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DIVISION S-8-NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Dynamics of Physical Organic Matter Fractions During De-inking Sludge Decomposition

^a Département de phytologie, Centre de Recherche en Horticulture, Université Laval, Ste-Foy, QC, Canada, G1K 7P4
^b Soils and Crops Research Centre. Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Ste-Foy, QC, Canada, G1V 2J3

angersd{at}em.agr.ca

	ABSTRACT

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Replenishment of soil C and N is essential for sustainable revegetation of minesoils. Our study investigated paper de-inking sludge as the organic amendment for revegetating an abandoned sandpit in Québec, Canada. Sludge was incorporated at 0 (check) and 105 Mg dry matter ha^-1 before seeding tall wheatgrass (Agropyron elongatum (Host) Beauv.). Nitrogen (at 315, 630 and 945 kg N ha^-1) and P (at 52.5 and 105 kg P ha^-1) were also applied to all plots. Distribution of C and N was determined periodically in two sizes (<53 µm and >53 µm) and two densities (<1.8 g cm^-3 and >1.8 g cm^-3) of soil fractions during 823 d. After 823 d, C concentrations were 43 and 69% of those of Day 5, for the low and high N rates, respectively. With time, the proportion of C in the heavy (>1.8 g cm^-3) fraction increased from 20 to 55%, but remained near 20% in the fine (<53 µm) fraction. Increasing N rates increased C conservation mainly in the coarse (>53 µm) fraction. The amount of N recovered in all fractions decreased after Day 86, in accordance with a previous litter bag study. Although inorganic N was positively correlated with total N in all fractions, the fine fraction was the best indicator of the size of the mineral N pool. Addition of sludge to the sandpit favored the restoration of C and N pools, and high levels of mineral N increased this effect. Residues became denser but remained relatively coarse during their decomposition.

	INTRODUCTION

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REPLENISHMENT OF SOIL C and N, which is essential for sustainable revegetation of minesoils, may be accelerated by the use of organic amendments. The fate of these amendments in minesoil, however, may differ from that in undisturbed soil. For example, microbial populations and activity may be different in minesoils, altering substrate decomposition (Visser et al., 1984). Also, decomposition of amendments in recently exposed minesoils may be affected by reduced macrofaunal activity. In addition, N availability may further affect decomposition of the organic amendments, which can be delayed (Cheshire and Chapman, 1996) or accelerated (Green et al., 1995) by added N in non-degraded soils. In a sandpit minesoil, a trend of delayed decomposition was observed late in the decay process with high N application rates (Fierro, 1998). A delayed decomposition or C conservation may enhance the long-term restoration of organic matter pools.

Soil physical fractionation has become a useful approach to characterize soil organic matter quality and dynamics. Light and particulate fractions are usually very sensitive to changes in management practices (Janzen et al., 1992; Cambardella and Elliott, 1992) and are assumed to represent intermediate pools between undecomposed residues and humified matter (Gregorich and Janzen, 1996). On the other hand, the fine and heavy fractions would contain more processed organic matter (Hassink, 1995a). Accordingly, several studies, under various soil and climatic conditions, have found positive correlations between light fraction and N mineralization (Janzen, 1987; Hassink, 1995b; Barrios et al., 1996). Studies with isotopically labelled plant residues have also shown the usefulness of physical fractionation in characterizing the short-term fate of residue-derived C and N in soil (Hassink and Dalenberg, 1996; Aita et al., 1997).

A better understanding of decomposition patterns and of the fate of residue-derived C and N in soil fractions should lead to improved management of organic amendments in minesoils. In the present study, paper de-inking sludge was used as an organic amendment for revegetating an abandoned sandpit (Fierro et al., 1998). De-inking sludge is composed of wood fiber but also contains fillers, ink and the chemicals used to dissociate these materials from the pulp fiber (NCASI, 1992). In a previous litter bag study, Fierro (1998) characterized in situ patterns of sludge decomposition and of N and P accumulation and release; in the present study we further investigate the fate of decomposing sludge in soil physical fractions. The specific objectives were to (i) determine the distribution of C and N in particle-size and density fractions during sludge decomposition in the minesoil, (ii) assess the effect of increasing rates of mineral N on C conservation, and (iii) assess the relationship between physical fractions and mineral N content.

	Materials and methods

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Study Site and Plot Establishment
The study site was an abandoned sandpit 20 km south of Québec city, at Saint-Lambert-de-Lévis, Québec, Canada (46°34' N, 71°13' W). The average annual temperature is 4°C and total precipitation is 1200 mm. Before mining, the surface soil was a Beaurivage sandy loam (Laplante, 1962) classified as an Haplorthod. The minesoil was exposed after removing the topsoil and mining subjacent layers of sand down to a depth of about 2 m. The minesoil was composed of 94% sand (determined by wet sieving at 53 µm). The minesoil had initial average total C and N contents of 1.3 and 0.2 g kg^-1, respectively, and a pH (H₂O) ranging from 4.7 to 6.0. A more detailed description of the minesoil is given in Fierro (1998).

In a one-time operation (25 July 1994), raw paper de-inking sludge was mechanically incorporated using a rotovator into the surface layer (0.21 m) of the minesoil at rates of 0 (control) and 105 dry Mg ha^-1. Three rates of N (315, 630 and 945 kg N ha^-1, which correspond to 3, 6, and 9 g kg^-1 sludge) and two rates of P (52.5 and 105 kg P ha^-1, which correspond to 0.5 and 1.0 g kg^-1 sludge) were applied by broadcasting urea and single superphosphate prior to sludge incorporation to both the control and the amended soils. De-inking sludge was obtained from the Daishowa Inc. paper mill (Québec City, Canada) (Table 1) . Levels of sludge, N, and P were selected based on results of greenhouse trials, and corresponded to a range within which plant growth was adequate (Fierro et al., 1997).

Soil Sampling, Fractionation, and Analysis
Soil was sampled (0 to 0.2 m depth) at six dates during a 27-mo period (i.e. 5, 25, 86, 452, 655, and 823 d after sludge and fertilizer incorporation). Days 86 and 452 corresponded to the beginning of the two winters throughout which this study was conducted. Field-moist samples (500 g) obtained from three to five combined cores (5-cm diam) per plot, were passed through a 6-mm sieve, and included all sludge particles. After sieving, samples were air-dried to constant weight and stored in sealed plastic bags until fractionation and analysis. Subsamples of field-moist soil were kept at 4°C for a maximum of 48 h until extraction and analysis of inorganic N.

The method for particle-size fractionation was adapted from Feller (1979). Twenty-five grams of air-dried soil and 100 mL of deionized water were shaken with glass beads for 12 h (reciprocating shaker, 180 cycles min^-1). The mixture was washed through a 53-µm sieve with deionized water. The two fractions obtained were dried at 50°C and weighed. Preliminary tests on fresh residues showed that 90 ± 2% of the residues were recovered on a 50-µm sieve after this treatment (Chantigny et al., 1999). For density fractionation, the general approach was that of Gregorich and Ellert (1993), but adapted to this study where the bulk of soil organic matter was the sludge. The two main modifications were (i) the use of a denser (1.8 g cm^-3) solution of sodium polytungstate needed for the flotation of all sludge particles and, (ii) an additional resuspension and centrifugation of the floating fraction (mainly sludge) in 0.01 M CaCl₂ in order to wash out the significant amount of sodium polytungstate that was absorbed and retained by the sludge.

Total C and N contents in fine (<53 µm) and light (<1.8 g cm^-3) fractions and in whole soil were determined by dry combustion (CNS-1000 analyser, LECO Co., St Joseph, MI) after finely grinding the samples with a bead grinder (Retsch mod. MM2, Brinkmann Instruments Ltd., Rexdale, Ontario). Deinking sludges contained only a small quantity of inorganic C (<3% of their total C content, Chantigny et al., 1999) and as the pH of the soil samples from both the control and the amended soils were <7 (Fierro et al., 1999), the total C concentrations were assumed to represent organic C.

Total C and N contents in coarse (>53 µm) and heavy (>1.8 g cm^-3) fractions were determined as the difference between amounts in whole soil and in fine and light fractions, respectively. For determination of inorganic N of whole soil, field-moist subsamples were extracted with 2 M KCl (soil:solution ratio of 1:5), shaken for 1 h and gravity filtered through Whatman No. 42 paper. Filtrates were stored at -20°C and analyzed colorimetrically for NH⁺₄–N and NO⁺₃–N plus NO⁺₂–N (Cd reduction) on an autoanalyser (Technicon Instruments Corp., Tarrytown, NY). Soil moisture corrections were done after oven drying subsamples at 105°C for 24 h. Inorganic N refers to the summation of NH⁺₄–N and NO⁺₃–N plus NO⁺₂–N.

Statistical Analyses
Analyses of variance (ANOVA) were performed using the GLM procedure in the SAS statistical package (SAS Institute, 1988) on a per-date basis. Data sets for C and N in density fractions were analyzed by two-way (N x P) ANOVA because treatments without sludge yielded insufficient material for chemical analysis. All other data sets were analyzed by three-way (sludge x N x P) ANOVA. Variances were homogeneous for all data sets with the exception of C concentrations in the whole soil and coarse fraction for which a logarithmic transformation was performed. In addition, a separate repeated measures ANOVA was conducted using the REPEATED statement (SAS Institute, 1988) on all data sets, in order to evaluate the effect of time. The sphericity test was rejected when P < 0.001. Because P application had no effect on any of the variables analyzed here, values presented are means of P treatments, unless otherwise specified.

	Results

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Distribution of Soil and Carbon within Size and Density Fractions
Carbon concentration of the whole soil receiving sludge changed with time, as indicated by a significant time effect in the repeated measures ANOVA. At the end of the study, C concentrations were 43 and 69% of those of Day 5, for the low and high N rates, respectively (Table 2) . From Day 25 onwards, the high N rate resulted in greater C concentration than with the low rate. As indicated by the sludge-by-N interaction, this difference was significant for the last two sampling dates and was greatest at the last date (823 d). In the absence of sludge, the C concentration of whole soil was initially low and decreased slightly with time.

View larger version (20K): [in this window] [in a new window]   Fig. 1 Relative distribution of total (A) carbon and (B) nitrogen in selected size and density fractions in the de-inking sludge amended minesoil. Means of N and P treatments . LSD for differences between dates

For both density fractions, a significant (P < 0.01) decrease of N with time was observed from Day 86 (Table 5). The amount of N present in the light fraction tended to be larger with the high N rate at all times. However, this N effect was not statistically significant, probably owing to the high variability observed. The proportion of whole soil N contained in the heavy fraction remained near 65% (Fig. 1B).

The C/N ratio differed between fractions and decreased in the order coarse, light, heavy and fine fraction (Fig. 2) . Dynamics of C/N in the coarse and heavy fractions were similar to whole soil, and differed from light and fine fractions. Inorganic N concentration of whole soil was correlated with the amount of N in the physical fractions (Table 6) ; these relationships were obtained over a very large range of inorganic N (0.3–180 mg kg^-1). The strongest correlation was with the fine fraction, followed by the heavy, coarse and light fractions.

View larger version (23K): [in this window] [in a new window]   Fig. 2 Carbon-to-N ratios in physical fractions and whole soil in the de-inking sludge amended minesoil. Means of N and P treatments (n = 16)

	Discussion

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At the end of the study (day 823), the proportion of whole soil C that had been lost from Day 5 forward corresponded roughly to the relative mass of sludge lost in a litter bag decomposition study conducted in the same experiment (Fierro, 1998). Similar to whole soil C and for the same reasons, changes in the mass of the light fraction was a suitable indicator of sludge decay. The mass of the light fraction remaining at the end of the study was 36% of its initial mass (Table 3), which corresponded roughly to the total mass of sludge remaining in the litter bags, which was about 43% of the initial mass (Fierro, 1998).

Carbon conservation with the high N application rate was observed only as a trend in the litter bag study (from Day 452 onwards) (Fierro, 1998). In contrast, the present study provided clear evidence of the N effect on C conservation. The high N rate resulted in about 25% more C remaining in whole soil at Day 655, and 65% more C remaining at Day 823 compared with the low N rate. The explanation of this effect is probably a combination of the following: (i) repression of lignolytic enzymes (sludge is a lignin-rich substrate) by added N late in the process when lignin is decomposed (Bremer et al., 1991), (ii) increased formation of recalcitrant products by urea reacting with lignin and other polyphenolic compounds (Skene et al., 1997), and (iii) reduced microbial yield efficiency under N limitation (Paustian et al., 1992). Also, significantly lower soil temperature (data not shown) may have slowed decomposition due to increased shading from a significantly larger plant biomass obtained with the high N rate (Fierro, 1998). The effect of added N on C conservation was also noticed in the physical fractions, especially in the coarse fraction.

In the presence of sludge, the C and N pools in the macroorganic matter were initially much larger than in the microorganic matter, but the difference decreased with time, largely at the expense of the coarse fraction. A similar trend has been reported during early phases of decomposition in agricultural soils amended with cereal straw (Hassink and Dalenberg, 1996; Aita et al., 1997). However, in those tracer studies the amount of straw-derived C increased rapidly in the microorganic matter as decomposition proceeded, probably indicating an accumulation of young soil organic matter which included microbial biomass and partially decomposed and fragmented straw. In the present study, such an increase of C (mostly sludge-derived, as discussed above) in the fine fraction was not observed. In fact, the amount of C in this fine fraction was rather constant with time which suggests that the rates of incorporation and decomposition were fairly similar (Aita et al., 1997). It is also likely that a large proportion of the organic matter present in this fraction was fine material originating directly from the sludge since shortly after sludge incorporation (day 5) the amount of C was already four to five times that of the unamended soil.

Carbon initially incorporated into the fine fraction represented about 10% of total soil C at that time, and probably originated from water-soluble C and the small powder-like component of raw sludge. Also, the C/N ratio of this fine fraction, even if apparently steady, was too high (20) to be that of a highly processed and more stable microorganic matter, which usually has a C/N ratio of about or less than 10 (Cambardella and Elliott, 1992; Aita et al., 1997). Since the relative mass of the fine fraction and its C concentration were rather constant and low during sludge decomposition, changes in the macroorganic matter (coarse fraction) reflected the dynamics of the whole soil organic matter; the amount of C present in the coarse fraction at the end of the study were 33 and 50% of their initial amounts for the low and high N rates, respectively.

Residue-derived C remaining in the soil is usually expected to be gradually transferred from labile to more stabilized pools (Hassink and Dalenberg, 1996). As sludge decomposition proceeded, the remaining C was increasingly found in the heavy fraction, at the expense of the light fraction. This transfer was not observed between size fractions, as earlier mentioned. Therefore, as sludge decomposed in the minesoil, the remaining material gradually densified but without a significant net fragmentation in fine particles. This explains why after more than 823 d in the minesoil, the sludge was still visually recognizable (but darker in color) even though more than half the initial mass was already lost at that time. One factor that may have rapidly increased the density of sludge particles as decomposition proceeded was the high mineral coating of sludge fibers, mainly originating from the significant clay component of sludge (clay is used as paper filler). Particulate organic matter is found to be coated with clays in soil (Besnard et al., 1996), and clay may protect particulate organic C from decomposition (Skene et al., 1996; Franzluebbers and Arshad, 1997). An additional possible explanation for the rather unusual distribution of decomposing sludge is the fact that no macrofauna were observed (nor expected) in this recently exposed minesoil. Reduction in particle size is partly imputed to the activity of soil macrofauna (Swift et al., 1979).

Nitrogen availability may be related to the amount of N in the most dynamic N pools, such as the light fraction (Janzen, 1987; Barrios et al., 1996; Hassink, 1995b). However, other studies have found that the heavy fraction could also be an important source of mineral N (Sollins et al., 1984; Boone, 1994). The present study showed that mineral N content in the minesoil was related to pools in all the fractions. One interpretation of this relationship is that all fractions were a significant source of mineral N, at least through the duration of this study. This is supported by the observed decrease in the amount of N of all fractions from Day 86 which corresponded to the onset of net N mineralization from sludge decomposing in litter bags (Fierro, 1998).

Findings of Boone (1994) suggest that the heavy fraction is the primary source of N in coarse-textured mineral soils. In our study, also on a coarse-textured soil, the different strength of the relationship between mineral N content and physical fractions suggested that the best indicator of the size of the mineral N pool was the amount of N in the fine fraction.

In summary, the negligible initial content of organic matter in the minesoil allowed the determination of the fate of C and N within soil physical fractions during de-inking sludge decomposition, without the use of isotopic tracers. As sludge decomposed in the minesoil, the remaining material became denser but without a significant net fragmentation in particles <53 µm. This is different from what is usually observed for crop residues, and was explained by the presence of clays in the de-inking sludge which may contribute to protect it from fragmentation, and to the absence of macrofauna. More C, mostly from the macroorganic matter pool, remained in the minesoil with the highest N application rate. This effect of increasing applied N on C conservation has practical implications for the management of lignocellulosic residues in a degraded soil, where the restitution of sufficient and stable C and N pools is the key for the restoration of ecosystem function.

	ACKNOWLEDGMENTS

 
We are grateful to J. Goulet and P. Jolicoeur for their technical assistance. Thanks are also extended to the Ministère de l'Éducation du Québec and to Consejo Nacional de Ciencia y Tecnología (Mexico) for a joint postgraduate scholarship to A. Fierro. Financial support to C.J. Beauchamp was provided by Les Composts du Québec, Inc. and Daishowa, Inc. This study was also partly funded by the Matching Investment Initiative Program of Agriculture and Agri-Food Canada. We also thank H.H. Janzen and M. Bolinder for their comments on an early draft of the manuscript.

Received for publication January 22, 1998.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (9) Citing Articles via Google Scholar Google Scholar Articles by Fierro, A. Articles by Beauchamp, C. J. Search for Related Content PubMed Articles by Fierro, A. Articles by Beauchamp, C. J. GeoRef GeoRef Citation Agricola Articles by Fierro, A. Articles by Beauchamp, C. J.