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DIVISION S-2—SOIL CHEMISTRY

Carbon Sequestration in Clay Mineral Fractions from ¹⁴C-Labeled Plant Residues

^a Dep. of Agronomy, Iowa State University, Ames, IA 50011
^b USDA-ARS Appalachian Farming Systems Research Center, 1224 Airport Rd., Beaver, WV 25813
^c USDA-ARS National Soil Tilth Lab., 2150 Pammel Dr., Ames, IA 50011

^* Corresponding author (jgonzalez{at}afsrc.ars.usda.gov).

	ABSTRACT

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An understanding of organic C dynamics in soils is necessary to develop management options to enhance soil organic C sequestration. The objective of this research was to study the distribution of newly formed humic materials into mineralogically distinct clay-size fractions of a silt loam soil. Oats (Avena sativa L. cv. Ogle), grown under simulated no-tillage conditions, were pulse labeled with ¹⁴CO₂. After senescence, the labeled surface residue was removed and replaced with unlabeled residue and the labeled roots were allowed to decompose in the soil for 360 d. The soil clay fraction (<2 µm) was separated into coarse, medium, and fine clay size-fractions (0.2–2.0, 0.02–0.2, and <0.02 µm, respectively) by centrifugation. X-ray diffraction indicated that quartz, illite, and kaolinite were the dominant mineral phases in the coarse clay fraction while smectite was the dominant mineral phase in the fine clay fraction. The organic C content in the coarse and fine clay fractions (3.70 and 3.93%, respectively) was similar. Scintillation analysis indicated an increase in ¹⁴C specific activity in all fractions after 360 d of incubation. For both sampling times, 0 and 360 d, the highest ¹⁴C specific activity occurred in the fine clay fraction (847.2 and 1529 Bq g^-1 C, respectively), whereas the lowest ¹⁴C specific activity occurred in the coarse clay fraction (565.8 and 770.9 Bq g^-1 C, respectively). The results suggest that new humic materials are preferentially forming or accumulating on smectite surfaces.

Abbreviations: POM, particulate organic matter • SEM, scanning electron microscopy • SOM, soil organic matter • XRD, X-ray diffraction

	INTRODUCTION

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THE SOIL C cycle has a substantial influence on the ability of soils to supply air, water, and nutrients to growing plants. Furthermore, concerns about the effect of increasing concentrations of greenhouse gases in the atmosphere on global climate have increased interest in the soil C cycle with a focus on the potential for increasing soil C sequestration. The transformation of plant residues into stabilized clay-humic complexes is a key process influencing both soil quality and whether a soil is a net sink or source of C to the atmosphere. This study was undertaken to further our understanding of the role of soil clay minerals in the formation of humic substances.

Carbon distribution and its biochemical properties have been extensively studied on soil clay size fractions. The coarse clay fractions typically contain more aromatic and recalcitrant or humified organic matter, whereas fine clay fractions contain more labile or less humified organic matter (Anderson and Paul, 1984; Anderson et al., 1981; Catroux and Schnitzer, 1987; Christensen, 1986; Christensen, 1987; Christensen and Sorensen, 1985; Laird et al., 2001; Oades, 1995; Turchenek and Oades, 1979). Typically the C/N ratio decreases with particle size (Anderson et al., 1981; Catroux and Schnitzer, 1987; Turchenek and Oades, 1979). In addition, it has been observed that the amount of organic matter per unit surface area is highest in the silt fraction followed by the coarse clay fraction and lowest in the fine clay fraction (Broersa and Lavkulich, 1980; Kahle et al., 2002). The above findings are focused on particle size; however, there is little information on the relationship between mineralogy and the chemistry and bioavailability of soil organic matter (SOM).

In a study of the factors controlling C content across a range of New Zealand soils, no correlation was found between soil clay content and soil organic C (r² = 0.01) whereas a correlation was found between the soil organic C content and dissolved Al and allophone (r² = 0.57) (Percival et al., 2000). In seven soils from Germany containing from 14.9 to 21.2% clay content (dominated by illite, approximately 80 to 86%), the clay content explained 43% of the organic C content variability, moreover the cation exchange capacity + specific surface area of C-free samples explained 90% of the organic C variability (Kahle et al., 2002). In another study, it was reported that the organic C was preferentially sequestered in smectite rich sediments on continental slopes compared with those clay fractions dominated by chlorite (Ramson et al., 1998). In near-shore sediments, the organic matter was associated with fine clays composed mainly of phyllosilicates (illite/mica, Fe-rich chlorite, and minor amounts of kaolinite and vermiculite) (Bock and Mayer, 2000). By contrast, the amount of organic matter in the clay-size fraction was independent of the clay mineralogy in six kaolinite- and six smectite-dominated soils from around the world (Wattel-Koekkoek et al., 2001). However, in this study it was found that kaolinite- and smectite-associated SOM is biochemically different. The kaolinite-associated SOM was enriched in polysaccharide products, whereas smectite-associated SOM was enriched in aromatic compounds as observed by pyrolysis-gas chromatography/mass spectroscopy (GC/MS) and nuclear magnetic resonance (NMR). The above studies do not offer any evidence of the role of clay minerals on SOM dynamics and the formation of new humic substances.

The retention of ¹⁴C increased with clay content among soils (4 to 34% clay content) incubated for 4 yr with ¹⁴C-labeled cellulose (from barley straw [Hordeum vulgare L.) (Sorensen, 1981). When ¹⁴C-labeled-glucose, -hemicellulose, and -straw were added to a soil, the ¹⁴C activity increased with clay content only for the ¹⁴C-labeled-glucose and -hemicellulose amendments, whereas the ¹⁴C specific activity decreased with clay content in the ¹⁴C-labeled hemicellulose and straw amendments (Christensen and Sorensen, 1985). The highest ¹⁴C activity was associated with the clay fraction (<2 µm) after 5- to 6-yr incubation, whereas the silt and sand fraction contained the lowest ¹⁴C activity. At the end of the experiment (18 yr) the ¹⁴C specific activity was similar in both clay and silt fractions (Christensen and Sorensen, 1985). In a different approach, Saggar and coworkers (1996)( 1999) have reported that clay mineralogy rather that clay content influences mineralization and stabilization of microbial metabolites from ¹⁴C-labeled substrate. In a 35-d study, mineralization of ¹⁴C-labeled glucose was higher in soils with lower clay content and lower mineral surface area (Saggar et al., 1999). Similarly, in a 5-yr study of ¹⁴C-labeled ryegrass (Lolium hybridum Hausskn.) amended soils, the ¹⁴C activity was higher in soils with the highest surface area (vermiculitic and smectitic soils), whereas the lowest ¹⁴C activity was found in low surface area soils (kaolinitic and amorphous minerals) (Saggar et al., 1996).

The present study is a follow-up of work recently reported by Laird et al. (2001) and Laird (2001). Laird et al. (2001) investigated relationships between clay mineralogy and the chemistry of humic materials separated from the Ap horizon of a Webster soil (Fine-silty, mixed, mesic Typic Hapludoll). A relatively aggressive physical fractionation procedure was used to isolate the coarse (0.2–2.0 µm), medium (0.02–0.2 µm), and fine (<0.02 µm) clay fractions from the soil. The procedure separated mineralogically distinct fractions as the coarse, medium, and fine clay fractions were dominated by quartz, a low-charged interstratified illite/smectite phase, and smectite, respectively. While all of the fractions contained similar levels of organic C (52–70 g C kg^-1 clay), the chemistry of the humic substances was substantially different among the clay fractions. The coarse clay fraction had a higher C/N ratio, stronger carboxyl and O-alkyl ¹³C-NMR peaks and lower levels of extractable amino acids, fatty acids, monosaccharides, and amino sugars than humic substances associated with the fine clay fraction. Analysis of the clay-humic complexes by scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (Laird 2001) revealed diffuse, filamentous films covering basal surfaces of 2:1 phyllosilicates in the medium and fine clay fractions and discrete particles (1–2 µm) of high-density metal-humic complexes in the coarse clay fraction. The results of Laird et al. (2001) and Laird (2001) indicate the existence of at least two distinct phases of mineral-associated SOM and suggest that clay mineralogy may contribute to the formation and/or stabilization of these materials. However, no evidence of the dynamics of new C additions to the clay-humic complexes was presented.

The samples used in the present study were from a previous study by Gale and Cambardella (2000). Gale and Cambardella (2000) grew oats in large pots under simulated no-tillage conditions and periodically pulse labeled the crop with ¹⁴C by exposing the growing plants to ¹⁴CO₂. Several times during a 360-d post-harvest incubation, pots were destructively sampled and the level of ¹⁴C-activity in various aggregate-size classes was quantified. Although the work was primarily focused on investigating mechanisms for aggregate formation and stabilization, the results also provide clear evidence for movement of new C from plant material through particulate organic matter (POM) and eventually into newly formed humic material associated with minerals in the silt + clay fraction. During the post-harvest incubation, 56.1% of the total ¹⁴C in the pots on Day 1 (day of harvest) was mineralized to ¹⁴CO₂. During the same period, ¹⁴C activity associated with the coarse roots + large POM decreased from 74.2 to 9.7%, while ¹⁴C activity associated with the clay + silt fraction increased from 17.1 to 23.9% of total ¹⁴C in the pots on Day 1.

Availability of the ¹⁴C-labeled silt + clay fraction samples from the study of Gale and Cambardella (2000) provided us with an opportunity to further investigate the role of clay mineralogy in the stabilization of SOM. The specific objective of the study was to determine whether organic C in newly formed humic materials is uniformly distributed in the silt + clay fraction or is preferentially associated with either discrete C-rich particles in the coarse clay fraction or the diffuse humic materials associated with surfaces of 2:1 phyllosilicates in the medium and fine clay fractions.

	MATERIALS AND METHODS

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Gale and Cambardella (2000) grew oats in two growth chambers using large pots filled with Monona soil (fine silty, mixed, mesic Typic Hapludoll). The plants of one chamber were pulse labeled with ¹⁴CO₂ 15, 20, 26, 31, 37, 46, and 52 d after emergence. After senescence (118 d after emergence), the ¹⁴C-labeled and unlabeled plants were cut off at the soil surface; the residue was dried at 50°C for 36 h and then returned to the pots. The ¹⁴C-labeled residue was placed on the soil surface of pots containing unlabeled roots and the unlabeled residues were placed on the soil surface of pots containing ¹⁴C-labeled roots. The pots were periodically sampled during 360 d of post-harvest incubation. During each sampling, the soil was separated into various aggregate-size classes to study the relationship between the incorporation of new C into SOM and the formation of soil aggregates (Gale and Cambardella, 2000). The finest fraction studied by Gale and Cambardella (2000) was the silt + clay fraction (<53 µm). To separate the silt + clay fraction, the soil was shaken for 18 h in a sodium hexametaphosphate solution and then the suspension was passed through a 53-µm sieve and rinsed with distilled water. The silt + clay fraction (material passed through the 53-µm sieve) was captured in a pan and dried at 70°C for 36 h.

For the present study, the silt + clay fractions from the pots containing unlabeled surface residue and ¹⁴C-labeled roots and soil were dispersed in distilled water by shaking and the clay (<2.0 µm) and silt fractions were separated by sedimentation. The clay fraction was further separated into coarse (0.2–2.0 µm), medium (0.02–0.2 µm), and fine (<0.02 µm) clay size fractions using a sonication-centrifugation-decantation technique (Laird et al., 1994). The various clay-humic fractions were Ca-saturated by washing in 0.5 M CaCl₂ four times, dialyzed against distilled water and freeze-dried.

The mineralogy of the various particle-size fractions separated from the Monona soil was analyzed by X-ray diffraction (XRD). About 100 mg of Ca-saturated clay were slurried in 95% ethanol, oriented on glass slides by the paste method, and dried above a saturated solution of Mg(NO₃)₂ (54% relative humidity). Glycerol-solvated samples were prepared as above, except that glycerol was added to the ethanol. The oriented samples were analyzed between 2 and 32° 2 with a Siemens D5000 X-ray difractometer (Siemens, Munich, Germany) using Cu K radiation. The ¹⁴C activity was measured in each fraction by combusting the samples in a Harvey Biological Oxidizer, model OX500 (R. J. Harvey Instrument Corp., Hillsdale, NJ). The ¹⁴C-CO₂ released during oxidation was trapped in Harvey's ¹⁴C cocktail and counted with the liquid scintillation analyzer. Organic C and total N content in each fraction was determined by dry combustion using a Carlo Erba NA1500 NSC elemental analyzer (Haake Buchler Instruments, Paterson, NJ). Elemental analysis (Al, Ca, Fe, K, Mg, Na, and Si) of the clay samples was performed with an inductively coupled plasma-atomic emission spectrometer (ICP-AES) using the suspension nebulization technique (Laird et al., 1991a).

Scanning electron micrographs (SEM's) of the clay-humic complexes were obtained using a JEOL JSM-5800 LV Scanning Electron Microscope (JEOL U.S.A., Inc., Peabody, MA) operated at 10 KeV. To prepare the samples, 50 mg of the various Ca-saturated samples were sonicated for 30 s at 40W in 15 mL of distilled water. Then, 50 µL of suspension were transferred to an Al-SEM stud covered with aluminum foil. The samples were dried in a desiccator and then sputter coated with gold-palladium.

	RESULTS

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The XRD patterns of the Ca-saturated clay fractions of the Monona soil are shown in Fig. 1 . The fine clay fraction is dominated by interstratified smectite/illite, which is indicated by broad 14.5- (54% relative humidity) (Fig. 1) and 18.0-Å (glycerol solvated) (figure not shown) XRD peaks. Further evidence for the interstratified smectite/illite includes the lack of a distinct 10-Å, illite XRD peak (Fig. 1) and the presence of relatively high amounts of K in the Ca-saturated fine clay fraction (Table 1). The nature of the interstratified smectite/illite in soils has been discussed previously (Laird and Nater, 1993; Laird et al., 1991b).

View larger version (28K): [in this window] [in a new window]   Fig. 1. X-ray diffraction patterns of silt and clay fractions from a Monona soil (S/I: smectite/illite; I: illite; K: kaolinite; Q: quartz; F: feldspars).

Total C and total N content for the various fractions are presented in Table 2. Because no carbonates were present in this soil (Gale and Cambardella, 2000), the total C content is assumed to equal the organic C content. The organic C content in the clay fractions ranged from 30.6 to 39.3 g C kg^-1 clay (medium and fine clay fraction, respectively). The total N content ranged from 3.4 g N kg^-1 clay in the coarse clay fraction to 4.1 g C kg^-1 clay in the fine clay fraction. Interestingly, the silt fraction contained much lower C and N content (4.45 and 0.50 g kg^-1 silt, respectively). The C/N ratios were similar in all fractions and ranged from 8.5 in the medium clay fraction to 11.0 in the coarse clay fraction.

View larger version (102K): [in this window] [in a new window]   Fig. 2. Scanning electron micrographs of the silt and clay fractions of a Monona soil; (a) silt, (b) coarse clay, (c) medium clay, and (d and e) fine clay (low and high resolution, respectively).

	DISCUSSION

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The separation of the clay fraction from the silt fraction by sedimentation gave, as expected from Stoke's Law, >2-µm uniform microaggregates (figure not shown). In the fine clay fraction, the observed large platy particles (quasicrystals) (Fig. 2d) do not exist in the soil. They were formed when smectites in the clay fraction were flocculated with Ca²⁺ (Laird et al., 2001). In the coarse clay fraction (Fig. 2b), quasicrystals are not formed nor expected since this fraction is dominated by quartz. However, some microaggregates survived the sonication-centrifugation steps of the fractionation (figure not shown). Because of the morphology and because the XRD analysis indicated that the coarse clay is dominated by quartz (Fig. 2b), these granular and round particles observed in the coarse clay are assumed to be quartz. On other hand, the platy particles observed in the coarse fraction are assumed to be either kaolinite or illite (Laird, 2001). From the results of SEM, mineralogical and elemental analyses of the soil clay fractions, it is clear that the fine and coarse clay fractions have completely different mineralogy and particle morphology, as observed previously (Laird, 2001; Laird et al., 2001).

Soil clay content has been used to determine whether clays influence soil organic C content and the chemical character of soil organic C. A poor correlation between clay content and total soil C in New Zealand grassland soils was found by Percival and coworkers (2000). Conversely, for two Australian soils, the soil with the higher clay content (54%) contained the higher organic C content (32 mg C g^-1 soil) and had a more highly processed nature of carbon (Baldock and Skjemstad, 2000). Whereas the soil with the lower clay content (11%) contained the lower C content (18 mg C g^-1) and a greater plant-like chemical structure of the organic C (Baldock and Skjemstad, 2000). Thus, clay content is only one of the variables influencing both soil organic C content and the chemical nature of SOM. Clay mineralogy also plays an important role in the stabilization of soil organic C (Furukawa, 2000; Laird et al., 2001; Ramson et al., 1998; Saggar et al., 1999; Saggar et al., 1996). Soil organic C appears to be preferentially sequestered in smectite-rich continental sediments of California and Washington (Ramson et al., 1998). By using electron energy-loss spectroscopy, it was observed that the organic matter within clay aggregates coexists with Ca, suggesting that Ca-smectites, rather than Ca-illite or Ca-kaolinite are preferentially associated with the organic matter of aquatic sediments from the Jourdan River (Furukawa, 2000). In our study, it is apparent from the ¹⁴C specific activity and the mineralogy data that newly formed humic substances either accumulate on or are formed on smectite surfaces.

	ACKNOWLEDGMENTS

 
We thank Dr. J. Gale and Dr. C. Cambardella for the mineral fraction of the Monona soil, Dr. M. Thompson for the support on the XRD analysis, T. Pepper and J. Manila for the support with the SEMs, P. Fleming for the support with the elemental analysis, and Dr. T. Moorman and B. Douglas for the support with the ¹⁴C analysis.

Received for publication December 16, 2002.

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This Article Abstract Figures Only Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (10) Citing Articles via Google Scholar Google Scholar Articles by Gonzalez, J. M. Articles by Laird, D. A. Search for Related Content PubMed Articles by Gonzalez, J. M. Articles by Laird, D. A. GeoRef GeoRef Citation Agricola Articles by Gonzalez, J. M. Articles by Laird, D. A. Related Collections Soil Organic Matter Carbon Sequestration Soil Chemistry