaher (2001). The C/N, C/P, and C/K ratios of these three types of paper sludge were adjusted to 30, 60, and 130, respectively, to avoid the immobilization effect (Zaher, 2001). Both the primary–secondary and deinking–secondary sludge mixes contained 20% secondary sludge (and the rest as primary or deinking sludges) with a high N content so as to reduce their C/N ratio.

Air-dried aggregates of a radius of approximately 6 mm were selected. They had been previously incubated covered at 20°C for 2 wk, after the addition of paper sludge (the period of time required to reach peak in dry aggregate stability). The intraaggregate air pressure was measured during immersion of air-dried aggregates in distilled water, and images were collected simultaneously to observe swelling, loss of matter, and air released. All treatments were replicated three times for each soil (total of 72 dry aggregates).

Model's Data
Several parameters are required for conceptual models describing pressure evolution Pp(t) in a spherical aggregate. Some of these parameters were measured and some were initially derived from initial conditions, specifically h0, P0, a, f, rg, and Ve following rapid wetting. The Kns, {alpha}, and hf were determined empirically from the fitted models to experimental data (see below).

Immersion of aggregates at 1-cm depth was rapid (<1 s), and therefore h0 was set to 10 mm. Aggregate diameters were measured twice with vernier caliper (largest and shortest diameters) to determine a mean radius (a), and a mean radius was calculated for each treatment. Estimates of (f = {theta}s – {theta}i) were obtained from the volumetric water content of 2-g samples of aggregate at saturation after a slow prewetting at –0.1 kPa of water potential, under a preestablished vacuum for 48 h. No slaking was observed with this prewetting treatment. Initial water contents of the air-dry aggregates were also determined gravimetrically. Estimates of bulk density were obtained by the kerosene method following Monnier et al. (1973) and used to convert gravimetric to volumetric water contents. The radius of swollen aggregates was calculated from images recorded after different times t, as follows. First, the surface of the aggregate S(t) was determined from:

[22]
where S(t) is the surface of the aggregate, D(t), the image area in pixels, and k a proportionality constant relating the real size to the image size calculated from the initial surface recorded immediately after immersion (S0) using the relationship:

[23]
where a represents the mean aggregate radius before immersion and D0 is the image size immediately after immersion. The extent of swelling ({zeta}), was calculated as:

[24]
The swelling rate for the different aggregates evolved linearly between 0 and 8 s following immersion:

[25]
with ß being the swelling rate per second. From Eq. [22] to [25], the radius after swelling was then calculated at each time t. This radius was calculated from:

[26]
Observations of cumulative volume of air expelled [Ve(t)] between 0 and 8 s also showed a linear increase in time (see appendix). Therefore, the analytical form of the relationship was described by:

[27]
where Ve(t) represents the volume of air expelled at time t, and {gamma} = the rate of expel. Finally, P0 in all equations was set at the atmospheric pressure.

The evolution of the internal pressure of the aggregates during their immersion in water was monitored using an experimental setup designed for this purpose. As shown in Fig. 1 , type 23G* one-inch (2.54-cm) needles (Terumo Medical Corporation, Elkton, MD) were fixed to the surface of the aggregates using drops of liquid paraffin. Each needle was then connected to a high-sensitivity sensor (PX26-005DV, Omega Engineering, Inc., Stamford, CT), coupled to a data acquisition device (CR10X, Campbell Scientific, Logan, UT). The aggregates were then submerged in water at a depth of 1 cm. Using a filmed light emitting diode, the data acquisition on the CR10 was synchronized with the image-taking process. The images were then transferred to a computer for analysis. The output voltage at the tip of the sensor was recorded (mV) and converted to units of pressure (kPa) to obtain the intraaggregate pressure relative to atmospheric pressure.



View larger version (21K):
[in this window]
[in a new window]
  		Fig. 1. Schematic view of the experimental setup to measure internal pressure.

 
The aggregates were simultaneously filmed for 1 min using a video camera, which recorded 30 images s–1 and digitized at a rate of four images per second. Using the database editing software Adobe Premiere version 5.0 for Mac OS (Adobe Systems, Inc., San Jose, CA), the image corresponding to the beginning of the immersion of the aggregate in water (t = 0) was visualized and the files corresponding to the succeeding images were compressed to 1 image s–1. The image treatment software NIH Image (v. 1.62 for Mac OS, National Institutes of Health, Bethesda, MD) was used to quantify the volume of air released, the quantity of matter lost, and the changes in the aggregate surface area (swelling) in 1-s intervals, from 0 to 8 s.

To estimate the different factors controlling water entry into the aggregates of the two soils used in this study, a theoretical simulation was performed using the experimental results. The two models were used to compare the evolution of the recorded internal pressure [Pa(t)] in the different aggregates with the pressure predicted [Pp(t)] using (Eq. [6] and [12]). The solution to Eq. [6] and [12] and the estimation of Kns, hf, and {alpha} with MathcadPlus v. 6.0 (Mathsoft Corp.) were performed using the data of air release and swelling obtained experimentally. These factors were estimated with the least square methods (smaller sum square error) calculated between Pa(t) and the Pp(t) obtained with the two models.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 APPENDIX A
 REFERENCES
 
The internal pressure measurements and simultaneous images of the aggregates during rapid wetting clearly showed the slaking of the aggregates after different immersion times. Figure 2 presents a number of these images from the two soils with and without organic soil amendments, and shows less slaking in the two amended treatments.



View larger version (57K):
[in this window]
[in a new window]
  		Fig. 2. Image recorded of individual aggregates in the silty-clay loam soil with (a) no and (b) 24 Mg ha–1 of deinking–secondary sludge applications, or in the clay loam soil with (c) no and (d) 24 Mg ha–1 deinking–secondary sludge application, 3 s after immersion in water.

 
The internal air pressures generally reached a peak within the first 4 s after immersion (Fig. 3) . In all aggregates, the internal pressure increased to a maximum level and then dropped over the first 8 s of immersion. The maximum pressure attained differed according to soil type and the type and dose of the soil amendment. After 2 wk of incubation, the maximum pressure attained decreased with increasing the quantity of paper sludge added. After the maximum pressure was reached, the importance of the subsequent decrease in pressure diminished as the dose of paper sludge increased.



View larger version (35K):
[in this window]
[in a new window]
  		Fig. 3. Evolution of the pressure in both soils: (a) silty-clay loam and (b) clay loam, without and with sludge application at different rates.

 
Image analysis and the statistical analysis of the different data collected from the series of repetitions were used to evaluate aggregate swelling and the amount of air released from each aggregate. The rate of swelling of the supposedly spherical aggregates was relatively linear between 0 and 8 s of wetting and the addition of paper sludge and the addition of composts significantly reduced the rate of swelling. Additionally, the quantity of air released diminished with the addition of paper sludge. These features are discussed in details elsewhere (Zaher, 2001).

Models were fitted to the air pressure data to quantify the magnitude of the different processes (Fig. 4 and 5) . These figures indicate that both models correspond to the general evolution of intraaggregate pressure following rapid wetting in water. The tendency to underestimate the peak in the control is observed for both models and soils. It may be the result of a preferential water flow in small fractures resulting in faster compression than predicted in the treated soils, a feature already observed (Caron et al., 1998). Also, the lack of fit observed for the early time (t < 2s) results from a low number of points relative to the whole data set over 8 s. Despite many attempts, a better fit for the control could not be obtained with such modeling approaches, except diminishing the number of points at the later time. This would obviously affect the data, but we chose not to remove these points, as it affected only the low organic matter treatment and did not change the trends observed (see below).



View larger version (26K):
[in this window]
[in a new window]
  		Fig. 4. Examples of the best fit lines obtained with Model 1 (air escape) and Model 2 (no air escape) in the silty-clay loam for (a) the control, and for an application of (b) 8, (c) 16, and (d) 24 Mg ha–1 of deinking secondary sludge; (e) application of 18 Mg ha–1 of primary–secondary sludge; (f) represents the 24 Mg ha–1 compost application.

 


View larger version (26K):
[in this window]
[in a new window]
  		Fig. 5. Results of the best fit for Model 2, without air escape, as sludge addition increases in the silty-clay loam.

 
For both soils, the results obtained using Model 2, without air release, are closer to real intraaggregate conditions because the overall sum of squares was always lower in the interval of good fit (2 s < t < 8 s), as seen in Tables 1 and 2. Also, the air release from the aggregates following wetting occurs in fact intermittently and the fraction of air released is relatively small in relation to cumulative infiltration (see the Appendix). In this case, air may be released at the same time as a pneumatic potential is generated at the wetting front. The model without air release could be used alone to describe the process, taking into account the fraction of air released at each instant and indeed, such a model better fits the observed data.


View this table:
[in this window]
[in a new window]
  		Table 1. Estimates of hf, Kns, and {alpha} obtained with both models for different sludge application rates in the silty-clay loam soil after 2 wk of incubation.

 

View this table:
[in this window]
[in a new window]
  		Table 2. Estimates of hf, Kns, and {alpha} obtained with both models for different sludge application rates in the clay loam soil after 2 wk of incubation.

 
The precision of the model improved with the quantity of sludges applied (Fig. 5). The results for Kns, {alpha}, and hf estimated with both models (Tables 1 and 2) show that these properties tend to evolve in the same way in the aggregates amended with paper sludge.

For both soils and both models tested, a decrease in Kns, an increase in {alpha}, corresponding to a decrease in the rate of water entry, as the wetting front moves toward the aggregate center, and an increase in hf were observed with the application of paper sludge in comparison to the control soils that received no amendment. The lack of fit observed for the control will not change that conclusion, as a better fit for the early part will only increase the differences between the treatments. The values for hf, Kns, and {alpha} in the aggregates amended with the deinking–secondary and the primary–secondary sludge mixes were similar to but greater than those obtained for the aggregates amended with compost. These results indicated that the protective effect of these two sludge treatments on soil stability is superior to that of the composted deinking sludge.

These tables showed that the possible physical effect of the organic matter is to reduce water entry by increasing the matric potential at the wetting front (Sullivan, 1990), and/or the obstruction of pores by organic matter or by air bubbles (Sullivan, 1990; Caron et al., 1996). The increase in hf is consistent with observations of Dinel and Gregorich (1995) and Wright et al. (1999) and attributing increased stability to a decrease in soil wettability (increase in hydrophobicity) (Le Bissonnais, 1989; Sullivan, 1990; Chenu et al., 2000). The increase in the matric potential at the wetting front may result from an increase of the apparent contact angle. However, the deposition or formation of organic material onto pore surface will also change the rugosity of the pore system (surface roughness), and hence the apparent contact angle. These results are consistent with the work of Dinel et al. (1991a)(1991b) and Wright and Upadhyaya (1998), who postulated that the hydrophobicity of organic matter plays a major role in the reduction of water entry into the soil. However, this consistency may only be apparent only, because most studies on hydrophobicity do not consider pore clogging and changes in surface roughness. The results of the present study are in keeping with the conclusions of many authors who noted that, in the presence of organic matter, greater amounts of air were trapped in the aggregates (Foster et al., 1983; Sullivan and Koppi, 1987; Sullivan, 1990) since air is often distributed nonuniformly at a microscopic level. The result of this irregular distribution of air is a partial or complete occlusion of the pore system (Philip, 1957; Emerson and Bond, 1963; Sullivan, 1990).

For both soils in this study, the matric potentials hf attained with Model 2 are lower than those attained with Model 1 (Tables 1 and 2). The difference is due in particular to the added effect of the pneumatic potential introduced into the theoretical formulation of Model 2 without air release. The pneumatic potential exerts some resistance to water entry, resulting in a decrease in the capillary force causing the water intake. The decrease in Kns of the two soils following the addition of organic matter in the form of paper sludge may be attributed as much to pore occlusion as to the increased of the rugosity of the pore system. In both cases, the rate of water entry is slowed and following the addition of organic matter, the intraaggregate pressures decreased. Direct measurements of intraaggregate pressure and the quantities of air released show that both of these decreased with the addition of organic matter, which suggests the movement of the wetting front was inhibited to a greater extent than could be accounted for by escape of entrapped air.

The results presented in Tables 1 and 2 also show that the reduction in near saturated hydraulic conductivity ({alpha}) was greater with the addition of paper sludge. This reduction in conductivity hampers the movement of the wetting front as it advances toward the center of the aggregate. It cannot be attributed to increased swelling since, on the contrary, swelling was observed to decrease with the addition of organic matter (Zaher, 2001). The lower conductivity may be due to increased occlusion of the pore network as water moves to the center of the aggregate. A hypothesis is that organic matter, whether debris, hyphae, or polymers, alone or in combination, act as a nucleus for aggregation and at the center of stable aggregates (Puget et al., 1995; Six et al., 1998), which slows the rate of water entry toward the center and protects aggregates from the stresses caused by contact with water (Caron et al., 1996, 1998).

In the light of the results of this study, Fig. 6 illustrates a plausible interrelation between organic matter and structural stability, where organic matter and air entrapment cause pore occlusion, alter the rugosity of the pore surface and lessen the pressure buildup in the aggregates following their immersion in water. The presence of organic matter sorbed onto pore surfaces increases surface roughness and hydrophobicity and increases hf. Meanwhile, if organic matter is the nucleus for aggregation and located in the center of the aggregate, it would result in a higher {alpha}-value, because Kns decreases faster as water invades a clogged-pore system than an unclogged one, consistent with our model's results. Finally, a pore network clogged, either by air bubbles or sorbed organic matter, will have a lower Kns value than an open one, consistent also with previous observations made (Caron et al., 1996).



View larger version (33K):
[in this window]
[in a new window]
  		Fig. 6. Conceptual model of the main factors and properties controlling aggregate stability when an aggregate is suddenly wet. It includes the rate of pressure buildup P(t)], the near saturated hydraulic conductivity (Kns), the potential at the wetting front [hf(t)] and the rate of loss of hydraulic conductivity as water enters the pore space ({alpha}).

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
APPENDIX A
 REFERENCES
 
The quantitative and simultaneous measurements of intraaggregate pressure, swelling, and air release from aggregates subjected to rapid wetting in water highlight the effect of organic matter in improving structural stability. Aggregates rupture rapidly in contact with water, within the first 8 s of immersion, and the rupture occurs at the periphery of the aggregates. The two factors controlling aggregate disintegration, namely pressure buildup and swelling, are significantly reduced by the addition of organic matter. The results of this study also indicate that organic matter plays a role in soil stability by improving cohesion and contributing to a decrease in water entry. The change in these two aggregate properties brought about by the addition of organic matter leads to a decrease in swelling and intraaggregate pressure.

The theoretical approach developed indicated that addition of organic matter brought about an increase in hf and {alpha} and a decrease in Kns. These results confirm that organic matter affects aggregation by slowing water entry into the aggregate. The two mechanisms of action of the organic matter in reducing the rate of water entry are occlusion and the increase in the rugosity and hydrophobicity of the pore space by the carbonaceous fractions.


   APPENDIX A
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX A
REFERENCES
 
Figure A1 illustrates the evolution of the percentage of air released for the two soils, along with the fitted linear models. It is seen that the proportion of air released remained significant but small in the first 8 s. This percentage evolves linearly with time, whatever the soil or the sludge amendment.



View larger version (23K):
[in this window]
[in a new window]
  		Fig. A1. Evolution of the percentage of air released in the first 8 s following immersion for (a) the silty-clay loam and (b) the clay loam.

 
Table A1 describes the variables used in this study.


View this table:
[in this window]
[in a new window]
  		Table A1. Variables used in this study.

 

   ACKNOWLEDGMENTS
 
The authors are grateful to the Natural Sciences and Engineering Research Council of Canada and to Daishowa, Inc. for their financial support. The laboratory assistance of L. Trépanier and E. Reid is gratefully acknowledged.

Received for publication September 22, 2003.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 THEORY
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 APPENDIX A
 REFERENCES
 




This article has been cited by other articles:


Home page
 Soil Sci.Home page
Z. Hafida, J. Caron, and D. A. Angers
Pore Occlusion by Sugars and Lipids as a Possible Mechanism of Aggregate Stability in Amended Soils
Soil Sci. Soc. Am. J., October 29, 2007; 71(6): 1831 - 1839.
[Abstract] [Full Text] [PDF]

Home page
 Vadose Zone JHome page
A. J.M. Smucker, E.-J. Park, J. Dorner, and R. Horn
Soil Micropore Development and Contributions to Soluble Carbon Transport within Macroaggregates
Vadose Zone J., May 17, 2007; 6(2): 282 - 290.
[Abstract] [Full Text] [PDF]

This Article
Right arrow Abstract Freely available
Right arrow Figures Only
Right arrow Full Text (PDF) Free
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (3)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Zaher, H.
Right arrow Articles by Ouaki, B.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Zaher, H.
Right arrow Articles by Ouaki, B.
Agricola
Right arrow Articles by Zaher, H.
Right arrow Articles by Ouaki, B.
Related Collections
Right arrow Structure and Properties
Right arrow Soil Physics
Right arrow Pore-Scale Modeling
Right arrow Macroporous/Aggregated Media

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
The SCI Journals Agronomy Journal Crop Science
Vadose Zone Journal Journal of Plant Registrations
Journal of Natural Resources
and Life Sciences Education		 Journal of
Environmental Quality