Decomposition of de-inking paper sludge in agricultural soils as
characterized by carbohydrate analysis
Martin H. Chantigny
a,*, Denis A. Angers
a, Chantal J. Beauchamp
ba
Agriculture et Agroalimentaire Canada, Centre de recherche et de deÂveloppement sur les sols et les grandes cultures, 2560 Hochelage Blvd, Ste-Foy, Que., Canada G1V 2J3
b
DeÂpartement de phytologie, Faculte des Sciences de l'Agriculture et de l'Alimentation, Universite Laval, Quebec, Que., Canada G1K 7P4
Accepted 26 February 2000
Abstract
Increasing amounts of de-inking paper sludge (DPS) are available from paper mills, and could be used to improve soil fertility because of their high organic matter content. Our aim was to use chemical fractionation and carbohydrate characterization to determine the transformation and decay rates of DPS in dierent soils when large loading rates are applied. DPS was added to a well-drained silty clay loam (Typic Dystrochrept) and a poorly-drained clay loam (Typic Humaquept) at rates of 0 (control), 50 or 100 Mg dry matter haÿ1
. Soil samples were obtained periodically during 726 days after sludge incorporation. Soil organic matter was fractionated into hot-water extractable (HWC), mild-acid extractable (MAC) and strong-acid extractable carbohydrates (SAC), and acid-resistant carbon (ARC). The MAC fraction mostly contained hemicellulosic sugars, whereas SAC fraction included most cellulosic glucose. The contribution of microbial saccharides to the dierent carbohydrate fractions increased during DPS decomposition. The carbohydrate composition indicated that the chemical fractions re¯ected the net balance between disappearance of sludge carbohydrates and appearance of newly synthesized microbial carbohydrates. The MAC, SAC and ARC fractions in DPS-amended soils, had relative degradabilities of SAC > MAC > ARC. The sludge used, appeared to decompose according to a two-phase pattern, with an initial rapid-decay phase mostly determined by SAC and ARC disappearance (mean residence time 0.1 and 0.3 year, respectively), and a second slow-decay phase: largely characterized by ARC disappearance (mean residence time 8.5 years). DPS decomposed more slowly at the highest application rate, presumably because the capacity of soil microbes to decompose C was temporarily limited by nutrient de®ciency. Chemical fractionation and carbohydrate analysis proved useful to study quantitatively and qualitatively the decomposition and transformation of wood-derived residues in agricultural soils. Crown Copyright 72000 Published by Elsevier Science Ltd. All rights reserved.
Keywords:Decomposition; Cellulose; Hemicellulose; Lignin; Chemical fractionation; Paper sludge
1. Introduction
An increasing proportion of recycled ®bers is used in newspaper making. This results in the production of large amounts of de-inking paper sludge (DPS). Con-sidering its high organic matter content, DPS is an interesting amendment to improve (TreÂpanier et al.,
1996; Chantigny et al., 1999, 2000) or restore (Fierro et al., 2000) soil fertility and biological functioning. Chantigny et al. (1999) found that DPS markedly improved macroaggregate stability. They also reported that 2 years after incorporation, the improvement in soil physical properties was still highly signi®cant even though 60% of the added sludge had been lost.
DPS is mostly (about 90% of dry matter) com-posed of short wood ®bers, and thus contains
lig-nin, cellulose and hemicellulose as major
components. There is a general agreement that the
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* Corresponding author. Tel.: +1-418-657-7980; fax: +1-418-648-2402.
initial phase of fresh organic matter decomposition in soil is rapid and mostly determined by free cellu-lose (holocellucellu-lose) disappearance, whereas the sec-ond phase of slow decomposition is driven by the degradation of cellulose encrusted with lignin (ligno-cellulose) (Berg et al., 1984; Melillo et al., 1989).
The hemicellulosic fraction of plant residues is
much smaller than the cellulosic one, and is often included with cellulose in the holocellulose fraction (Ryan et al., 1990; Melillo et al., 1989). This suggests that both types of polysaccharides would behave similarly during decomposition (Berg et al., 1984). However, it has been argued that part of the hemicellulose may be more recalcitrant than cellu-lose because of its heterogeneity in sugar compo-sition and polymer structure (Alexander, 1977). The proportion of remaining material accounted for by lignin-like compounds increases with time during or-ganic matter humi®cation (Melillo et al., 1989). Herman et al. (1977) also suggested that the rate of lignin loss during decomposition of various organic residue is directly linked to the initial lignin content of the residues. Lignin is considered as the most recalcitrant fraction of plant tissues (Aber et al., 1990; Ryan et al., 1990) and as a precursor of soil humic substances. Some authors have argued that the amount of humic material formed from the de-composition of a given organic residue would be proportional to its initial lignin content (De Haan, 1977; Melillo et al., 1982).
Plant residues are generally rapidly degraded once in the soil (Alexander, 1977), and are partly minera-lized but are also partly transformed into new mi-crobial metabolites (Murayama, 1984; Voroney et al., 1989). Hence, fractionating soil organic matter to study the decomposition of its components can only provide estimates of net decay rate and mean
residence time because changes recorded in the
dierent fractions with time re¯ect the net result between rates of plant residue disappearance and appearance of microbial metabolites (Aita et al., 1997). Mannose and galactose are mostly of mi-crobial origin in soils, whereas xylose and arabinose
mostly arise from plant residues (Oades and
Wagner, 1971; Cheshire, 1977), and the monosac-charide composition of dierent carbohydrate frac-tions should get enriched in microbial sugars as decomposition of fresh organic matter proceeds.
Our goal was to use chemical fractionation and carbohydrate analysis of the fractions (i) to determine quantitative and qualitative changes in soil organic matter following the application of large amounts of DPS, and (ii) to estimate the decay rates of DPS and its dierent components (hemicellulose, cellulose, lig-nin) in agricultural soils.
2. Materials and methods
2.1. Field sites
A ®eld study was undertaken on two soil types; a well-drained Tilly silty clay loam (®ne, mixed, frigid, Typic Dystrochrept) and a poorly-drained Kamour-aska clay loam (®ne, mixed, frigid, Typic Humaquept). In the autumn of 1994, DPS was applied on main plots at rates of 0 (control), 50 or 100 Mg haÿ1
(dry matter basis). Each DPS rate was replicated four times on each soil type for a total of 12 main plots on each soil type. The fresh DPS was ®rst pulverized to <6 mm, uniformly spread on the soil surface of each plot, and buried by rototilling to obtain a worked surface layer of 20 cm. In spring 1995, each main plot was subdivided into three sub-plots for a total of 36 exper-imental plots on each soil type. The sub-plots were cropped to either alfalfa (Medicago sativa L.), red clo-ver (Trifolium pratenseL.) or bromegrass (Bromus iner-mis L.) for the 1995 and 1996 growing seasons. Details of the study site, soil types and DPS properties were reported by Chantigny et al. (1999). Brie¯y, the silty clay loam had 17 g C kgÿ1
2.2. Soil sampling and analyses
Experimental plots were sampled four times from July to October 1995 and four times from June to Sep-tember 1996. At each sampling date, ®ve soil samples were collected in each plot. Soil samples were sieved at 6 mm in the ®eld and mixed to obtain one representa-tive soil sample per plot. Mesh size of 6 mm was the minimum opening to be used for DPS to pass the sieve without being forced through it. Soil samples were air-dried and stored in plastic bags until analyzed.
Total soil C content was determined by dry combus-tion and the values were presented in detail by Chan-tigny et al. (1999). Soil organic matter was also chemically fractionated using a series of acid-hydroly-sis steps of increasing strength. Each hydrolyacid-hydroly-sis type was performed on a dierent soil sub-sample. Hot-water extractable carbohydrates (HWC) were obtained by incubating 2 g of ®nely ground soil (<0.15 mm) with 30 ml of distilled water in the 50-ml centrifuge tubes at 858C for 24 h. After the incubation period, the hydrolysates were cooled to room temperature and centrifuged (16,000 g) for 10 min. Each hydrolysate was then acidi®ed to yield monosaccharides by mixing 1 part of 18 M H2SO4to 35 parts of supernatant in a
carbohydrate contents. Insoluble carbohydrates were extracted by two dierent methods, according to Lowe (1993). In the ®rst case, 2 g of ®nely ground soil were incubated with 30 ml of 0.5 M H2SO4 at 858C for 24
h. The extracts were then centrifuged and neutralized as described for HWC. In the second case, 2 g of ®nely ground soil were ®rst hydrolyzed with 8 ml of 12 M H2SO4, for 2 h at room temperature in the 50-ml
cen-trifuge tubes. The soil and acid were gently mixed with a glass rod to ensure that all soil sub-sample was mois-tened with the acid. Afterwards, slurries were trans-ferred into 250-ml centrifuge bottles with 184 ml of distilled water to dilute the acid to 0.5 M. The slurries were then incubated and treated as described in the ®rst case.
The carbohydrate concentration in the dierent hydrolysates was determined using an automated alka-line±ferricyanide method and glucose as a standard (Cheshire, 1979), and was corrected for carbohydrate contamination from ®lter papers. Since a dierent soil sub-sample was used for each type of hydrolysis, 0.5
M H2SO4 extracts included the HWC fraction,
whereas 12 M H2SO4 extracts included the 0.5 M
H2SO4 fraction. Therefore, the dierence in
carbo-hydrate content between 0.5 M H2SO4 and HWC was
de®ned as the mild-acid extractable carbohydrate (MAC) fraction. Similarly, the dierence in carbo-hydrate content between 0.5 and 12 M H2SO4
hydroly-sates was de®ned as the strong-acid extractable carbohydrate (SAC) fraction. The HWC, MAC and SAC contents were all expressed on a C basis, based on 40% C in carbohydrates.
Total organic C content of 12 M H2SO4
hydroly-sates was measured by wet oxidation and IR-CO2
detection using a soluble C analyzer (model DC-180, Dohrmann, Santa Clara, CA). The acid-resistant car-bon (ARC) fraction was calculated by subtracting total organic C content in 12 M H2SO4 hydrolysates
from total soil organic C content. The initial amount of material present in all above-mentioned fractions was quanti®ed in soils and fresh DPS used in our study (Table 1). Since large amounts of DPS were added to the soils investigated, the contribution of DPS to the dierent measured C fractions was esti-mated by subtracting the values obtained in DPS-amended soils from the values recorded in the una-mended controls. These values were used to model DPS decomposition in soil.
To determine the origin and characterize the nature of carbohydrates present in the dierent hydrolysates, soil samples collected at day 370 were analyzed for their content of arabinose, galactose, glucose, mannose and xylose by liquid anion-exchange chromatography with pulsed amperometric detection (Model DX-500, Dionex, Sunnyvale, CA, USA) according to Martens and Frankenberger (1991). The only modi®cation to the method was that the extracts were neutralized to pH = 7 with 0.1 M NaOH and ®ltered (Whatman #42) prior to puri®cation through solid-phase extrac-tion columns. Arabinose, xylose and glucose values were corrected as ®lter papers contributed small amounts of these sugars.
2.3. Statistical analyses
The experimental design was analyzed as a split± split-plot with ®eld sites as the main plots, DPS amendment rate as the subplots, and crop species as the sub±sub-plots. Analyzes of variance were per-formed separately for each sampling date as described by Gomez and Gomez (1984) for experiments repli-cated in space, and using the GLM procedure of SAS (SAS Institute, 1989). The ANOVA revealed no signi®-cant crop eect on any measured soil parameters. Therefore, only the soil type and DPS eects are dis-cussed in the text (Table 2). Non-linear regression ana-lyzes were performed to determine the decay rate constant and mean residence time of the dierent measured C fractions and whole DPS. Standard devi-ation of the mean for monosaccharide concentrdevi-ations and sugar ratios in soil was calculated as proposed by Bevington (1969).
3. Results
3.1. Carbohydrate characterization of the dierent soil hydrolysates
The impact of DPS amendment on soil monosac-charide content was estimated by subtracting the con-tent recorded in unamended soils from those measured in DPS-amended soils (Table 3). Addition of DPS to the soil had a positive eect on the monosaccharide Table 1
Initial amounts of C (g kgÿ1)a present in dierent organic matter fractions for the soils and DPS used during the study, as expressed on a soil dry matter basis
C fraction Silty clay loam Clay loam DPS
Total C 17.0 (0.0) 18.9 (0.0) 387.3 (0.8)
HWCb 0.2 (0.0) 0.3 (0.1) 10.5 (0.7)
MAC 1.2 (0.2) 1.2 (0.2) 46.1 (1.1)
SAC 1.5 (0.2) 1.3 (0.1) 127.4 (4.0)
ARC 10.5 (0.5) 11.6 (0.2) 203.2 (5.1)
a
The imbalance in the sum of C fractions versus Total C is due to non-saccharide C extracted by the acids, which was not accounted for. Values in parentheses are standard deviation.
b
Table 2
Summary of analysis of variancea on the eect of soil type (Site) and de-inking paper sludge amendment rate (DPS) on hot-water extractable carbohydrates (HWC), mild-acid extractable carbohydrates (MAC), strong-acid extractable carbohydrates (SAC) and acid-resistant carbon (ARC) fractions in a clay loam and a silty clay loam
Fraction Sampling datesb
271 307 342 370 596 645 683 726
HWC
Site (S) 0.403 0.530 0.004 0.051 0.081 0.152 0.480 0.251 >
DPS (D) 0.001 0.003 0.032 < 0.001 < 0.001 0.001 0.004 < 0.001
DLinc < 0.001 0.001 0.018 < 0.001 < 0.001 < 0.001 0.001 < 0.001
DQuad 0.215 0.515 0.215 0.921 0.002 0.102 0.153 0.833
MAC
Site (S) 0.461 0.378 0.960 0.123 0.372 0.470 0.580 0.258
DPS (D) < 0.001 < 0.001 < 0.001 0.001 < 0.001 < 0.001 < 0.001 < 0.001
DLin < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
DQuad 0.188 0.264 0.172 0.894 0.099 0.036 0.123 0.599
SAC
Site (S) 0.641 0.562 0.262 0.090 0.172 0.180 0.076 0.096
DPS (D) < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
DLin < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
DQuad 0.082 0.004 0.020 0.172 0.259 0.074 0.048 < 0.001
ARC
Site (S) 0.807 0.995 0.584 0.341 0.281 0.284 0.825 0.892
DPS (D) < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
DLin < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 < 0.001
DQuad 0.719 0.570 0.830 0.798 0.879 0.443 0.936 0.995
aPvalues. There were no signi®cant (P> 0.05) interaction between Site and DPS eects, except for HWC on day 645, and MAC and SAC on
day 726.
bTime after DPS amendment (days). cD
Lin, linear response of carbohydrate fraction to increasing DPS amendment rate; DQuad, quadratic response of carbohydrate fraction to
increasing DPS amendment rate.
Table 3
Contributionaof DPS to the monosaccharide composition of a silty clay loam and a clay loam, 370 days after DPS incorporation
Fraction DPS rate (mg haÿ1)
Arab
(mg C kgÿ1dry soil) Gal
(mg C kgÿ1dry soil) Glu
(mg C kgÿ1dry soil) Man
(mg C kgÿ1dry soil) Xyl
(mg C kgÿ1dry soil)
Silty clay loam
HWCc 50 1 (0) 5 (1) 20 (17) 4 (1) 3 (3)
100 6 (2) 11 (2) 55 (36) 19 (6) 11 (4)
MAC 50 134 (77) 170 (84) 159 (62) 244 (89) 276 (81)
100 143 (13) 180 (13) 166 (40) 326 (26) 362 (26)
SAC 50 82 (12) 152 (25) 2129 (827) 352 (198) 222 (58)
100 134 (68) 337 (96) 5915 (1803) 1250 (403) 723 (177)
Clay loam
HWC 50 4 (5) 6 (8) 59 (14) 14 (11) 6 (2)
100 7 (2) 16 (2) 111 (15) 30 (1) 19 (1)
MAC 50 84 (86) 83 (59) 85 (74) 152 (37) 138 (61)
100 106 (76) 150 (96) 149 (36) 295 (197) 324 (170)
SAC 50 125 (47) 48 (30) 1263 (202) 278 (64) 159 (43)
100 128 (4) 104 (52) 4481 (1143) 847 (420) 399 (179)
a
Estimated as the dierence between values recorded in DPS-amended and control soils. Values in parentheses are standard deviation.
b
Ara, arabinose; Gal, galactose; Glu, glucose; Man, mannose; Xyl, xylose.
c
content of all measured carbohydrate fractions. As expected the eect of DPS was larger in soil amended
with 100 than 50 Mg DPS haÿ1
. The relative abun-dance of monosaccharides in the HWC fraction was glucose > mannose > xylose3galactose > arabinose.
The amount of monosaccharides present in the MAC fraction was larger than in HWC, especially for man-nose and xylose which were the most abundant
mono-saccharides in the MAC fraction. The highest
monosaccharide contents were found in the SAC frac-tion, in which glucose was much more abundant than any other monosaccharide. In the MAC fraction of DPS-amended soils, glucose accounted for an average of 15% of total monosaccharides, whereas this pro-portion increased to an average of 71% in the SAC fraction. More than 90% of total extractable glucose was recovered in the SAC fraction. In general, 40± 60% of total extractable arabinose, galactose, mannose and xylose were found in the MAC fraction of DPS-amended soils, whereas the rest was recovered in the SAC fraction.
On average, the ratio of (galactose + mannose) to (arabinose + xylose) was about 1.0 for MAC, 1.8 for SAC and 2.0 for HWC fraction in DPS-amended soils collected on day 370 of the study (Table 4). This ratio was close to 1.0 in control soils collected on day 370 of the study, and in fresh DPS, irrespective of the carbohydrate fraction considered.
3.2. Dynamics of soil C in chemical fractions
The HWC content was signi®cantly (P< 0.01) lar-ger in DPS-amended soils than in the unamended con-trols throughout the study (Table 2). Moreover, the amount of carbohydrates found in this fraction ¯uctu-ated around a mean value for the duration of the study (Fig. 1). In the silty clay loam, the mean values were 220 mg C kgÿ1
for the unamended control, 271
for the 50 Mg DPS haÿ1
treatment, and 327 for the
100 Mg DPS haÿ1
treatment. Corresponding values for the clay loam were 256, 325 and 358 mg C kgÿ1
, respectively. Despite that HWC values tended to be smaller in the silty clay loam than in the clay loam, signi®cant (P < 0.05) dierences between soil types were only found on the third sampling date (Table 2).
The MAC, SAC and ARC contents were all signi®-cantly (P< 0.001) larger in DPS-amended than in con-trol soils, and although the dierences decreased with time (Fig. 2), the DPS eect on these fractions remained signi®cant (P< 0.001) until the end of the study (Table 2). Even though SAC and ARC contents tended to decrease faster in the clay loam than in the silty clay loam (Fig. 2b and c), dierences between soil types were not signi®cant (P> 0.05) at any sampling date (Table 2). The MAC, SAC and ARC contents all appeared to change according to a two-phase pattern with an initial phase of rapid decrease followed by a phase of slow disappearance. From day 271 to 726 of the study, the dierence in MAC content between
Fig. 1. HWC content in a silty clay loam and a clay loam amended with DPS. Arrows on the graphs indicate winter periods when soils were not sampled.
Table 4
Ratio of (galactose + mannose) to (arabinose + xylose) recorded in dierent carbohydrate fractions of fresh de-inking paper sludge, and in a silty clay loam and a clay loam at day 370
Material type DPSarate (Mg haÿ1) HWC MAC SAC
Fresh DPS ± 1.1 (0.1)b 1.0 (0.0) 1.0 (0.1)
Silty clay loam 0 1.0 (0.1) 0.8 (0.0) 1.0 (0.1)
50 2.2 (0.2) 1.0 (0.1) 1.7 (0.1) 100 1.8 (0.1) 1.0 (0.0) 1.9 (0.1)
Clay loam 0 1.1 (0.1) 1.0 (0.0) 0.9 (0.2)
50 2.0 (0.3) 1.1 (0.1) 1.7 (0.1) 100 1.8 (0.0) 1.0 (0.2) 1.8 (0.3)
a
DPS, de-inking paper sludge; HWC, hot-water extractable carbo-hydrates; MAC, mild-acid extractable carbocarbo-hydrates; SAC, strong-acid extractable carbohydrates.
b
DPS-amended and control soils was reduced by 78% in soils amended with 50 Mg haÿ1
, and by 73% in soils amended with 100 Mg haÿ1
(Fig. 2a). During the same time interval, the dierence in SAC content between DPS-amended and control soils decreased by 99% and 84%, respectively (Fig. 2b), whereas corre-sponding decreases in ARC were 50% and 44% (Fig. 2c).
3.3. Composition of residual carbon
The ARC represented the largest fraction of organic matter in both soils and in fresh DPS, followed in
decreasing order by SAC, MAC and HWC fractions (Table 1). In the control soils, the proportions of these C fractions remained roughly constant for the duration of the study (data not shown). However, in DPS-amended soils the composition of residual C markedly changed as DPS decomposed (Fig. 3). From day 271 to 726 of the study, the proportion of residual organic C accounted for by the ARC fraction increased from about 500 g kgÿ1
in both DPS-amended soils to 850 g kgÿ1
in soils treated with 50 Mg DPS haÿ1
, and to 700 g kgÿ1
in soils treated with 100 Mg DPS haÿ1
. In the meantime, the proportion of residual organic C accounted for by the SAC fraction decreased from about 350 g kgÿ1
in both soils to almost 0 in soils
amended with 50 Mg DPS haÿ1
, and to 150 g kgÿ1
in
soils amended with 100 Mg DPS haÿ1
. The pro-portions of residual organic C recovered as HWC and MAC remained virtually unchanged throughout the study, ranging from 5 to 20 g kgÿ1
and from 50 to 80 g kgÿ1
, respectively.
3.4. Modeling of DPS decomposition patterns
Non-linear regression analyses were performed on total soil organic C (data from Chantigny et al., 1999),
Fig. 3. Proportion of C derived from DPS accounted for by HWC (.), MAC (T), SAC (Q) and ARC (W) as DPS decomposed in a clay loam (open symbols) and a silty clay loam (closed symbols). Arrows on the graphs indicate winter periods when soils were not sampled.
MAC, SAC and ARC data to model DPS decompo-sition in soils (Table 5). Temperature is a determining factor of organic matter decomposition in the cool humid area where our study took place (Rochette et al., 1991; Fierro et al., 2000). Moreover, C losses are small during winter periods when soil temperature is close to 08C (Fierro et al., 2000; Chantigny et al., 1999). Therefore, the data obtained for each C fraction (values in DPS-amended soil corrected for the una-mended control) were regressed against accumulated degree-days, according to Honeycutt et al. (1988). In our case, accumulated degree-days were calculated as the sum of daily mean air temperature from day 271 to 726 of our study. A two-compartment exponential decay function was used to model changes in C con-tent for all measured C fractions (Table 5).
When total soil organic C contents were considered, the regression analysis ascribed 27% of the initial C content (CFD/(CFD + CSD); Table 5) to a fast-decay
compartment in soils amended with 50 Mg DPS haÿ1
, whereas this proportion was 34% in soils amended
with 100 Mg DPS haÿ1
. The remaining C (73% and 66%, respectively) was included in the slow-decay compartment. In soils amended with 50 Mg DPS haÿ1
, 62% of the initial MAC content was ascribed to the fast-decay compartment, whereas it was 53% in soils treated with 100 Mg DPS haÿ1
. According to the re-gression analysis, an average of 28% of the initial SAC content was decomposing rapidly in both soils, whereas the remaining 72% was more recalcitrant ma-terial. In the case of the ARC fraction, 36% of the
in-itial content was included in the fast-decay
compartment in soils amended with 50 Mg DPS haÿ1
, whereas this proportion was 42% in soils amended with 100 Mg DPS haÿ1
.
The SAC fraction had the highest net decay rate
constants, whereas ARC had the lowest, and total
or-ganic C and MAC showed intermediate values
(Table 5). The dierent C fractions showed larger decay rates in soils amended with 50 than 100 Mg DPS haÿ1
. According to the decay rate constants, and given that an average of 2400 degree-days are accumu-lated each year in the area of the study, the mean resi-dence time of C for the fast-decay compartment ranged from 0.11 year for SAC to 0.28 year for MAC and ARC fractions. The mean residence time of C in the slow-decay compartment ranged from 1 year for SAC, and 8.5 year for the ARC fraction.
4. Discussion
4.1. Monosaccharide composition of acid hydrolysates
In plant material, hemicellulose mostly contains xylose and arabinose, and smaller amounts of glucose, galactose and mannose, whereas cellulose is exclusively composed of glucose (Alexander, 1977). Lowe (1993) proposed that 0.5 M H2SO4would hydrolyse all
non-cellulosic polysaccharides in soil, whereas 12 M H2SO4
would include cellulose. Therefore, it could be inferred that in our study MAC should have included all the arabinose, galactose, mannose and xylose, whereas SAC should have contained only glucose. Our results indicated that SAC comprised about 40±60% of the total extractable hemicellulosic saccharides, indicating that it did not represent exclusively cellulose coming from DPS. On the other hand, glucose present in the SAC fraction represented more than 70% of total monosaccharides present in this fraction, and more than 90% of total extractable glucose. Hence, SAC
Table 5
Non-linear regression parameters and coecients obtained by modelingaof the decay of dierent C fractions in soils amended with DPS
Fraction DPS (mg haÿ1) CFD(g kgÿ1) CSD(g kgÿ1) kFD(ddÿ1) kSD(ddÿ1) MRTFDb(year) MRTSD(year) R2c
Total Cd 50 4.8 12.8 0.0025 (80) 0.00016 (25) 0.17 2.6 0.96
100 14.3 27.5 0.0018 (133) 0.00013 (76) 0.23 3.2 0.82
MAC 50 0.8 0.5 0.0019 (58) 0.00012 (108) 0.22 3.5 0.91
100 1.6 1.4 0.0015 (60) 0.00010 (92) 0.28 4.2 0.94
SAC 50 1.3 3.7 0.0039 (36) 0.00040 (37) 0.11 1.0 0.88
100 4.5 10.4 0.0026 (146) 0.00031 (32) 0.16 1.3 0.93
ARC 50 3.6 6.4 0.0015 (107) 0.000054 (167) 0.28 7.7 0.79
100 8.8 12.4 0.0015 (100) 0.000049 (184) 0.28 8.5 0.77
aC fraction data were regressed using a two-compartment exponential decay function: Crem
CFDeÿkFDdd
CSDeÿkSDdd where C
rem, is the
amount of C remaining from a given fraction; CFDand CSD, the amounts of C ascribed to the fast-decay (FD) and slow-decay (SD)
compart-ments, respectively;kFDandkSD, the decay rate constants of the SD and FD compartments, respectively; dd, degree-days accumulated during a
given period of time. Values in parentheses are coecient of variation (standard deviation/mean).
b
MRTFDand MRTSD, are the mean residence times of C ascribed to FD and SD compartments, respectively, assuming that 2400 degree-days
are accumulated each year.
c
R2, coecient of determination of the regression function.
d
was considered as an acceptable index to estimate the decomposition rate of cellulose from DPS in soil.
Similarly, MAC did not represent total hemicellulose coming from DPS since it contained only 40±60% of the total extractable hemicellulosic sugars. Oades et al. (1970) and Murayama (1984) proposed the use of 2.5 M H2SO4to extract hemicellulosic saccharides. Hence,
the acid strength used in our study was likely too low to extract all the hemicellulosic saccharides and about half of these saccharides were recovered in the SAC fraction. Nevertheless, since more than 80% of total monosaccharides present in the MAC fraction were accounted for by hemicellulosic sugars, we assumed that MAC fraction mostly re¯ected the dynamics of hemicellulosic saccharides in DPS-amended soils.
In soils, arabinose and xylose are considered to be mostly of plant origin, whereas mannose and galactose would be mainly of microbial origin (Oades and Wagner, 1971; Cheshire, 1977). Furthermore, the ratio of (galactose + mannose) to (arabinose + xylose) would be near 0.5 for fresh plant material, and close to 2.0 for microorganisms (Oades and Wagner, 1971; Murayama, 1984). The ratios of (galactose + man-nose) to (arabinose + xylose) found in HWC, MAC and SAC fractions were close to 1.0 for both una-mended soil and also for fresh DPS, indicating that they were already colonized by microbes. A value of about 1 can therefore be inferred for DPS-amended soils at time of DPS application. After 370 days, this ratio was close to 2.0 for HWC and 1.8 for SAC frac-tion, indicating that the proportion of microbial sac-charides increased in these carbohydrate fractions as DPS decomposed in soil. The presence of microbial saccharides in HWC and SAC fractions indicates that
during decomposition, part of added DPS was
resynthesized as new microbial polysaccharides
released in soil, as previously suggested by Murayama (1984) and Voroney et al. (1989) for crop residues. This ®nding demonstrates that once an organic ma-terial is added to the soil, chemical fractionation of soil organic matter can no longer provide isolation of pure plant-derived carbohydrates such as cellulose or hemicellulose, and consequently, such terminology should be used only to simplify the discussion.
4.2. Decomposition of de-inking paper sludge in soil
Non-linear regression analysis of total organic C data reported by Chantigny et al. (1999) revealed that about one-third of DPS-C was decomposing at a rapid rate whereas the major part was decomposing more slowly. This ®nding is in agreement with previous stu-dies which have demonstrated that complex organic materials such as crop residues (Voroney et al., 1989; Aita et al., 1997) or forest litters (Melillo et al., 1989; Aber et al., 1990) often decompose according to a
two-compartment decay model. Fierro et al. (2000) also modelled DPS decomposition in a sandy soil with a two-phase exponential decay function, but obtained smaller decay rate constants than ours even though both studies were carried out under similar climatic conditions and with similar DPS. The discrepancy would thus depend on dierences in experimental con-ditions. Fierro et al. (2000) studied DPS decomposition using litter bags, whereas we incorporated DPS by mixing it with the soil. In addition, Fierro et al. (2000) worked with a degraded mine soil (>95% sand, <1.5 g C kgÿ1
), and they fertilized this soil with N. All these factors regulate the rate of organic matter de-composition (Fog, 1988; Seastedt et al., 1992).
A large proportion of water-extractable C is readily available to microbes in soil and must be replaced sev-eral times a year to maintain its level (McGill et al., 1986). As mentioned previously, microbial carbo-hydrates dominated the HWC fraction at day 370, and HWC contents ¯uctuated around a mean value for the duration of the study. Therefore, HWC measurements re¯ected an instantaneous equilibrium between the amount of carbohydrates entering this pool and their consumption rate, and the HWC fraction must have been continuously replenished through a gradual depo-lymerization of cellulose and hemicellulose from DPS, and microbial synthesis of carbohydrates.
phase of ARC were similar to those calculated for MAC, which suggests that part of ARC was as labile as MAC. Alternatively, the large amount of ARC pre-sent in fresh DPS stimulated lignolytic micro¯ora and may have resulted in accelerated ARC degradation. Herman et al. (1977) reported that the rate of lignin loss was directly related to the initial lignin content of the degraded material. Our ®ndings indicated that the relative degradability of the investigated C fractions was HWC > SAC > MAC > or = ARC, and there-fore, the resistance of organic molecules to acid
hy-drolysis did not directly re¯ect their biological
recalcitrance.
The slow phase of decomposition estimated for all investigated C fractions could be due either to (1) some chemical heterogeneity in the hydrolyzed poly-mers of a given fraction, (2) a gradual condensation of decaying polymers with recalcitrant molecules such as humic acids, or (3) microbial degradation followed by resynthesis and release of more recalcitrant metab-olites. Another possibility would be that those carbo-hydrate fractions became physically protected against degradation due to DPS encrustation with mineral soil particles. Mineral particles had been shown to physi-cally interfere with biological degradation of organic matter (Ou and Alexander, 1974; Ladd et al., 1996). Chantigny et al. (1999) found that wood ®bers present in DPS were rapidly encrusted with mineral particles during the initial phase of decomposition in soil, which slowed their decomposition.
Some authors have reported that the initial phase of plant residue decomposition was mostly explained by free cellulose (holocellulose) disappearance, whereas the second phase was determined by the decomposition of cellulose encrusted with lignin and lignin itself (lig-nocellulose) (Berg et al., 1984; Melillo et al., 1989). In our study, the initial phase of rapid DPS decompo-sition was mainly explained by SAC and ARC
disap-pearance, whereas the second phase of slow
decomposition was mostly accounted for by ARC dis-appearance. Hence, according to previous ®ndings (Berg et al., 1984; Melillo et al., 1989), the SAC frac-tion contained most of the holocellulose coming from DPS, whereas the ARC fraction mainly comprised lig-nocellulose.
The composition of DPS changed as it decomposed in soil. The proportion of residual DPS accounted for by the dierent C fractions revealed that MAC dynamics were similar to whole DPS, whereas SAC degraded more rapidly and ARC degraded less rapidly than DPS as a whole. Two years after DPS had been applied, 40% of initially added C remained in the soil (Chantigny et al., 1999) of which ARC was by far the dominant fraction. Estimated mean residence times for the slow-decay compartment of ARC indicated a rela-tively rapid disappearance of DPS (<10 years) under
our conditions. However, given the short duration of this study (2 years), the mean residence time for ARC, which contained recalcitrant material such as lignin, might have been underestimated. Some humic material must be formed from decomposing DPS since it in-itially contains large amounts of lignin (>50%). Assuming that the amount of humic material derived from fresh organic matter is proportional to its initial lignin content (De Haan, 1977; Melillo et al., 1982), DPS would have a great potential to contribute stable C as humic substances in soil.
Although unexpected, both decay rate constants and the composition of degrading DPS consistently indi-cated faster decay rates in the soil amended with 50 than 100 Mg DPS haÿ1
. The soil can be envisioned, as a C-limited environment, since microbial activity gen-erally responds positively to fresh C input. However, C-rich amendments such as DPS can immobilize large amounts of soil N and P (Fierro et al., 1997, 2000). Therefore, applying DPS on the soil likely created nutrient limiting conditions to microbial activity and C decomposition, when application rates exceeded 50 Mg haÿ1
.
5. Conclusions
We found that chemical fractionation of soil organic matter could be used to study the decomposition of speci®c fractions of organic material such as cellulose, hemicellulose and lignin. However, one must be aware that changes in the investigated organic matter frac-tions were the result of the dierence in the rate of in-corporation and degradation of plant and microbial carbohydrates `¯owing through' each fraction. There-fore, decay rates and residence times derived with the proposed approach only provide information about the relative or net decomposition rates of dierent or-ganic matter components. In combination with mono-saccharide analysis, the present approach provided insights into the microbial degradation of DPS carbo-hydrates in soil. Fractionation of soil organic matter with increasing extractant strength is relatively rapid and easy to perform, and could be standardized with appropriate acid strengths to allow for direct compari-sons among dierent experiments.
In our study, the decomposition pattern of DPS in soil was fairly comparable to crop residues with an in-itial phase of rapid C losses followed by a second phase of slow C losses. From our results, we assumed that the initial phase of decomposition was mostly explained by the decomposition of holocellulose and
lignocellulose. The second slow-decay phase was
remained in soil, mainly as acid-resistant C, whereas holocellulose had almost completely disappeared. In both soils, decomposition rates of DPS were greater
when adding 50 than 100 Mg haÿ1
likely due to a shortage of soil nutrients.
Acknowledgements
Funding of this project was provided by the Daishowa, QueÂbec City, Canada and by the Matching Investment Initiative of Agriculture and Agri-Food, Canada. We thank Jean Goulet for assistance in ®eld work, and Sylvie CoÃte and Johanne Tremblay for their assistance in carbohydrate analyses.
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