Characterization of organic matter in an interlayer clay-organic complex from soil by pyrolysis methylation-mass spectrometry

f~,!

ELSEVIER

Geoderma 69 (1996) 105-118

Characterization of organic matter in an interlayer clay-organic complex from soil by pyrolysis methylation-mass spectrometry H.-R. Schulten

a

p. Leinweber

b, B.K.G.

Theng c

~ lnstitut Fresenius, Chemical and Biological Laboratories, hn Maisel 14, 65232 Taunusstein, Germany b Institute for Spatial Analysis and Planning in Areas of Intensive Agriculture (LTPA), University of Vechm, Driverstrasse 22, P.O. Box 1 553, 49 364 Vechta, Germany L.Manaaki Whenua, Landcare Research, Private Bag 11 052. Pahnerston North, New Zealand

Received 20 April 1995: accepted 22 August 1995

Abstract The composition of organic matter in clay-organic complexes isolated from a New Zealand Spodosol has been investigated using the novel analytical combination of (a) labscale (500 mg) off-line pyrolysis, (b) direct inlet (5 mg) off-line methylation with tetramethylammonium hydroxide (TMAH) and pyrolysis-field ionization mass spectrometry (Py-FIMS), and (c) Curie-point pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) with off-line and conventional on-line derivatisation with TMAH. In complex I the organic matter was associated with both external and interlayer surfaces of a swelling clay. Complex II was obtained by removing the external portion with H202, leaving the interlayer organic matter essentially intact. For both complexes, a high proportion of the carbon (6486%) and nitrogen (76-91%) was volatilized during pyrolysis. Analysis of the pyrolysates showed mono- and dicarboxylic acids, alkanes, alkenes, n-alkylmonoesters, and N-containing compounds to be major constituents. Thermal methylation of the material, prior to (off-line) and during (on-line) pyrolysis gave rise to a wide range (between C6 and C35) of methyl esters of aliphatic and aromatic acids, methoxy derivatives of phenols and phenolic acids. In accord with previous ~3C-nuclear magnetic resonance (NMR) spectroscopic measurements, the organic matter in both complexes was highly aliphatic in composition. Although there was little difference between complex I and complex II in the composition of the aliphatic constituents, complex II was greatly depleted in lignin-derived aromatics. In addition, the interlayer material in complex II had a relatively low thermal stability.

1. Introduction Interlayer complexes of clay with organic matter ( O M ) in soil have only rarely been detected because their formation is conditional on the soil being acidic and rich in swelling 0016-7061/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD10016-706 I (95)00054-2

106

H.-R. Schulten et al. / Geode rma 69 (1996) 105-118

clays. Theng et al. (1986) have reported the occurrence of such a complex in the clay fraction of an acid Spodosol with a smectitic clay mineralogy from Maungatua, New Zealand. Using J3C-NMR spectroscopy they were able to show that the OM was composed of a polymethylene chain containing carboxyl and hydroxyl groups. Radiocarbon dating put the age of the interlayer material at nearly 7000 years B.P. (Theng et al., 1992). Its preservation in the surface horizon of a contemporary soil was ascribed to physical protection within crystals of the swelling clay as well as to structural recalcitrance. Solid-state ~3C-NMR spectroscopy, with cross polarization (CP) and magic angle spinning (MAS), has provided valuable information on the gross composition of OM in soil fractions and whole soils (Oades et al., 1987; Wilson, 1987). However, the technique does not lend itself to fine structure analysis of the organic constituents in soil. The applicability of ~3C CP/MAS NMR spectroscopy may be further limited by the lack of spectral resolution especially for samples of low carbon content, or with appreciable amounts of paramagnetic iron species (Skjemstad et al., 1994). Pyrolysis-field ionization mass spectrometry (Py-FIMS), on the other hand, is not subject to these limitations (Schulten and Leinweber, 1995). As such, the technique is well suited for detailed structural analysis of complex mixtures of organic compounds, including OM in and from soil (Schulten et al., 1986; Bracewell et al., 1989; Hempfling et al., 1991; Schnitzer and Schulten, 1992). Moreover, Py-FIMS has also been effectively used to assess the changes that occur in the composition of OM during soil development, or imposed by certain soil management (Schuiten and Leinweber, 1991, 1993; Leinweber et al., 1993) or caused by temporal variations in long-term field experiments (Leinweber et al., 1995; Schulten et al., 1995). Using Py-FIMS, Righi et al. (1995) were able to detect alkylaromatics, carbohydrates, phenols, lignins, lipids and N-containing compounds in the fine clay fraction from the C horizon of an acid soil. However, the OM in the corresponding fraction from the A 1 and Bw horizons consisted largely of alkylaromatics and N-containing compounds that were relatively resistant to oxidative degradation. They suggested that the chemically stable components were selectively sorbed, or even intercalated by the clay. The extent to which this process occurred would depend on the type of clay present in a given horizon within the soil profile. That association with clays and minerals can influence the thermal behaviour of soil OM during pyrolysis has been demonstrated by Schnitzer et al. (1994) for fulvic acid. The capability of Py-FIMS for soil OM analysis can be further improved by combination with off-line pyrolysis, pyrolysis methylation, and gas chromatography/mass spectrometry (Py-GC/MS). By these means, Schulten and Sorge (1995) were able to detect a wide range of polar organic compounds in whole soils, including carboxylic acids, aromatic alcohols, amines, and amides. Here we report on the application of this suite of instrumental techniques to characterize the clay-associated OM separated from the Maungatua humic silt loam. The analysis of polymethylene structures in the sample is of particular interest because these may have derived from natural waxes and aliphatic biopolymers present in the plant cover. Since the OM has persisted for nearly 7000 years, its mass spectrum can serve as a "finger-print" of past vegetation. A further objective is to differentiate between the OM attached to external

H.-R. Schulten et al. / Geoderma 69 (1996) 105-118

107

clay surfaces and the interlayer material in terms of composition and thermal stability (volatility) during pyrolysis.

2. Materials and methods 2.1. Clay-organic complexes

The complexes were isolated from a soil sample, taken at a depth of 90-160 mm within the A21g horizon of the Maungatua humic silt loam, a Typic Placaquod, in Otago, New Zealand. The location, cover vegetation and climate of the sampling site have been described by Molloy and Blakemore (1974). Complex I refers to the clay ( < 2/xm) fraction, obtained by dispersing the air-dry soil in water with an ultrasonic probe, and separating the material by sedimentation under gravity (Theng et al., 1986). The clay is a regularly interstratified mica-beidellite (Churchman, 1978) in which the OM is partly attached to external crystal surfaces, and partly intercalated. Complex II was obtained by treating complex I twice with 30% H202 at room temperature, and washing repeatedly with deionized water. The external OM was presumed to have been preferentially removed (oxidized) by this treatment, leaving the interlayer component essentially intact (Theng et al., 1992). 2.2. Off-line pyrolysis

Off-line pyrolysis was carried out by placing about 500 mg of the air-dry sample, contained in a reaction tube, in a specially designed oven. The material was heated (from room temperature) to 973 K at a heating rate of 2-3 K s- ~. The CO, evolved was measured by conductometric analysis. When the pyrolysis was completed, the tube was slowly removed from the oven, and allowed to cool down under nitrogen. The carbon and nitrogen content of the sample, before and after pyrolysis, was determined by dry combustion and semimicro-Kjeldahl analysis (Bremner, 1965), respectively. The standard deviation in weight loss, and C and N values, between replicate determinations (n = 2) was < 5%. 2.3. Methylation

For off-line methylation, the complexes were dried, milled and thoroughly mixed approx. 1:1 with a 25% w/w aqueous solution of tetramethylammonium hydroxide (TMAH). After heating at 523 K under ambient pressure for 10 min, the methylated materials were subjected to direct (in-source), temperature programmed and indirect flash (Curie-point) pyrolysis. The pyrolysis products were analysed by FIMS and GC/MS as described previously (Schulten and Sorge, 1995). In addition, the routinely used on-line methylation was carried out by mixing the samples with TMAH and methylation inside the Curie-point pyrolyzer at 1043 K (Challinor, 1989, 1991, 1994). 2.4. Pyrolysis-mass spectrometry

About 5 mg of each complex was heated under high vacuum from 323 to 973 K at a rate of 0.5 K s-~ using a modified direct introduction system with electronic temperature-

108

H.-R. Schulten et al. /Geoderma 69 (1996) 105-118

a)

1.6

1.4 143 1.2-l

424

1.0 0.8 ~

0.6

~-

0.4 0.2

"" t-

._o > ° --~ n,'

20

100

200

300

400

500

600

700

800

43

1.6 1.4 1.2 1.0

900

b)

1

396

0.8 0.6 0.4 0.2 20

100

200

300

400

500

600

700

800

900

m/z

Fig. 1. Summedand averaged pyrnlysis-fieldionization ( sum.ey) mass spectra of (a) the untreated complex ( I ), and (b) the H202-treated complex (11) separated from the clay fraction of a Spodosol from Maungatua, New Zealand. programming (IGT Instrumente- and Ger~ite-Technik GmbH, D-53804 Much, Germany). The thermal degradation products were ionized at an ion-source potential of + 8 kV using a Finnigan MAT 731 mass spectrometer (Schulten, 1987, 1993). About 60 magnetic scans were recorded for the mass range from m / z 16 to m / z 1000 (corresponding to 60 single spectra). Following three replicate measurements, in total 180 mass spectra were averaged to give a surL, ey spectrum (see Fig. I). The assignment of the various signals to specific compounds has been described elsewhere (Schnitzer and Schulten, 1992). 2.5. Curie point pyrolysis-methylation-gas chromatography/mass spectrometr 3,

Complexes I and II and their respective methylated derivatives were thermally degraded in a Curie-point pyrolyzer (type 0316, Fischer, D-53340 Meckenheim, Germany). The method differed from that of Challinor (1989) in that the total heating time was 9.9 s, and the final temperature was 573 K. These conditions were used in order to optimize the analysis of lipids, notably fatty acids. The pyrolysis products were separated in a Varian 3700 gas chromatograph (Varian, D-64289 Darmstadt, Germany) equipped with a 30 m capillary column (DB 5 ) of 0.32 mm inner diameter and coated with a 0.25 mm thick film. The starting and finishing temperature was set at 308 and 518 K, respectively, and the heating rate at 10 K min -~. Mass analysis was performed in a Finnigan MAT 212 mass spectrometer coupled with the gas chromatograph (Finnigan MAT, D-28127 Bremen, Germany). The conventional electron ionization mode was used under the following conditions: 3 kV accelerating voltage, 70 eV electron energy, 2.0 kV multiplier, 1.1 s/mass

109

H.-R. Schulten et aL / Geoderma 69 (1996) 105-118

Table 1 Some properties of clay-organic complexes separated from the Maungatua humic silt loam before treatment (complex I) and after treatment (complex II) with H202 Samples

C ( %)

Complex I 22.0 Complex II 11.8

N ( %)

0.59 0.25

C/N ratio

37.2 47.4

Off-linepyrolysis Weight loss (%)

Py-FIMS TII ( X 106) b weight loss Residual Residual (%) mg ~ mg C Ca Nb sample

23.0 16.3

36.0 13.5

23.7 9.1

21.2 15.9

26.4 24.3

120.0 205.9

aThese values refer to the contents in carbon and nitrogen in the residue after pyrolysis, and expressed as a percentage of the C and N contents in the original samples. ~1"II= total ion intensity, expressed as counts per mg of material. decade scan speed, and m / z 48-451 mass range. The mass spectra were compared with those of the National Institute of Science and Technology (NIST; 40,000 spectra), Wiley ( 145,000 spectra), and in-house collection obtained from pyrolysis of model compounds ( > 5000 spectra).

3. Results and discussion 3.1. Organic matter volatilization and ion intensities

Table 1 lists the carbon and nitrogen contents of the complexes showing that 46% C and 68% of N were lost from complex I by treatment with H202. By comparison, complex 1 lost only 23%, and complex II 16% of its weight during off-line pyrolysis. Similar losses of material were recorded during (on-line, in-source) Py-FIMS. The close similarity in weight-loss measured by off-line and in-source pyrolysis has also been observed by Leinweber and Schulten (1995) for 32 samples of soil OM, particle-size fractions, and whole soils. On this basis, the amounts of C and N, volatilized during Py-FIMS may be estimated from the corresponding values measured by off-line pyrolysis. In the present instance, complex I contained about 36% C and 24% N, and complex II 14% C and 9% N after completion of off-line pyrolysis (Table 1 ). We therefore assumed that during Py-FIMS about 64% and 86% of the carbon, and 76% and 91% of the nitrogen had volatilized from complex I and II, respectively. Although these proportions fell in the upper range of values reported by Leinweber and Schulten (1995), they were consistent with the measured total ion intensity (TII) values which were also uncommonly large for soil OM (Table 1 ). However, the interlayer material was apparently more volatile than the OM in complex I. This observation reflects differences between the two complexes in OM composition, notably in the frequency distribution of long-chain fatty acids, to which we shall refer later. 3.2. Pyrolysis-field ionization mass spectrometry

The Py-FI mass spectra for complexes I and II (Fig. la, b) show similar features. Molecular ions ranging from m / z 18 (water) up to m / z > 900 were detected. Signals in the

110

H.-R. Schulten et al, /Geoderma 69 (1996) 105-118

Table 2 Methyl esters of carboxylic acids identified in methylated clay-organic complexes separated from the Maungatua humic silt loam by Curie-point Py-GC/MS with the aid of the mass spectra library of NIST and Wiley. Peak No.

MW Purity CAS

Molecular formula C

H

O

Compounds N

12a lla 10a 9a 5a 3a 2a la 4a 6a 7a 8a

214 242 270 298 326 354 382 410 438 466 494 522

653 268 770 684 812 559 887 9t8 776 750

111-82-0 124-10-7 112-39-0 112-61-8 1120-28-1 929-77-1 2442-49-1 5802-82-4 55682-92-3 629-83-4

13 15 17 19 21 23 25 27 29 31 33 35

26 30 34 38 42 46 50 54 58 62 66 70

2 2 2 2 2 2 2 2 2 2 2 2

Dodecanoic acid, methyl ester Even-number Tetradecanoic acid, methylester monocarboxylic Hexadecanoic acid, methylester acids Octadecanoic acid, methyl ester Eicosanoic acid, methyl ester Docasanoic acid, methyl ester Tetracosanoic acid, methyl ester Hexacosanoic acid, methyl ester Octacosanoic acid, methyl ester Tricontanoic acid, methyl ester Dotricontanoic acid, methyl ester Tetratricontanoic acid, methyl ester

9b 8b 6b 4b 2b lb 3b 5b 7b 8b

256 284 312 340 368 396 424 452 480 508

894 687 375 852 803 863 590

7132-64-1 1731-92-6 1731-94-8 6064-90-0 2433-97-8 55373-89-2 55429-72-6

16 18 20 22 24 26 28 30 32 34

34 36 40 44 48 52 56 60 64 68

2 2 2 2 2 2 2 2 2 2

Pentadecanoic acid, methyl ester Odd-number Heptadecanoic acid, methyl ester monocarboxylic Nonadecanoic acid, methylester acids Heneicosanoic acid, methyl ester Tricosanoic acid, methyl ester Pentacosanoic acid, methyl ester Heptacosanoic acid, methyl ester Nonacosanoic acid, methyl ester Eicosanedioic acid, methyl ester Tritriconanoic acid, methyl ester

8d

328

19

36

4

6d

342

20

38

4

5d

356 552

21

40

4

7d

370

22

42

4

2d

384

23

44

4

3d

398 676

24

46

4

10d

412

25

48

4

lid

426

26

50

4

12d

440

27

52

4

13d

454

28

54

4

14d

468

292

56

4

Heptadecanedioic acid, dimethyl Aliphatic ester Octadecanedioic acid, dimethyl dicarboxylic ester Nonadecanedioic acid, dimethyl acids ester Eicosanedioic acid, dimethyl ester Hemicosanedioic acid, dimethyl ester Docosanedioic acid dimethyl ester Tricosanedioic acid, dimethyl ester Tetracosanedioic acid, dimethyl ester Pentacosanedioic acid, dimethyl ester Hexacosanedioic acid, dimethyl ester Heptacosanedioic acid, dimethyl ester

22399-98-0

H.-R. Schulten et al. / Geoderma 69 (1996) 105-118

Peak No.

MW Purity CAS

Molecular formula C

H

O

15d

482

30

58

4

16d

496

31

60

4

17d

510

32

62

4

7c 9c 6c

144 964 146 793 124 805

624-49-7 106-65-0 55683-21-1

6 6 8

8 10 12

4 4 1

8c 4c lc

136 929 134 942 166 950

93-58-3 637-69-4 5368-81-0

8 9 9

8 10 10

2 1 3

10c

171 952

827-16-7

6

9

3

llc 12c 5c

202 854 216 848 196 853

1732-09-8 1732-10-1 2150-38-1

10 11 10

18 20 12

4 4 4

2c

192 920

832-01-9

11

12

3

111

Compounds N Octacosanedioic acid, dimethyl ester Nonacosanedioic acid, dimethyl ester Tricontanedioic acid, dimethyl ester

3

2-Butenedioicacid, dimethylester Butanedioic acid, dimethyl ester 2-Cyclopenten-l-one 3,4,5trimethyl ester Benzoic acid, dimethyl ester l-Ethenyl-4-methoxybenzene 3-Methoxy-benzoic acid, methyl ester 1,3,5-Triazine -2,4,6-trione, 1,3,5, trirnethyl ester Octanedioic acid, dimethyl ester Nonanedioicacid,dimethylester 3,4-Dimethoxybenzoic acid, methyl ester 3-(4-Methoxyphenyl)-2propenoic acid, methylester

MW=molecular weight; Purity=agreement between library and measured mass spectrum (100% agreement = 1000). CAS = Chemical Abstracts Service, Registry numbers (as far as available)

lower mass range (m/z 57, 59, 70, 83, 85) are assigned to N-containing compounds, including amino acids (m/z 57, 70) (Sorge et al., 1993). In the higher mass range, the intense signals at m/z 266, 280, 294, 308 and 322 were indicative of n - C l 9 to n-C23 alkenes while those at m/z 340, 354, 368, 382, 396, 410, 424, 438,452, 466, 480 are due to n-C22 to n - C 3 2 monocarboxylic acids. The relatively weak, but distinct signals at m/z 648, 676, 704, 732 and 760 are assignable to homologous n-C44.48.5o.52alkyl-monoesters.

3.3. Methylationpyrolysis-gas chromatography/mass spectrometry. Analyses of the methylated products by Py-GC/MS provided complementary information on chemical composition. The abundance of fatty acids in both complexes was indicated by the detection of methyl esters of aliphatic even- (n-C12_34) and odd-numbered (n-C3, n-C15_33 ) monocarboxylic acids together with dimethyl esters of dicarboxylic acids (nC4,8,9, n-C17_30 ) and methyl esters of aromatic carboxylic acids (Table 2). Some of these compounds have not been documented by, or are not available from, commercial libraries. Other compounds, not seen in the Py-FI mass spectra, were detectable by methylation and Py-GC/MS. These were short-chain dicarboxylic acids, derivatives of aromatic carboxylic acids and, unexpectedly, of a triazine derivative (Table 2).

112

H.-R. Schulten et al. / Geoderma 69 (1996) 105-118

438

4.0 3.5 3.0

41o I 192

2.0

1.5

i

159

3!5

¢/) ¢.

.e.

,

100

/

~1 .................

200

t 494

354 ]il

!~6i1[ I

466

J

382]

!, ,

a)

I .........

300

400

r ']1 , i I ;' '1 ~~tI1 :'l~ ....i i

500

600

'1

700

800

900

¢..

.9 Q) ._> n-

b)

2.0

1.5

201

438 i 466

131

1.0

= 0.5

100

410 1

192

I L k49'

IjijlLi 20t

300

400 m/z

500

600 >

700

800

900

Fig. 2. Pyrolysis-fieldionizationmass spectra (a) complexl, and I b) complexII afterthermal methylation.The spectra were integratedfor the temperaturerange 373-573 K. Methylation with Py-GC/MS confirmed the presence of n-C22 to n-C32 monocarboxylic acids as Py-FIMS had indicated (Fig. 1). Moreover, the interpretation of Py-GC/MS data was supported by Py-FIMS of methylated samples, shown in Fig. 2. Here, the prominent signals at m / z 354, 382, 410, 438, 466 and 494 may be identified with methyl esters of nC22,24,26,28,3o,32 monocarboxylic acids, respectively. These compounds also gave intense signals in the spectra of non-methylated samples (Fig. I ). Methylated species with m / z 166, 171, 192, 196 in the Py-FI mass spectra (Fig. 2) were also detected by Py-GC/MS (Table 2). 3.4. Interlayer organic matter

Fig. 3 shows the difference Py-FI mass spectrum obtained by subtracting the spectrum of complex II from that of complex I. Positive signals denote that the corresponding compounds were more abundant in complex I than in complex II. These signals were due to phenols, lignin monomers ( m / z 94, 108, 210), lignin dimers ( m / z 262, 286, 300, 312, 314, 326, 328, 330, 342, 356), n-C27_31 carboxylic acids ( m / z 410, 424, 438,452, 460), nC46.48.5o,52alkyl-monoesters ( 676,704, 732,760), and other unidentified species (474, 496, 503 ). Presumably these compounds were preferentially removed (oxidized) by treatment with H202. This suggestion is consistent with complex I being approximately 1000 years younger in radiocarbon age than complex II since the former is likely to be contaminated by modem carbon (Theng et al., 1992). The l ignin-derived compounds, with which complex I is enriched, probably originate from existing plant roots and partially decomposed plant remains in the soil horizon.

113

H.-R. Schuhen et al. / Geoderma 69 (1996) 105-118 0.4

474

I--

4.~ 14~

o~ 0.3 G) 0 eI--

0.1

'- -0.3

.~

c

~

0.2

_0.4j 20

~

94 i

146

210

~

424

~,~1.L a,z

LP'x~I J[

704 6e6

~' ~

200

~ ~3

slol~__ 414L,.I II

43

100

/

300 m/z

~

405

400

500

600 >

I 732

700

760

652

800

900

Fig. 3. Difference Py-FI mass spectrum obtained by subtracting the spectrum of complex II from that of complex I. Negative intensity signals indicate relative enrichments of compounds in complex II (after treatment with H202 ).

Conversely, complex II was relatively enriched in compounds that gave a negative signal. These may be identified with N-containing compounds (m/z 59, 71,85), n-C22_26 carboxylic acids (m/z 340, 354, 368, 396 and 410), aromatic esters (m/z 376, 404,432,460), another homologous series of compounds (m/z 420, 434, 448), and many high-molecular weight species (m/z 495-750). That the//-C22_26 carboxylic acids were apparently more resistant to H20 2 oxidation than their longer chain (n-C27.31) counterparts was a little surprising. Fig. 4 shows the abundance of methyl esters of monocarboxylic and dicarboxylic acids relative to the most intense signal in the Py-GC mass spectra of methylated samples. For complex I the signal in question was due to the methyl ester n-C26 monocarboxylic acid (m/z 410), and for complex II to that of n-C22 monocarboxylic acid (m/z 354). In other words, these acids were most abundant among species with even number C atoms (Fig. 4a). For the odd number series, carboxylic acids with less than 25 C atoms predominated (Fig. 4b). Although their frequency distribution was asymmetric, the dicarboxylic acids in complex II also tended to be of shorter chain than those in complex I (Fig. 4c). The Py-GC/MS data of the methylated samples were in overall agreement with those obtained by Py-FIMS, confirming that there were differences in chain length distribution between externally adsorbed and intercalated OM. The relative abundance of shorter chain fatty acids in complex II could partly arise from the oxidation of hydroxyl groups to carboxyl which were then decarboxylated. The presence of hydroxyl groups in the (polymethylene) structure has been indicated by J3C-CP/MAS NMR spectroscopy (Theng et al., 1986). The ability of clay to catalyze fatty acid decarboxylation is also well documented (Almon and Johns, 1976; Theng, 1982). X-ray diffractometry showed that the interlayer OM was extensively degraded when complex II was heated from 473 to 573 K in air (Theng et al., 1992). On this evidence, most of the intercalated material would be expected to volatilize in this temperature range. Fig. 5 shows the Py-FI mass spectrum of complex II recorded at 473-573 K. Signals in the low mass range of the spectrum (m/z 57, 70, 71, 83, 85) may again be assigned to N-containing compounds, and those in the range from m/z 300 to 550 to n-C2o to n-C37 monocarboxylic acids. The presence of n-C2z to n-C26 acids was also detected by Py-FIMS of the methylated samples as well as by Py-GC/MS. The intense peak at m/z 396 is probably due to a combination of n-C22 carboxylic acid and/3-sitosterol. In addition, n -

H.-R. Schulten et al. / Geoderma 69 (1996) 105-118

114

100 9O 80 70 60 5O 40 30 2O 10 1:2 14 100 r - 90! 80

16

18

20

22

24

26

28

30

32

b)

J •Compiex;" 7 , [ ] Complex II

;oOj

34

~

_. 1[

50. 30

2o!

i

!

10

L,LI 15

17

19 21

23

25

27

29

31

33

100

I,O~q , ~ c ° ~ P 80

c)

lex' II 70~ ' ~ Complex II

60 5O 4O 3O 2O 10

/

17 18 19 20 21 22 23 24 25 26 27 28 29 30

C - number

>

Fig. 4. Relative abundance of carboxylic acids in complexes I and II. (a) Methyl esters of even-numbered n-C~2_ 34 monocarboxylic acids; (b) methyl esters of odd-numbered rt-C]5_33 monocarboxylic acids; (c) dimethyl esters of rl-ClT_30 dicarboxylic acids.

C~w45 alkanes, volatilized between 473 and 573 K, gave rise to signals from m/z 224 to 616. Signals at m/z 374, 388,402, 416, 430, 444, 458, 472, 486, 500, 514, 528, 542, 556, 570, 584 and 598 possibly originated from n-C22 to n-C38 diols. A homologous series of aromatic esters gave signals at m/z 348, 376, 390, 404, 418,432, 446, 460, 474, 488, 502, 516, 530, 544 (Hempfling and Schulten, 1989). Alkylmonoesters produced signals at m/z 620 (C42), 648 (C44), 676 (C46), 704 (C48),732 (C5o), 760 (Csz), 788 (C54) Signals at I-

5!

43

3~ e--

c

2

~

,0

03

58 185

~ .:1

I1191J

"~ I'Y

L,

20

. . . . . . L ,!a~a . . . . .

100

340

i

200

•a

=

300 m/z

424 10 448 I ~

I

400

~~

7134

"1L

500

.-,

676 I 732 ; ,,, t, i= ~,~.~ ~ i

600 >

700

800

,

T]

900

Fig. 5. Pyrolysis-field ionization mass spectrum of complex II (H202-treated) integrated for the temperature range 473 to 573 K.

H.-R. Schulten et al. / Geoderma 69 (1996) 105-118

115

m/z 48, 376, 390, 404, 418, 432, 446, 460, 474, 488, 502, 516, 530 and 544 are due to yet unidentified aliphatic compounds. Apart from the presence of aromatic esters among the pyrolysis products, the data confirm that the interlayer OM is highly aliphatic in composition and structure. The apparent absence from the 13C NMR spectrum of carbon in aromatic structures (Theng et al., 1986, 1992) might be due to interference from paramagnetic iron compounds within the clay lattice.

4. General discussion

Solid state ~3C CP/MAS NMR spectroscopy has previously indicated that the OM in the clay fraction of the Maungatua Spodosol is made up of a substituted polymethylene chain (Theng et al., 1986). Py-FIMS and pyrolysis methylation with GC/MS have now shown unequivocally that this material is essentially aliphatic in composition. In addition, the polymethylene structures can largely be identified with long-chain fatty acids. Although clay-associated OM in soils is generally more aliphatic than aromatic (Oades et al., 1987, Golchin et al., 1994), the degree of aliphaticity of the Maungatua sample is uncommonly high. The mass spectral patterns ( "fingerprints" ) are also exceptional in that signals from carbohydrates, phenols, lignin monomers, and lignin dimers are much fewer and weaker than would be expected from clay--organic complexes of soil origin (Schulten and Leinweber, 1991 ; Leinweber and Schulten, 1992; Schulten et al., 1994). Evidence is accumulating that the aliphatic constituents of OM in soil and sediment derive from natural waxes and aliphatic biopolymers present in protective envelopes of vascular plants (Rullkrtter and Michaelis, 1990). Closely related similarly polymethylene compounds have been detected in soil OM (humic substances), sedimentary OM (kerogen), as well as in cuticles and barks of several angiosperms (Nip et al., 1986; Schulten et al., 1986; Tegelaar et al., 1989a, b; Schulten, 1994). Because of their intrinsic biostability, such compounds are selectively preserved in the environment. In line with this evidence, we propose that the OM in the clay fraction of the Maungatua soil was mainly derived from epicuticular waxes of tussock grass (Chionochloa), the dominant vegetation of the area. That waxy compounds are present in the epicuticles of Chionochloa leaves has been reported by Connor and Purdie (1981). We further presume that these compounds moved down from the litter layer rather than formed in situ as a result of microbial activity. Such a process has been suggested by Sorge et al. (1994) to account for the enrichment of long-chain aliphatics of the clay fraction from the Bh horizon of a Canadian Spodosol. These compounds were extractable with hydrocarbon solvents (Schulten and Schnitzer, 1990), and their mass spectral pattern resembled that of the Maungatua sample (Fig. 1). How polymethylene compounds, which are sparingly soluble in water, are translocated within the soil profile is open to question. The process may involve association with watersoluble organic macrospecies, such as fulvic acids, as Theng et al. (1992) have suggested. On coming into contact, and interacting, with clay at a certain depth (here at the A21g horizon), these compounds are immobilized and preserved. Since the interlayer OM has been preserved for about 7,000 years, the other question that arises is whether the Maungatua Range was covered with tussock grass at that time. The

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dominant vegetation could then well have been silver beech as this tree species was of widespread occurrence in the region. However, pollen analysis of a nearby site indicates that silver beech forest would have been rare 7,000 years ago, especially at the altitude of the sampling site (370 m). Here the vegetation would likely to have been tussock grass, and a kind of subalpine scrub. If that was the case, the main plant species contributing to the waxy component in the clay-organic complexes would have consisted of Chionochloa, Dacrophyllum, Phyllocladus alpinus, and Daco'dium bidwilii (M. McGlone, pers. comm u n . ).

5. Conclusions The combination of pyrolysis with FIMS and GC/MS is a powerful tool for the qualitative and quantitative analysis of OM in particle-size fractions and whole soils. Py-MS is not influenced by the presence of organic radicals and paramagnetic metal cations in the sample, and hence is more versatile than ~3CCP/MAS NMR spectroscopy. Besides giving a detailed picture of OM quality in terms of molecular-chemical structures, the mass spectrum of selectively preserved OM in soil can serve as a finger-print of past vegetation (Hempfling and Schulten, 1989). Pyrolysis or thermally induced methylation, in conjunction with Py-FIMS and Py-GC/ MS, has not been previously used to characterize OM in soil fractions. Its application here has enabled the detection of methyl esters of aliphatic mono- and dicarboxylic acids and benzenecarboxylic acids, methoxy benzenes (from phenols) and benzenecarboxylic acid methylesters (from phenolic acids). The results of pyrolysis methylation and mass spectrometry accord with previous J3C NMR spectroscopic analysis showing that the OM in the clay fraction of the Maungatua soil is predominantly aliphatic. Here the OM is partly attached to external clay surfaces, and partly intercalated into clay crystals. The aliphatic constituents of the external and interlayer OM is surprisingly similar in composition. However, the intercalated material is depleted in lignin-derived aromatic compounds. The occurrence of aromatic structures was not indicated by13C NMR spectroscopy presumably because of interference from paramagnetic Fe3+ species.

Acknowledgements Financial support for this work was granted by the Deutsche Forschungsgemeinschafl, Bonn-Bad Godesberg, Germany (projects Schu 416/3; Schu 416/18-2). Technical assistance by C. Dornieden and M. Uchtmann (ISPA Vechta), and R. Miiller (Fresenius Institute, Taunusstein) is gratefully acknowledged. B.K.G. Theng is grateful to the Foundation of Research, Science and Technology, New Zealand, for funding (contract CO9314), to Drs. M. McGlone, W. Lee, and K.R. Tate for helpful discussions, and to Miss J. Williams for typing assistance.

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