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Soil Science Society of America Journal 63:1214-1221 (1999)
© 1999 Soil Science Society of America

DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Aggregation and Organic Matter Decomposition in Soils Amended with De-Inking Paper Sludge

Martin H. Chantignya, Denis A. Angersb and Chantal J. Beauchampc

a Agriculture and Agri-Food Canada, Soils and Crop Research and Development Centre, 2560 Hochelaga Blvd., Ste-Foy, PQ, Canada, G1V 2J3
b Agriculture and Agri-Food Canada, Soils and Crop Research and Development Centre, 2560 Hochelaga Blvd., Ste-Foy, PQ, Canada, G1V 2J3
c Centre de Recherche en Horticulture, Faculté des Sciences de l'Agriculture et de l'Alimentation, Université Laval, Québec, Canada, G1K 7P4

chantignym{at}em.agr.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusion
 REFERENCES
 
De-inking paper sludge (DPS) has been traditionally disposed of by burning or landfilling, but could be used as an organic amendment in agricultural soils. Our objective was to assess the impact of DPS incorporation on organic matter and aggregation of a clay loam (Typic Dystrochrept) and a silty clay loam (Typic Humaquept). Whole soil C, particulate (>53 µm) and light fraction (density <1.8 Mg m-3) C, and water-stable aggregation were measured periodically during a 3-yr period after a single application of DPS at rates of 0 (control), 50, and 100 Mg ha-1. Microscopic observations of water-stable aggregates were also performed. Adding DPS increased whole soil C content, which remained greater than in the control for the duration of the study. After 2 yr, about 40% of the initial material remained in the soil. The proportion of residual C attributed to DPS and present in the particulate fraction remained constant at 70 to 90% during the first 2 yr of the study, whereas the proportion of residual C present in the light fraction decreased from >95% for fresh DPS to <50% after 2 yr. One year after incorporation of DPS, the proportion of water-stable aggregates >1 mm was 2 to 6 times larger in amended soils than in the control. This effect was still statistically significant after 3 yr. Microscopic observations revealed that DPS formed into clusters of wood fibers which became encrusted with mineral particles. We hypothesized that this encrustation provided physical protection to the decaying DPS which remained particulate (>53 µm) in size and progressively densified to >1.8 Mg m-3. As a result, water-stable macroaggregates were formed with DPS as a central core.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results
 Discussion
 Conclusion
 REFERENCES
 
APPLICATION OF VARIOUS PAPER MILL SLUDGES on agricultural soils has been proposed for years as an economic and environmental alternative to burning or landfilling (Dolar et al., 1972; Das and Jena, 1973). Previous studies have focused primarily on the impact of these residues on crop performance (Bellamy et al., 1995; Fierro et al., 1999c) and soil nutrient cycling (Zibilske, 1987; Simard et al., 1998), but less attention has been paid to their impact on soil structure and organic matter, which are critical properties for soil productivity and conservation.

Physical fractionation of organic matter based on either particle size or density has proven useful to determine the fate and decomposition of crop residues in soil (Christensen, 1992). It has been found that the light fraction of crop residues rapidly decomposes (Shields and Paul, 1973), and that residue particle size decreases with a large proportion of the residual material becoming heavier because of close association with the clay- and silt-size fractions (Christensen, 1992; Aita et al., 1997). DPS is mainly composed of short wood fibers and kaolin clays used during paper-making processes, and inks and chemicals used for paper recycling purposes. Further, freshly milled DPS readily forms millimeter-size clusters of low density. Therefore, it was hypothesized that particulate and light fraction assessments would be useful to characterize the fate and decomposition pattern of DPS in agricultural soils.

Microorganisms first colonize the surface of the substrate and process the easily available C fraction at a relatively rapid rate during the initial phase of decomposition of a particulate organic substrate (Ladd et al., 1996). This initial flush of activity may lead to extracellular polysaccharide production and the binding of soil particles (Golchin et al., 1997; Ladd et al., 1996). As a result, new aggregates are formed with particulate organic matter as the central core. It is also believed that particulate organic matter occluded within soil aggregates is physically protected against decomposition (Golchin et al., 1997; Angers and Chenu, 1997).

Few studies have dealt with the use of DPS as an organic amendment or soil conditioner. Fierro et al. (1999b) studied the decomposition of DPS in a degraded sandy soil and found that the decaying substrate progressively densified but without significant fragmentation into particles <53 µm. Using similar material, Trépanier et al. (1996) found an increase in soil aggregate stability when adding 18 Mg DPS ha-1 to a silty loam cultivated to potato (Solanum tuberosum L.). However, it was found that loading rates up to 18 Mg DPS ha-1 only had a transient effect and had to be renewed each year to sustain soil stability. The fate of DPS after burial in agricultural soils, and the mechanisms by which DPS may improve soil aggregation are unknown. Further, DPS application rates larger than 18 Mg ha-1 have not been tested as a means to produce larger and prolonged improvement in soil aggregate stability. The aims of this study were (i) to determine the impact of one large addition of DPS on soil organic matter and water-stable aggregation, and (ii) to determine the physical transformations undergone by DPS as characterized by its particle size and density fractions.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results
 Discussion
 Conclusion
 REFERENCES
 
Experimental Set-Up
The field sites used were located near Québec City, Canada (Lat. 46.50° N, Long. 71.20° W). The soil type of one field site was a well drained Tilly clay loam (fine, mixed, frigid, Typic Dystrochrept), whereas the other field was a poorly drained Kamouraska silty clay loam (fine, mixed, frigid, Typic Humaquept). The sites are in a cool and humid area with yearly mean precipitation of 1200 mm almost uniformly distributed throughout the year. The soil is generally snow covered from early November to late April with soil temperature around 0°C (Ouellet et al., 1975). The growing season begins in early May and concludes by the end of September with a daily mean soil temperature near 20°C (Ouellet et al., 1975), and heat accumulation between 2200 and 2400 degree-days.

DPS was obtained from the Daishowa, Inc., paper mill company located at Québec City, and was similar to the material used and described by Trépanier et al. (1996) and Fierro et al. (1999b). Briefly, the sludge used was generated mostly from the recycling of newspaper, which was made of black spruce [Picea mariana (Mill) BSP] fibers. Before its use, to ensure uniform mixing with the soil, DPS was pulverized to obtain a fluffy granular material <6 mm. At the time of application, the sludge had a bulk density of 0.224 Mg m-3 and a water content of 0.52 kg kg-1. Selected properties of DPS and soils are reported in Table 1 . De-inking paper sludge was applied at rates of 0 (control), 50, and 100 Mg ha-1 (dry basis) in autumn 1994 to plots of 5 by 7 m in size. The material was uniformly spread on the soil surface and buried by rototilling to obtain a worked surface layer of 20 cm thick. In the spring of 1995, the worked soil layer had settled to 15 cm and did not significantly change thereafter. Alfalfa (Medicago sativa L.), cv. Saranac, red clover (Trifolium pratense L.), cv. Florex, and bromegrass (Bromus inermis L.), cv. Baylor, were established in pure stands, and grown for the first two growing seasons of the experiment (1995 and 1996). Each spring, the soils were fertilized with P and K according to crop requirements and soil analyses (CPVQ, 1994). In the spring of 1995, an additional 0.23 kg P was added to each Mg DPS incorporated to counteract P immobilization (Fierro et al., 1999c). In the autumn of 1996 (Day 741 of the study), soils were plowed under to a depth of 20 cm and sweet corn (Zea mays L.), cv. Delectable, was grown on all plots in the growing season of 1997, and fertilized with 120 kg N ha-1. The experiment was laid out as a split-plot replicated four times on each soil type with DPS application rates as the main plots and crop species as the sub-plots.


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  		Table 1 Selected properties of the soils and de-inking paper sludge used during the study, as expressed on a dry matter basis (± std. dev.)

 
Soil Sampling and Analyses
Soil samples were collected during the growing seasons of 1995, 1996, and 1997, but were not sampled during the winter periods of 1994, 1995, and 1996 when the ground was snow-covered. More precisely, soil samplings were performed at Day 271, 307, 342, 370, 596, 645, 683, 726, 973, and 1075 after DPS was applied to the soil. On each sampling date, five soil cores (5-cm diam.) were collected in each plot to a depth of 15 cm, which corresponded to the whole soil layer affected by the added sludge. The soil cores were sieved at 6 mm in the field and pooled to make one representative soil sample per plot. Mesh size of 6 mm was the minimum opening to be used for DPS to completely pass the sieve without being forced through it. Soil samples were air dried and stored in plastic bags until analyses were performed.

Whole soil C was determined by dry combustion (LECO CNS-1000, Leco Corp., St. Joseph, MI) on soil samples ground to pass a 0.5-mm sieve. There were no carbonates in the soils used. Furthermore, less than 3% of total DPS-C was present as carbonates, which was likely lost soon after application to our acid soils. Therefore, whole soil C measurements were considered to represent organic C contents.

The light fraction was determined on the basis of the method proposed by Gregorich and Ellert (1993) with some modifications dictated by the particular nature of DPS. Ten grams of air-dried soil (<6 mm) were placed in 100-mL polypropylene tubes with 40 mL of sodium polytungstate solution. Preliminary results showed that the proportion of fresh DPS recovered as light material increased with increasing density of polytungstate solution from 1.2 to 1.8 Mg m-3. A solution of 1.8 Mg m-3 was used since >95% of fresh DPS was recovered as light material with no significant gain at higher solution density. Tubes were agitated for 60 min on a reciprocal shaker. The internal side of each tube was then washed with a small quantity of sodium polytungstate solution, and tubes were centrifuged at 1000 g for 15 min. The floating material was recovered and filtered on a 0.45-µm filter membrane, and then transferred to another 100-mL tube and shaken for 30 min with 50 mL of 0.01 M CaCl2 solution. Preliminary tests indicated that this "washing-shaking" step was necessary to remove polytungstate salts retained by DPS materials, and reduced the error in estimating the weight of material recovered to <2%. Tubes were centrifuged and the floating material was recovered on a filter membrane as described above. The material remaining on the membrane was rinsed with 50 mL of deionized water and dried at 65°C until constant weight. Carbon present in the light fraction (LFC) was determined as described for whole soil C.

Particulate organic carbon (POC) was assessed according to Feller (1979) with some modifications. Briefly, 25 g of air-dried soil (<6 mm) were shaken (reciprocal shaker) for 16 h in a 250-mL plastic centrifuge bottle with 100 mL deionized water and 10 glass beads (6-mm diam.). The material was then passed on a 53-µm sieve to separate particulate (>53 µm) from fine organic matter. We tested this procedure with fresh DPS and found that 90 ± 2% of initial material was recovered as >53-µm particles. Carbon content of the fine fraction was determined as described above, and POC was calculated as the difference between whole soil C and fine fraction C (Cambardella and Elliott, 1992).

At the end of each growing season, water-stable aggregation was measured by wet sieving with slaking (Angers and Mehuys, 1993). Only soil samples collected in alfalfa plots were analyzed by this procedure. One hundred grams of air-dried soil (<6 mm) were put on the top of a nest of sieves with decreasing openings (1000 and 250 µm). The sieves were totally immersed in water for 10 min with an apparatus similar to that described by Bourget and Kemp (1957). The water containing material <250 µm was then poured onto a 53-µm sieve to yield 53- to 250-µm and <53-µm fractions. Soil fraction recovered on each sieve was weighed, corrected for sand content (Kemper and Rosenau, 1986), and expressed as a percentage of total dry soil. Soil passing the 53-µm sieve was recovered by centrifugation (15 min at 1000 g). Size classes obtained were >1 mm, 250 to 1000 µm, 53 to 250 µm, and <53 µm.

Water-stable aggregates were also observed by stereomicroscopy (Model M8, Wild Heerbrugg, Switzerland). Ten 4- to 5-mm aggregates were collected from each wet-sieved sample, and air dried. The surface of intact aggregates was first observed (80x magnification), and the aggregates were then broken apart to expose the interior and observed again (100x magnification).

Statistical Analyses
Analyses of variance were performed separately for each site and each sampling date by the GLM procedure of SAS (SAS Institute, 1989). There was no consistent crop effect on any measured soil parameters except for LFC measurements. However, since the mean square fraction attributed to the crop effect on LFC was always very small compared with the fraction attributed to the DPS effect, only the DPS effects are presented in the present paper. Contrast analyses were performed to assess for the linear or quadratic response of measured parameters to DPS loading rates. When significant (P < 0.05), the response of studied parameters to DPS amendment were always linear, and therefore, this is not further mentioned in the text.


   Results
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
Discussion
 Conclusion
 REFERENCES
 
Whole Soil Carbon
Increased soil C due to added DPS was statistically significant (P < 0.01) until the end of the study. At the first sampling date (Day 271), the silty clay loam amended with 100 Mg DPS ha-1 had a C content of 62 g kg-1 dry soil, whereas the clay loam had 58 g kg-1 (Fig. 1) . The C contents of the control soils were subtracted from those of the amended soils to estimate the amount of C attributable to DPS. At Day 271, the contribution of DPS was 44 g C kg-1 in the silty clay loam and 39 g C kg-1 in the clay loam for soils amended with 100 Mg DPS ha-1, and 18 and 17 g C kg-1, respectively for soils treated with 50 Mg DPS ha-1.



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  		Fig. 1 Whole soil C content of a silty clay loam and a clay loam at different times following addition of de-inking paper sludge (DPS). The response to DPS loading rates was statistically significant (P < 0.01) for the duration of the study. First, second, and third arrow indicate winters of 1994, 1995, and 1996, respectively. Soil samples were not collected during the winter periods

 
Patterns of C losses from the applied sludge were similar in both soils and with both application rates. From Day 271 to 342, the amount of soil C attributable to DPS decreased by 44% in the silty clay loam, and 46% in the clay loam (Fig. 1). Afterwards, C losses were much slower, and at Day 726, 60% of the initial C present at Day 271 had been lost in the silty clay loam, whereas 62% had been lost in the clay loam. A sudden and marked decrease in soil C content was observed from Day 726 to 973 in DPS-amended soils, and occurred after soil tillage at Day 741.

Particulate Organic Carbon
Addition of DPS increased POC content of both soils (Fig. 2) , and the effect was significant (P < 0.01) until Day 726. At the first sampling date and compared with the control soil, adding 100 Mg DPS ha-1 increased POC by 41 g kg-1 in the silty clay loam, and 38 g kg-1 in the clay loam. These values were 16 g kg-1 in both soils amended with 50 Mg DPS ha-1. As for whole soil C, POC content decreased rapidly from Day 271 to 342 and more slowly from Day 596 to 726. A large decrease in POC was also found between Day 726 to 973 subsequent to soil tillage. More than 70 to 90% of the C attributed to DPS was recovered as POC until Day 726 (Table 2) . This proportion decreased following soil tillage in fall 1996.



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  		Fig. 2 Particulate organic carbon (POC) as measured at different times following de-inking paper sludge (DPS) amendment to a silty clay loam and a clay loam. The response to DPS loading rates was statistically significant (P < 0.01) from Day 271 to 726 of the study. First, second, and third arrow indicate winter of 1994, 1995, and 1996, respectively. Soil samples were not collected during the winter periods

 

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  		Table 2 Proportion of whole soil C content attributed to de-inking paper sludge (DPS) and accounted for by particulate organic C (POC) and light fraction (LFC), in a silty clay loam and a clay loam at different time following DPS incorporation

 
Light Fraction Carbon
The LFC (density <1.8 Mg m-3) increased following DPS addition to the soil (Fig. 3) , and the effect was significant (P < 0.01) until Day 726. At the first sampling date, the silty clay loam amended with 100 Mg DPS ha-1, contained 23 g kg-1 more LFC than the control, whereas in the clay loam the difference was 25 g C kg-1. Corresponding values for soils amended with 50 Mg DPS ha-1 treatment were 11 and 12 g C kg-1, respectively. In DPS-amended soils, LFC contents decreased similarly for both amendment rates, but LFC losses were faster than whole soil C. As a result, the proportion of whole soil C derived from DPS and recovered in the light fraction decreased from >95%, for fresh DPS, to <50% at Day 726, and <30% at the end of the study (Table 2).



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  		Fig. 3 Carbon present in the light fraction (LFC) as measured at different times following de-inking paper sludge (DPS) amendment to a silty clay loam and a clay loam. The response to DPS loading rates was statistically significant (P < 0.01) from Day 271 to 726 of the study. First, second, and third arrow indicate winter of 1994, 1995, and 1996, respectively. Soil samples were not collected during the winter periods

 
Soil Aggregation
The proportion of aggregates >1 mm significantly (P < 0.001) increased following DPS addition in both soils (Fig. 4) . This increase in the large macroaggregates was accompanied by a decrease in the 53- to 250-µm size fraction, and to a lesser extent, a decrease in the 250- to 1000-µm size fraction. At Day 370, the proportion of aggregates >1 mm in the silty clay loam was 2.2 times larger in the 50 Mg DPS ha-1 treatment than in the control, and 3.2 times larger in the 100 Mg DPS ha-1 treatment (Fig. 4a). Corresponding values for the clay loam were 3.3 and 5.9, respectively (Fig. 4d). At Day 726, the difference between the control and DPS-amended soils had decreased but was still significant (P < 0.001) in both soils (Fig. 4b and e). In control soils, the proportion of aggregates >1 mm was 1.7 times larger at Day 726 than at Day 370. Although less obvious than in the previous dates, the effect of DPS addition on soil structure was still significant (P < 0.05) at Day 1075 in both soils (Fig. 4c and f). The decrease in the amount of aggregates >1 mm, between Day 370 and 1075 of the study, was accompanied by a recovery in the 53- to 250-µm and 250- to 1000-µm size fractions.



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  		Fig. 4 Water-stable aggregation of a silty clay loam and a clay loam at Day 370, 726 and 1075 after de-inking paper sludge (DPS) was applied. The response to DPS loading rates was statistically significant (P < 0.05) for the duration of the study

 
Microscopic Observations of De-Inking Paper Sludge and Soil Aggregates
Fresh DPS material tends to aggregate into millimeter size clusters formed by the enmeshment of wood fibers embedded with clays (Fig. 5a, b) . At the first sampling date (Day 271), DPS clusters were not much altered, but cluster surfaces were already partly encrusted with soil particles (not illustrated). Wood fibers located inside these clusters appeared similar to fresh DPS (see Fig. 5b). At Day 370, many water-stable aggregates contained DPS as a central core (Fig. 5c). Wood fibers were still detectable at the surfaces of these aggregates, but DPS was extensively covered with soil particles. Internal wood fibers were easily recognizable and undecomposed (Fig. 5d). From Day 726 onward, water-stable aggregates containing DPS had a smoother surface than at Day 370 (Fig. 5e and g). Although less abundant and of shorter length than at Day 370, internal wood fibers were still present in most water-stable aggregates at Day 726 (Fig. 5f). At Day 1075, wood fibers were hardly observed when water-stable aggregates were broken apart, indicating that most of the original fiber structure had been decomposed (Fig. 5h).



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  		Fig. 5 Stereomicroscopy photographs of de-inking paper sludge (DPS) and soil aggregates at different times after DPS was added to the soil. (a), intact and (b) broken cluster of fresh DPS; (c) intact and (d) broken water-stable aggregate at Day 370 after DPS amendment; (e) intact, and (f) broken water-stable aggregate at Day 726; (g) intact and (h) broken water-stable aggregate at Day 1075. WF, wood fibers; SP, soil particles; C, clays, and inks present in fresh DPS; white bar on the lower left corner of each image indicates 1-mm width

 

   Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
Conclusion
 REFERENCES
 
Decomposition Patterns of De-Inking Paper Sludge
Amending the soil with DPS increased its C content and the effect lasted for the duration of the study (3 yr). The material appeared to decompose rapidly until Day 342, and rather slowly thereafter. Using the same material in a soil restoration experiment, Fierro et al. (1999a) found that the rapid decomposition phase of DPS ended about 100 d after burial. The difference between both studies is mostly due to the inclusion of the 1994 winter period in our study, which represented >200 d with snow cover and soil temperatures around 0°C (Ouellet et al., 1975). Carbon losses are small during winter periods (Fierro et al., 1999a). Since C decomposition in soil is largely controlled by temperature, both studies could be compared on the basis of heat accumulation (Honeycutt et al., 1988). We calculated the amount of degree-days (base temperature = 0°C) accumulated during the first 342 d of our study and found that the rapid phase of decomposition ended after 1700 degree-days were accumulated. The decomposition curve presented by Fierro et al. (1999a) indicated that about 1500 degree-days had accumulated during the first 100 d of his study. The small difference between the values (200 degree-days more in our study) may be due to the fact that Fierro et al. (1999a) measured soil temperature, whereas we calculated degree-days based on air temperature. However, the high C-to-N ratio of DPS (219-to-1), and the absence of N fertilization in our experiment, might have delayed the early phase of DPS decomposition until legume crop contributed significant amounts of N. A lack of N has been found to hinder labile C decomposition (Fög, 1988).

At the end of the first growing season (Day 370), the residual DPS constituted a recalcitrant C pool which decomposed slowly with a mean residence time of >15 yr (Fierro et al., 1999a). Therefore, in our study, the sudden decreases in whole soil C, POC, and LFC found from Day 726 to 973 were mostly attributable to the effect of soil tillage performed at Day 741. The residual C was located in the top 15 cm of soil and was plowed under to 20-cm depth so that some dilution occurred. Part of the effect could also be due to stimulation of decomposition induced by tillage. An effect of N fertilization in spring 1997 is to be discarded since this was performed after Day 973.

At the first sampling date (Day 271), only 50 to 70% of C attributed to DPS was accounted for by LFC in both soils. Since >95% of fresh DPS was comprised of light material, part of fresh DPS became heavier than 1.8 Mg m-3 shortly after incorporation into the soil. Furthermore, the proportion of residual DPS accounted for by light fraction decreased with time. In other words, as DPS decomposed, an increasing proportion of the remaining material had a density >1.8 Mg m-3, indicating progressive densification. Fierro et al. (1999b) reported that the density of DPS increased in a sandy soil (<5% clay), and argued that clays present in fresh DPS were mainly responsible for this phenomenon. In our case, light fraction losses appeared to be faster than in Fierro's study, indicating that encrustation with soil clays might have increased the densification rate of DPS.

The close relationship between whole soil C and POC dynamics in DPS-amended soils indicated that DPS decomposition was mostly linked to transformations in the particulate material present in fresh sludge. Furthermore, the rather constant proportion of whole soil C accounted for by POC during the first 2 yr of the study, indicated that decaying DPS remained mostly particulate (>53 µm) and that POC losses were more due to C mineralization than to a transfer to the fine (<53 µm) C fraction. This finding is different from what is generally reported on crop residue decomposition. During crop residue decomposition, particulate material is generally rapidly broken down into smaller-size fragments, resulting in increased clay- and silt-size C particles (<50 µm) at the expense of the sand-size fraction (Christensen, 1992; Aita et al., 1997). Encrustation with mineral particles such as observed in our study, would provide a physical protection to particulate organic matter against decomposition (Golchin et al., 1997; Ladd et al., 1996; Angers and Chenu, 1997), and partly explains our finding that there were no net fragmentation of the decaying DPS into particles <53 µm during decomposition. Fresh DPS is composed of short wood fibers partly embedded with clays, which also provides intrinsic physical recalcitrance to this material (Fierro et al., 1999b). Finally, the relatively high lignin content of the DPS we used (about 50%; unpublished data) renders it recalcitrant to most organisms unable to decompose lignin. Therefore, both the physical and chemical nature of DPS would explain that its decomposition pattern was different than what is commonly observed with crop residues.

Aggregate Formation and Stabilization
The improvement in soil aggregation found within 1 yr in DPS-amended soils was greater than what is reported on perennial cropping systems in eastern Canada (Angers, 1992; Carter et al., 1994; Chantigny et al., 1997). In our study, we found a positive cropping effect on soil aggregation as reflected by the increase in water-stable aggregation between Day 370 and 726 in control soils. By comparison, improvement in soil aggregation due to DPS was 1.3 to 3.5 times larger than this cropping effect.

When wet-sieving air-dried DPS, we found that >90% of this material remained on the 1-mm sieve. Hence, part of the increase in the proportion of aggregates >1 mm was due directly to the mass of water-stable DPS clusters. This effect can be roughly estimated by calculating the ratio of DPS mass remaining on the 1-mm sieve to the total soil mass (corrected for the control) recovered on the same sieve. For example, soils amended with 100 Mg DPS ha-1 received an equivalent of 116 g DPS kg-1. Assuming that 90% of the material would remain on the 1-mm sieve, 104 g DPS kg-1 would be recovered on this sieve. In the silty clay loam amended with 100 Mg DPS ha-1, the total soil mass recovered on the 1-mm sieve at Day 370 was 457 g kg-1 larger than the control soil. If we assume that the sludge used lost 50% of its initial weight during the first year (Fierro et al., 1999a), the direct mass effect of DPS would account for less than 12% of the total increase in soil aggregates >1 mm. In the clay loam, this effect would not exceed 8%. Therefore, the direct mass effect of DPS was likely minor and about 90% of the increase in the proportion of aggregates >1 mm was due to the aggregation of soil particles from smaller size fractions, and especially the 53- to 250-µm fraction.

Trépanier et al. (1996) also found a positive effect of DPS on aggregation of a silty loam, but this effect was only small and transient, and disappeared within 1 yr following DPS amendment. In our case, the DPS effect on soil aggregation decreased with time, but was still significant 3 yr after DPS application. Seemingly, the lower amounts of DPS (18 Mg DPS ha-1) used by Trépanier et al. (1996) only induced a brief effect on soil aggregation. Hence, larger loading rates provide larger and longer-lasting improvement in soil physical properties. Because of the large C-to-N ratio of this material, high loading rates might be appropriate for instance when cropping to legumes in which case early N immobilization has no consequences on crop growth (Allahdadi et al., 1998).

Visual inspection of water-stable aggregates confirmed that DPS was directly involved in soil aggregation, acting as the central core of many macroaggregates. De-inking paper sludge was already associated with soil particles shortly after the rapid decomposition phase had started, confirming LFC data showing that the light fraction of fresh DPS underwent a rapid densification due to encrustation with mineral particles. The increasing encrustation visually noticed between Day 271 and 1075 also confirmed light fraction data pointing to a progressive densification of DPS during decomposition.

A large increase in microbial biomass and activities is found when adding DPS to the soil (Tardif, 1996). In the present study, we noted visually that surface wood fibers disappeared more rapidly than internal wood fibers. We suggest that soil microbes rapidly colonized the surface of DPS clusters, and degraded exposed wood fibers to produce polysaccharides or other polymers which bound soil particles (Ladd et al., 1996). As a result, new aggregates were formed with particulate DPS as the central core. Golchin et al. (1997) proposed a model for soils in which organic matter plays a major role in aggregate formation and stabilization. In this model, encrustation of free particulate organic matter with mineral particles leads to the formation of macroaggregates with organic matter as a central core. The stability of those macroaggregates would only be transient as their organic core decomposes rapidly, and microaggregates are soon formed from the breakdown of these macroaggregates. Golchin et al. (1997) also proposed that microaggregates derived from broken down macroaggregates are more stable because their organic core is better protected against decomposition. On a longer-term period, however, the organic core slowly decomposes and the microaggregates become unstable. Considering the LFC and POC data and the images presented here, we suggest that macroaggregate formation and stabilization in DPS-amended soils occurred as proposed by Golchin et al. (1997). However, in our study, the DPS effect on stable macroaggregates was not transient and lasted for up to 3 yr.

Although soil fauna was present in our soils, its influence on DPS decomposition and soil aggregation was not studied. However, the decrease in the amounts of aggregates >1 mm between Day 370 and 1075 of the study, and the associated increase in the 250- to 1000- and 53- to 250-µm size classes might have been partly caused by faunal activity and/or organic core decomposition, which progressively reduced the size of macroaggregates and DPS clusters.


   Conclusion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusion
REFERENCES
 
Physical fractionation of soil organic matter, microscopic observations, and soil aggregation assessments all indicated that DPS rapidly formed clusters of wood fibers which became encrusted with mineral particles. This encrustation likely contributed to the physical recalcitrance of encased wood fibers. As a result, DPS clusters mostly remained particulate, and acted as the central core of water-stable macroaggregates. The addition of DPS was efficient at increasing soil C content. However, a most interesting feature of the sludge used was its capacity to rapidly (within 1 yr) form large amounts of water-stable macroaggregates (>1 mm) in clay soils. Under similar environmental conditions, loading rates between 50 and 100 Mg DPS ha-1 are expected to improve soil aggregation and organic matter content for at least 3 yr.


   ACKNOWLEDGMENTS
 
Funding of this project was provided by the Daishowa, Inc, Québec City, Canada, and by the Matching Investment Initiative of Agriculture and Agri-Food Canada. We thank Jean Goulet for assistance in field work and Patrice Jolicoeur, Pauline St-Onge-Audesse, and Sylvie Côté for their assistance in laboratory analyses.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results
 Discussion
 Conclusion
 REFERENCES
 




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