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a Dep. of Soil and Water Sciences, Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew Univ. of Jerusalem, P.O. Box 12, Rehovot 76100, Israel
b Environmental Science Graduate Program, The Ohio State Univ., 100 W 18th Ave., Columbus, OH 43210
* Corresponding author (chefetz{at}agri.huji.ac.il)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CPMAS, cross polarization magic angle spinning • DAME, dicarboxylic acid dimethyl esters • FAME, fatty acid methyl esters • GC, gas chromatography • G, guaiacyl structures HA, humic acid • HS, humic substances • MS, mass spectroscopy • NMR, nuclear magnetic resonance • OM, organic matter • P, p-hydroxyphenyl structure • Py-GC/MS, pyrolysis-GS/MS • SOM, soil organic matter • S, syringyl type structure • TMAH, tetramethylammonium hydroxide • TMAH-GC/MS, TMAH thermochemolysis-GC/MS
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Chemical characterization of soil particle-size fractions has been suggested as a useful approach to analyzing pools of SOM and humification processes in soils (Amelung et al., 1999; Piccolo et al., 1997; Christensen, 1992; Piccolo and Mbagwu, 1990). However, little analytical information is available about the nature of SOM in different particle-size fractions and its relation to the humification process in soils.
Thermochemolysis in the presence of TMAH was recently applied to analyze bulk agricultural soils (Chefetz et al., 2000a). This technique was shown to provide detailed structural information on building blocks of natural macromolecules present in soils without the application of any extraction procedures. The TMAH-thermochemolysis technique is a highly selective method of cleaving ester and certain ether linkages in macromolecular OM (Hatcher and Minard, 1996; Hatcher et al., 1995). The TMAH technique is a chemolytic procedure that hydrolyzes and methylates ester and ether linkages, and assists in the depolymerization and methylation of lignin (Filley et al., 1999). This technique has been used to characterize HAs (Martin et al., 1995), lignin, peat, and wood (Clifford et al., 1995; McKinney and Hatcher, 1996), cuticular plant material (del Rio and Hatcher, 1998; McKinney et al., 1996), composted OM (Chefetz et al., 2000b), and HSs (Fabbri et al., 1996; Hatcher and Clifford, 1994). Tetramethylammonium hydroxide thermochemolysis overcomes the limitations of conventional pyrolysis products because it assists in converting polar products to less polar derivatives that are more amenable to chromatographic separation. Moreover, this procedure avoids decarboxylation and produces the methyl esters of carboxylic acids and methyl ethers of hydroxyl groups, rendering many of the polar products volatile enough for GC analysis.
In this study, we employed advanced analytical techniques such as cross polarization magic angle spinning (CPMAS) 13C NMR, TMAH thermochemolysis-GC/MS (TMAH-GC/MS), and Py-GC/MS to analyze bulk SOM and corresponding HAs obtained from size fractions of agricultural and calcareous soil samples. The objective of this work was to apply these techniques to investigate the humification processes in a particle-size fraction from an agricultural soil.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Carbon, Nitrogen, and Total Saccharide Determination, and Humic Acid Extraction
Total saccharides were extracted using acid hydrolysis and determined according to the method described by Ashwell (1966). Before HAs were extracted, CaCO3 was removed from the soil samples with 1 M HCl followed by washing with distilled water and freeze-drying. Humic acids were extracted from each soil-size fraction using the procedure recommended by the International Humic Substances Society (Swift, 1996). In short, the acid-washed soil aggregate-size fractions were extracted with 0.1 M NaOH. The alkaline supernatant was acidified to pH 2 with 6 M HCl to obtain the HA fraction, which was then purified with an HF-HCl mixture. Carbon, H, and N were measured with an 1108 Elemental Analyzer (Fisons Instruments, Milan, Italy).
Carbon-13 Nuclear Magnetic Resonance Spectroscopy
Solid-state 13C NMR spectra with CPMAS were obtained for the HA samples using a Chemagnetics M-100 NMR spectrometer (Chemagnetics, Inc., Fort Collins, CO. The spectrometer was operated at a 1H frequency of 100 MHz and a 13C frequency of 25 MHz. Pertinent experimental parameters were as follows: a contact time of 1 ms; a recycle delay time of 0.8 s, a sweep width of 14 kHz (562.5 ppm), and a line-broadening of 30 Hz. Samples were spun at a frequency of 3.5 kHz at the magic angle (54.7° to the magnetic field). Contact time of 1 ms was determined to be optimum for all types of C functionalities.
Tetramethylammonium Hydroxide Thermochemolysis-GC/MS
A total of 1 to 2 mg of organic C is required for the TMAH-thermochemolysis analyses. Therefore, 50 to 100 mg of oven dried soil samples; and 1 to 2 mg of HA samples were weighed and placed in glass tubes. A 200-mL aliquot of TMAH (25% in methanol; Aldrich, Milwaukee, WI) was added to tubes containing soil or HA samples and gently mixed before the methanol was evaporated under a stream of N2. Then, the tubes were sealed under vacuum and subsequently placed in an oven at 250°C for 30 min. After cooling, the tubes were cracked open, an internal standard (1951 ng of n-eicosane) was added and the inside surfaces of the tubes were extracted (three times) using ethyl acetate. The combined extracts were reduced to 
25 µL under a stream of N2. Gas chromatographic analyses were performed with a Hewlett-Packard 6890 GC (Hewlett-Packard, Palo Alto, CA) equipped with a 15-m fused silica capillary column coated with chemically bound DB-5 (0.25-mm i.d., film thickness 0.1 mm; Supelco, Bellefonte, PA). Samples (1 µL) were injected using a Hewlett-Packard 7683 series autoinjector (Hewlett-Packard, Palo Alto, CA) with a split ratio of 5 and a front inlet temperature of 310°C. Helium was used as the carrier gas at a flow rate of 1 mL min-1; electronic flow control was set for constant flow. The GC oven temperature was programmed from 40 to 300°C at rate of 8°C min-1. The GC was directly coupled to a Pegasus II (Leco Corporation, St. Joseph, MI) time-of-flight mass spectrometer by a deactivated fused silica transfer-line heated to 300°C. Mass spectra from 33 to 700 m/z were accumulated at a rate of 9 scans s-1. Peaks were assigned by comparison with the library of the National Institute of Standards and Technology (version 1.6, National Institute of Standards and Technology, Gaithersburg, MD), by analysis of fragmentation and by comparison with standards.
Pyrolysis-Gas Chromatography/Mass Spectroscopy
Pyrolysis-GC/MS was performed with a Carlo Erba Mega 500 series gas chromatograph (Carlo Erba, Milan, Italy) operating in a split mode (20:1), equipped with a CDS analytical pyroprobe-2000 controller, a CDS AS-2500 pyrolysis autosampler, and a 30-m fused silica capillary column coated with chemically bound Rtx-50 (0.25-mm i.d., film thickness 0.25 µm). The interface temperature was held at 273°C. Helium was used as a carrier gas with flow rates of 2 mL min-1 through the column and 20 mL min-1 through the split at a head pressure of 65 kPa. The following oven temperature program was used: initial temperature 40°C (held for 1 min), heating rate 8°C min-1 to a final temperature of 320°C (held for 15 min). The gas chromatograph was connected to a Kratos MS-25 RFA mass spectrometer (Kratos Analytical Instruments, Manchester, UK) operating at an electron impact potential of 8.011 x 10-18 J (50 eV) with a mass range of 40 to 510 m/z and a cycle time of 0.7 s (electron beam current 120 µA, source temperature 250°C).
Humic acid samples (0.3 mg) were weighed and transferred on top of a minimal amount of silica wool placed on top of a solid fused silica spacer inside a quartz tube. The tube was dropped by the pyrolysis autosampler into the pyrolysis chamber, which was flushed with He gas prior to pyrolysis at 70 mL min-1 for a period of 6 s. After the chamber was automatically connected with the GC column by a six-port valve, the pressure was allowed to equilibrate for 6 s. The pyrolysis chamber was subsequently heated to 615°C at a rate of 5°C ms-1 and was held at this temperature for 15 s. After pyrolysis, the chamber was flushed with the carrier gas for 21 s. Data acquisition and analysis were performed using a Dart/Kratos Mach 3 data system (Kratos Analytical Instruments, Manchester, UK). Pyrolysis products were identified based on their mass spectra and GC retention times (van der Kaaden et al., 1984; Pouwels et al., 1989).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The HA analytical data suggest different OM degradation and humification mechanisms with aggregate-size fraction. However the data are limited with respect to the chemical composition and humification pathways in soils. Therefore, the chemical properties of the SOM and HAs in two aggregate-size fractions (>250 and <2 µm) were analyzed with advanced analytical techniques to better understand the overall humification process in agricultural soils.
Chromatograms of the TMAH-GC/MS products released from the >250 and <2 µm soil aggregate-size fractions are presented in Fig. 1 . Table 2 lists the main compounds identified in the chromatograms, which can be classified into the following major groups: lignin-derived and nonlignin-derived aromatic compounds, heterocyclic N, FAMEs, and DAMEs. In addition to these, different compounds arising from proteins and polysaccharides were also detected.
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Lignin-derivatives of benzenecarboxylic acids (G6 and S6), phenylacetic acid (P24), 4-methoxyphenylpropenoic acid (P18), methyl benzoate, methyl phenylpropanoate, and methyl phenylacetate were detected in the TMAH-GC/MS chromatograms. Their detection highlights the advantage of using the TMAH technique over conventional pyrolysis; decarboxylation is avoided and carboxyl groups are protected by methylation.
The ratio of acid-containing derivatives to aldehyde-containing derivatives (acid/aldehyde ratio) for lignin derivatives is commonly used to assess the degradation stage of the lignin. High acid/aldehyde ratios represent a developed stage of lignin side-chain oxidation by microorganisms. Hatcher et al. (1995) have suggested that benzenecarboxylic acids in TMAH products are predominantly derived from lignin units where the 
C of the side chain has been oxidized to a carboxyl group. The soil samples studied here yielded only one major phenolic aldehyde (P4), whereas G4 was detected only in the >250-µm aggregate-size fraction and S4 was not detected at all. With decreasing aggregate size, the relative intensity of the P4 peak decreased compared with its corresponding acid (P6), resulting in an increase of the acid/aldehyde ratio from 1.25 to 1.95. These changes suggest further oxidation of lignin units with decreasing aggregate-size fraction.
Two unmethylated lignin-derived compounds (2-methoxyphenol and 2,6-dimethoxyphenol, peak 11 and 27, respectively) were detected in the TMAH-GC/MS chromatograms of the <2- and >250-µm soil samples, but not in the corresponding HA samples. The presence of these compounds suggests incomplete methylation that may have resulted from interaction between the soil mineral matter (e.g., clay and oxides) and the TMAH or TMAH thermochemolysis products. Therefore, these effects should be further investigated before applying this technique as a standard method of bulk soil characterization.
The main difference between the TMAH-GC/MS chromatograms of the soil fractions is that the >250-µm chromatogram reveals more lignin-derived compounds and their relative intensities are more pronounced than in the <2-µm chromatogram. Similar observations (i.e., decreasing content of lignin and increasing acid/aldehyde ratio) have been reported with soil depth and with decreasing soil particle size for a savanna soil (Guggenberger et al., 1994). This general trend is in agreement with the 13C NMR data reported by Tarchitzky et al. (2000), suggesting that the SOM in the >250-µm aggregate-size fraction consisted predominantly of lignin- derived structures. During humification, these structures are further transformed to aromatic humic-like structures, where lignin side chains are oxidized and methoxy groups removed. Thus some of the characteristic lignin peaks are lost. Our data indicate the fact that grass-type lignin plays an important role in the formation of SOM in agricultural soils.
Aromatic NonLignin-Derived Compounds
Aromatic compounds not directly related to lignin structures were also identified in the soil chromatograms. Among these, the presence of benzaldehyde, methoxymethylbenzene, 1-cyclohexyl ethanone, phenyl-methylbenzoate, 1,4-dimethoxybenzene, phenylmethylethanoate, phenyl-methylpropanoate, 1,2,4-trimethoxybenzene, and 1,3,5-trimethoxybenzene is worth noting. The last compound has been previously reported as a major TMAH-thermochemolysis product of cutan, a resistant biopolymer found in the cuticles of some plants (McKinney et al., 1996). The origin of other aromatic compounds is unknown but could result from a progressive oxidation stage of lignin units.
Lipids
Under TMAH-thermochemolysis conditions, soil samples from both size fractions yielded saturated, unsaturated, and branched FAMEs of varying C-chain length (from C8 to C32), with the most intense peaks being from C16 and C18 FAMEs. In addition, long-chain DAMEs (C20 to C28) and C9 DAME were also detected. These fatty acids were suggested to originate from the plant waxes, cutan, and cutin. A strong even-over-odd predominance was observed for this series. Challinor (1991) reported that transesterification of triglycerides and other lipids can form FAMES under conditions of TMAH thermochemolysis. The presence of even-numbered long-chain fatty acids and dicarboxylic acids in soils may be primarily because of an input of aboveground plant aliphatic biopolymers such as cutin and cutan (del Rio et al., 1998; McKinney et al., 1996), and plant root aliphatic biopolymers such as suberin (Nierop, 1998). Short-chain fatty acids and dicarboxylic acids (C4-C9) can arise from microbial activity and odd-C-numbered and branched-chain fatty acids are commonly used as bacterial biomarkers. A series of FAMEs was reported as major compounds released from HS (del Rio and Hatcher, 1998). Nierop (1998) reported that pyrolysis of soil samples reveals alkane and alkene pairs, which can be attributed to suberin present in roots. These authors suggested that these compounds arise from higher plants and are chemically bound through ester bonds to humic macromolecules. Several studies have shown that aliphatic biopolymers can be selectively preserved in soils with little or no alteration (Almendros et al., 1996); and as humification proceeds, these aliphatic moieties tend to accumulate (Zech et al., 1997). The relative intensity of the long-chain FAMEs and DAMEs increased with decreasing aggregate size, highlighting the importance of aliphatic biopolymers in the bulk structure of SOM.
Nitrogen-Containing Compounds
Tetramethylammonium hydroxide thermochemolysis of the studied soil samples yielded heterocyclic N products such as pyrroles, imidazoles, pyrazoles, pyrimidines, pyrazines, and indoles (Table 2). A similar series of heterocyclic N structures was identified in Py-GC/MS chromatograms of several soil samples (Schulten and Schnitzer, 1998). In contrast, 15N NMR spectroscopic studies of soils and HAs have revealed that most of the organic N (80–85%) prevails in SOM as amide and peptide structures (Knicker and Hatcher, 1997; Kögel-Knabner, 1997). Some of the heterocyclic N compounds identified after pyrolysis or TMAH thermochemolysis of soil samples have been thought to originate from biological precursors such as plant and microbial residues. On the other hand, it has been reported that heterocyclic N compounds can form under pyrolysis conditions (Hendricker and Voorhees, 1998; Chiavari and Galletti, 1992). Thus, the origins of some the heterocyclic N compounds identified in the TMAH-GC/MS chromatograms cannot be specified with certainty. The most intense peaks in the bulk soil chromatograms were N-containing compounds such as 1-methyl-4-nitro-1H-imidazole, 4,6-dimethyl-3,5-dioxo-2,3,4,5 tetrahydrotriazine, and 4-methoxy-6-methyl-2-pyrimidinamine. Specific protein markers (amino acids and the heterocyclic N compounds) were identified in both TMAH-GC/MS chromatograms (Fig. 1). These compounds are 1-methyl, pyrrolidine-2,5-dione, 3-phenyl-methylpropanoate, 3-phenyl-methyl-(2-propenoate), and methylated amino acids (Phe, hydroxyproline, Lys, norleucine, Val, Trp, and Tyr). It is likely that the TMAH thermochemolysis cleaves peptide bonds in proteinaceous materials and methylates the released amino acids.
Humic Acids Analyses
The term SOM encompasses all OM fractions present in soil, including crop residues (litter), plant residues in varying stages of decomposition, microbial biomass, dissolved OM, and stable humus. Above, we examined structural properties of the bulk SOM fractions and discerned major decomposition and humification processes. However, a more detailed view of humification in soil can be accomplished by studying the structure of HAs in different aggregate-size fractions.
Solid-state 13C-NMR spectra of the HAs extracted from different aggregate-size fractions are presented in Fig. 2 . The CPMAS 13C-NMR spectra of all HAs are similar and exhibited peaks at 32 ppm (paraffinic carbons), 56 ppm (methoxy carbons), 72 ppm (alkyl-O carbons), a shoulder at 105 ppm (anomeric C of carbohydrates), 130 ppm (C-substituted aromatic carbons), a small peak at 152 ppm (O-substituted aromatic carbons) and 172 ppm (carboxyl and amide carbons; Kögel-Knabner, 1997; Zech et al., 1997). The NMR spectra of the HAs displayed high levels of aromatic carbons (aromaticity of 35–42%) and paraffinic carbons, but low levels of lignin-type carbons (peaks at 56 and 152 ppm). However, the main change between the spectra was a relative reduction of the 56 and 152 ppm peaks (assigned to lignin structures) with decreasing of the size fraction. This trend is in agreement with the TMAH-thermochemolysis data for the soil aggregate fractions. Comparison between the HA 13C NMR spectra (Fig. 2) and the spectra of the corresponding bulk soil (Tarchitzky et al., 2000) suggests that the HAs contain fewer carbohydrates than the SOM from which they originated.
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The chromatograms of the TMAH-GC/MS products released from the >250- and <2-µm HA samples are presented in Fig. 3 , and a list of the major peaks identified is provided in Table 2. The main classes of products released by the TMAH thermochemolysis procedure were similar to those released from the soil fractions (Fig. 1). However, the major peaks in the chromatograms of HAs were short-chain DAMEs (C4 and C5, peaks 36 and 39, respectively), 1-methyl, pyrrolidine-2,5-dione, a series of lignin-derived compounds and a homologous series of long-chain FAMEs (C12 to C32).
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Similar trends were observed in the Py-GC/MS chromatograms (Fig. 4 ; peak identification is listed in Table 3). Compared with the TMAH-GC/MS data, the Py-GC/MS data exhibited relatively lower levels of lignin-derived peaks for HAs and did not contain any benzenecarboxylic acid compounds. The intensity of the phenol peak (LG1, the major lignin-derived peak in the Py-GC/MS chromatograms) decreased slightly with decreasing aggregate-size-fraction as compared with decreases in the other lignin-derived peaks. Therefore, it is concluded that phenol was generated not only from lignin but also from melanoidin-type compounds and amino acids (Tyr) under pyrolytic conditions (van Heemst et al., 1999; Peulve et al., 1996). The total level of lignin-derived compounds obtained from the >250-µm HA Py-GC/MS chromatogram was 15.6%, while this group of compounds represented only 5.2% of the total chromatogram area in the <2-µm HA Py-GC/MS chromatogram. The pyrolysis data support the TMAH-GC/MS data: both suggest that lignin units can be incorporated into HA structures, but the lignin signal is weaker in the smaller aggregate-size fraction. As the size fraction decreases, the lignin structures are subject to further side-chain oxidation, resulting in a relative increase of aromatic acids and a loss of the characteristic lignin peaks from the pyrolysates and TMAH thermochemolysis products of the HA macromolecule. This trend is suggested to present a humification process.
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Lipids
The series of long-chain DAMEs (C16 to C30) and long-chain FAMEs (C12 to C32) produced by TMAH thermochemolysis of the HAs are presented in a selective ion chromatogram (74 m/z) of the <2-µm HA in Fig. 5
. The distribution pattern shows a strong even-over-odd C-number predominance and maxima at C16 and C18. Unsaturated FAMEs C16, C18, and C20 were also detected. A similar pattern of FAMEs has been reported for peat HA (del Rio et al., 1998) and soil HAs (Grasset and Ambles, 1998). Typical bacterial activity products (C15 branched and C17 FAMEs) were present in both HA samples (Fig. 3 and 5), but with a higher predominance in the <2-µm HA sample (Fig. 3b). In addition to FAMEs, both samples yielded long-chain DAMEs and 1,3,5-trimethoxybenzene (peak no. 30; Fig. 3). The presence of these compounds has been reported previously by several authors studying TMAH thermochemolysis of higher plant cuticular materials (del Rio and Hatcher, 1998; McKinney et al., 1996). The relatively high abundance of C16 and branched C15 homologs, relative to C18, C20, and C22 FAMEs (Fig. 5), suggests a significant contribution of both aboveground cuticular material and residues from microbial activity, rather than suberin, to the HA structure. These compounds may be chemically bound (ester linkages) to the humic macromolecule. The predominance of FAMEs over lignin-derived peaks (Fig. 3) is in contrast with recent data for a pyrolysate of a NaOH-soluble soil fraction (Nierop et al., 1999). These differences could be because of the different origins of the soil samples (forest vs. agricultural soils). Fatty acids and alkanes were also present in the Py-GC/MS chromatograms (Fig. 4) at higher relative intensity in the HA extracted from the <2-µm aggregate-size fraction. The presence of this aliphatic biopolymer as a major component of the HA structure is supported by the characteristic peak at 32 ppm in the NMR spectra (Fig. 2) and is suggested to be an important fraction of the HA structure.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The data collected in this study suggest that the coarser aggregate-size fractions contain mainly fresh OM. Therefore, their corresponding HAs contain more lignin-derived units and their origin is strongly linked to plant biopolymers (lignin and cuticular materials). With decreasing aggregate size, polysaccharide compounds diminish (microbial utilization), and more oxidation products of lignin units are observed for the bulk SOM. Less significant changes were noted for the HA fractions, which probably become more similar because of the humification and decomposition processes. With decreasing aggregate size: (i) the abundance of lignin units decreases and they are present mostly as their highly oxidized subunits; and (ii) the level of fatty acids originating from microbial activity increases. Therefore, with decreasing aggregate-size, signatures of plant biopolymers in the HA decrease substantially as a result of polysaccharide utilization, lignin side-chain oxidation, and incorporation of microbially derived units into the humic macromolecule. These processes are suggested to be part of the humification process in agricultural soils.
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ACKNOWLEDGMENTS |
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Received for publication December 4, 2000.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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