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Composting and storage of organic household waste with dierent litter amendments. I: carbon turnover Y. Eklind * , H. Kirchmann Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden Received 31 May 1999; received in revised form 8 December 1999; accepted 14 December 1999 Abstract Composting of source-separated organic household wastes is becoming a more common practice in several countries. Carbon decomposition dynamics during composting are important for an overall understanding of the process. We investigated over 590 days losses of organic C and decomposition of C constituents in artificial organic household waste mixed with six dierent litter amendments; straw, leaves, hardwood, softwood, paper and sphagnum peat. Litter addition was necessary to achieve an aerobic process. Samples were analysed for dry matter, ash, organic C, volatile fatty acids, and lignin, cellulose and hemicellulose fractions. Calculated by first-order kinetics, residual amounts of dry matter were 22–63% and of organic C 11–61%, and both amounts were highest in the peat mixture and lowest in the control without litter addition. Rate constants for dry matter and organic C de- composition were highest in the leaf mixture and lowest in the control. The initial lignin content in the mixtures was highly cor- related (R 2 0:91) with the residual amount of organic C. A lag phase, of varying length, in lignin decomposition was present in some but not all cases. Cellulose decomposition was slower in leaf, hardwood and softwood mixtures than in paper and straw mixtures. The results showed that the characteristics of litter amendments greatly influence the composting process. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Carbon loss; Litter amendments; Cellulose; Hemicellulose; Lignin; Volatile fatty acids; Decomposition rate 1. Introduction Separation of organic household wastes at source is becoming a more common practice in several European countries and composting is one biological option for treating this waste before it is used in agriculture or horticulture. However, composting of source-separated organic household wastes often presupposes that litter material is added to act as a bulking agent to improve structure and enhance aeration, to adsorb excess liquids, and to provide microorganisms with an extra energy source to balance the normally high N content. Litter materials added can vary considerably in both physical and chemical properties, such as bulk density, water holding capacity, composition of C constituents (as re- viewed by Jenkinson, 1981 and Lynch, 1987), and thereby may have dierent eects on composting. There have been earlier studies of the eects of dierent litter additives on the regulation of bulk density in composts (Liao et al., 1995), on N losses (Brink, 1995) as well as content and form of N in the compost produced (Martin et al., 1993; Meyer and Sticher, 1983) and properties related to plant growth (Kobayashi et al., 1994). Carbon decomposition dynamics during composting are important for an overall understanding of compo- sting, as C compounds provide the energy for the de- gradation process. Volatile fatty acids are a type of C compound that may be formed during the composting process (Kirchmann and Wid en, 1994), and since they are phytotoxic can cause problems in cultivation (DeVleeschauwer et al., 1981). Changes in C compo- nents in relation to organic C have been used to define the degree of decomposition or bio-maturity of com- posted city refuses (Inoko et al., 1979; Harada et al., 1981), grass compost (Mahmood et al., 1987) and farmyard manure (Levi-Minzi et al., 1986). The aim of the present study was to investigate the mass loss and C turnover during composting, matura- tion and storage of household waste mixtures with dif- ferent litter amendments. Results of the N turnover of the same experiment were described by Eklind and Kirchmann (2000). Bioresource Technology 74 (2000) 115–124 * Corresponding author. 0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 0 4 - 3

Composting and storage of organic household waste with different litter amendments. I: carbon turnover

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Composting and storage of organic household waste with di�erentlitter amendments. I: carbon turnover

Y. Eklind *, H. Kirchmann

Department of Soil Sciences, Swedish University of Agricultural Sciences, P.O. Box 7014, S-750 07 Uppsala, Sweden

Received 31 May 1999; received in revised form 8 December 1999; accepted 14 December 1999

Abstract

Composting of source-separated organic household wastes is becoming a more common practice in several countries. Carbon

decomposition dynamics during composting are important for an overall understanding of the process. We investigated over 590

days losses of organic C and decomposition of C constituents in arti®cial organic household waste mixed with six di�erent litter

amendments; straw, leaves, hardwood, softwood, paper and sphagnum peat. Litter addition was necessary to achieve an aerobic

process. Samples were analysed for dry matter, ash, organic C, volatile fatty acids, and lignin, cellulose and hemicellulose fractions.

Calculated by ®rst-order kinetics, residual amounts of dry matter were 22±63% and of organic C 11±61%, and both amounts were

highest in the peat mixture and lowest in the control without litter addition. Rate constants for dry matter and organic C de-

composition were highest in the leaf mixture and lowest in the control. The initial lignin content in the mixtures was highly cor-

related (R2 � 0:91) with the residual amount of organic C. A lag phase, of varying length, in lignin decomposition was present in

some but not all cases. Cellulose decomposition was slower in leaf, hardwood and softwood mixtures than in paper and straw

mixtures. The results showed that the characteristics of litter amendments greatly in¯uence the composting process. Ó 2000

Elsevier Science Ltd. All rights reserved.

Keywords: Carbon loss; Litter amendments; Cellulose; Hemicellulose; Lignin; Volatile fatty acids; Decomposition rate

1. Introduction

Separation of organic household wastes at source isbecoming a more common practice in several Europeancountries and composting is one biological option fortreating this waste before it is used in agriculture orhorticulture. However, composting of source-separatedorganic household wastes often presupposes that littermaterial is added to act as a bulking agent to improvestructure and enhance aeration, to adsorb excess liquids,and to provide microorganisms with an extra energysource to balance the normally high N content. Littermaterials added can vary considerably in both physicaland chemical properties, such as bulk density, waterholding capacity, composition of C constituents (as re-viewed by Jenkinson, 1981 and Lynch, 1987), andthereby may have di�erent e�ects on composting. Therehave been earlier studies of the e�ects of di�erent litteradditives on the regulation of bulk density in composts

(Liao et al., 1995), on N losses (Brink, 1995) as well ascontent and form of N in the compost produced (Martinet al., 1993; Meyer and Sticher, 1983) and propertiesrelated to plant growth (Kobayashi et al., 1994).

Carbon decomposition dynamics during compostingare important for an overall understanding of compo-sting, as C compounds provide the energy for the de-gradation process. Volatile fatty acids are a type of Ccompound that may be formed during the compostingprocess (Kirchmann and Wid�en, 1994), and since theyare phytotoxic can cause problems in cultivation(DeVleeschauwer et al., 1981). Changes in C compo-nents in relation to organic C have been used to de®nethe degree of decomposition or bio-maturity of com-posted city refuses (Inoko et al., 1979; Harada et al.,1981), grass compost (Mahmood et al., 1987) andfarmyard manure (Levi-Minzi et al., 1986).

The aim of the present study was to investigate themass loss and C turnover during composting, matura-tion and storage of household waste mixtures with dif-ferent litter amendments. Results of the N turnover ofthe same experiment were described by Eklind andKirchmann (2000).

Bioresource Technology 74 (2000) 115±124

* Corresponding author.

0960-8524/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 0 - 8 5 2 4 ( 0 0 ) 0 0 0 0 4 - 3

2. Methods

2.1. Materials used for composting

Based on a survey of the di�erent types of materialssorted into the compostable fraction in households(Nilsson et al., 1993), a standardized, arti®cial, organichousehold waste was prepared consisting of potatoesand carrots, chopped to about 1 cm3, meat meal andbone meal (commercial products), mixed in proportionsof 65%, 15%, 13% and 7% of dry matter, respectively.The potatoes and carrots were ecologically grown to besure to exclude residues of pesticides, and used withoutshoots. The carrots were unwashed and therefore had ahigh ash content (Table 1).

The litter additives used were straw from winter wheatof up to 10 cm length; autumn leaves, mostly consisting ofAcer platanoides and Aesculus hippocastanum, dried andcut with a lawnmover into 2±3 cm2 pieces; shavings fromBetula spp. (hardwood); shavings from Pinus silvestrisand Picea abies (softwood), up to 2±3 cm2 in size; driedand milled pulp of waste paper; and sphagnum peat; boththe latter in lumps of up to 2� 3� 6 cm (Table 1). Litterwas added so that the same ratio of litter C to household

waste C in all litter-amended mixtures was achieved.Moreover, the intention was that the mixtures shouldhave initial C to N ratios within normally recommendedlimits, resulting in initial C to N ratios of 22±34 and alitter C proportion of about 50% of the organic C. Arti-®cial, organic household waste without additives wasused for control. The initial water content was 58±65% ofthe fresh weight in the litter-amended mixtures and 74%in the control. Dry bulk densities of the mixtures were0.09±0.19 kg dmÿ3, with the lowest values in straw andthe highest in leaf mixtures, and 0.43 kg dmÿ3 in thecontrols without litter additives. The dry bulk densitieswere determined by weighing dried material after uni-form compaction in a 1 l beaker.

2.2. Experimental performance

Composting was carried out in 125 l, insulated bins,rotatble around their horizontal axes, octagonal incross-section and with ventilation holes in the side walls.The bins were ®lled to the same volume (about 90% oftotal volume) and as the litter amendments had di�erentbulk densities, di�erent total initial weights were used(Table 2). They were then placed in a climatic chamber

Table 1

Properties of materials used in the experiment

Material Dry matter (% of fw) Ash (% of dm) Carbon (% of dm) Nitrogen (% of dm) C to N ratio

Raw materials

Potatoes 19.6 5.1 41.3 1.37 30

Carrots 10.8 43.2 24.8 0.82 30

Meat meal 93.4 42.9 28.0 7.64 4

Bone meal 95.2 46.0 26.3 7.37 4

Arti®cial household

wastea

27.8 19.2 36.2 2.84 13

Litter amendments

Straw 95.6 8.5 43.2 0.47 92

Leaves 91.7 30.8 36.5 1.13 32

Hardwood 94.6 0.4 47.2 0.07 638

Softwood 93.5 0.7 47.3 0.06 736

Paper 95.8 6.9 43.8 0.10 438

Peat 54.1 2.0 48.2 0.92 52

a Consisted of the wastes mentioned (potatoes, carrots, meat meal and bone meal; 65%, 15%, 13% and 7% of dry matter, respectively).

Table 2

Proportions of arti®cial household waste and litter amendments used to achieve the same litter C addition, expressed in total amounts of dry matter

per bin. Control was arti®cial organic household waste without litter addition

Waste mixture Arti®cial household

waste (kg DM)

Amendment (kg DM)

Straw Leaves Softwood Hardwood Paper Peat

Household waste + Straw 2.86 2.60 ± ± ± ± ±

Household waste + Leaf 5.30 ± 6.16 ± ± ± ±

Household waste + Softwood 6.22 ± ± 4.98 ± ± ±

Household waste + Hardwood 6.18 ± ± ± 4.82 ± ±

Household waste + Paper 4.18 ± ± ± ± 4.06 ±

Household waste + Peat 4.00 ± ± ± ± ± 3.66

Control 18.58 ± ± ± ± ± ±

116 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124

with an ambient temperature of 17±23°C. Daily openingof lids and manual rotation of the bins (4.5 turns) wasdone to favour aeration and homogenous conditions.Temperature was measured daily in the centre of themixture masses until it reached the ambient tempera-ture.

Samples were taken from the decomposing materialsnine times during the experimental period (0, 7, 14, 21,35, 70, 106, 177 and 590 days). Samples were pooledfrom 10 subsamples of about 250 ml and kept at )24°Cfor later analysis. Sampling and all following analyseswere done in duplicate. The excess of pooled materialnot needed for analysis was immediately returned to thebins. On each sampling occasion, the mixture masseswere weighed, and the water content was determinedand adjusted by water addition to about 50% of thewater holding capacity. Initial water holding capacitiesof the straw, leaf and control mixtures were 3.17 g H2Ogÿ1 dry matter; of hardwood mixture 3.78 g H2O gÿ1; ofsoftwood mixture 3.31 g H2O gÿ1; of paper mixture 4.56g H2O gÿ1 and of peat mixture 7.13 g H2O gÿ1 drymatter. After reaching ambient temperature, the com-post masses were kept in the bins to maintain controlledconditions also during maturation. On day 177, thecompost masses were removed from the bins, placedinto open plastic bags and stored indoors at about 17°Cuntil day 590. The relatively long experimental periodwas choosen to check chemical changes in compostmaterial also during maturation and long-term storage.The materials were turned and water was added severaltimes during storage to compensate for evaporation.

2.3. Chemical analyses

Ashing was done at 550°C. Organic C in mixtureswas measured by dry combustion and IR determinationof CO2 evolved (LECO analyser, USA). Cellulose andlignin fractions were determined using the acid deter-gent ®bre (ADF) and permanganate method as de-scribed by Goering and Van Soest (1970). Neutraldetergent ®bre (NDF) was also determined by Goeringand Van Soest's method, but modi®ed by using trieth-ylene glycol instead of ethylene glycol monoethylether.In addition, amylase was added to the ND solutionbefore boiling (Jeraci and Van Soest, 1990). The hemi-cellulose fraction was calculated by subtraction of NDFfrom ADF values. All these analyses were carried outon dried (60°C) samples.

Total N in mixtures was measured by the Kjeldahlmethod (Kjeltec, Tecator, Sweden). In samples con-taining signi®cant amounts of nitrate and/or nitrite, to-tal N was determined by a modi®ed Kjeldahl methodwhere the sample is pretreated with salicylic acid andthiosulphate (Bremner and Mulvaney, 1982). Presenceof volatile fatty acids (VFA) was used as an indicator ofanaerobic conditions in the waste mixtures. VFA con-

centrations were determined using high pressure liquidchromatography (HPLC): 10 g frozen material wasthawed and extracted with 40 ml deionized water inbottles. Samples were shaken for 30 min at 6°C andcentrifuged for 20 min at 3000 revolutions minÿ1. Oneml of the solution was put in Eppendorf tubes, 50 ll 1MH2SO4 was added and the samples were centrifuged for20 min at 12 000 revolutions minÿ1. Two hundred ll ofthe solution was used for the HPLC analysis. All theseanalyses were performed on wet, thawed material.

2.4. Calculations and statistical analysis

Data on C constituents were expressed on a C-basisassuming the following C content: lignin 63% C, cellu-lose 45.5% C and hemicellulose 58% C (Swift et al.,1979). Losses of dry matter and organic C, and the de-composition dynamics of lignin, cellulose and hemicel-lulose over time were ®tted to the following exponentialdegradation function by the least-squares technique us-ing the SAS procedure NLIN (SAS Institute, 1985)

M � Mo � �100ÿM0� eÿkt;

where M is the remaining mass (%), M0 the potentialresidual mass (%), k the decomposition rate (dayÿ1) andt is time (days).

When a lag phase was present the function givenabove was ®tted from the last measured point beforedecomposition started, although the period could onlybe approximated and could lie between two measuredpoints.

The potential residual amount, M0, represents a re-calcitrant part of the organic matter or C constituentstudied, that does not degrade during the particular timeperiod. As a result, in composting and incubationstudies the degradation function will not reach zero as itdoes in long-term soil studies.

The values of M0 and k, respectively, were comparedby t-test, using the standard errors of the parameterestimates received from the NLIN procedure. The timeneeded for 50% degradation of the organic C and Cconstituents, the so-called half-life, was calculated usingthe decomposition rate constants derived from thementioned function. Correlations between di�erentquality and decomposition variables were tested with thesoftware JMP (SAS Institute, 1989).

3. Results

3.1. C to N ratios, and concentrations of C constituentsand volatile fatty acids

The initial C to N ratio was 13 in the control and 22±34 in the mixtures with litter additives. Only a smallchange in the C to N ratio, from 28 to about 20, was

Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124 117

found in the peat mixture during composting, matura-tion and storage whereas a considerable change wasapparent in the straw mixture, from 28 to 7 (Table 3).

The control had low initial concentrations of ligninand cellulose (0.9% and 2.9% of ash-free dry matter,respectively), whereas hemicellulose concentration was14.5% (Table 4). In the mixtures with litter amend-ments initial lignin concentrations accounted from7.3% to 20.8% of ash-free dry matter, cellulose from11.3% to 33.8%, and hemicellulose from 12.9% to26.4%. Concentrations of lignin and hemicellulose in-creased during the experimental period but not theconcentration of cellulose. After 590 days of decom-position, concentrations of lignin varied between 8.3%and 36.0%, of cellulose between 3.8% and 35.5% and ofhemicellulose between 18.0% and 51.2% of ash-free drymatter.

Concentrations of volatile fatty acids (VFA) were lowduring the whole experimental period in mixtures withlitter amendments. Only traces of acetate and lactatewere recorded during the ®rst weeks. In the leaf mixturefor example, the highest total concentrations of VFAwas on a dry matter basis 1.5 mg VFA gÿ1 compost onday 14. In the same mixture, the C present in VFAconstituted only 0.2% of the organic C present. In thecontrol, however, in addition to acetate and lactate,butyrate, propionate and also formate were found. Thehighest total concentration of VFA in the control was15 mg VFA gÿ1 on a dry matter basis, and 2.2% on a Cbasis. However, no VFAs were recorded on day 70 orlater in the control.

3.2. Dry mass loss and decomposition rates

Dry mass losses during composting, maturation andstorage are shown in Fig. 1. The dry mass losses after590 days were largest in the control (about 80%), fol-lowed by the hardwood-, paper-, straw- and softwood-amended mixtures (about 70%), the leaf-amended mix-ture (about 50%) and the peat-amended mixture (about40%) (Table 3). The correlations between losses of drymass and organic C were very high (R2� 0.98±1.0).

Potential residual amounts (M0) of dry matter variedbetween 22% and 63%, of organic C between 11% and61%, and were highest in the peat-amended mixture andlowest in the control (Table 5). The rate constants (k) fordry mass losses did not di�er between the mixtures withadded litter. However, the decomposition rate constantfor organic C was signi®cantly higher in the leaf-amended mixture than in the straw- and hardwood-amended mixtures (Table 5). The control had the lowestdecomposition rate constant, which was signi®cantlydi�erent from those of the litter-amended mixtures. Allmixtures showed the largest mass losses during the ®rstweek, with the greatest loss in the leaf mixture.

Correlations between descriptors of initial organicmatter quality of waste mixtures and measures of de-composition are given in Table 6. Signi®cances are usedto indicate the strength of the relationship only; theyshould not be interpreted in a strict statistical sense.Initial lignin content of the mixtures (in percent of ash-free dry matter) were very highly correlated with thepotential residual amount of total C (R2 � 0:91), as re-

Table 3

Changes in mass and composition of the household waste mixtures after 590 days

Type of

mixture

Mass (kg dm) Ash (% of dm) Organic C

(% of dm)

Organic C

(% of ashfree dm)

Tot N (% of dm) C to N ratio

Initial Finala Initial Final Initial Final Initial Final Initial Final Initial Final

Straw 5.46 1.76 13.2 42.6 40.0 26.6 46.1 46.4 1.4 4.1 28 6

Leaf 11.46 5.44 21.0 50.7 36.6 25.0 46.3 50.8 1.7 2.8 22 9

Softwood 11.20 3.84 9.2 27.0 41.7 35.6 46.0 48.8 1.3 2.0 32 18

Hardwood 11.00 3.12 9.6 33.7 41.2 32.7 45.6 49.4 1.2 2.4 34 14

Paper 8.24 2.52 12.5 41.2 39.2 28.2 44.9 47.8 1.3 2.2 30 12

Peat 7.66 4.68 10.6 16.8 42.5 39.2 47.6 47.1 1.5 2.0 28 20

Control 18.58 4.52 19.2 64.0 36.2 17.3 44.8 48.0 2.8 3.8 13 5

a Corrected for mass removal caused by sampling.

Table 4

Initial and ®nal concentrations (590 days) of carbon constituents in the household waste mixtures

Type of compost Lignin (% of ashfree dm) Cellulose (% of ashfree dm) Hemicellulose (% of ashfree dm)

Initial Final Initial Final Initial Final

Straw 7.3 18.9 25.7 8.8 26.4 27.4

Leaf 16.5 27.0 11.3 15.0 14.7 18.6

Softwood 17.3 33.8 32.6 26.9 15.9 24.0

Hardwood 13.7 33.4 26.9 20.8 16.9 26.9

Paper 12.2 22.9 33.8 31.2 12.9 26.9

Peat 20.8 27.6 14.9 23.1 19.6 22.4

Control 0.9 8.3 2.9 5.4 14.5 51.2

118 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124

ceived from the degradation function. When using dataof organic matter loss from one single sampling occasiononly, as Chandler et al. (1980) did for anaerobic de-gradation, the correlation with initial lignin content waslower (day 106, R2 � 0:81; day 590, R2 � 0:88). Theinitial lignin content was not correlated to organicmatter loss at the beginning of the composting period.Use of descriptors of organic matter quality that includeN content, as described by Melillo et al. (1982) andHerman et al. (1977), did not give a better correlationwith the potential residual amount of total C than whenthe lignin content only was used. However, substitutionof soluble carbohydrates in Herman's expression with

cellulose C gave a better correlation than initial amountsof decomposable C (de®ned in this paper as [totalorganic C)(lignin C + cellulose C + hemicellulose C)] ´100/total organic C).

The decomposition rate constants for organic C andthe ash-free dry mass loss on day 21 were chosen todescribe the most intense phases of composting, duringwhich the breakdown of the most easily decomposableC fraction was supposed to dominate. The initialamount of decomposable C was correlated to neither ofthose variables. The `initial amount of decomposable C'was thus found to be a bad predictor for decompositionduring the most intense phases of composting.

Fig. 1. Dry mass loss from the household waste mixtures during composting, maturation and storage.

Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124 119

Table 5

Rates (k) for mass loss and organic carbon decomposition and potential residual amounts (M0) during 590 days derived with a ®rst-order degra-

dation function. Standard error in parenthesis. Values labelled with the same letter do not di�er signi®cantly at the 5% probability level

Type of compost Dry mass loss Organic carbon decomposition

k (dayÿ1) M0 (%) R2 k (dayÿ1) M0 (%) R2

Straw 0.044 a 37.85 d 0.968 0.040 b 30.03 d 0.948

(0.0057) (2.35) (0.0069) (3.59)

Leaf 0.223 a 56.60 ab 0.861 0.126 a 40.86 bc 0.885

(0.098) (2.48) (0.035) (3.27)

Softwood 0.067 a 50.56 bc 0.823 0.065 ab 46.63 b 0.829

(0.021) (4.03) (0.020) (4.30)

Hardwood 0.045 a 38.28 d 0.915 0.039 bc 33.11 cd 0.915

(0.0096) (3.67) (0.0083) (4.21)

Paper 0.080 a 41.20 cd 0.943 0.077 ab 35.58 bcd 0.922

(0.014) (2.51) (0.016) (3.28)

Peat 0.067 a 62.56 a 0.931 0.061 ab 61.46 a 0.950

(0.013) (1.82) (0.0096) (1.61)

Control 0.024 b 24.16 e 0.99 0.0225 c 12.02 e 0.991

(0.0029) (2.44) (0.0026) (2.81)

Table 6

Correlations between descriptors of initial organic matter quality of the waste mixtures and measures of decomposition

Descriptor (x) Measure of decomposition (y) R2

Lignin (% of ash-free dm) Potential residual amount of organic C 0.91���

Rate constant (organic C) ns

Lignin C (% of organic C) Potential residual amount of total C 0.89��

Lignin (% of ash-free dm)a Ash-free dry mass loss

day 590 0.88��

day 200 0.87��

day 100 0.81��

day 75 0.72�

day 50 ns

Lignin/N (both in % of ash-free dm)b Potential residual amount of organic C 0.80��

Lignin C/N (% of organic C/% of ash-free dm) Potential residual amount of organic C 0.77��

Rate constant (organic C) ns

Lignin/N (both in % of ash-free dm) Ash-free dry mass loss day 70 0.83��

Ash-free dry mass loss day 21 ns

[Organic C)(Lignin C+Cellulose C+Hemicell. C)]*100/Organic Cc Potential residual amount of organic C ns

(% of organic C)

(Lignin * C:N)(Initial decomposable C)ÿ1=2 (dm)d Decomposable amount of dry mass ns

(Lignin * C:N)(Initial decomposable C)ÿ1=2d

(ash-free dm) Decomposable amount of organic C 0.72�

(Lignin * C:N)(Initial cellulose C)ÿ1=2 (dm)e '' 0.90��

(Lignin * C:N)(Initial cellulose C)ÿ1=2 (ash-free dm)e '' 0.89��

*** p < 0.001, ns�not signi®cant.** p < 0.01.* p < 0.05.a Adapted from Chandler et al. (1980).b Adapted from Melillo et al. (1982).c Referred to as initial decomposable C in this paper.d Adjusted for ash content using an equation from Herman et al. (1977) but substituting soluble carbohydrates with expression c.e Adjusted for ash content using an equation from Herman et al. (1977) but substituting soluble carbohydrates with cellulose C.

120 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124

3.3. Decomposition of C constituents and half-life time

The potential residual amounts of C constituents(hemicellulose, cellulose and lignin) varied more thanthe decomposition rate constants between di�erentmixtures (Table 7). Decomposition of the C constituentsin the peat-amended mixture ®tted a linear function,whereas decomposition in the other mixtures was ex-ponential. Consequently, k values of C constituents inthe peat mixture could not be compared with the othermixtures (Table 7). The potential residual amount ofhemicellulose was signi®cantly higher in the paper andsoftwood mixtures than in the hardwood and strawmixtures and the control. The decomposition rate con-stant for hemicellulose was not signi®cantly di�erentbetween the treatments.

The control had a lag phase regarding cellulose de-composition, lasting 21±35 days; and the hardwoodmixture also showed a short lag phase in cellulose de-composition. Potential residual amounts of cellulosewere signi®cantly higher in the leaf and paper mixturesthan in the straw mixture. The decomposition rateconstants for cellulose were signi®cantly higher in thepaper and straw mixtures than in the leaf and softwoodmixtures. The decomposition rate constant of hemicel-lulose was higher than, or similar to that of cellulose.However, measured over the whole experimental periodcellulose was decomposed to a greater extent with alower amount remaining.

Lignin decomposition had a lag phase in most, butnot all, of the mixtures. The potential residual amountof lignin di�ered between 32% in the paper mixture and67% in the softwood mixture. The decomposition rateconstant for lignin was 0.004 dayÿ1 in the paper mixtureand 0.052 dayÿ1 in the softwood mixture. However, thedi�erences were not signi®cant.

The time taken to reach 50% decomposition of totalC was shortest for the leaf mixture (15 days), whereasthe peat mixture did not reach 50% decompositionwithin the 590 days (Table 8). The leaf mixture had theshortest half-life time for hemicellulose (26 days),whereas the half-life time for cellulose was shortest inthe paper mixture (18 days). Moreover, the paper mix-ture was the only one where 50% lignin decompositionwas reached within the 590-day experimental period(after 336 days).

4. Discussion

Dry mass losses of waste mixtures with di�erent litteradditives showed considerable di�erences and werelowest in the peat mixture. The resistant nature of peatwas also shown in soil organic matter studies (Kirch-mann et al., 1994). The higher biodegradability ofhardwood over softwood in the present compost study

was in accordance with results from soil decompositionstudies with wood and bark from a number of treespecies (Allison, 1973).

The leaf mixture had a high decompostion rate con-stant for dry matter and organic C and a short half-lifefor organic C. This was not explained by its content oflignin, cellulose or hemicellulose; but may also have beencaused by its content of more easily available constitu-ents not analysed in this study. A large surface area ofthe leaves may also explain the rapid decomposition.

The potential residual amounts for the hemicelluloseand lignin fractions varied between the mixtures withdi�erent litter amendments, that is, the amounts avail-able for degradation di�ered. No signi®cant di�erencesin decomposition rate constants of the hemicellulose andlignin fractions could be found in this study. However,the cellulose fraction had di�erent decomposition rateconstants in the di�erent composts. The low decompo-sition rate constant and long half-life for cellulose in theleaf, hardwood and softwood mixtures compared topaper and straw mixtures, could have been caused byphysical protection or chemical coupling of cellulose tolignin structures in the former (Berg et al., 1984). It isknown from forest soils that cellulose and hemicellulosecan be conserved in such a way for a very long time(Berg et al., 1982). The process of paper productionresults in a destruction of the chemical bounds betweenC constituents in wood, which can explain the highercellulose biodegradability of paper.

A lag phase in lignin decomposition was found in thepaper-, leaf-, hardwood- and straw-amended mixtures.Vinceslas-Akpa and Loquet (1994) also found a lag phaseof about 30 days for lignin when composting maplewastes. Lignin breakdown during composting was ini-tially high in a study by Michel et al. (1993) using plantmaterials, but low and only measurable during the initialperiod in a study using cattle manure carried out byGodden and Penninckx (1987). Horwath and Elliott(1996) showed that combined mesophilic and thermo-philic conditions during the composting process favourlignin breakdown compared to mesophilic conditionsonly.

The decomposition rate constant of hemicellulosewas in some mixtures higher than, and in some mixturessimilar to, that of cellulose. Similar rates of cellulose andhemicellulose decomposition were found during abench-scale composting study using leaves mixed withgrass (Michel et al., 1993) and during decomposition ofcereal straw in soil (Harper and Lynch, 1981). The cel-lulose was decomposed to a greater extent than hemi-cellulose in the present study, which Hammouda andAdams (1987) also found when composting grass, hayand straw with added inorganic N; but not when com-posting unamended straw.

The initial lignin content showed a strong correlationwith the potential residual amount of organic C, over

Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124 121

Tab

le7

Dec

om

po

siti

on

of

lign

in,

cell

ulo

sean

dh

emic

ellu

lose

Cd

uri

ng

590

days

des

crib

edb

ya

®rs

to

rder

deg

rad

ati

on

fun

ctio

n.

Sta

nd

ard

erro

rin

pare

nth

esis

.V

alu

esla

bel

led

wit

hth

esa

me

lett

erd

on

ot

di�

ersi

gn

i®ca

ntl

ya

tth

e5

%p

rob

ab

ilit

yle

vel

Typ

eo

f

com

po

st

Hem

icel

lulo

seC

Cel

lulo

seC

Lig

nin

C

k(d

ayÿ1

)M

o(%

)R

2L

ag

ph

ase

(days)

k(d

ayÿ1

)M

o(%

)R

2L

ag

ph

ase

(days)

k(d

ayÿ1

)M

o(%

)R

2L

ag

ph

ase

(days)

Str

aw

0.0

43

a2

9.2

4c

0.9

49

00.0

35

a16.6

4b

0.8

97

00.0

12

a54.1

7ab

0.7

93

0±7

(0.0

073

)(3

.46

)(0

.0076)

(5.4

7)

(0.0

051)

(7.5

9)

Lea

f0

.09

7a

45

.57

ab

0.8

70

00.0

13

bc

42.1

3a

0.8

70

00.0

18

a52.2

5ab

0.8

57

7±14

(0.0

27)

(3.3

4)

(0.0

051)

(7.7

6)

(0.0

074)

(6.2

7)

So

ftw

oo

d0

.05

1a

52

.20

a0

.90

30

0.0

05

c19.0

4ab

0.9

02

00.0

52

a66.8

4a

0.6

65

0

(0.0

12)

(3.1

0)

(0.0

016)

(12.2

2)

(0.0

24)

(4.2

6)

Ha

rdw

oo

d0

.04

9a

42

.09

b0

.94

80

0.0

08

c15.3

5b

0.9

83

7±14

0.0

11

a50.0

2ab

0.8

45

7±14

(0.0

080

)(2

.65

)(0

.0010)

(4.6

8)

(0.0

040)

(7.3

6)

Pa

per

0.0

55

a5

4.3

0a

0.8

74

00.0

81

a35.2

7a

0.8

75

00.0

04

a31.5

6b

0.9

71

21±35

(0.0

14)

(3.2

8)

(0.0

22)

(4.2

4)

(0.0

012)

(9.2

1)

Pea

ta0

.06

8)

0.6

48

00.0

26

)0.0

61

00.0

51

)0.1

41

0

(0.0

12)

(0.0

093)

(0.0

15)

Co

ntr

ol

±b

00.0

45

a17.6

2b

0.9

89

21±35

±c

(0,0

081)

(2,7

3)

aP

eat

com

po

stw

as

®tt

edto

ali

nea

rd

eco

mp

osi

tio

nfu

nct

ion

,k

(%d

ayÿ1

).b

Co

uld

no

tb

tted

toa

ny

fun

ctio

n.

cV

ery

sma

llin

itia

lco

nce

ntr

ati

on

(see

Ta

ble

4).

122 Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124

the 590 days decomposition period. Use of qualityvariables with the initial N content included did notimprove the correlation, as it did in decompositionstudies in soil by Melillo et al. (1982) and Herman et al.(1977).

For anaerobic degradation, Chandler et al. (1980)concluded that the lignin content was the predominantfactor in determining the extent of substrate decompo-sition, based on experiments at 35°C for 90±120 days.Haug (1993) argued that the equation by Chandler et al.(B � 0:830ÿ �0:028�X , where B is the biodegradablefraction and X is the lignin content, expressed as apercentage of the ash-free dry matter) could be appli-cable also to aerobic composting conditions. This seemsto be the case, but the time when degradation is deter-mined plays an important role for the intercept of theequation, which gives the size of the non-degradablefraction. The same value (0.83) was obtained in ourstudy for similar degradation times (106 days), buthigher values were obtained with longer times (177 and590 days) as the organic matter was degraded further.

Obviously, the initial lignin content is a very impor-tant factor in determining the biodegradability of dif-ferent materials. However, which quality variable givesthe best prediction of the biodegradability depends onthe time of decomposition, as shown for C mineraliza-tion in soil decomposition studies (Mtambanengwe etal., 1998). The initial decomposable C gave the bestprediction of C mineralization from plant litter duringone month, whereas an expression modi®ed from Her-man et al. (1977), including lignin C, C to N ratio, andinitial decomposable C gave the best prediction of Cmineralization during periods longer than one month(Mtambanengwe et al., 1998).

The NDF and ADF methods, originally developedfor determining ®bre fractions of forage, have also beenused earlier in another compost study (Atkinsson et al.,1996). The methods can be very useful for lignin deter-mination of waste mixtures to estimate the biodegra-dability, as discussed above. However, applying themethods to decomposing instead of undecomposedmaterial may result in serious miscalculations, since it is

unclear in which fraction the humic substances, formedduring composting, will end up. Organic matter formedduring composting can be extracted together with thesolubles and thus be identi®ed as hemicellulose, or oxi-dized by the permanganate and thus be identi®ed aslignin. The risk for such false identi®cation may increasewith decomposition time.

Aerobic conditions predominated in the mixtureswith litter amendments, as indicated by the small con-centrations of VFAs, occuring only at the beginning ofthe composting period. In contrast, the conditions in thecontrol were partly anaerobic for at least one month,with a large VFA concentration, low pH and a fairly lowtemperature (Eklind and Kirchmann, 2000). This wasmost likely due to a poor structure of the material.

A ratio of less than 0.35 of hemicellulose C pluscellulose C to organic C has been suggested as charac-terizing biomaturity of city refuse composts (Harada etal., 1981; Inoko et al., 1979). This maturity index wasnot found to be relevant to composts in this study, sincethe ratio of C in hemicellulose plus cellulose to totalorganic C generally increased during composting andnever fell below 0.35, despite the long period of de-composition. Only the leaf compost had ratios below0.35, but this was already so from the start of thecomposting process.

5. Conclusions

Litter amendments greatly in¯uence the decomposi-tion process during composting, maturation and stor-age. All tested litter types enabled an aerobiccomposting process and minimized VFA formation,whereas no litter resulted in partly anaerobic conditions.The half-life of organic C was shortest in leaf and papermixtures, and longest in the peat mixture. The initiallignin content was the most important factor describingthe biodegradability of the waste mixtures measuredover the 590 days experimental period. However, asubstantial portion of lignin was degraded during theexperimental period, despite its resistant nature.

Table 8

Time for 50% degradation for total carbon and carbon constituents of household waste mixtures with di�erent litter amendmentsa

t1=2 (days)

Type of compost Total Carbon Hemicellulose C Cellulose C Lignin C

Straw 31 29 26 nr

Leaf 15 26 153 nr

Softwood 42 nr 205 nr

Hardwood 35 41 112 nr

Paper 19 nr 18 336

Peat nr nr nr nr

Control 37 ±b 21 ±c

a nr�not reaching 50% degradation within the 590-day composting time.b Could not be ®tted to any function.c Very small initial concentration.

Y. Eklind, H. Kirchmann / Bioresource Technology 74 (2000) 115±124 123

Acknowledgements

The Swedish Council for Forestry and AgriculturalResearch provided ®nancial support for this investiga-tion. Ulf Olsson is thanked for statistical advice and JanPersson and Ernst Witter for valuable comments on themanuscript.

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