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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Using Paper De-inking Sludge to Maintain Soil Structural Form

Field Measurements

^a Département des Sols et de Génie Agroalimentaire, FSAA, Université Laval, Laval, Québec, Canada G1K 7P4

jean.caron{at}sga.ulaval.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
A high level of organic matter in soils is crucial to maintain structural stability but organic matter sources differ in their effectiveness in stabilizing structural units. Objectives of this study were, first, to determine the optimal rate of sludge and fertilizer application to improve soil physical properties, and second, to investigate a possible correlation between hydraulic conductivity and structural stability measurements. A 4-yr field study (1994–1997) was conducted on three different soil types to evaluate the effect of different amounts of de-inking secondary paper sludge on the soil physical properties. The soil physical properties we monitored were structural stability, water desorption characteristics, bulk density, and saturated hydraulic conductivity. Structural stability was increased by 17% in silty clay soil (SCS) and 15% in loamy soil (LS), but decreased by 35% in sandy loam soil (SLS). Results suggest that the effect of sludge application (SA) is short-lived and that an annual application of sludge is necessary to obtain a year-to-year effect on structural stability. Measured bulk density dropped significantly in the SCS (4–10%) and in the LS (1–6%). A significant increase in air capacity and available water values revealed that SA increases both transmission and storage pores in the SCS. Field-saturated hydraulic conductivity (K_fs) was increased in the SCS, but decreased in the SLS and the LS. A good correlation was observed between structural stability and hydraulic conductivity measurements in the SCS and the LS.

Abbreviations: LS, loamy soil • SA, sludge application • SCS, silty clay soil • SLS, sandy loam soil • WAS, wet aggregate stability

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
STRUCTURAL STABILITY is a very important soil physical property that describes the ability of the soil to retain its arrangement of solids and voids when exposed to different stresses (Kay, 1990). The stability of the pores, also known as soil poral system stability, is important in controlling the storage of water available to plants and the transmission of water and air through the soil (Oades, 1984).

Organic matter plays a fundamental role in the stabilization of soil aggregates and the formation of pores (Tisdall and Oades, 1982; Oades, 1984). Organic matter is a labile soil component but is also a renewable resource (Carter and Stewart, 1996). Decreasing soil organic matter content usually leads to the degradation of soil physical properties, especially soil structural stability. The result of a loss of structural stability is a reduction in the total porosity and changes in the pore size distribution (Collis-George and Laryea, 1971; Kemper et al., 1988; Or, 1996). Consequently, the soil will generally have smaller pores and a lower infiltration rate (Collis-George and Greene, 1979; Kemper et al., 1988; Or, 1996). A reduced infiltration rate may induce soil erosion by increasing runoff (Le Bissonnais et al., 1995) and consequently lead to a decrease in soil productivity.

The detrimental impact of a reduction in organic matter in maintaining the stability of the soil poral system during water infiltration and water flow into the soil is well-known. The soils under intensive continuous cultivation are in a net deficit of C and require external sources of organic matter to equilibrate their deficient C balance. With the aim of increasing the organic matter level in agricultural land, there has been a growing interest in the advantages of using waste and organic residues in agriculture during the last decade (Cline and Chong, 1991; Martens and Frankenberger, 1992). Industrial waste rich in C, such as paper de-inking sludge from recycled papers, may provide an interesting solution to soil degradation problems (Cline and Chong, 1991; Trépanier et al., 1996a). While rich in C, these sludges may cause N immobilization, and field trials need to be conducted to determine optimum sludge application rates (Cline and Chong, 1991; Trépanier et al., 1996a).

In recent years, many studies have been carried out to verify the effect of organic matter on soil physical properties. Most of these studies have focused on improving soil structural stability and acknowledged its possible impact on infiltration and water storage processes, but did not measure it. Measuring structural stability may be useful to predict erosion losses due to increased runoff and sediment transport (Le Bissonnais, 1988; Le Bissonnais and Arrouays, 1997). Indeed, a stability criterion linked to saturated hydraulic conductivity could be used to predict infiltration and runoff near saturation following different rainstorm events (Le Bissonnais, 1988; Le Bissonnais and Arrouays, 1997). Stability parameters have the advantage of being easier to measure than the total decrease in water storage and infiltration in the field. Linking stability measurements to the infiltration process is a difficult task since information on the wetting rate is critical to assess the importance of structural destabilization during rainfall or irrigation (Ghavami et al., 1974; Caron et al., 1996). Also, the fact that aggregates are confined when submitted to rapid wetting (Kay et al., 1994) and that uneven wetting may occur (Sullivan, 1990) further complicates that task. However, research efforts in the field should be aimed at establishing links between stability and the infiltration process (Loch and Foley, 1994; Or, 1996). This paper presents the results of a field research study on the application of a fresh mixture of de-inking and secondary sludges in potato and barley productions, two C-deficient production systems.

The objectives of this study were, first, to determine the optimum rate of de-inking and secondary sludge application on three soil types to improve soil physical properties, and second, to investigate possible relationships between soil hydraulic conductivity and soil structural stability measurements.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Sludge Characteristics
Three different types of sludge (de-inking sludge, secondary sludge, and primary sludge) are produced by Daishowa Inc. (a paper company located in Quebec City, Canada). The de-inking process produces a waste by-product called de-inking sludge, which contains mainly paper fibers, clay particles, and residual inks (Brouillette et al., 1996; Trépanier et al., 1996b). Brouillette et al. (1996) reported that de-inking sludge produced by Daishowa Inc. consists of cellulose (39%), hemicellulose (11%), and lignin (23%). De-inking sludge has a C/N ratio of about 300 (rich in C and poor in N) and N must be added to compensate for the immobilization effect generally associated with sludge application (Cline and Chong, 1991; Trépanier et al., 1996a). Secondary sludge (C/N 20) is waste by-product, rich in N and already inoculated with microbes that decompose cellulose wood fiber products. They were added to decrease the C/N ratio of the de-inking sludge. A mix composed of 85% de-inking sludge and 15% secondary sludge was thus used as a sludge amendment, with a C/N ratio of about 260. The proportion of secondary to de-inking sludge was determined by the average production of both sludges at the mill. Primary sludge is a waste by-product consisting mainly of undecomposed wood fibers that cannot be used in the paper fabrication process. This type of sludge was not used in this study.

Brouillette et al. (1996) characterized de-inking sludge in order to identify compounds that might cause contamination problems in agriculture. More than 150 organic and inorganic components were analyzed, including heavy metals, polychlorinated biphenyls, and hydrocarbons. The results showed no contamination problems except a high level of Cu, which ranged from 102 to 191 mg kg^-1. Repeated applications of de-inking sludge may therefore have to be limited in the long term because of the potential accumulation of this metal.

Experimental Setup
A 4-yr study (1994–1997) was performed on three different sites, involving four experimental designs, with representative soils and crops. One site was on a Tilly silty clay soil under barley (Hordeum vulgare L. cv. Chapais) production located in St-Augustin; the second site was on a l'Atrée sandy loam soil with a first experimental design under barley and a second experimental design in potato (Solanum tuberosum L. cv. Kennebec) production located in St-Pierre, Île d'Orléans. The third site was on a Bedford loamy soil under potato (Solanum tuberosum L. cv. Superior) production located in Ste-Croix. All sites were in the province of Québec, Canada. The characteristics of the soils of the different sites are summarized in Table 1 .

The evolution of the soil physical properties was monitored during the application year, as well as during subsequent years (spring 1994 to summer 1997), to determine if there were any residual effects. The soil physical properties monitored were the soil structural stability, water desorption characteristics, bulk density, and saturated hydraulic conductivity.

Soil Structural Stability Measurements
The structural stability of soil aggregates was determined 11 different times using the wet-sieving method (Angers and Mehuys, 1993) between August 1994 and September 1997. Measurements were carried out on composite samples formed by mixing four sub-samples taken in the top 15-cm layer of soil in each experimental plot. A 20-g (oven-dry weight basis) sample of field-moist, 2- to 4-mm aggregates was placed onto the top 1-mm sieve of a wet-sieving apparatus, which was filled with distilled water and submerged with a vertical stroke of 35 mm. The apparatus was operated at a speed of 30 cycles per min for a 10-min period. The soil remaining on the 1-mm sieve was oven-dried and weighed, and the sand or gravel content measured after dispersion. The wet aggregate stability (WAS) was expressed as

(1)

The initial water content of the field-moist sieved aggregates was determined gravimetrically. Regression analysis between the WAS value and water content was performed on the whole data set to better estimate the water content effect on structural stability. Statistical analyses were then performed on the residuals of the regression model per individual time, that is, on the adjusted WAS values (WAS_adj), through the following equation:

(2)

where ß is the slope of the WAS- regression estimated by least squares, _obs is the gravimetric water content of the observation, and _avg is the average water content of all treatments and sampling times. This procedure, called the adjusted Y method (Sokal and Rohlf, 1969), is analogous to covariance analyses and reduces the error term variance increased by a covariate. It is sometimes more appropriate than a straight covariance analysis if on a restricted subset of data, the known covariate effect is poorly estimated. This adjustment therefore reduces effects of water content on aggregate stability values, which tend to mask treatment effects in the statistical analyses (Perfect et al., 1990; Caron et al., 1992a, 1992b).

Soil Water Desorption Curves
The soil water desorption curves were obtained for samples collected in August 1994, 1995, and 1996. No measurements were carried out in 1997 for residual effects since, as shown later, no differences were observed in 1996. Metal sampling cylinders, 30 mm high with a 62.5-mm i.d., were used to take undisturbed soil cores, and two samples were taken in the first 15 cm of the soils of each experimental plot. These cores were obtained at or near field capacity to avoid soil shattering (dry soil) or compaction (wet soil) during sampling. Excavated cores were trimmed flush with the end of the cylinder, capped using nylon cloth and rubber bands, and wrapped in plastic bags to prevent evaporation and to provide protection during transport. Soil cores were stored at 4°C to reduce microorganism activity before soil water desorption determinations.

In the laboratory, the cores were placed in the vacuum chamber and distilled water was added from underneath by small increments until the cores were saturated. After saturation, the volumetric soil water contents at different matrix potentials (0, -1, -5, -33, -100, and -1500 kPa) were determined using a tension table and pressure plate apparatus.

Air-filled porosity and available water were estimated from the soil water desorption curves. The air-filled porosity (the volume of air present in the soil after saturation and drainage to a potential of -33 kPa) was calculated from the difference between the total porosity and the volumetric water content at a potential of -33 kPa. Available water for plant growth was calculated from the difference between a potential of -33 kPa and a potential of -1500 kPa. The soil bulk density was measured using the Blake and Hartge method (Blake and Hartge, 1986a). The total porosity was calculated using the following equation:

(3)

The particle density was measured using the pycnometer method (Blake and Hartge, 1986b).

Hydraulic Conductivity Measurements
Saturated Hydraulic Conductivity Determination (K_fs)
Two infiltration measurements per plot were made in August 1994, 1995, and 1996, using a constant head pressure infiltrometer (Reynolds and Elrick, 1990). A wood board and a hammer were used to drive a 10-cm diam. ring into the soil to a depth of 5 cm. The ring was thin-walled (1 mm thick) and beveled to a sharp cutting edge at the base to reduce resistance, soil compaction, and shattering during the insertion process. The infiltrometer consisted of a Mariotte bottle and a water outlet tube placed into the ring to maintain a constant water head at the soil surface. Water was ponded at the soil surface with a constant head of about 14 cm in the ring to obtain a steady-state rate. The field-saturated hydraulic conductivity was then calculated as follows (Reynolds and Elrick, 1990; Reynolds, 1993):

(4)

where K_fs (cm s^-1) is the field-saturated hydraulic conductivity, ß (cm^-1) is a soil texture–structure parameter, G is a dimensionless shape factor, A (cm²) is the cross-sectional area (cell constant) of the infiltrometer reservoir, R (cm s^-1) is the steady rate of fall of the water level in the infiltrometer reservoir, a (cm) is the inside radius of the ring, and H (cm) is the steady pressure head at the infiltration surface (set by the height of the air tube and determined by measuring the height of water in the standpipe). The shape factor (G) was obtained from (Reynolds, 1993):

(5)

where d (cm) is the depth of ring insertion into the soil. This method presents advantages: it provides an in situ determination of field-saturated hydraulic conductivity in the unsaturated zone and allows working with an undisturbed soil.

Difference in K_fs as a Stability Measurement
The difference between the saturated hydraulic conductivity under rapid and slow wetting was used to estimate the in situ structural stability. In this method, two different measurements of saturated hydraulic conductivity (slow and rapid wetting) were performed on five treatments. The treatments consisted of (i) control with fertilizer, (ii) compost, (iii) sludge (8 dry-t ha^-1 at 1.2% N), (iv) sludge (16 dry-t ha^-1 at 1.2% N), and (v) sludge (24 dry-t ha^-1 at 1.2% N). These measurements were performed in August 1995 and August 1996, about 1 yr after each sludge application.

For the slow-wetting measurements, a thin layer of sand (about 5 mm) was applied on the soil surface to obtain good contact between the soil surface and the porous bottom surface of the infiltrometer. Then, a tension infiltrometer with a -10-cm height of water potential was used to slowly wet the soil surface during a 48-h period. Thereafter, the tension infiltrometer was replaced by a ring, and a constant head pressure infiltrometer with a head of 14 cm was used to measure the hydraulic conductivity (K_fs-slow).

For the rapid-wetting measurements, 30 cm away from the first measurement, a thin layer of sand was also applied and a ring inserted, and a constant head pressure infiltrometer with a head of 14 cm was then used to directly measure the hydraulic conductivity (K_fs-rapid). In this case, rapid water entry into the dry soil led to aggregate disintegration and a reduced infiltration rate. Consequently, the hydraulic conductivity values measured under rapid wetting were lower than those found for slow wetting.

Obviously, the initial water content of the soil is an important parameter that can affect the results of these measurements. Thus, to obtain maximum differences between hydraulic conductivity values and to reduce the effect of water content, the measurements were performed when the soil was initially dry. The initial gravimetric water contents during the hydraulic conductivity measurements for different soil types were 0.24 ± 0.02 kg kg^-1 in the silty clay soil, 0.19 ± 0.02 kg kg^-1 in the loamy soil, and 0.09 ± 0.02 kg kg^-1 in the sandy loam soil. The differences between the hydraulic conductivity values for slow and rapid wetting were used to evaluate the effect of the treatments on structural stability. The SAS system for Windows, release 6.12 (SAS Institute, 1996), was used to analyze the results according to a completely randomized block design with different rates of sludge application and mineral fertilizer, compost, and fertilized and unfertilized control plots as treatments for the different parameters.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Wet Aggregate Stability
The effect of different rates of sludge application on the wet aggregate stability (WAS) for all three soil types and different sampling dates is presented in Fig. 1 and 2 . In these figures, each value of WAS is an average of nine observations (three fertilization rates and three repetitions) for the 8, 16, and 24 dry-t ha^-1 treatments, and an average of three observations (three replicates) for the control with fertilizer since fertilization at 0.8, 1.2, and 1.6% had no effect on aggregate stability of any given sludge treatment. The results of the statistical analyses for the WAS at 11 sampling dates between August 1994 to September 1997 are presented in Table 3 .

View larger version (35K): [in this window] [in a new window]   Fig. 1 Variation in wet aggregate stability with sludge applications and sampling dates for (a) silty clay soil under barley production, and (b) loamy soil under potato production. *, **, *** Significant at P 0.10, P 0.05, and P 0.01, respectively. SE is standard error. The arrows represent the time at which sludge application occurred

View larger version (31K): [in this window] [in a new window]   Fig. 2 Variation in wet aggregate stability with sludge applications and sampling dates for a sandy loam soil (a) under barley production, and (b) under potato production. *, **, *** Significant at P 0.10, P 0.05, and P 0.01, respectively. (-) is negative effect. SE is standard error. The arrows represent the time at which sludge application occurred

The first soil reaction to the presence of sludge amendment was detected for the silty clay soil in May 1995 (Fig. 1a), 7 mo after the first sludge application. The beneficial effect of sludge application on wet aggregate stability was observed again in July 1995 and continued until August of the same year. This beneficial effect was observed up to 11 mo after treatment. The differences between the treatments were consistent with the amounts of sludge applied, as revealed by a significant positive correlation between stability and the rate of sludge application. Indeed, results of linear contrast between control and 8, 16, and 24 dry-t ha^-1 of sludge applications showed that WAS and sludge application rate were linearly correlated in the silty clay soil in May 1995 and in July 1995 . The maximum level of wet aggregate stability was obtained with an application of 24 dry-t ha^-1 of sludge in July 1995. This treatment resulted in a 17% increase in wet aggregate stability, compared with the control treatment.

No differences were observed in September or November of 1995. Two subsequent increases in wet aggregate stability were observed in May 1996 and July 1996 after the second sludge application. No trend was observed in September or November 1996. In 1997, structural stability measurements were performed at two different dates in May and September to quantify any residual effects of previous applications, since no application was made that year. No differences were observed between the treatments.

In the case of the loamy soil (Fig. 1b), a beneficial effect was detected in September 1995, 1 yr after the first sludge application. The effect lasted for about 2 mo. The results revealed a trend toward higher stability with higher application rates. No difference was observed in May, July, or November 1995. Wet aggregate stability was significantly increased by about 15%, compared with the control treatment following the second sludge application (24 dry-t ha^-1 sludge) in July 1996. No significant changes were observed among the other dates in 1996 or in 1997.

The results showed that the effect of the sludge application was detected for only about 9 mo following each application in both the silty clay and the loamy soils. The beneficial effect of sludge application on aggregate stability is therefore short-lived at these application rates. An annual application or higher rates of sludge would thus be necessary to obtain a year-to-year effect on structural stability. Ram and Zwerman (1960) and MacRae and Mehuys (1985) reported similar results after an application of green manure in soils. These results agreed with those of Tisdall and Oades (1982), who found that compared with readily decomposable substrates, recalcitrant substrates (e.g., cellulose and hemicellulose) slowly increase water stability and persist for 8 to 12 mo.

Unexpected results were obtained with the sandy loam soil (Fig. 2). Wet aggregate stability significantly decreased following sludge application in July 1995, November 1996, and even in September 1997 under barley production, as well as in July 1996 and November 1996 under potato production. These results show a consistent trend toward lower aggregate stability with higher application rates. No significant changes between the control and the other treatments were observed for other dates. Wet aggregate stability was significantly reduced by 35%, relative to the control treatment. The decrease in aggregate stability for the sandy loam soil was probably due to (i) increased decomposition of the sludge amendment and (ii) destruction of (organic matter)–(Fe or Al)–(mineral particle) structuring linkages.

Indeed, this sandy loam soil was well-aerated, resulting in more important microbial activity and more rapid decomposition than in the two fine-textured soils. Allison (1973) and MacRae and Mehuys (1985) reported similar results. This increased microbial activity may lead to the decomposition of soil organic matter through a priming effect, even in the (organic matter)–(Fe or Al)–(mineral particle) structuring linkage, which in turn would adversely affect aggregate stability.

Also, chelating agents might have been produced by the decomposition of the sludge, therefore forming ligands with Fe and Al ions and removing them from the organic matter–metal–clay linkage. This phenomenon may not have been as important in the more clayey soils since the cation-exchange capacity of clay is much greater and the number of linkages to break are logically more important.

This decrease in aggregate stability for the sandy loam soil is consistent with the findings of Reid et al. (1982). They hypothesized that the decrease in aggregate stability of a sandy loam caused by maize roots was mainly due to (i) the removal of Fe and Al ions acting as bridges between clay surfaces and organic materials, and (ii) changes in the concentrations of various cations released from the soils into the solutions used for aggregate stability analyses.

Data for all three sites also reflected the important temporal variability observed in structural stability. Many studies have addressed that specific question as early as that of Yoder (1936) and Hénin (1948), with some of the latest focusing on the roles of water content (Perfect et al., 1990; Caron et al., 1992a, 1992b), roots and biomass (Perfect et al., 1990), production and sorption of binding agents (Webber, 1965; Caron et al., 1992a), freezing, water content, time and tillage (Bullock et al., 1988), and age hardening (Kemper et al., 1987; Caron and Kay, 1992). These studies all indicate that structural stability is a complex process resulting from multiple interactions between chemical, physical, and biological factors. The effects observed here, following sludge applications, were most likely attributed to the production of binding (silty clay and loamy soils) and dispersive (sandy loam soil) substances through microbial decomposition, as indicated above. However, observed effects may also result from interactions with environmental factors such as temperature, water content, and the production of biomass. These effects may be modified by the treatments themselves and may affect the time and the amount of binding or dispersive substances produced, as well as their sorption onto surfaces, and may explain the time-dependant response to the applied treatments. However, the lack of associated measurements regarding temperature, moisture regime, binding–dispersive agents, and biomass production in the field prevent us from further investigating these aspects.

Of interest is the fact that for the silty clay and loamy soils, differences were observed mainly in the summer months, despite the correction for water content. This suggests that the stabilizing material is produced or becomes efficient only in the summer. This has implications for erosion studies, as higher stability might be desirable in early spring and late fall. This leaves limited hope to diminish erosion losses in the fall or in the spring with such a conservation strategy. The negative effect was not observed in the laboratory (Nemati, 1999). In that specific case, this suggests that the observed effect involved a treatment interaction with environmental conditions met in the field only.

The statistical analyses showed no significant effect of N fertilizer on wet aggregate stability for the different sampling dates and soil types (data not shown). These results suggest that any differences in structural stability found after sludge and fertilizer applications could most likely be attributed to the sludge treatment alone. Compost applications also did not induce any significant changes on wet aggregate stability for the different sampling dates and soil types. This result was probably due to the lack of readily decomposable organic residues in the compost, compared with the de-inking and secondary sludges. These residues would have already been decomposed during the composting process.

Soil Water Desorption Curves
The results of the bulk density measurements for the silty clay and loamy soils are presented in Fig. 3 , and the results of the air capacity and available water measurements for the silty clay soil are presented in Fig. 4 . Initial measurements of these parameters before the first sludge application (in October 1994) showed no differences between plots, confirming the absence of treatment effects before sludge application. In August 1995, 1 yr after the first sludge incorporation, measured bulk density dropped significantly in the silty clay soil (Fig. 3a) and in the loamy soil (Fig. 3b). The sludge treatments (8, 16, and 24 dry-t ha^-1) decreased the mean bulk density of the silty clay soil by 4, 7, and 10% and the loamy soil by 1, 2, and 6%, respectively, compared with the control with fertilizer. No significant differences were observed in the sandy loam soil under barley and potato production. Powers et al. (1975) suggested that decreased bulk densities following organic applications were a result of a dilution effect of the added amendments on the denser mineral fraction of the soil. This dilution effect probably did not play a role in the initial decrease in the bulk density measurements (August 1995) since compost applied at similar rates produced no significant bulk density changes. The effect of sludge application on soil bulk density was most likely due to the production of binding substances that aggregated soil particles and produced a more open soil structure, which consequently led to an increase in soil total porosity and a decrease in soil bulk density. Similar results have been reported by MacRae and Mehuys (1985). The total soil porosity, which was inversely related to soil bulk density, was significantly increased in the silty clay soil and in the loamy soil in August 1995. Here again, no significant differences were observed in the sandy loam soils (data not shown).

View larger version (46K): [in this window] [in a new window]   Fig. 3 Variation in soil bulk density (a) in a silty clay soil, and (b) in a loamy soil with sludge applications. Bars represent the least significant differences (LSD) between treatments at P 0.10. Treatments not marked by the same letter are significantly different

View larger version (45K): [in this window] [in a new window]   Fig. 4 Variation in soil physical parameters (a) changes in air capacity, and (b) changes in available water with sludge applications for a silty clay soil. Bars represent the least significant differences (LSD) between treatments at P 0.10. Treatments not marked by the same letter are significantly different

Bulk density, total porosity, air capacity, and available water measured in August 1996 showed that the second sludge addition (October 1995) did not induce significant changes in these parameters (Fig. 3 and 4). This may have been due to faster sludge residue decomposition. No significant effect of N fertilizer was observed on soil bulk density, total porosity, air capacity, or available water in any of the three soil types.

As with WAS, the effects observed were variable, being significant only in 1995. The same positive effects were observed in a companion laboratory study (Nemati, 1999), supporting the view that the differences observed in the field were real, but may sometimes remain undetected because of the influence of environmental factors.

Hydraulic Conductivity Measurements
Field Saturated Hydraulic Conductivity (K_fs)
The ability of a soil to transmit water depends on the arrangement of the soil particles, as well as on the stability of the pores. Since the addition of organic amendments usually leads to an increase in aggregation and porosity, K_fs is also expected to be greater in more stable soils. The results of the K_fs measurements for the loamy and the sandy loam soils are presented in Fig. 5 .

View larger version (33K): [in this window] [in a new window]   Fig. 5 Variation in saturated hydraulic conductivity (a) in a loamy soil, and (b) in a sandy loam soil (potato) with sludge applications. Bars represent the least significant differences (LSD) between treatments at P 0.10. Treatments not marked by the same letter are significantly different

A general increase in K_fs was observed with increasing sludge application rates in the loamy soil (Fig. 5a) and in the silty clay soil in 1995, which was consistent with the observed increase in wet aggregate stability. The 24 dry-t ha^-1 treatment had the highest hydraulic conductivity, followed by the 16 dry-t ha^-1 and the 8 dry-t ha^-1 treatments. Increased hydraulic conductivity, resulting from the application of organic materials, has been reported by MacRae and Mehuys (1985) and Martens and Frankenberger (1992). They found that the application of organic amendments greatly increased soil water infiltration, and that the increases were directly related to the quantity of organic material applied.

In September 1996, the loamy soil showed a significant decrease in K_fs with increasing sludge application rates (Fig. 5a), which might again be due to decomposing fibers or kaolinite found in paper sludge and clogging some of the pores. No significant effect was observed at the other sites. No significant effects of N fertilizer between 0.8 and 1.6% or compost application (24 dry-t ha^-1) were observed on K_fs for any of the three soil types.

Difference in K_fs as a Stability Measurement
The results of the statistical analyses for wet aggregate stability (WAS) and Dk_fs are presented in Table 4 . In 1995, sludge treatments had a significant effect on WAS and Dk_fs in both soils (Table 4). Compared with the control, wet aggregate stability was significantly higher in both the amended silty clay soil and the amended loamy soil, and Dk_fs values were significantly lower in the amended silty clay soil and the amended loamy soil. These results indicate that the stability of the pore network during water entry into the soil was significantly improved by sludge applications in both soils. Generally, the higher the sludge application rate, the more stable the soils and the lower the Dk_fs, as shown by the significant treatment effects for the silty clay and loamy soils (Table 4). In 1996, a significant effect for applied treatments was observed only for Dk_fs in the silty clay soil. No significant effect was observed for WAS or Dk_fs for the other soil types or measuring dates.

View larger version (22K): [in this window] [in a new window]   Fig. 6 Relationship between wet aggregate stability and the difference between saturated hydraulic conductivity for rapid and slow-wetting measurements (a) in a silty clay soil, and (b) in a loamy soil. *, **, *** Significant at P 0.10, P 0.05, and P 0.01, respectively

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The results of these experiments show that sludge application linearly improved the physical properties of silty clay and loamy soils. Higher sludge application rates produced better physical conditions compared with the control in both the silty clay and loamy soils. The results also indicate that sludge application deteriorated the physical properties in the sandy loam soil, and that applications of de-inking sludges for this soil type should be avoided, unless additional work is conducted to investigate the consequences of this stability loss and the long-term effect of these materials on physical properties. The soil physical property measurements revealed that sludge application increased the stability of the soil poral system during water infiltration by increasing both transmission and storage pores in the silty clay soil. For application rates up to 24 dry-t ha^-1 in the silty clay and loamy soils, this effect lasted for about 9 mo. An annual application of sludge at these rates would therefore be necessary to obtain a year-to-year effect on physical properties. The rate of N fertilization (0.8–1.6%) had no significant effect on soil physical properties. Results also showed that N rates between 0.8 and 1.6% had no significant effect on potato and barley yield (unpublished), while NO₃ losses to the environment increased with an increasing N percentage in the sludges (Trépanier et al., 1998). Therefore, a 0.8% N rate should be preferred in order to reduce environmental pollution and the cost of supplemental N following sludge application. Finally, while composting reduces the risk of N immobilization, it also decreases to a large extent the beneficial effect of sludge applications on soil structure. Therefore, composting these residues before their application does not appear to be advantageous in terms of soil structure.

The results obtained from physical property measurements revealed that a good relationship could be established between WAS and Dk_fs (the difference between K_fs measurements under rapid and slow wetting) in the silty clay and loamy soils. A general decrease in Dk_fs was observed with increasing WAS for both soils. This relationship could be used in erosion models since changes in hydraulic conductivity can be predicted from saturated hydraulic conductivity and WAS.

	ACKNOWLEDGMENTS

 
The authors are grateful to Daishowa Inc. and NSERC (Natural Sciences and Engineering Research Council of Canada) for their financial support. Special thanks are extended to Farzaneh Shishehgarha, Gilles Grenier, and Rosanne Chabot for their field and laboratory assistance. Thanks are also extended to the Iranian Ministry of Agriculture for the financial support accorded to M. R. Nemati.

Received for publication January 4, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 

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