cold and hot water–extractable organic matter as indicators of litter decomposition in forest...

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1436-8730/06/0102-76

76 DOI: 10.1002/jpln.200521711 J. Plant Nutr. Soil Sci. 2006, 169, 76–82

Cold and hot water–extractable organic matter as indicators of litterdecomposition in forest soilsDirk Landgraf1*, Peter Leinweber2, and Franz Makeschin3

1 Research Institute of Post Mining Landscapes, Brauhausweg 2, D-03238 Finsterwalde, Germany2 Institute of Land Use, University of Rostock, Justus von Liebig Weg 6, D-18051 Rostock, Germany3 Institute of Soil Science and Site Ecology, Faculty of Forest, Geo-, and Hydrosciences, Dresden University of Technology,

P.O. Box 1117, D-01735 Tharandt, Germany

Accepted November 11, 2005 PNSS P171/1B

SummaryMild extractions were used as indicators of easily decompo-sable organic matter (OM). However, the chemical composi-tion of extracted OM often remained unclear. Therefore, thecomposition of cold and hot water–extractable OM was inves-tigated in the O horizons (Oi, Oe, Oa) of a 170 y old beechstand (Fagus sylvatica) in the Ore Mtns., SE Germany. Tosimulate litter decomposition, the O horizon samples wereincubated for 1 week under defined conditions. Cold- andhot-water extracts were analyzed and chemically character-ized by pyrolysis–field ionization mass spectrometry (Py-FIMS). The C and N concentrations were always lower in thecold-(C: 2.69 to 3.95 g kg–1; N: 0.14 to 0.29 g kg–1) than in thehot-water extracts (C: 13.77 to 15.51 g kg–1; N: 0.34 to0.83 g kg–1). The C : N ratios of both extracts increased withincreasing depth. Incubation increased the concentrations ofC and N in all water extracts, while C : N ratios of extractsdecreased. The molecular-chemical composition of cold andhot water–extracted OM revealed distinct differences. Gener-

ally, cold water–extracted OM was thermally more stablethan hot water–extracted OM. The mass spectra of the hotwater–extracted organic matter revealed more intensive sig-nals of carbohydrates, phenols, and lignin monomers. Addi-tionally, the n-C28 fatty acid and the n-C38–to–n-C52 alkylmonoesters clearly distinguished the hot- from the cold-waterextract. A principle-component analysis visualized (1) altera-tions in the molecular-chemical composition of cold- and hot-water extracts due to previous incubation of the solid O hori-zon samples and (2) a decomposition from the Oi to the Ohhorizon. This provides evidence that the macromorphologicallitter decomposition was reflected by the chemical compo-sition of water extracts, and that Py-FIMS is well-suited toexplain at the molecular level why OM decomposability iscorrelated with water-extracted C.

Key words: soil organic matter / hot-water extract / cold-waterextract / humus mineralization / analytical pyrolysis

1 Introduction

Labile soil C pools have been suggested as sensitive indica-tors of soil organic-matter (SOM) changes (Powlson et al.,1987; Larson and Pierce, 1994; Evans et al., 2001). Mild oxi-dation methods, extraction with weak acid, and extractionswith cold and hot water were commonly used as chemicalmethods for the determination of labile soil C (Khanna et al.,2001). Cold water–extraction (CWE) methods were intro-duced in the late 1980s to estimate easily mineralizable SOMin grassland (Corre et al., 1999) and forest soils (Davidson1987; van Ginkel et al., 1994; Jandl and Sollins 1997). Thehot-water extraction (HWE) was introduced by Bronner andBachler (1979) to estimate mineralizable N in sugar-beetcropping in Austria. Since then, this method widely has beenused for predictions of the mineralizable-C and mineralizable-N pools in arable soils (Behm, 1988; Körschens et al., 1990;Schulz, 1990; Körschens et al., 1998; Sparling et al., 1998;Gregorich et al., 2003). To the best of our knowledge, there isonly one published study on the molecular-chemical compo-sition of OM in HWE from an arable soil (Leinweber et al.,1995). In this study, C-13–nuclear magnetic–resonance(13C NMR) spectra and pyrolysis–field ionization mass spec-tra (Py-FIMS) in good agreement clearly showed the predo-minance of carbohydrates in the HWE, and indicated pep-tides as accompanying compounds, both probably originating

from microbial biomass and rhizodeposits. So far, neitherCWE nor HWE from forest soils (i.e., O layers) have beenstudied for their molecular composition in detail. However,detailed knowledge of the chemical composition of water-extractable OM appears necessary if these methods shall beestablished as simple and low-cost indicators of soil qualitychanges.

The objectives of the present study were to investigate (1)the composition of CWE and HWE OM from O horizons of aforest soil and (2) how these fractions reflect litter decomposi-tion in incubation experiments in the sequence of the Oi, Oe,and Oa horizon.

2 Materials and methods

The forest site is located in the Ore Mtns., SE Germany (for-est district “Heinzebank”, 850–900mm annual precipitation,8.9°C–9.5°C mean annual temperature). Humic-horizon sam-ples were taken under a beech stand (Fagus sylvatica) withan approximate age of 170 y being under further investigationof forest-conversion study (Koch and Makeschin, 2004).Representative samples from the Oi, Oe, and Oa horizonwere collected in fall of 2000 by pooling at least 3 to 5 sub-samples from an area of 400 cm2 and replicating this fourtimes at an area of about 1600 m2. These composite sampleswere pooled to give one sample per horizon. Moisture con-tent was determined by weight loss after drying at 65°C for* Correspondence: Dr. D. Landgraf; e-mail: [email protected]

96 h. A portion of each sample was air-dried (40°C) for physi-cal and selected chemical analyses. The pH was measuredon air-dry samples in 0.01 M CaCl2 solution (soil : solutionratio = 1:2.5). Mineral N was extracted by 0.01 M CaCl2 solu-tion (soil : solution ratio = 1:10) and determined with an con-tinuous-flow analyzer (Skalar Model SANplus). Organic C andtotal N were determined by dry combustion using a CNS ana-lyzer (Foss-Heraeus, Model vario EL). Table 1 shows basicchemical properties of the samples.

Nine replicated fresh subsamples of each composite-horizonsample were incubated according to the Stanford and Smith(1972) method as modified by Beck (1983). Cold-waterextracts from pre- and postincubated samples were obtainedby shaking a 10 g aliquot of a field-moist sample with 100 mLdeionized water horizontally at 180 rev min–1 for 24 h. Aftershaking, the suspension was centrifuged at 4000 rev min–1

for 10 min, and extracts were separated from soil by carefuldecantation. Hot-water extracts were obtained by boiling a 5 galiquot of field-moist sample in 100 mL de-ionized water for60 min. After cooling down to room temperature, 2 mL CuSO4solution was added. The suspension was centrifuged at4000 rev min–1 for 10 min. Extracts were separated from soilby decantation. The concentrations of C (TOC) and N (TON)in aliquots of the CWE and HWE were determined by a CNAnalyzer (Jena Analytics). The remaining extracted materialwas bulked for each of the treatments, freeze-dried, andstored at –5°C for further analyses.

The C and N concentrations reported are averages of dupli-cated analyses of each of the nine replicates and areexpressed on a moisture-free basis. Separation of means bythe U-test (Mann-Whitney) and the Partial CorrespondenceAnalysis for classification of the HWE and CWE were per-formed by the program Statsoft Statistica®, Version 6.

For pyrolysis–field ionization mass spectrometry (Py-FIMS)about 5 mg of a sample were thermally degraded in the ionsource of a modified Finnigan MAT 731 high-performancemass spectrometer. The sample was heated in high vacuumfrom 100°C to 700°C at a heating rate of approximately 10 Kper magnetic scan (three replicates). After 19 min of totalregistration time, about 60 magnetic scans were recorded forthe mass range 16 to 1000 Dalton (single spectra). Thesewere combined to obtain one thermogram of total ion inten-sity and an averaged mass spectrum. In addition, for each ofthe single scans, the ion intensities of marker signals for eightselected classes of chemical compounds in SOM were calcu-lated. All Py-FIMS data were normalized per mg sample. Fordetailed descriptions of the Py-FIMS methodology and of sta-tistical evaluations of sample weight and residue, volatilizedmatter, and total ion intensities (TII) see Sorge et al. (1993a).A principal-component analysis (PCA) was applied to dis-close differences between the samples as derived from thePy-FI mass spectra. The principal components (PC) com-prised of selected m / z values with highest variance weight,i.e., the ratio of interclass variance of a variable to interclassvariance (Kowalski and Bender, 1972).

3 Results and discussion

Table 2 shows that the C and N concentrations were alwayshigher in the HWE than in the CWE. This was valid for thepre- and postincubation samples. In the Oi horizon, the con-centration of HWE C was higher by factor 4 than the concen-tration of CWE C. This factor increased to about 6 in the Oahorizon. However, there were no clear depth gradients in theconcentrations of CWE and HWE C and N. The C : N ratioswere wider in the HWE than in the CWE, and C : N ratios ofpre- and postincubation samples increased with increasingdepth from the Oi to the Oa horizon.

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Table 1: Chemical properties of the O horizons of a beech stand in the Ore Mtns., SE Germany.

Horizon pH value Corgg kg–1

Nt Corg : Nt Nmin-Ng kg–1

Oi 5.7 ± 0.2 477.4 ± 1.8 17.2 ± 0.12 27.8 0.11 ± 0.03Oe 5.2 ± 0.1 479.3 ± 1.3 23.7 ± 0.01 20.3 0.10 ± 0.01Oa 4.8 ± 0.3 344.3 ± 0.6 18.1 ± 0.00 19.0 0.08 ± 0.02

Table 2: Concentrations of hot and cold water–extracted C and N and C : N ratios of pre- and postincubation extracts from the O horizons in abeech stand.

Horizon extractionmethod

Cpre-

Cpost-

Npre-

Npost-

C : N ratiopre-

C : N ratiopost-

incubation incubation incubation

g kg–1

Oi cold 3.71 (±0.20) 8.51 (±0.30) 0.29 (±0.02) 1.12 (±0.07) 12.8 7.6hot 14.63 (±0.42) 21.37 (±2.00) 0.83 (±0.02) 1.57 (±0.15) 17.6 13.6

Oe cold 3.95 (±0.21) 8.55 (±1.28) 0.22 (±0.02) 1.11 (±0.19) 18.0 7.7hot 13.77 (±0.20) 27.02 (±7.36) 0.66 (±0.01) 1.99 (±0.55) 20.9 13.6

Oa cold 2.69 (±0.14) 3.69 (±0.28) 0.14 (±0.02) 0.40 (±0.03) 19.2 9.2hot 15.51 (±0.34) 23.62 (±5.91) 0.34 (±0.02) 1.50 (±0.42) 45.6 15.8

J. Plant Nutr. Soil Sci. 2006, 169, 76–82 Cold/hot water–extractable OM as indicators of litter decomposition 77

The concentrations of CWE C and N in Tab. 2 were muchhigher than values reported by Zsolnay and Steindl (1991),Jandl and Sollins (1997), and Gregorich et al. (1996). This isexplained by the general larger C and N contents in the Olayers of our experimental site as compared to mineral soilsstudied by the authors mentioned above. However, the con-centration ranges of HWE C (about 14 to 27 g kg–1) of thepresent study were similar to concentrations reported byChodak et al. (2003) for Oi layers in beech stands in centraland N Germany (28 to 30 g kg–1). As for CWE C, the C con-centrations were generally much higher in HWE from O layersof forest soils than in HWE from arable mineral soils (0.5 to0.9 g kg–1; Leinweber et al., 1995). The increase in C : Nratios from the Oi to the Oa horizon was surprising since theC : N ratios of solid SOM generally decrease with increasingdepth (Chodak et al., 2003; Zhong and Makeschin, 2003),reflecting the advanced decomposition of organic residues(Millard, 1996; Sanger et al., 1998). This indicates that water-extractable fractions from O horizons do not reflect the pre-ferential C mineralization occurring in solid OM or undergodifferent chemical transformations that finally result in resi-dual N enrichments compared to the horizons above them.

Incubation significantly increased the concentrations of C andN in all water extracts (Tab. 2). The factors increased be-tween 1.4 (Oa, CWE) and 5.0 (Oi, CWE). These values con-tinuously became smaller with profile depth in case of CWEC (2.3 in Oi to 1.4 in Oa) and larger in case of HWE N (1.9 inOi to 4.4 in Oa). The C : N ratios of extracts decreased due toincubation. Factors of decrease became larger with increas-ing profile depth for HWE (1.3 in Oi to 2.9 in Oa). The presentresults confirmed Zhong and Makeschin (2003) who reportedincreasing N concentrations with incubation time. Jussy et al.(2004) countersigned this report with results obtained in sixforest soils under mountainous climate. To the best of ourknowledge, the effects of incubation on different water-extractable C and N fractions were not investigated so far.

One basic idea of the present experiment was to simulatelitter decomposition in the sequence of O horizon layers byaerobic incubation. The results, especially the C : N ratios ofthe water extracts, indicated differences in OM changes out-side in the field and inside in incubation experiments. Thedecrease in C : N ratios of all extracts after incubation indi-cated that extracted OM reflects the decomposition of readilymobilizable C-containing compounds which were trans-formed to CO2 and the relative enrichment of either microbialN-containing metabolites or nondecomposable N-containingcompounds. Since these processes are well established forthe sequence of O layers in forest soils (Kögel, 1987), thetendency of wider C : N ratios with increasing profile depthindicated additional inputs of C-rich OM to the water-extracta-ble fraction. One explanation can be the impact of soil fauna–derived, N-free lipids which is probably stronger in the fieldthan in the incubated samples. For instance, fauna samplescontained about 1 to 2 lg kg–1 (enchytraeids and acari) to33 lg kg–1 (nematodes) of saturated alkanoic acids (Jandl etal., 2005). Furthermore, the slower decomposition at lowertemperatures can be another possible explanation for thedisagreement between extracted OM from humus profile andpre- and postincubated samples.

For a deeper insight into OM transformations, the molecular-chemical composition of CWE and HWE OM was investi-gated by pyrolysis–field ionization mass spectrometry(Py-FIMS). Figure 1 shows the thermograms of total ionintensity (upper right) and summed and averaged Py-FI massspectra of CWE and HWE OM. The most prominent signalsin the lower-mass range were assigned to carbohydrates(m / z 84, 96, 110, 126) and N-containing compounds such asheterocycles and peptides (m / z 43, 59, 70, 95). Phenols andlignin monomers were indicated by prominent signals atm / z 94, 108, 124, 138, and 168. In higher-mass range, sig-nals of alkylaromatics (e.g., m / z 232, 234) and lipids (e.g.,m / z 256, 272, 286, 340, 392, 396, 424, 592, 620, 648, 676,704, 732) predominated. Among the lipids, a homologuesseries of saturated n-C10 (m / z 172)–to–n-C36 (m : z 536)alkanoic acids was clearly discernible. The most prominentmembers were n-C16 (m / z 256), n-C22 (m / z 340), n-C26(m / z 396), and n-C28 (m / z 424). A homologous series of n-C38–to–n-C52 alkyl-monoesters with predominance of theeven over the odd C-numbers is indicated by the signals be-tween m / z 564 and 760.

Visual comparison of the TII thermograms (upper right) andmass spectra in Fig. 1 shows distinct differences betweenCWE and HWE OM. The TII thermogram of CWE OM indi-cated a large proportion of OM volatile only above 400°C.Generally, CWE OM was thermally more stable than HWEOM. In the mass spectra, the signals of carbohydrates and ofphenols and lignin monomers (see above for examples ofm / z) were more intensive in the HWE than in the CWE.Furthermore, in the higher-mass range, the very intensivem / z 424 (n-C28 fatty acid) and a homologous series of then-C38–to–n-C52 alkyl monoesters clearly distinguished theHWE from the CWE (Fig. 1).

These differences also became obvious by the quantitativeevaluation of ten important compound classes of SOM(Tab. 3). Absolute and relative intensities indicated that morecarbohydrates and phenols and lignin monomers were solu-bilized by boiling the samples. Furthermore, the HWE OMcontained more heterocyclic N-containing compounds andpeptides than the CWE.

The Py-FI mass spectra and proportions of compoundclasses can be compared with water extracts and dissolvedorganic matter (DOM) from forest and arable mineral soils,peat, and with aquatic humic substances. The predominanceof lipids and alkylaromatics in the present water extractsagrees with Py-FIMS of fulvic acids from fen peat soils (Lein-weber et al., 2001a). Groundwater below a sprinkled woodstorage was enriched in lignin and tannin subunits as well aslipid-derived structures (Sorge et al., 1993b). Furthermore,there is partial agreement with detailed investigations ofDOM fractions by Guggenberger et al. (1994) who assignedabout 25%–33% of OM in DOM fractions to alkyl structures,depending on the acidity and hydrophobicity of the fraction.Aquatic humic substances from a lake rich in fulvic acids froma surrounding bog peat had similar proportions of alkylaro-matics but more phenols and lignin monomers at the expenseof lipids than the present water extracts (Schulten, 1999). Insummary, CWE were more similar to water extracts and


78 Landgraf, Leinweber, Makeschin J. Plant Nutr. Soil Sci. 2006, 169, 76–82

DOM from forest and arable mineral soils, peat, and withaquatic humic substances than HWE.

DOM from Oi and Oa horizons of another forest soil after 90 dof incubation was relatively enriched in carbohydrates, phe-nols and lignin monomers, N-containing compounds andpeptides at the expense of lignin dimers, lipids, and sterols(Kalbitz et al., 2003). Furthermore, DOM from the rhizo-sphere of mineral soils was rather similar to the hot-waterextracts but contained more peptides (3%–6% of TII)

(Melnitchouck et al., 2004). To summarize this, the strongerthe influence of microorganisms or microbial processes onCWE, either temporarily due to long-term incubation (Kalbitzet al., 2003) or spatially due to selective sampling of rhizo-deposits (Melnitchouck et al., 2004), the more the CWEbecome similar to the HWE. This, in turn, indicates a signifi-cant microbial contribution to HWE OM. This corresponds toGregorich et al. (2003) who found a greater biodegradabilityin HWE than in CWE. Thus, the present Py-FI mass spectra,along with similar previous investigations, directly explain that


1.4 2.5

100 200 300 400 500 600 700 800 900m / z

Ion inte

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TII

Temperature in °C

Inte

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620648

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Figure 1: Thermograms of total ion intensity (TII, upperright) and summed and averaged pyrolysis–field ioniza-tion mass spectra of (a) cold and (b) hot water–extracted organic matter from three O horizons of abeech stand in the Ore Mtns., SE Germany. Thethermograms and spectra represent means for ninesingle determinations from three horizon samples.

Table 3: Absolute contents (106 counts mg–1) and relative abundance (% of total ion intensity) of ten important compound classes of SOM; car-bohydrates = CHYD, phenols + lignin monomers = PHLM, lignin dimers = LDIM, lipids = LIPID, alkylaromatics = ALKY, heterocyclic nitrogen–containing compounds = NCOMP, sterols = STEROL, peptides = PEPTI, suber = suberin, FATTY = free fatty acids.

Extracts Units CHYDR* PHLM* LDIM LIPID ALKY* NCOMP STEROL PEPTI** SUBER FATTY* TII VM

Cold water 106 counts mg–1 1.740 2.735 2.348 6.513 4.388 2.823 2.869 1.201 0.364 2.018 56.866 48.8% TII 3.1 4.8 4.1 11.5 7.7 5.0 5.0 2.1 0.6 3.5

Hot water 106 counts mg–1 3.724 3.607 1.055 4.025 3.615 2.640 1.338 1.311 0.133 1.527 40.519 55.2% TII 8.1 8.9 2.6 9.9 8.9 6.5 3.3 3.2 0.3 3.8


HWE C was correlated with microbial biomass C (e.g., Spar-ling et al., 1998; Ghani et al., 2003) because it extracts micro-bial metabolites and decomposition products.

The second obvious difference between CWE and HWE ofthe present study was a larger mass range covered alongwith a greater intensity of n-alkyl monoesters in the latter(Fig. 1). Supercritical water extracts of various soils (tempera-ture: 150°C to 250°C, pressure: 17.2 MPa) did not containn-alkyl monoesters but n-C38–to–n-C44 alkenes, which wereinterpreted as indicators of the corresponding alcohols(Schnitzer et al., 1991). However, supercritical carbon dioxideextracts from the A horizon of an arable and the Bh horizon ofa forest soil contained the same homologous series of n-alkylmonoesters (Schulten and Schnitzer, 1991) as detected inthe present study. Long-chain n-alkyl monoesters were alsodetected in solid samples of plant residues (e.g., Gregorich etal., 1996) and of O horizons (Beyer et al., 1993) and ascribedto be typical constituents of plant waxes. Thus, the extractionof n-alkyl monoesters with boiling water and identification byPy-FIMS offers an opportunity to trace the fate of plant waxesin forest soils.

Although the aerobic incubation of solid soil samples alteredthe molecular-chemical composition of CWE and HWE, the

sequence of O horizons was clearly discernible by a princi-pal-component analysis (PCA) (Fig. 2). In the CWE (Fig. 2a),the OM decomposition in the profile was reflected bydecreasing values of PC 1 which explained about 36% of var-iance between the Py-FI mass spectra. The PC 3, whichexplained about 8% of sample variance, contributed strongerto the separation of extracts from pre- and postincubatedsamples. In the HWE (Fig. 2b), OM decomposition in the pro-files was reflected by increasing values for PC 1 of this sam-ple set. The effects of the incubation were stronger reflectedby PC 2.

The PCA gave the impression of gradual and directed chang-es in OM composition of the extracts down the humus profile.In all samples, the data point triangles for the Oe horizonwere aside a straight line from the Oa to the Oi. This indicatesa discontinuity in OM changes which was reflected strongerby PC 2 and 3 than by PC 1, irrespectively on the extractionmethod. Similar patterns were obtained for the evaluation ofPy-FIMS data from daily sampled, short-time, compostingexperiments (Leinweber et al., 2001b). This agreement indi-cates that Py-FIMS is sensitive enough to reflect differencesin the composition of transforming solid OM (e.g., in compost-ing experiments) and DOM extracted from solids by cold andhot water. A better understanding of OM transformations in


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Figure 2: Principal-component analysis of pyrolysis–fieldionization mass spectra of (a) cold and (b) hot water–extracted OM from three O horizons of a beech stand in theOre Mtns., SE Germany.


the sequence of O horizons requires the complete evaluationof the Py-FI mass spectra of all individual horizon samplesand of the derived data sets for compound classes, molecu-lar-weight distributions, and thermal properties.

4 Conclusions

(1) Because of the statistically proved differences in elemen-tal composition and Py-FI mass spectra, the present metho-dological approach is well-suited to detect effects of OMdecomposition in a humus profile and due to aerobic incuba-tion of solid OM on the water-extractable fractions.

(2) Cold and hot water extracted differently composed anddifferently stable OM fractions from O layers of forest soils.Therefore, a basis is laid to understand why these fractionsreflect OM decomposability and microbial processes in soilsas reported by some authors but only based on regressionand correlation analyses.

(3) The present results indicate that the HWE fraction of Cand N contained more easily available substances such ascarbohydrates, phenols and lignin monomers, and organic Ncompounds and therefore probably is a better predictor ofeasily decomposable OM than the CWE.

Acknowledgment

We thank the two reviewers for constructive comments whichhelped us to improve the manuscript.

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cold and hot water–extractable organic matter as indicators of litter decomposition in forest...

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