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DIVISION S-2-SOIL CHEMISTRY

Characterization of Soil Organic Matter Fractions of Tundra Soils in Arctic Alaska by Carbon-13 Nuclear Magnetic Resonance Spectroscopy

^a Agricultural and Forestry Experiment Station, Univ. of Alaska, 533 E. Fireweed, Palmer, AK 99645 USA
^b Institute of Soil Science and Soil Geography, Univ. of Bayreuth, 99440 Bayreuth, Germany

Corresponding author (pnxyd{at}uaa.alaska.edu)

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Soil organic matter (SOM) was extracted with 0.1 M NaOH resulting in an extractable fraction (EF) and a nonextractable fraction (humin). The SOM of the EF was separated into six fractions: humic acid (HA), fulvic acid (FA), low-molecular-weight acids (LMA), low-molecular-weight neutrals (LMN), hydrophobic neutrals (HON) and hydrophilic neutrals (HIN). Liquid-state and solid-state ¹³C nuclear magnetic resonance (NMR) were applied to characterize the EF and the humin as well as the whole soils, respectively. The distribution of C species was calculated on the basis of relative integrated peak area. The liquid-state ¹³C NMR spectra of the extractable organic fractions demonstrated that O-alkyl C was concentrated in the low-molecular-weight fractions such as LMN and LMA, while the aromatic–unsaturated C was predominantly in the HA and FA. The solid-state ¹³C NMR of the whole soils showed that the Site 2 soil (Oe and O/A horizons) contained more O-alkyl C and less alkyl C content than the Site 1 (Oa1 and Oa2) and 3 (Cf horizon) soils; the Site 3 Cf horizon exhibited spectra similar to those of the Site 1 soil, supporting the theory that organic matter in the Cf horizon could originate in the Oa horizon and was translocated by cryoturbation. Spectra of the humin, which make up 53 to 76% of total C, exhibited trends similar to those of the whole soils. Humin appeared to possess greater alkyl C and less O-alkyl C content than whole soils; aromatic, carboxyl–carbonyl C contents seemed the same. Although cross-polarization, magic angle spinning (CPMAS) ¹³C NMR spectra intensities are nonquantitative, the spectral differences between humin and HAs and FAs indicated significant differences in their composition. The humin contained much higher alkyl C and lower aromatic–unsaturated as well as carboxyl–carbonyl C contents than the HA and FA. The results suggested that the humin fraction was different chemically from the HA and FA. It consisted of a large proportion of paraffinic carbons that may derive from algal or microbial sources.

Abbreviations: BP, before present • CPMAS, cross-polarization, magic angle spinning • EF, extractable fraction • FA, fulvic acid • HA, humic acid • HIN, hydrophilic neutrals • HON, hydrophobic neutrals • LMA, low-molecular-weight acids • LMN, low-molecular-weight neutrals • NMR, nuclear magnetic resonance • SOM, soil organic matter

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
TUNDRA ECOSYSTEMS alone contain 191 x 10¹⁵ g of soil C that represent 12% of the global soil C pool (Billings, 1987). Recent measurements of CO₂ flux in arctic Alaska indicate that tussock and wet sedge tundra are now sources of atmospheric CO₂. If the current efflux of CO₂ from the arctic tundra continues, these ecosystems could be a significant source of atmospheric CO₂ further increasing concentration and becoming a positive feedback to climate change (Oechel et al., 1993). The quantity and quality of SOM play an important role in terrestrial C cycle, which influences global climate. Since not all SOM participates equally in the transformation processes, the more bioavailable C is the more easily affected it will be by global climate change. Identifying the chemical characterization and bioavailability of SOM is imperative in studying dynamics of SOM and predicting the feedbacks of SOM to the global climate change. In our SOM study, several different approaches were used to study characteristics and bioavailability of SOM in arctic soils. The chemical characterization of SOM was determined by ¹³C NMR and pyrolysis-GC/MS techniques as well as by SOM fractionation (Ping et al., 2000). The bioavailability of the SOM was evaluated by laboratory incubation methods at optimum temperature and moisture. Microbial biomass of the SOM was also measured to compare bioavailable C. This paper summarizes the ¹³C NMR results of the organic matter study in arctic soils.

Soil organic matter is a complex series of related but different molecules with varying physical properties, chemical structures, and different functional groups. To characterize SOM, it is often removed or extracted from the soil, although some techniques do not require this process (Swift, 1996). The traditional fractionation scheme is one that extracts SOM with dilute NaOH and then separates the extract into HAs, FAs, and humin fractions on the basis of solubility. However, dilute NaOH extracts not only humic substances, but also many nonhumic materials such as LMA and saccharides (Thurman and Malcolm, 1989). Thus, what are commonly referred to as HAs and FAs are actually HA and FA fractions (Clapp et al., 1993). A modified fractionation scheme (Ping et al., 1995) extracted SOM with 0.1 M NaOH, with the extractable SOM containing soluble humified and nonhumic components fractionated into HA, FA, HON, LMA, LMN, and HIN by passing the extractable SOM through tandem columns of XAD8/XAD4 resins (Thurman and Malcolm, 1981; Ping et al., 2000). There are different ways of characterizing SOM. Many chemical, physical, and spectroscopic methods have been used to study soil humic substances in an attempt to determine the composition and general structure of the component macromolecules. However, because of the heterogeneous and polydisperse nature of humic substances, and the complexity of the inter- and intramolecular reactions, it is often difficult to interpret the results of these studies (Swift, 1996).

Nuclear magnetic resonance spectroscopy including solution and solid-state NMR spectroscopy provides a powerful tool for studying humic substances. The most common type of NMR used in study of humic substances is solid-state CPMAS-NMR. Solution NMR spectroscopy has greater resolution, and problems associated with quantification are better understood. However, the solid-state NMR is often preferred, because many humic substances are solids and the properties of the molecules may not be the same in solution as in the solid state (Wilson, 1989). Solid-state NMR does not depend on solubility of the sample, thereby allowing the examination of insoluble soil fractions as well as bulk soil samples (Schmidt et al., 1997). There are limitations to the quantitative reliability of ¹³C CPMAS-NMR spectra, mainly due to differences in the cross-polarization dynamics of carbons in different environments (Preston et al., 1997). For example, carbons cross-polarize more slowly if they are remote from H, and carbons lose magnetization more rapidly if they are close to paramagnetic centers such as Fe³⁺. For quantitatively reliable spectra in solution NMR, it is necessary to ensure complete ¹³C T₁ relaxation between pulses and to suppress the nuclear Overhauser enhancement by the inverse-gated decoupling technique (Preston and Blackwell, 1985); in solid-state NMR, proton spin lattice relaxation times in the rotating frame are measured by varying the contact time, and the degree of protonation of C atoms is determined by the dipolar dephasing technique (Gillam et al., 1987).

The objectives of this study were (i) to characterize the organic matter contained in the different extractable organic fractions, the nonextractable fraction (humin), and the whole soils by ¹³C NMR; (ii) to compare the differences among the fractions, the humin fraction, and the whole soil; and (iii) to relate the chemical composition to the biological properties of the SOM in tundra soils.

	MATERIALS AND METHODS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Description of Study Sites and Soil Analysis
Sites sampled and classification of soils are presented in Table 1, and selected properties of the soils are presented in Table 2. Site 1 soils are from the arctic coastal plain. The landform is characterized by frost polygons and thaw lakes; drainage is poor to very poor. Site 2 and 3 soils are from the arctic foothills, which are glaciated uplands, characterized by rolling hills with imperfect drainage. Soil samples were kept in a cooler in the field and frozen before shipping. Samples were thawed and homogenized by hand before extraction and fractionation. Soil subsamples were homogenized by hand and air-dried before total C and N analysis using a LECO 1000 CHN analyzer (Leco Corp., St. Joseph, MI).

View this table: [in this window] [in a new window]   Table 2. Selected properties of the whole soils and the humin fraction.

Carbon-13 Nuclear Magnetic Resonance Spectroscopy
Liquid-state ¹³C NMR spectra of freeze-dried organic fractions were obtained on a Bruker Avance DRX 500 NMR spectrometer (11.7T; Bruker, Billerica, MA) at a resonance frequency of 125.77 MHz. Samples of 150 mg were dissolved in 3 mL of 0.5 M NaOD solution. At a 1.84-s pulse delay and inverse-gated decoupling, 13800 to 17000 scans were recorded for each sample. Spectra were obtained at a temperature of 290 K, in 10-mm sample tubes, using a 0.16-s acquisition time, and a 100-Hz line-broadening factor.

Since proton decoupling causes changes in the population distribution of the C energy levels, the C nuclei in close proximity to protons are more affected than those more remote, resulting in an enhancement of the ¹³C signals (Wilson, 1987; Preston and Blackwell, 1985). To reduce this enhancement effect (Overhauser effect), spectra with essentially quantitative intensity distributions were obtained with inverse-gated decoupling, 45° pulse, and delays of 1 to 2 s (Preston and Blackwell, 1985).

Solid-state ¹³C NMR was conducted at Munich Technical University, Germany. Spectra of the whole soils and the humin fractions were obtained on a Bruker DSX 200 operating at a frequency of 50.3 MHz using zirconia rotors of 7-mm o.d. with KEL-F-caps. The CPMAS technique (Schaefer and Stejskal, 1976) was applied during magic-angle spinning of the rotor at 6.8 kHz. A contact time of 1 ms and a ramped 90° ¹H-pulse width of 4.7 µs were used for all spectra to circumvent mismatch of the Hartmann-Hahn match (Peerson, 1993). The ¹³C-chemical shifts were calibrated to tetramethylsilane (0 ppm). Between 10000 and 400000 scans were accumulated using a pulse delay of 400 ms (Fründ et al., 1989; Knicker and Lüdemann, 1995). Prior to Fourier transformation, a line broadening between 20 and 75 Hz was applied. Relative C distribution was determined by integration routine supplied with the instrument software.

In general, the ¹³C NMR spectra of natural organic matter are divided into regions corresponding with specific chemical classes: unsubstituted alkyl C (alkanes, fatty acids), 0 to 35 ppm; N-alkyl C, quaternary C, 35 to 50 ppm; methoxyl C (amino acids, peptide, protein C), 50 to 60 ppm; aliphatic C-O (notably carbohydrates), 60 to 108 ppm; aromatic–unsaturated C (unsubstituted and alkyl substituted), 108 to 145 ppm; phenolics, 145 to 160 ppm; carboxyl–carbonyl C (including the carboxylate ion, COO^-, esters, ketones, aldehydes), 160 to 220 ppm (Kaiser et al., 1997; Stevenson, 1994).

	RESULTS AND DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Liquid-State Carbon-13 Nuclear Magnetic Resonance Spectroscopy
Extractable Organic Fractions
The liquid-state ¹³C NMR spectra of the distinct extractable organic fractions are shown in Fig. 1 and 2 . An estimate of the amounts of different C types was obtained by the integration of the ¹³C NMR spectra (Table 3). In general, the spectra of disparate organic fractions isolated from the Oa1 and Oa2 horizons in Site 1 had prominent peaks in the resonance areas of alkyl C (0–50 ppm), O-alkyl C (60–108 ppm), aromatic–unsaturated C (108–145 ppm), and carboxyl C (160–190 ppm). Minor peaks were found for methoxyl C (50–60 ppm), O-aryl C (145–160 ppm), and carbonyl C (190–220 ppm). The large differences in composition among the distinct fractions were apparent from spectra in Fig. 1 and 2. The spectrum of the HA from the Oa1 horizon had a very intense broad carboxyl C peak at 177.5 ppm, two intense narrow aromatic–unsaturated C peaks at 119.4 and 129.9 ppm, one broad intense and one moderately intense O-alkyl C peaks at 71.6 and 61.7 ppm, respectively, one moderately intense methoxyl C peak at 56.6 ppm, and two moderately intense alkyl and N-alkyl C peaks at 30.8 and 38.9 ppm (Fig. 1). The spectrum of the HA from the Oa2 horizon looked very similar, except that it showed somewhat less intense alkyl C and more intense aromatic C peaks (Fig. 2). The FA from both the Oa1 and Oa2 horizons demonstrated some differences from the HA. Specifically, the FA had larger N-alkyl and smaller O-alkyl as well as alkyl C peak areas than the HA (Fig. 1 and 2). Small differences were also observed between the FA from the Oa1 and from the Oa2 horizons. A narrower N-alkyl C peak and a less intense O-alkyl C peak were apparent in the spectrum of the FA from the Oa1 horizon. Spectra of the LMA from the Oa1 and the Oa2 horizons demonstrated large differences from those of the HA and FA spectra. Two very large intense peaks at 70.1 ppm (O-alkyl C peak) and 177.4 ppm (carboxyl peak), one very weak aromatic–unsaturated C peak at 129.6 ppm, and one small N-alkyl C peak at 38.5 ppm were apparent in the spectrum of the LMA from the Oa1 horizon. The LMA spectrum of the Oa2 horizon was very similar but exhibited a less intense carboxyl C peak. A very intense broad O-alkyl C peak at 76.5 ppm dominated over alkyl C as well as aromatic–unsaturated and carboxyl C peaks in the spectrum of LMN from the Oa1 horizon, suggesting that the major component of the LMN comprised carbohydrate material (45%), which was readily decomposed by microbes. HON spectrum (Oa1 horizon) showed a very intense and a moderately intense alkyl C peaks at 25.5 and 32.3 ppm, respectively, one weak aromatic–unsaturated C peak and a very narrow intense carboxyl C peak at 179.8 ppm. The narrow carboxyl peak suggested the single type carboxyl C in the HON fraction. The very intense alkyl peak may represent an unknown organic contaminant in the HON (Malcolm et al., 1995).

View larger version (27K): [in this window] [in a new window]   Fig. 1. Liquid-state 13C nuclear magnetic resonance (NMR) spectra of HA, FA, LMA, HON, and LMN derived from alkali extracts of the Oa1 horizon in Site 1 soil. HA, humic acids (14700 scans); FA, fulvic acids (13801 scans); LMA, low-molecular-weight acids (17000 scans); HON, hydrophobic neutrals (14001 scans); LMN, low-molecular-weight neutrals (14220 scans)

View larger version (28K): [in this window] [in a new window]   Fig. 2. Liquid-state 13C nuclear magnetic resonance (NMR) spectra of HA, FA, and LMA derived from alkali extracts of the Oa2 horizon in Site 1 soil. HA, humic acids (16710 scans); FA, fulvic acids (14001 scans); LMA, low-molecular-weight acids (13760 scans)

The chemical composition of the organic materials contained in the organic fractions in the two different horizons is similar, and several trends were evident. O-alkyl C was concentrated in the low-molecular-weight fractions such as LMA and LMN, while aromatic–unsaturated C content was greater in HA and FA. Carboxyl–carbonyl C constituted the greatest contribution to HA, FA, and LMA structure, while these carbons appeared as smaller components in the neutral fractions such as LMN and HON (Table 3). Differences in the alkyl C content were small (except for HON from the Oa1) and variable. The increase of O-alkyl C content and the decrease of carboxyl–carbonyl C contents from FA, HA, LMA to LMN may indicate a decrease of the humification degree. Microbes decompose O-alkyl C (mostly carbohydrates) as energy or C sources to form amino acids, amino sugars, and polyphenols as precursor materials for the formation of humic substances. Amino acids as well as amino sugars react with polyphenols and are oxidized into quinones to form high-molecular-weight nitrogenous humates (Paul and Clark, 1996). Increase of carboxyl–carbonyl C is due to the oxidative cleavage of lignin side-chains, and other oxidizable functional groups, such as aldehydes, and primary alcohol structures during degradation (Hayes, 1991; Zech et al., 1992). The changes in aromatic–unsaturated C are due to selective preservation, since there was no net synthesis of aromatic C during the decomposition of O-alkyl C (Baldock et al., 1992).

An increase in the aromatic as well as carboxyl–carbonyl C contents and the decrease in the O-alkyl C contents from LMN, LMA to HA, and FA agreed with the incubation results (Dai et al., 2000). Dai et al. (2000) indicated that LMN had the largest CO₂ evolution, followed by HON and LMA. Humic acid and FA had the lowest CO₂ evolution. A good negative correlation between the CO₂ evolution and the content of carboxyl–carbonyl C was observed. However, no correlation of the CO₂ evolution and the percentage of the O-alkyl C was found (Dai et al., 2000). These results indicated that the carboxyl–carbonyl C content might be an indicator for the bioavailability of the organic matter in tundra soils.

Solid-State Carbon-13 Nuclear Magnetic Resonance Spectroscopy
Whole Soils
Solid-state CPMAS ¹³C NMR was used to determine the chemical composition of the whole soil and the humin fraction. The spectra of the whole soils are presented in Fig. 3 . Common characteristics in the NMR spectra for tundra soils were that alkyl and O-alkyl C peaks dominated over aromatic–unsaturated as well as carboxyl–carbonyl C peaks. The spectrum of the Site 1 Oa1 horizon consisted of a very large intense alkyl C peak at 31.7 ppm, an intense O-alkyl C peak at 72.3 ppm accompanied by a small peak for anomeric C at 103.7 ppm, a moderately intense carboxyl C peak, and two weak aromatic–unsaturated C peaks at 130.2 and 152.2 ppm. The spectrum of the Oa2 horizon differed little, except for a less intense alkyl C peak at 32.5 ppm and a more intense O-alkyl C peak at 72.2 ppm. The spectra of the Site 2 soil exhibited large differences from those of the Site 1 soil. Large, intense O-alkyl C peaks at 73.5 and 72.4 ppm dominated the spectra of Oe and O/A horizons. Peaks for aromatic and carboxyl C were small and moderately intense, respectively. Some differences between the Oe and O/A horizons were also noted. The Oe horizon spectrum exhibited a larger O-alkyl C peak and a less intense alkyl C peak than that of the O/A horizon from the same site. Aromatic–unsaturated and carboxyl C peaks of the O/A horizon also appeared somewhat more intense than those of the Oe horizon spectrum. The Site 3 Cf horizon exhibited a spectrum similar to that of the Oa1 horizon from Site 1 except that the carboxyl C and aromatic–unsaturated C peaks of the Cf horizon were smaller than those of the Oa1. A larger amount of O-alkyl and smaller amount of alkyl C in the Site 2 soil than in the Site 1 soil suggested that the organic matter in the Site 2 soil was younger and less humified than that in the Site 1 soil due to the larger amount of O-alkyl C and smaller amount of alkyl C in the Site 2 soil than in the Site 1 soil (Zech et al., 1992; Hempfling et al., 1987). Similar spectra of the Site 3 Cf horizon and the Site 1 Oa1 horizon may indicate that the organic matter in the Site 3 Cf horizon originated from the Oa horizon due to cryoturbation.

View larger version (22K): [in this window] [in a new window]   Fig. 3. Cross-polarization, magic angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectra of the whole soils from Site 1 (Oa1 and Oa2 horizons), Site 2 (Oe and O/A horizons), and Site 3 (Cf horizons) 8829 and 13499 scans for the Site 1 soil Oa1 and Oa2 horizons, respectively; 15353 and 55168 scans for the Site 2 soil Oe and O/A horizons, respectively; and 115534 scans for the Site 3 soil Cf horizon

View this table: [in this window] [in a new window]   Table 4. Carbon percentages of the whole soil and the humin fraction from solid-state CPMAS 13C NMR (Fig. 3 and 4).

View larger version (21K): [in this window] [in a new window]   Fig. 4. Cross-polarization, magic angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) spectra of the humin fractions from Site 1 soil (Oa1 and Oa2 horizons), Site 2 soil (Oe and O/A horizons), and Site 3 soil (Cf horizon). 150917 and 61612 scans for the Site 1 soil Oa1 and Oa2 horizons, respectively; 18825 and 80194 scans for the Site 2 Soil Oe and O/A horizons, respectively; and 199912 scans for the Site 3 Cf horizon

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The chemical composition of the organic matter contained in the EF revealed several trends. O-alkyl C was concentrated in the low-molecular-weight fractions such as LMA and LMN, while aromatic–unsaturated C comprised a greater contribution to HA and FA. Carboxyl–carbonyl C was greater in HA, FA, and LMA but smaller in the neutral fractions such as LMN and HON. A decrease of carboxyl–carbonyl C contents and an increase of O-alkyl C content from FA, HA, LMA to LMN were observed. Incubation results (Dai et al., 2000) also showed that the low-molecular-weight fractions were more bioreactive than the HA and FA. A good negative correlation between the carboxyl–carbonyl C contents and the CO₂ evolution was observed. These results indicated that the HA and FA were more decomposed and more humified than the low-molecular-weight fractions, and the carboxyl–carbonyl C contents may be used as an indicator for the bioavailability of the EF in tundra soils.

Chemical composition in the whole soils determined by CPMAS ¹³C NMR revealed that the organic matter in the Site 2 soil (Oe and O/A horizons) demonstrated more O-alkyl C content and less alkyl C content than that in the Site 1 (Oa1 and Oa2 horizons) and Site 3 (Cf horizon) soils, indicating that the Site 2 soil was less humified than the Site 1 and 3 soils (Zech et al., 1992; Hempfling et al., 1987). However, the chemical composition determined by CPMAS ¹³C NMR did not exhibit any relationship with the CO₂ evolution (Dai et al., 2000) of the organic materials contained in the whole soils or the humin fractions.

The chemical composition in the humin fraction revealed similar trends as those in the whole soils. However, differences between the whole soil and the humin were noted. The humins contained relatively more alkyl carbons but fewer O-alkyl carbons than the whole soils; the aromatic–unsaturated, carboxyl–carbonyl C contents appeared constant. Differences of the chemical composition between the humin and the EF were visually significant. Alkyl and O-alkyl carbons were dominant in humins, whereas HA and FA structure were dominated by aromatic–unsaturated and carboxyl–carbonyl carbons. The results suggested that the humin was different chemically from HA and FA. Humin apparently was comprised of a large proportion of paraffinic carbons that might derive from algal or microbial sources (Hatcher et al., 1985; Ping et al., 1998).

	ACKNOWLEDGMENTS

 
The authors gratefully acknowledge the field assistance of J.M. Kimble, and the lab assistance of G.J. Michaelson. Financial support was provided by the ARCSS-LAII program of the National Science Foundation, office of Polar programs.

Received for publication November 24, 1999.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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