DIVISION S-2—SOIL CHEMISTRY

Characterization of Dissolved Organic Matter Extracted from CompostedMunicipal Solid Waste

Benny Chefetz, Patrick G. Hatcher, Yitzhak Hadar, and Yona Chen*

ABSTRACTThe properties and transformation of dissolved organic matter

(DOM) extracted from municipal solid waste compost at five stagesof composting were studied using a chemical fractionation scheme.We fractionated the DOM into hydrophobic or hydrophilic neutrals,acids, and bases, and obtained solid-state cross polarization magicangle spinning (CPMAS) "C-nuclear magnetic resonance (NMR)spectra of the unfractionated DOM and of the different fractions.The hydrophobic acid (HoA) fraction was the dominant hydrophobicfraction, exhibiting a moderate increase during composting. The hy-drophobic neutral (HoN) fraction increased sharply while the hy-drophobic bases (HoB) fraction decreased during composting. Thehydrophilic neutrals (HiN) represented the major fraction of the hy-drophiles until 120 d of composting, decreasing thereafter by 38%.The "C-NMR spectra of the unfractionated DOM revealed an increas-ing level of aromatic structures in the residual DOM with compostingtime. The "C-NMR spectra of the HoA fraction suggested a pol-yphenol-humic structure, whereas the HoN spectra exhibited strongaliphatic features. The spectra of the HiN fraction confirmed its poly-saccharide nature and the hydrophilic bases (HiB) fraction containedmainly proteins and carbohydrate—amino complexes. A comparisonbetween the "C-NMR spectra of HoA and fulvic acid (FA) indicatedthe former to be the most soluble FA fraction. The steady DOMconcentration and the relative decrease of HiN as the HoA and HoNfractions increased indicates that DOM at the final stage of compostingcontained less bioavailable organic matter and more macromoleculesrelated to humic substances. The constant level of DOM observedduring the curing and maturation stages represents a steady-statesituation during which the chemical composition is continuallychanging.

COMPOSTS AND MANURES of various origins are fre-quently used as soil amendments. After a short

period of decomposition, they become part of the soilorganic matter (OM). The amount and the propertiesof OM change dramatically during composting. Approx-imately 50% of the OM becomes fully mineralized,mostly due to the degradation of easily degradable com-pounds such as protein, cellulose, and hemicellulose,which are utilized by microorganisms as C and Nsources. The residual OM contains newly formed macro-molecules along with nondegradable OM, jointly form-ing the humic substance (HS) — the main stable fractionof mature compost. Although the transformation of OMduring composting has been widely studied (Aoyama,

B. Chefetz and Y. Chen, Dep. of Soil and Water Science, and Y.Hadar, Dep. of Plant Pathology and Microbiology, Faculty of Agricul-tural, Food and Environmental Quality Sciences, Hebrew Univ. ofJerusalem, P.O.B. 12, Rehovot 76100, Israel; and P.G. Hatcher, FuelScience Program, 405 Academic Activities Building, PennsylvaniaState Univ., University Park, PA 16802. Received 27 Feb. 1997. Cor-responding author (yonachen@agri.huji.ac.il).

Published in Soil Sci. Soc. Am. J. 62:326-332 (1998).

1991; Chefetz et al., 1996; Chen and Inbar, 1993; Chenet al., 1989; Ciavatta et al., 1993; Frimmel et al., 1994;Jimenez and Garcia, 1992), most of the reports havefocused on NaOH-extracted HS.

Composting of agricultural and municipal solid wastes(MSW) is defined as a biological treatment in whichaerobic microorganisms utilize OM as a substrate (Zuc-coni and de Bertoldi, 1987). Since microorganisms areonly active at the solid-liquid interface of the compost,a study of this interface is essential for a better under-standing of C and N cycles during composting (Lynch,1993). Moreover, characterization of the compositionof the DOM, the active OM fraction, may be a betterindicator of the overall transformation of the OM. Inaddition, DOM in compost is well known to enhanceplant growth due to its maintenance of Fe, Mn, Cu, andZn in solution at sufficient levels (Chen et al., 1994). Insoils, DOM is related to C and N availability (Gresselet al., 1995; Stevenson, 1994; Zech et al., 1994), metalcomplexation, and enhanced plant growth (Chen andAviad, 1990).

A determination of DOM composition is difficult be-cause it contains a mixture of low molecular weightcompounds (free amino acids and sugars) and chemi-cally heterogeneous macromolecules (enzymes, amino-sugar complexes, polyphenols, and HS). Thus, prelimi-nary fractionation is essential to better understandingthe variety of different physicochemical and biologicalcompounds (Leenheer, 1981). The objectives of thisstudy were to investigate the transformation of DOMfractions during MSW composting, using solid-state 13C-NMR and determine whether the plateau concentrationlevel of DOM recorded during compost maturation re-sults from its steady composition or from OM transfor-mation and a steady-state situation.

MATERIALS AND METHODSDissolved Organic Matter Extraction

Aqueous compost extracts were prepared by placing 1 kgcompost (wet weight) in 10 L of distilled water, and shaking(125 rpm) for 2 h at room temperature. The suspension wasthen centrifuged (10 000 X g for 30 min) and the supernatantfiltered through a 0.45-jjim membrane filter (Supor-450, Gel-man Sciences, Ann Arbor, MI). The concentration of dissolvedorganic carbon (DOC) was measured immediately after ex-

Abbreviations: CPMAS, cross polarization magic angle spinning;DOC, dissolved organic carbon; DOM, dissolved organic matter; FA,fulvic acid; HA, humic acid; HiA, hydrophilic acid; HiB, hydrophilicbase; HiN, hydrophilic neutral; HoA, hydrophobic acid; HoB, hy-drophobic base; HoN, hydrophobic neutral; HS, humic substance;MSW, municipal solid waste; OM, organic matter.

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CHEFETZ ET AL.: DISSOLVED ORGANIC MATTER EXTRACTED FROM COMPOST 327

traction or fractionation using a total C monitor (TCM 480,Carlo Erba Instruments, Milan, Italy).

Preparative FractionationPreparative DOM fractionation was performed according

to Leenheer (1981) with some modifications. Step 1: 10 L ofthe compost-water extract were pumped with a peristalticpump at a flow rate of 10 to 15 pore volumes per hour througha glass column containing 300 mL of Amberlite XAD-8 resin(Sigma Chemical Co., St. Louis, MO). Following the sample,500 mL of distilled water were passed through the resin. Toobtain the organic fraction that had adsorbed onto the resin(HoB), it was flushed with a 0.25 pore volume of 0.1 M HC1,followed by 1.5 pore volumes of 0.01 M HC1.

Step 2: The sample effluent was combined with the distilledwater effluent. This solution was acidified to pH 2.0 with 6 MHC1 then cycled over three connected columns containingAmberlite XAD-8 resin, cation exchanger Amberlyst 15(Merck, Darmstadt, Germany), and anion exchangerAmberlyst A-21 (Merck), respectively. The flow rate was 10pore volumes per hour, the hydrophobic/hydrophilic break(/Co5r) of the XAD-8 resin was 20 to 25, and the amount ofion exchangers was calculated as 150% of the ion concentra-tion estimated by electrical conductivity. Following the sam-ple, 1500 mL of distilled water were passed through the com-bined columns. The effluent contained the HiN fraction.

Step 3: The HoA fraction was displaced from the XAD-8resin with a 0.25 pore volume of 0.1 M NaOH, followed by1.5 pore volumes of distilled water. The HoA was desaltedby loading onto a protonated cation exchanger Amberlyst 15glass column.

Step 4: The hydrophobic neutral (HoN) fraction was de-sorbed from the XAD-8 resin by Soxhlet extraction with fivepore volumes of MeOH. The excess methanol was removedby vacuum-rotary evaporation at 40°C.

Step 5: The Hi A fraction, which had adsorbed onto theanion exchanger Amberlyst A-21, was released by washingthe column with two pore volumes of 1 M NaOH. Protonationof this fraction was performed by the same method used forthe HoA.

Step 6: The HiB fraction was desorbed from the cationexchanger Amberlyst 15 column by elution with one porevolume of 0.1 M NH4OH. Excess NH^ was removed by vac-uum-rotary evaporation at 40°C. All of the fractions werefreeze-dried for further analysis.

The following is a simplified scheme of the fractionationprocedure:Hydrophobic fraction, sorbed to XAD-8 (Ho):

Ho Base (HoB), desorbed with 0.1 M HC1Ho Acid (HoA), desorbed with 0.1 M NaOHHo Neutrals (HoN), desorbed with MeOH

Hydrophilic fraction, not sorbed to XAD-8 (Hi) (obtainedafter sample elution through XAD-8 column):

Hi Base (HiB), sorbed to H+ cation exchange resin, de-sorbed with 0.1 M NH4OH

Hi Acid (HiA), sorbed to anion exchange resin, desorbedwith 1.0 M NaOH

Hi Neutrals (HiN), remaining in deionized-water solutionafter all column separation.

Fulvic Acid ExtractionFor FA studies, 20 g of compost were extracted with 200

mL of 0.1 M NaOH under N2. The supernatant was collectedfollowing centrifugation at 15 000 X g for 30 min and theresidue was resuspended in 0.1 M NaOH. This procedure

was repeated eight times. The combined supernatants wereacidified with 6 M HC1 to pH 1 and left at room temperaturefor 24 h. The supernatant containing the extracted fulvic frac-tion was separated from the humic acid (HA) by centrifugation(2500 X g for 30 min). The FA was further purified by adsorp-tion to Amberlite XAD-8 resin following the same methodused to purify the HoA fraction.

Carbon-13 Nuclear MagneticResonance Spectroscopy

Solid-state "C-NMR spectra with CPMAS were obtainedusing a Chemagnetics M-100 NMR spectrometer, located atPennsylvania State University. The spectrometer was oper-ated at a 'H frequency of 100 MHz and a 13C frequency of 25MHz. Pertinent experimental parameters were the following:a contact time of 1 ms; a recycle delay time of 0.8 s; a sweepwidth of 14 kHz (562.5 ppm) and a line broadening of 30 Hz.Dried samples (48 h at 65°C) were placed into a rotor andspun at a frequency of 3.5 kHz at the magic angle (54.7° tothe magnetic field).

RESULTSDissolved Organic Matter Concentration

and FractionationChanges in DOC concentration during composting

occurred in three phases: (i) a rapid decrease from theinitial concentration of 47.27 to 7.37 g kg"1 (dry weight)during the first 33 d; (ii) Days 33 to 105, when the DOCcontinued to decrease moderately to 3.75 g kg"1 (dryweight); (iii) Day 105 to the end of the experiment,during which the DOC concentration reached a plateau(3.75-3.67 g kg"1 dry weight). The question arising fromthis data is whether the DOC concentration plateaurecorded during maturation actually reflects a steadyDOM composition or a steady-state situation. A spec-troscopic study, starting with quantitative fractionationinto well-defined groups, was suggested as a logical ap-proach to answering this question.

The hydrophobic fractions increased from 30% of thetotal DOM after 47 d of composting to 40% after 161 dof composting (Fig. 1). The HoB fraction decreasedsharply from 13.9 to <1% of the total DOM after 105 dof composting, whereas the opposite trend was exhibitedby the HoN fraction, which increased to 18% on Day161 of composting. The HoA fraction remained thedominant hydrophobic fraction throughout the process.The HiN fraction was the major fraction of the totalDOM until 126 d of composting; thereafter, it decreasedby 38% (from 34.3 to 21.2% of total DOM), while HiAbecame the dominant hydrophilic fraction. The HiAand HiB fractions showed no consistent trends duringcomposting.

Solid-state Nuclear Magnetic ResonanceIn all experiments, the "C-NMR spectra were divided

into five regions (Hatcher et al., 1983). Region I (0-50ppm) was assigned to paraffinic C or carbons that arebound only to other carbons; Region II (50-112 ppm)was assigned to C-O and C-N bonds, as in carbohy-drates, alcohols, esters, and amines; Region III (112-163

328 SOIL SCI. SOC. AM. J., VOL. 62, MARCH-APRIL 1998

25

20 -

15 -

I 10-

40

35 -

30 -

g 25 -

S 20 -

15 -

»~ HiN

- HiA

••• HiB

f40 60 80 100 120 140 160 180 200

Composting time (days)Fig. 1. Fractionation of dissolved organic matter (DOM) extracted

from municipal solid waste compost at six stages of composting(47, 77, 105, 126, 161, and 187 d). Bars represent the standarddeviation of three replicates. Fractions include: hydrophobic neu-tral (HoN), acid (HoA), and base (HoB); hydrophilic neutral(HiN), acid (HiA), and base (HiB).

ppm) was assigned to aromatic and phenolic C; RegionIV (163-190 ppm) to carboxyl, ester, and amide Cgroups; Region V (190-215 ppm) to the carbonyl C.The 0 to 112 ppm region was calculated as aliphatic C,60 to 112 ppm as polysaccharide C, and 112 to 163ppm as aromatic C. Total aromaticity was calculated byexpressing aromatic C (112-163 ppm) as a percentageof the aliphatic C (0-112 ppm) plus aromatic C (Hatcheret al., 1981).

The CPMAS 13C-NMR spectra of the unfractionatedDOM are shown in Fig. 2. The major peaks were at 25ppm (microbial deoxy sugars or methyl C), 30 ppm(methylene C, proteins), 40 ppm (methyl C), 56 ppm(methoxyl C), 75 ppm (carbohydrates or aliphatic alco-hols), 105 ppm (anomeric C of polysaccharides), 115ppm (protonated aromatic C), 130 ppm (C-substitutedaromatic C), 145 ppm (C3 and C5 in etherified phenolicsyringyl units), 145 to 150 ppm (O-substituted aromaticC), and 175 ppm (carboxyl C) (Gutierrez et al., 1996;Kogel-Knabner et al., 1991; Wilson, 1981).

Distinct changes in the unfractionated DOM spectraduring composting were: a decrease of the 75- and 105-ppm peaks; an increase in the aromatic C peak (130ppm); and the development of the 145- and 115-ppm

250 225 200 175 150 125 100 75 50

Chemical shift (ppm)Fig. 2. Cross polarization magic angle spinning 13C nuclear magnetic

resonance spectra of unfractionated dissolved organic matter ex-tracted from municipal solid waste compost samples at five stagesof composting (47, 77, 105, 126, and 187 d).

peaks. The relative changes in the levels of C-containinggroups are presented in Table 1. The following changesshould be noted: (i) polysaccharides (60-112 ppm) de-creased by 16.2%, from 33.5 to 28.1% in 187-d compost,the C-O and C-N group area (70-98 ppm) decreasedby 19.5%; and alkyl-group compounds (0-50 ppm)stayed at a relatively constant level (31.2-33.1% of totalC) during composting; (ii) aromatic components, includ-ing aromatic and phenolic C (112-163 ppm) increasedby 51%, from 10.8% in the 47-d DOM to 16.5% ofthe total C in the mature compost; total aromaticityincreased by 48.2%, from 12.9 to 19.2% in the maturecompost.

Due to the empirical nature of the fractionation pro-cedure used, the fractions showed different structuralfeatures as detected by the solid-state 13C-NMR (Fig.3). The HoN fraction exhibited four well-resolved peaksat 25 to 30, 75, 130, and 178 ppm, and a shoulder at145 ppm. The alkyl C-containing group is the dominantstructure in the spectra (48.6^44.3%). The main changesin the HoN spectra during composting were: the appear-ance of the 75- and 145-ppm peaks, a 33% increase ofthe 60- to 98-ppm region (alcohols and aliphatic alco-hols), and a 12.4% increase of the aromaticity of theC-containing groups in the 187-d spectrum. Due to therelatively small amount of HoB fraction purified fromcompost from 80 d on, NMR spectra could only be

CHEFETZ ET AL.: DISSOLVED ORGANIC MATTER EXTRACTED FROM COMPOST 329

Table 1. Distribution of dissolved organic matter C-containing groups after 47, 77,105,126, and 187 d of composting of municipal solidwaste as determined by cross-polarization magic angle spinning 13C nuclear magnetic resonance.

Chemical shift

ppm0-5050-112112-145145-163163-190

Carbon-containing group

AlkylO-alkylAromaticPhenolicCarboxyl

47 d

31.242.07.33.5

16.4

77 d

28.544.76.82.9

17.6

% of total C

105 d

29.541.59.84.2

14.7

126 d

30.438.19.74.6

15.3

187 d

33.136.311.05.5

13.8

Change withtime

%+6.0

-13.6+50.2+54.1-15.4

detected with the 47-d sample. The HoB spectrum ex-hibited four well-resolved peaks at 30, 56, 75, and 175ppm. The HoB fraction proved to have high aromaticstructures making up 25% of the C-containing groups.In addition, this fraction contained 30% paraffinic Cand 29% carbohydrates and amines. The spectra of theHoA fractions (47 and 187 d) exhibited fewer aliphaticstructures than the HoN fraction and revealed thefollowing changes during composting: a decrease inthe 25- and 56-ppm peaks and a relative increase in thearomatic C-containing group region (10.4%) in the187-d spectrum.

The spectra of the HiN fraction exhibited four well-resolved peaks at 25,75,112, and 178 ppm. The configu-ration of the spectra indicated that this fraction containsmainly polysaccharides. The only difference betweenthe 47- and 187-d spectra was a 23% decrease of the 60-to 112-ppm region (from 55.6 to 42.8%). The presence ofa carbonyl peak (175 ppm) indicates that the polysaccha-rides are probably polyuronides.

The HiB spectra exhibited three dominant peaks at25,65, and 175 ppm and one poorly resolved peak at 130ppm. The solid-state NMR spectra of the HiB fractionindicated strong C-O and C-N C-containing groups anda strong carboxyl C. Moreover, the chemical property ofthis fraction as obtained in the fractionation procedureindicated that it contains mainly proteins or amino-sugar biopolymers. The main changes in the HiB spectraduring composting were: the appearance of peaks in thealkyl region (30 and 40 ppm) in the 187-d spectrumand a small increase in the aromatic region from 13.2to 15.9%.

The HoA fraction is usually considered to be identicalto the FA fraction (Grossl and Inskeep, 1996; Wershawet al., 1995) due to the similar extraction methods usedfor their purification (adsorption onto XAD-8 resin anddesorption by alkaline solution). To study the differ-ences between the water-soluble hydrophobic acid frac-tion (HoA) and the NaOH-soluble hydrophobic acidfraction (FA), their two spectra were compared (Fig.4). The two fractions showed similar NMR spectra, ex-hibiting peaks at 175,130,75, and 30 ppm. These spectrasuggest a mixture of aliphatic and aromatic structureswith a high amount of carboxylic C-containing groups.The main differences between them were the 150, 75,and 56 ppm peaks, which appeared distinctly in the FAspectrum and did not appear in the HoA spectrum.The aromatic-C-containing groups represent 25% of thetotal C in the FA spectrum and only 15% in the HoAspectrum. A similar trend of changes can be observedbetween the two fractions at the methoxy-C- and ano-meric-C-containing groups.

DISCUSSIONIn this research we followed the properties of DOM

during the composting of MSW in an attempt to under-stand DOM transformations during the curing stage ofcomposting. The properties of the DOC fractions ofcomposted MSW are compared, in this discussion, toDOC fractions of well-humified OM extracted fromsoils. The DOM fractionation results suggest the impor-tance of HoA and HoN as the main organic compounds

HoN fraction

HoB fraction

HoA fraction

HiN fraction

HiB fraction

250 200 150 100 50 0 -50

Chemical shift (ppm)

250 200 150 100 50 0 -50

Chemical shift (ppm)Fig. 3. Cross polarization magic angle spinning "C nuclear magnetic

resonance spectra of dissolved organic matter fractions (hydropho-bic neutral [HoN], acid [HoA], and base [HoB]; hydrophilic neu-tral [HiN] and base [HiB]) extracted from municipal solid wastecompost samples at two stages of composting (47 and 187 d). TheHoB fraction approached zero concentration from Day 77 on, andthe 187-d spectrum is therefore not shown. Spectra of the HiAfraction were poorly resolved due to high salt concentrations andare therefore not shown.

330 SOIL SCI. SOC. AM. J., VOL. 62, MARCH-APRIL 1998

250 200 150 100 50

Chemical shift (ppm)Fig. 4. Cross polarization magic angle spinning "C nuclear magnetic

resonance spectra of the fulvic acid (FA) and hydrophobic acid(HoA) fractions purified from 187-d municipal solid waste compost.

remaining in the liquid phase of the mature compost.The HoA fraction represented 22.4% of the total DOM,whereas in DOM from forest soil this fraction makesup 50 to 55% (Quails and Haines, 1991; Zech et al.,1994). McLaughlin et al. (1996) reported dominance ofthe HoA fraction (70-90%) in forest and mineral soilsolutions. Similar HoA fraction dominance has beenreported in lake DOC (Roila et al., 1994). The chemicalnature of the HoA fraction, as suggested by Quails andHaines (1991), consists of tannins, polyphenols, oxidizedpolyphenols, and humic-bound carbohydrates. The sug-gested polyphenol-humic structure of the HoA fractionexplains its higher proportion in soils exhibiting higherhumification indexes than those of compost (Chen etal., 1996). Moreover, the HiN fraction from compostrepresented 21 to 34% of the total DOM, whereas inthe aqueous soil solution this fraction represents only0 to 10% (Cook and Allan, 1992; Quails and Haines,1991; Zech et al., 1994). These differences can be ex-plained by the structure and formation of this fraction.The HiN fraction is composed mainly of poly- and oligo-saccharides resulting from plant-derived and microbialpolysaccharides (Guggenberger and Zech, 1994; Leen-heer, 1981; Stevenson, 1994; Zech et al., 1994). Thissuggested structure was validated by the solid-stateNMR spectra (Fig. 3). Carbohydrates represent themost easily degradable structure class of DOM (Garciaet al., 1991; Quails and Haines, 1991). Thus, the HiNfraction is expected to be dominant in biologically de-composed OM, whereas in ecosystems where slow deg-radation takes place (soils and lakes), HiN is expectedto represent a lower proportion of the DOM. The steadyconcentration of the HiN fraction during 120 d of com-posting could represent a steady-state condition inwhich the carbohydrates originate from both degradableOM and microbial biomass. The decrease in the HiNfraction at the last stage of composting (120 d on) repre-

sents deficient C availability (low levels of polysac-charides).

According to the literature, the HiA fraction consistsof organic compounds with a high degree of biologicaloxidation, or humic-like substances of low molecularweight (David et al., 1989; Quails and Haines, 1992;Wershaw et al., 1995). The amount of this fractionchanges only slightly during the composting process andrepresents a DOM percentage similar to that reportedfor soil DOM (Zech at al., 1994). At the last stageof composting, the HiA fraction became the dominanthydrophilic fraction, suggesting relatively low bioavail-ability to microorganisms active in compost degrada-tion. The HiB fraction contains mainly proteins, pep-tides, and amino-sugar polymers carrying a positivecharge at pH 2. The strong protein feature of this frac-tion is seen in the NMR spectra as high carboxylic andC-N peaks. No significant differences in HiB fractioncontent were observed during composting. We assumethat during the rapid decomposition process (until Day80) (Chefetz et al., 1996), the HiB fraction containsmainly active enzymes and proteins, while activity atlater stages of the process have been reduced.

In general, DOM concentration declines during com-posting (Inbar et al., 1993). Two important fractionsalso declined in their relative concentrations (HoB andHiN). However, the relative amount of HoN increasedfrom the very beginning of composting. In fact, it almostdoubled in concentration from Days 105 to 187. Basedon the fractionation procedure and NMR data, we con-clude that the HoN fraction consists of highly apolarcompounds with 48 to 44% aliphatic structures. It con-tains the least soluble organic molecules from the DOMand may be closely related to the HA fraction, whichis not soluble in water. According to Cook and Allan(1992) and Zech et al. (1994), the HoN fraction repre-sents 5 to 10% of the total DOM in soils. The contribu-tion this fraction makes to the mature compost is muchhigher (16-18%) than in soils. This may be due mainlyto the fact that this fraction represents an intermediatestage in the humification process and, thus, these apolarcompounds are more likely to be included.

Based on the studies of Guggenberger and Zech(1994), the HoA fraction exhibits a polymeric structurewith covalently linked carbohydrates. Similar featureswere observed in the compost HoA fraction, but con-trary to the suggestion of Wershaw et al. (1995), thisfraction was not identical to the compost or soil FA.The relative proportion of the HoA-fraction increaseduring composting indicated stability toward microbialdegradation. The presence of the methoxy C peak inthe 47-d HoA suggests that this fraction was originallythe soluble fraction of lignocellulose-degradation prod-ucts. Similar conclusions were reached by Grossl andInskeep (1996), who characterized the HoA fractionisolated from wheat (Triticum aestivum L.) straw. Acomparison of the HoA fractions isolated from soil,wheat straw, and compost indicated that the compostHoA fraction is at an intermediate stage between rawOM (lignocellulose complex) and that which has beendegraded and gone through humification processes (soilFA). Moreover, the differences between the HoA and

CHEFETZ ET AL.: DISSOLVED ORGANIC MATTER EXTRACTED FROM COMPOST 331

FA fractions revealed HoA to be the most soluble frac-tion of the compost FA.

Dissolved organic matter in compost at various stagesof decomposition contains heterogeneous organic com-pounds. Being the phase most readily accessible to mi-croorganisms, DOM is the organic fraction containingthe organic materials used as an energy source, bio-originating macromolecules (enzymes, polysaccharides,and proteins), breakdown products, and the repolymer-ized compounds eventually contributing to HS forma-tions. The proposed structure and formation of theDOM fractions enable a better understanding of thedifferences between the 13C-NMR spectra of unfraction-ated DOM at different stages of composting. The de-crease in carbohydrate-C-containing groups seen in the187-d NMR spectrum of the unfractionated DOM paral-lels the trend followed by the percentage of the HiNfraction. During composting, the easily degradable or-ganic compound decreased while the relative proportionof the resistant fraction increased. Thus, the aromatic-C-containing groups (aromatic C, protonated aromaticC, and etherified phenolic syringyl units: 130, 115, and145 ppm, respectively) increased during composting by>50%. Similar reduction of carbohydrate C was exhib-ited in composted and stockpiled manure extracts (Li-ang et al., 1996).

Monitoring the relative concentrations of the variousDOM fractions, and studying the chemical structure ofthese fractions, revealed that the plateau concentrationlevel of the DOM (Days 105-187) does not reflect asteady composition. The relative concentrations of theHoN, HoB, and HiN fractions change significantly dur-ing the curing stage of MSW composting.

CONCLUSIONSDissolved organic matter represents the most active

fraction of compost, both biologically and chemically.It is therefore the organic fraction most subject tochange, and as such directly reflects the OM transforma-tion process. In this study we used chemical fraction-ation and solid-state NMR methods to follow OM trans-formation during the cooling-down and curing phasesof MSW composting. Chemical fractionation and theNMR spectra of the unfractionated DOM indicated deg-radation of the HiN fraction, resulting in relatively highlevels of aromatic and aliphatic structures in the residualDOM. The increasing levels of the HoN and HoA frac-tions at the last stage of composting may represent thehumification process of the compost. The differencesbetween the FA and HoA fractions indicated that HoAis the most mobile and therefore the most active partof the FA fraction. The decrease in HiN from Day 120and the increase in HoN from roughly the same period,may be of relevance to plant-growth enhancement. Im-proved uptake of elements (increased HoN concentra-tion) or the removal of compounds toxic to plants (re-duced HiN concentration) could be the mechanismsinvolved. Monitoring the chemical and biologicalchanges in DOM during the composting process, in par-ticular during the maturation stage, can improve ourknowledge of the transformation of OM during the pro-cess of rapid humification.

ACKNOWLEDGMENTSWe greatly appreciate the partial financial support provided

by GIF and DFG.

332 SOIL SCI. SOC. AM. J., VOL. 62, MARCH-APRIL 1998