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


     

Published online 29 October 2007
Published in Soil Sci Soc Am J 71:1831-1839 (2007)
DOI: 10.2136/sssaj2006.0257
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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 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 Google Scholar
Google Scholar
Right arrow Articles by Hafida, Z.
Right arrow Articles by Angers, D. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Hafida, Z.
Right arrow Articles by Angers, D. A.
Agricola
Right arrow Articles by Hafida, Z.
Right arrow Articles by Angers, D. A.
Related Collections
Right arrow Structure and Properties

SOIL PHYSICS

Pore Occlusion by Sugars and Lipids as a Possible Mechanism of Aggregate Stability in Amended Soils

Zaher Hafidaa, Jean Caronb,* and Denis A. Angersc

a Département Sol Eau Biodiversité, École Nationale Forestière d'Ingénieurs, BP 511, Tabriquet, 11 000, Salé, Morocco
b Département des Sols et de Génie Agroalimentaire, Université Laval QC, G1K 7P4 Canada
c Centre de Recherche sur les Sols et les Grandes Cultures, Agriculture et Agroalimentaire Canada, Ste-Foy, QC, G1V 2J3, Canada

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


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Understanding the underlying mechanisms of structural stability and the contribution of specific organic fractions to such mechanisms is critical in designing new soil and water conservation strategies relying on organic amendments. The objective of this work was to study the role of neutral and uronic sugars and lipids in affecting key mechanisms (swelling rate, pressure evolution) involved in the stabilization of individual aggregates. A 48-wk incubation study was performed on a clay loam and a silty clay loam amended with either deinking–secondary sludges, primary–secondary sludges, or composted deinking sludges at rates ranging from 8 to 24 Mg dry matter ha–1. Different structural stability indices were measured during the incubation, along with CO2 evolved, neutral and uronic sugar, and lipid contents. Significant increases in all stability indices were measured for both soil types. These improvements were linked to a very intense phase of C mineralization and highly correlated with neutral and uronic sugars as well as lipid contents. Paper sludge amendments also resulted in significant decreases in maximum internal pressure of aggregates and aggregate swelling following immersion in water, two mechanisms affecting structural stability. Overall, the results suggest that reduction in maximum internal pressure induced by organic amendments probably resulted from increases in pore surface roughness and pore occlusion rather than an increase in surface wetting angles. This study also supports the view of a nonspecific action of the lipids and neutral and uronic sugars on aggregate stability to rapid wetting.

Abbreviations: DCF, dispersible clay fraction • Kns, aggregate near saturated hydraulic conductivity • WSA, water-stable aggregation of moist soil • WSAD, water stable aggregation of soil immersed air dried


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Aggregate stability is important to maintain agricultural productivity and environmental quality. The beneficial effect of organic matter on aggregate stability has been known for a long time; however, there is still much uncertainty and debate about the mechanisms that are responsible for this effect (Kay, 1998). Soil organic matter and mineral particles interact through complex chemical, physical, and biological mechanisms.

Swelling rate and pressure buildup resulting from water entering the aggregate have been identified as major contributing mechanisms to aggregate disruption when dry soil aggregates are immersed in water (Grant and Dexter, 1990; Chan and Mullins, 1994). The relative importance of each factor varied in the different studies. Using a modeling approach and measurements on undisturbed aggregates, Zaher et al. (2005) also confirmed that dry aggregate disintegration following sudden wetting was linked to both differential swelling and pressure buildup. They also concluded that organic matter addition (paper sludge) reduced pressure buildup by slowing water entry into the aggregate, possibly through a reduction in water entry and an increase in the roughness and hydrophobicity of the aggregate internal pore space.

The effects of organic amendments on aggregate stability are linked to their transformation by soil microbes and the stimulated microbial activities resulting from a new C input (Lynch and Bragg, 1985). Complex interactions between biological, chemical, and physical factors are involved. Soil microbial activity can promote aggregate stability through (i) a mechanical effect linked to physical enmeshment by fungal hyphae (Tisdall and Oades, 1982; Degens et al., 1996), and (ii) an aggregating effect induced by biomolecules exuded by the microbial colonies (Czarnes et al., 2000; Hallett and Young, 1999). Several studies have looked at the specific roles of selected organic fractions such as carbohydrates (e.g., Chantigny et al., 1997) and lipids (e.g., Bastos et al., 2005). Carbohydrates, by their capacity to form complexes with mineral surfaces, can improve structural stability by increasing cohesion and restricting swelling (Chenu et al., 1994; Cheshire et al., 2000), while lipids, by increasing hydrophobicity and reducing aggregate wettability, can reduce pressure buildup following immersion (Sullivan, 1990; Piccolo and Mbagwu, 1999).

To date, only a few studies have tried to simultaneously compare the role of neutral and uronic sugars and lipids (Monnier, 1965; Dinel et al., 1991). Moreover, evidence for the mechanisms involved has often been established following wetting of moist aggregates after various pretreatments, thus altering aggregate structure and resulting in manipulations on altered aggregates with which the rapid wetting process may not be involved anymore. To the best of our knowledge, such studies have never been performed on intact dry aggregates. Zaher et al. (2005) and Zaher (2001) have investigated the contributing mechanisms on dry intact aggregates suddenly immersed, and experimentally measured the magnitude of those mechanisms (pressure buildup and swelling rate) and stability indices (lost soil material from aggregates) following the sudden wetting of the aggregates in free water. They concluded that organic amendments would increase aggregate stability by restricting swelling and reducing internal pressure evolution, probably by clogging the internal pore space and restricting water entry, as a result of lower aggregate near saturated hydraulic conductivity (Kns) values as water moved into the aggregate. These studies also concluded that the restriction of water entry was probably a result of air entrapment and an increase in pore surface roughness rather than a true change in wetting angle. Hence, it could be hypothesized that specific substances like neutral and uronic sugars and lipids may create such effects, as these substances would be produced by microbial decomposition of applied organic amendments The objective of this study was to expand earlier work with additional stability measurements and correlate these different stability indices with uronic sugar, neutral sugar, and lipid amounts found in amended soils. Hence, this study aims at providing new insights into the specific role of neutral and uronic sugars and lipids in aggregate stabilization.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Setup
Two soil types from the Québec City (Canada) area (46°N, 71°W) were used in this experiment. One was a silty clay loam from the Tilly series (fine-silty, mixed, frigid Aquic Haplorthods) from St-Augustin and the other a clay loam from the Bedford series (loamy, mixed, nonacid, frigid Typic Humaquepts) located in Ste-Croix de Lotbinière (Table 1 ). Composite soil samples were collected along a transect from the surface horizon (0–20 cm) using a spade of each soil; samples were sieved directly in the field to collect the 0- to 6-mm fraction. The samples were put in plastic bags and brought to the laboratory to be stored at 4°C. Later, 200-g samples of moist soil were amended with three different types of paper sludge (Table 1) at different rates. The treatments were (i) three application rates of deinking–secondary sludge mix (8, 16, and 24 Mg dry matter ha–1), (ii) one application rate of primary–secondary sludge mix (18 Mg oven-dry matter ha–1) containing the same quantity of C as the 24 Mg ha–1 rate of deinking–secondary mix, (iii) one application rate of composted deinking sludge (24 Mg ha–1), and (iv) a control treatment that received no amendment. The rates of C applications corresponded to 1345, 2690, and 4035 mg C kg–1 dry soil for the 8, 16, and 24 Mg ha–1 of deinking sludge application, respectively, 4035 mg C kg–1 dry soil for the primary–secondary sluge application (18 Mg ha–1), and 2776 mg C kg–1 dry soil for the compost (24 Mg ha–1) treatment. The sludges were generated by a nearby plant and came from a stable production process, while the compost came from a composting site using the same sludges. The sludges were obtained from the plant and found to have a composition representative of the process. The sludges are composed primarily of wood fibers (essentially 39% cellulose, 11% hemicellulose, and 23% lignin), whereas the deinking sludge may also contain clay (aluminum silicates), ink residues, kaolinite, and charcoal black. Ammonium nitrate (34–0–0, N–P–K) was added to the deinking–secondary and primary–secondary sludges to lower the C/N ratio to <30. Phosphorus and K were also added to reach C/P and C/K ratios of 60 and 130, respectively, using superphosphate (0–46–0) and KCl (0–0–60) (Mustin, 1987).


View this table:
[in this window]
[in a new window]

  		Table 1. Physicochemical characteristics of the soils and the different types of paper sludge.

 
The deinking–secondary mixes were composed of 80% deinking and 20% secondary sludge, based on the average ratio observed at the factory. For the primary–secondary sludge mix, the applied rate was chosen to obtain the same C input as the deinking–secondary sludges, while maintaining the same 80:20 primary/secondary ratio.

Soil texture was measured using the hydrometer method (Day, 1965), and pH determined in a 1:1 soil/distilled water suspension (McLean, 1982). Organic C was determined by the Walkley–Black method (Nelson and Sommers, 1982) and total N by micro-Kjeldahl analysis (Bremner and Mulvaney, 1982).

The 200-g soil samples were incubated in 500-mL glass jars for 48 wk at 20°C, maintaining a water content corresponding to a matric potential of –33 kPa (380 g H2O kg–1 for the silty clay loam and 250 g H2O kg–1 for the clay loam) by weighing twice a week and adding the proper water volume to maintain the target weight. The corresponding water potential was based on the water desorption curve of the same soil measured independently. All treatments and two controls (an empty jar and a control soil without amendments) were replicated three times, for each incubation time, for a total of 21 jars per treatment. At different times, jars were removed and their incubated soils used for destructive analyses.

Soil Aggregate Stability Measurements
The water-stable aggregation (WSA) and dispersible clay fraction (DCF) of moist soils were measured after 0, 2, 4, 8, 12, 24, and 48 wk of incubation following Pojasok and Kay's (1990) method. This technique involves an assessment of the stability of macroaggregates and the dispersibility of clay using a combination of wet sieving and turbidimetric measurements. Five grams of incubated, moist, <6-mm aggregates was used for measurements of WSA. The aggregates were poured into 40 mL of distilled water in 100-mL test tubes, and then agitated for 5 min at 56 oscillations min–1 on a horizontal shaker. The suspension was then filtered through a 1-mm sieve and test tubes were washed with 80 mL of distilled water. All the filtrate was collected in a 125-mL Erlenmeyer flask and the amount of dispersible clay in the filtrate was determined by measuring transmittance at a 620-nm wavelength of a 0.1-mL sample collected at the 1-cm depth of the filtrate, sedimented long enough to collect the clay fraction according to Stokes' law. The DCF was expressed as a percentage of the total soil clay present in the soil. The soil remaining on the 1-mm sieve was oven dried and weighed, and the sand or gravel content measured after dispersion. The WSA was expressed as (Angers and Mehuys, 1993):

Formula 1[1]

The proportion of stable aggregates of soil immersed dry (WSAD) was also measured after 0, 2, 4, and 24 wk of incubation (Angers and Mehuys, 1993). A 10-g sample of air-dried, sieved to <6-mm aggregates was then placed on a nest of sieves (sized 2.0, 1.0, 0.5, and 0.25 mm). The aggregates were wetted by direct immersion in water at room temperature (20–22°C) and sieved for 10 min at a frequency of 30 cycles min–1 at a 40-mm amplitude. Each aggregate size fraction was oven dried, weighed, and expressed on an oven-dry basis. The WSAD was computed using Eq. [1], using only the corresponding material left on the appropriate sieve size for calculation. A correction was made for the presence of sand and coarse fragments in the stable aggregates (Eq. [1]).

Measurements of loss of soil material (i.e., small fragments detached from aggregates following immersion and recorded on the image taken) of suddenly immersed aggregates and air released following immersion were performed, as well as measurements of other factors contributing to stability (swelling rate and internal pressure) after 2 wk of incubation. Briefly, air-dry aggregates of a radius of approximately 6 mm were chosen. The intraaggregate pressure was measured using a pressure transducer in contact with aggregates following the rapid wetting of dry aggregates into distilled water. Image data were collected simultaneously, and other parameters such as swelling rate, loss of soil material, and air release were evaluated from the image taken. Further details can be found in Zaher et al. (2005) and Zaher (2001). The data from that study were used for correlation studies (see below).

Carbon Mineralization and Organic Fractions
Carbon mineralization was determined by titration after 0 and 3 d, and 1, 2, 4, 8, 12, 24, and 48 wk of incubation. A 20-mL 1 mol L–1 NaOH solution was titrated with HCl in excess of BaCl2, using phenolphthalein as an indicator. The various C fractions (neutral and uronic sugars and lipids) were measured after 0, 2, 4, 8, 12, 24, and 48 wk of incubation. Both neutral and uronic sugars were hydrolyzed using H2SO4. Carbohydrates were measured on sieved (0.15-mm) air-dried soils collected after the proper incubation time. Two grams of soil was mixed with 30 mL of 0.5 mol L–1 H2SO4 at 85°C for 24 h. For total neutral sugars, the extract was neutralized with NaOH. After filtration and centrifugation, the extract was then analyzed with the automated alkaline-ferricyanide method (Cheshire, 1979). The content was expressed as milligrams of D-glucose per kilogram of soil. The total uronic sugar content of the extract was determined by colorimetry according to Blumenkrantz and Asboe-Hansen (1973). An extract of 0.2 mL, to which was added 1.2 mL of 0.0125 mol L–1 solution of tetraborate in concentrated H2SO4, was placed in crushed ice for 10 min and then heated for 5 min in a water bath at 100°C. After cooling in an ice bath for 10 min, 20 µL of the m-hydroxydiphenyl reagent was added and, within 5 min, absorbance measurements were read at a 520-nm wavelength. The content was expressed as milligrams of sodium galacturonate per kilogram of soil.

Total lipids were analyzed using the method described by Bligh and Dyer (1959). One hundred gram samples of moist soil were extracted with a mixture of chloroform and methanol (volumes of water, chloroform, and methanol were in the proportions 0.8:1:2 and 1.8:2:2 before and after dilution, respectively). The homogenate was allowed to completely separate and clarify. The chloroform layer, which contained the purified lipid, was evaporated to dryness at 40°C. This dry residual was weighed and the quantity of lipids was expressed as milligrams of total extracted lipids per kilogram of dry soil.

Statistical Analysis
The SAS Version 6.12 system for Windows (SAS Institute, 1996) was used to analyze the data. After verifying that variances were homogenous, ANOVAs of the data were performed specifying a factorial treatment x time experiment, arranged in a randomized block design with respect to treatments and times, the different rates and sludge type combinations being considered as individual treatments. Treatments were compared using a protected LSD test when significant treatment effects were found and the statistical interaction of time with treatment (time x treatment) found to be not significant. Polynomial contrasts were used to identify dominant time (linear, quadratic, cubic, quartic, and quintic) effects, also in the absence of interactions. For each soil type, multiple correlations were used between the different measured parameters, by individual dates and for all dates confounded.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Aggregate Stability
Wet Sieving of Moist Aggregates
Temporal trends in WSA were typically the same for both soils (Fig. 1 ). The clay loam soil generally showed a lower proportion of large stable aggregates because of the lower clay content. The proportion of WSA for both soils varied with time of incubation. Amending the soil with paper mill residues had a significant effect on WSA at all sampling dates except the last one at 48 wk. Two peaks were observed, one at 2 wk and a smaller one at 8 wk.


Figure 1
View larger version (16K):
[in this window]
[in a new window]

  		Fig. 1. Effect of different paper sludge application rates and types on the amount of water-stable aggregates for (a) silty clay loam and (b) clay loam soils after different incubation times. Bars represent the LSD between treatments at P ≤ 0.05.

 
There was a linear relationship between C input and the WSA values at almost all dates, shown in Fig. 2 for WSA at 2 wk. The proportion of WSA was generally higher (several times significantly) with the deinking–secondary sludge mix (24 Mg ha–1) than with the sludge compost at the same amount of sludge applied, and not significantly different from the deinking–secondary sludge mix (16 Mg ha–1) at an equivalent C input. This behavior may be related to the compost stability, where readily labile material may have already been decomposed, an explanation supported by the C mineralization study (see below).


Figure 2
View larger version (10K):
[in this window]
[in a new window]

  		Fig. 2. Relationship between C input and the amount of water-stable aggregates for two soils after 2 wk of incubation. The regressions were performed for all the treatments, including the compost.

 
The effects of the residues were still significant 24 wk after addition (Fig. 1). This behavior is consistent with Tisdall and Oades (1982), who argued that slowly decomposable material like cellulose results in a stability increase persistent for months, in comparison to labile material like glucose, for which the effects on stability are temporary (a few weeks). The same trends were observed for sewage sludge amendments by Metzger and Yaron (1987) and by Sidi and Pansu (1990) following wheat (Triticum aestivum L.) straw incorporation as well as in earlier work (Baver, 1948; Monnier, 1965). In a field study, Chantigny et al. (1999) and N'Dayegamiye (2006) also illustrated the rapid and persistent effect of paper sludges on aggregate stability.

Dispersible Clay of Moist Aggregates
Dispersible clay behaved in the opposite direction relative to WSA (Fig. 3 ), consistent with Kay and Dexter (1990), who postulated that an increase in aggregate stability reduced surfaces exposed to clay dispersion because a higher proportion of stable aggregates would mean fewer aggregates of smaller size having a smaller external surface likely to generate clay fragments. The lower extent of swelling and fragmentation with sludge additions (Zaher, 2001) suggests, however, that sludge amendment acted also at the microaggregate level, increasing cohesion and hence decreasing dispersion, in addition to reducing the exposed surface area. The variation in dispersible clay during the 48 wk of incubation also showed that the effect of different sludge applications was transient.


Figure 3
View larger version (15K):
[in this window]
[in a new window]

  		Fig. 3. Effect of different paper sludge application rates and types on the dispersible clay fraction of (a) silty clay loam and (b) clay loam soils after different incubation times. Bars represent the LSD between treatments at P ≤ 0.05.

 
Wet Sieving of Dry Aggregates
While wet sieving of moist aggregates mainly emphasizes a cohesion effect on aggregation through mechanical abrasion by turbulent water and particle displacement on a nest of sieves (Hénin, 1938), the wet sieving of a dry aggregate integrates the additional effect of slaking and differential swelling. Following wet sieving, the distribution of the different aggregate sizes revealed a very similar trend to that observed for wet sieving of moist aggregates (WSA), the proportion of large-size aggregates revealed by WSAD increased with sludge amendment after 2 wk of incubation for both soil types (Fig. 4 ). The same effect, but with a lower significance level, was observed after 4 and 24 wk. Stability values for the WSAD >1 mm were lower than those observed for the WSA test, a fact probably related to the rapid wetting of a dry aggregate resulting in the additional slaking effect during the wet sieving test. Meanwhile, little change had occurred in the control between Days 0 and 15 for both soils, indicating that the changes observed in the other treatments were mainly attributable to microbial activity following sludge application (Fig. 4). After 2 wk, increases in aggregation occurred mainly in the fraction >2 mm, at the expense of the smaller size fractions for both soil types (all fractions <1 mm), which supports the concept of aggregate hierarchy (Oades and Waters, 1991) in these soils (Angers and Giroux, 1996).


Figure 4
View larger version (34K):
[in this window]
[in a new window]

  		Fig. 4. Wet aggregate size distribution of aggregates immersed after air drying of (a) silty clay loam and (b) clay loam soils after 2 wk of incubation with different sludge types and application rates. Bars represent the LSD between treatments at P ≤ 0.05.

 
Other Stability Parameters
The effects of paper sludge application rate on WSA, DCF, and WSAD were very consistent with other additional independent measurements of the stability parameters, notably slaking, swelling rate, and intraaggregate pressure buildup on the same studied soils (Zaher, 2001; Zaher et al., 2005). These studies showed a decrease in slaking of fine material and a reduction in the swelling rate following immersion with increasing sludge application rate. Concurrently, a decreasing amount of air release on immersion, a lower rate of swelling, and a lower maximum pressure recorded were observed with increasing sludge application rates (Zaher, 2001).

Carbon Mineralization
The peak of C mineralization was observed 3 d following the start of incubation for both soils (Fig. 5 ), in agreement with previous studies with similar materials (Zibilske, 1987; Honeycutt et al., 1988). This phase, which is characterized by a rapid mineralization of C, can be attributed to the presence of cellulosic material, which is the dominant fraction of these residues (Chantigny et al., 2000). The rapid decomposition of cellulose is also probably due to the fact that the wood fibers of these residues are short and very humid, and thus more vulnerable to microbial attack. Angers and Recous (1997) showed that crop residue decomposition increases with decreasing particle size and increasing specific surface area of residues. This peak of C mineralization corresponded to the period during which there were increases in WSA and in the different carbonaceous fractions (Fig. 1, 5, 6 , and 7 ), a feature already reported by Harris et al. (1966) and Vandevivere et al. (1990) for different organic materials and by Metzger et al. (1987) for sewage sludge. The decomposability of the organic material seemed to have a strong influence on the extent of stable aggregate formation and stabilization. The compost-amended soil had the lowest mineralization rate and the lowest effect on WSA. Correlation analyses indicated a strong relationship between mineralization rate and WSA for the silty clay loam (P = 0.001) but not for the clay loam (P = 0.10) (Fig. 8 ).


Figure 5
View larger version (13K):
[in this window]
[in a new window]

  		Fig. 5. Carbon mineralization in (a) silty clay loam and (b) clay loam soils during 48 wk of incubation.

 

Figure 6
View larger version (11K):
[in this window]
[in a new window]

  		Fig. 6. Evolution of (a) neutral sugars, (b) uronic sugars, and (c) lipids following paper sludge amendment of a silty clay loam soil. Bars represent the LSD of the time x treatment interaction at P ≤ 0.05.

 

Figure 7
View larger version (12K):
[in this window]
[in a new window]

  		Fig. 7. Evolution of (a) neutral sugars, (b) uronic sugars, and (c) lipids following paper sludge amendment of a clay loam soil. Bars represent the LSD of the time x treatment interaction at P ≤ 0.05.

 

Figure 8
View larger version (8K):
[in this window]
[in a new window]

  		Fig. 8. Relationship between mineralization rates (dc/dt) and water-stable aggregate content for both soils at 2 wk of incubation. The regressions were performed on all treatments, including compost.

 
Organic Fractions
Neutral Sugars
Neutral sugar content varied significantly during the incubation as a function of the organic amendments. Differences at time zero were due to the amount of sugars present in residues before incubation (Table 1). Two peaks were observed in the silty clay loam: a dominant one after 2 wk and a secondary peak after 8 wk of incubation (Fig. 6a), as confirmed by significant (P < 0.0001) higher order polynomial contrasts (Orders 1–4) in the time effect. The same significant treatment effect (P < 0.0001) was observed for the clay loam, with similar trends in peaks revealed by significant (P < 0.0001) higher order (1–6) polynomial contrasts (Fig. 7a). The absolute value, however, was lower in the clay loam than in the silty clay loam soil (maximum value at 3300 vs. 5300 mg D-glucose kg–1 dry soil), probably reflecting the lower initial organic C and carbohydrate content of the former (Table 1).

Increases in neutral carbohydrate concentrations seemed to be influenced by both the type of residue and the application rate. The lowest values were observed for the compost and the lowest rate of sludge addition. The small effect of the compost is consistent with the fact that this amendment did not induce much microbial activity, as seen in the mineralization data. Also, during the composting process, labile C undergoes microbial transformation (Robertson and Morgan, 1995) and there was, therefore, relatively less sugars being applied with the composted residues (Table 1).

Uronic Sugars
In general, variations in uronic sugar content were similar to those of neutral sugars, with a significant (P < 0.0001) increase with sludge application rate and a smaller but significant effect of compost in both soils. A difference was also observed at time zero and was possibly due to uronic sugars found in the residues. Primary–secondary sludge application resulted in larger uronic sugar content than the deinking–secondary sludge mix application, a factor possibly due to the higher uronic sugar content of this sludge mixture (Table 1). The second peak observed at 8 wk was less pronounced for uronic sugars in both soils even at the highest sludge application rate, as reflected by a less significant number of the higher order polynomial contrasts (Orders 1–4). As for neutral sugars, the average level was also higher for the silty clay loam soil than for the clay loam soil (maximum value at 1950 vs. 3350 mg sodium galacturonate kg–1 dry soil).

Lipids
Treatment effects on lipid content were similar to those of neutral sugars, with initial significant differences (P < 0.0001) in lipid content as a result of amendments, which were measured through the whole experiment. The first major peak at 2 wk was followed by a smaller one about 8 wk after the start of the incubation for the clay loam soil but could not be detected for the silty clay loam soil, where only a quadratic trend was found to be significant (P = 0.01) in the time effect, a factor possibly linked to a higher variability between replicates in this soil (Fig. 6c and 7c). Higher order polynomials (1–5) were all significant (P < 0.0001) in the clay loam soil. Lipid contents increased with sludge application rate, but the primary–secondary sludge mix ranked third in terms of total content, a factor possibly due to the higher lipid content of the deinking sludge compared with the primary sludge (Table 1). Again, the lipid content of the amended silty clay loam soil peaked at a higher value than that of the clay loam soil (2250 vs. 1550 mg extracted lipids kg–1 dry soil), a factor partly linked to the higher initial lipid content of the clay loam soil (Table 1).

Relationships between Organic Matter Fractions and Stability Parameters
Correlation analyses between stability parameters and organic fractions were performed at all dates (2, 4, 12, 24, and 48 wk). These relationships generally followed the same trends at all dates, and for the sake of clarity only correlations obtained after 2 wk of incubation (Tables 2 and 3 ) are presented. Correlation analyses were also performed on the amendment-induced sugar and lipid contents (i.e., increase in sugar and lipid contents relative to the initial amount detected at time zero, immediately after amendment) and stability parameters. These also showed the same trends, with even higher correlation coefficients. Despite this, it was decided to use correlation between uncorrected C fractions and stability parameters, to allow more appropriate comparisons with previously published studies.


View this table:
[in this window]
[in a new window]

  		Table 2. Correlation matrix between stability parameters and different organic fractions for two soil types 2 wk after paper sludge amendment (n = 18).

 

View this table:
[in this window]
[in a new window]

  		Table 3. Correlation matrix between some stability mechanisms and different organic fractions for two soil types 2 wk after paper sludge amendment (n = 18).

 
Correlations between C fractions and stability parameters consistently showed the same trends for both soil types. All three fractions were closely linked to the different stability parameters and none of the fractions really stood out from the others. Correlations were, in general, higher at the macroaggregate (WSAD and WSA) than the microaggregate scale (dispersible clay and lost soil material), reflecting the sensitivity of millimeter-size aggregate to organic matter management in these soils (Angers, 1998). When it came to the two mechanisms related to stability in these soils, namely swelling rate and maximum pressure, correlation coefficients were higher for the maximum pressure reached in the aggregate. Again, none of the fractions clearly stood out from the others for either soil type. The same observations were made for the swelling rate, where correlation coefficients were all significant and around the same values for neutral and uronic sugars and lipids.

The significant relationships between neutral and uronic sugars on the one hand and dispersible clay, lost soil material, and the swelling rate on the other hand suggest a role of carbohydrates in cohesion. The uronic and neutral sugars produced in the soil following microbial decay may have created links between microbial products and clay surfaces through van der Waals attraction for neutral sugars and cationic bridges, electrostatic attraction, or ligand exchange for uronic sugars, hence increasing cohesion (Hamblin and Greenland, 1977; Caron et al., 1992). The fact that these same fractions, well known to be dominantly hydrophilic, are related to macroaggregation and its associated mechanism (the maximum pressure reached) also suggests a second role for neutral and uronic sugars. Indeed, observations made earlier on the same treatments of lower Kns values in aggregates (Zaher et al., 2005), of a decreasing Kns as water penetrates aggregates (Zaher et al., 2005), and of a larger amount of entrapped air (Zaher, 2001) are consistent with this study for the action of sugars at the external surface of microbial colonies and decaying organic matter inside pores. These sugars, occupying the pore space and clogging it (Quirk and Williams, 1974; Caron et al., 1996), would result in a lower maximum pressure reached during rapid wetting (Zaher et al., 2005) as a result of a reduction in water entry into the aggregate.

In this study, the lipid fraction also showed the same close and positive relationships with all four stability indices and both contributing mechanisms (Table 3), in agreement with previous work (Capriel et al., 1990; Haynes and Swift, 1990; Dinel et al., 1992; Chenu et al., 2000). It is often argued that the strong correlation between stability and the lipid fraction is linked to the hydrophobicity of the lipid fraction (Dinel et al., 1991; Dinel and Gregorich, 1995). Increasing sludge application rate did result in a significant increase of the potential at the wetting front for the same soil treatments (Zaher et al., 2005). Combined with these new results on lipid measurements, increased potentials at the wetting front support the view of the existence of a true hydrophobic effect, associated with a change in the contact angle at the mineral–water interface brought about by lipids. Zaher (2001), however, showed that the use of a tensioactive solution did not affect the pressure evolution inside aggregates, which implies no change in the resulting potential at the wetting front, despite a change in wetting angle. This suggests that lipids may act by changing the surface roughness, as polysaccharides would also do. This common and nonspecific mechanism of action for both lipids and carbohydrates is consistent with the hypothesis that all three fractions would occupy pores on the internal surface of aggregates, which would slow down the rate of water entry and would increase the apparent water potential at the wetting front. This would also reduce the swelling rate of the aggregate because of reduced exposed internal surfaces in contact with the moving wetting front, relative to unamended aggregates, and result in a lower rate of wetting and a lower maximum pressure reached in the silty clay loam soil. This hypothesis is supported by the observations made by Chantigny et al. (1999) that, following deinking paper sludge addition, stable aggregates are formed by encrustation of small organic clusters by mineral particles. It is also analogous to the aggregation model of Golchin et al. (1997), which suggests that particulate plant residues serve as a nucleus of stable aggregate formation.

For the clay loam soil, changes in contact angle may also be involved to a limited extent, although the surfactant effect had a lower efficiency during the first 10 s of wetting, a critical period in the pressure buildup evolution (Zaher, 2001). The fact, however, that the use of a surfactant during wetting of an aggregate had not significantly changed either the pressure evolution or the swelling rate of immersed aggregates relative to the treatment in pure water (Zaher, 2001), combined with the correlation found here, suggests that changes in the contact angle also played a minor role in the overall mechanisms causing structural instability of the clay loam.

In this study, the first measurements were made 2 wk after the start of the incubation. Since changes in aggregate stability can occur rapidly after organic amendments (e.g., Abiven et al., 2007), some of the early changes may not have been captured. Correlations performed at all dates gave similar results, however, which further supports the hypothesis of nonspecific actions of carbohydrates and lipids.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study confirmed the beneficial effect of paper sludge amendment on aggregate stability. Results suggest a nonspecific but dominant role of the biologically mediated production of neutral and uronic sugars and lipids on aggregate stability and on the mechanisms controlling it. They support the view that all three fractions originating from decaying organic matter would occupy pores on the internal surface of aggregates. Such occlusion appears to play a dominant role during rapid wetting, with consequent effects on the pressure evolution and the swelling rate of the immersed aggregates, and hence their final stability under rapid wetting conditions.


   ACKNOWLEDGMENTS
 
We are grateful to the Natural Sciences and Engineering Research Council of Canada and to Daishowa Inc. for their financial support. The laboratory assistance of G. Thériault, F. Shisregara, L. Trépanier, and E. Reid is gratefully acknowledged. Thanks are extended to C. Chenu for helpful comments.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication July 10, 2006.


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





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 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 Google Scholar
Google Scholar
Right arrow Articles by Hafida, Z.
Right arrow Articles by Angers, D. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Hafida, Z.
Right arrow Articles by Angers, D. A.
Agricola
Right arrow Articles by Hafida, Z.
Right arrow Articles by Angers, D. A.
Related Collections
Right arrow Structure and Properties

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