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Soil Science Society of America Journal 66:1526-1530 (2002)
© 2002 Soil Science Society of America

DIVISION S-2—NOTES

Organic carbon at soil particle surfaces—evidence from x-ray photoelectron spectroscopy and surface abrasion

Wulf Amelunga, Klaus Kaiser*,a, Gerd Kammerera and Gustav Sauerb

a Institute of Soil Science and Soil Geography, University of Bayreuth, 95440 Bayreuth, Germany
b Institute of Physical Chemistry II, University of Bayreuth, 95440 Bayreuth, Germany

* Corresponding author (klaus.kaiser{at}uni-bayreuth.de)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Material and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
This study aimed at investigating the potential of x-ray photoelectron spectroscopy (XPS) for investigating soil organic matter at secondary soil particles. The XPS was applied to microaggregates of the A horizon of a Typic Haplustoll (<20-µm equivalent diameter, >53-µm maximum real diameter) and to the fine-earth fraction (<2 mm) of the Bs horizon of a Typic Haplorthod. Carbon and N, as well as Si (both samples), Ca (Haplustoll), and Al (Haplorthod) were detected. Removing the particle surface layer (<50 nm) by bombarding with Ar+ resulted in a strong reduction of the signals of C and N, while those attributed to inorganic components increased relatively. Consequently, in both soils, organic matter was concentrated at the surface of soil aggregates. We conclude that Ar+ sputtering followed by XPS analysis is a useful tool in identifying the accumulation of elements at the surfaces of soil particles.

Abbreviations: XPS, x-ray photoelectron spectroscopy


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Material and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 
ORGANIC MATTER may be chemically or physically stabilized in soil (review Christensen, 1992). Chemical stabilization involves sorption of organic matter at mineral particle surfaces (Nelson et al., 1994; Jandl and Sletten, 1999; Kaiser and Guggenberger, 2000), while enclosure of organic matter inside soil aggregates provides physical protection against decay (Elliott, 1986; Beare et al., 1994). To identify both processes it may be necessary to distinguish between organic matter located at surfaces or inside of soil particles. Amelung and Zech (1996) showed that the 0.5-mm surface fraction of peds (>2 mm) was depleted in organic matter, and that its lignin was more oxidized than that inside the peds. The authors concluded that better O2 supply at aggregate surfaces (Sexstone et al., 1985) favored microbial degradation of organic matter. Yet, it is unclear whether this finding remains also valid for smaller scales.

Peeling of aggregates for studying the chemical composition of their surface is restricted to aggregate sizes >2 mm (Kayser et al., 1994; Amelung and Zech, 1996; Santos et al., 1997). To work at smaller scales, a technique is required that selectively but representatively characterizes surface chemistry of particles. This can be achieved using XPS (Martin and Smart, 1987; Yuan et al., 1998; Barr et al., 1999; Arnarson and Keil, 2001). The XPS is well suited for the detection of compounds adsorbed to surfaces (Allen et al., 1999). Using Ar+ sputter-etching allows one to remove the surface of the particles selectively prior to additional XPS measurements (Ohama et al., 2000).

Our objective was to test the hypothesis that organic matter is located preferentially at the interiors of microaggregates versus the alternative hypothesis that it is located primarily at the surface of small soil particles. For this, we applied XPS to microaggregate fractions of a Mollic epipedon of the Russian steppe (where physical entrapment of organic particles within aggregates is suspected) and to the illuvial Bs horizon of a forest Spodosol (where preferential sorption of dissolved organic matter at microaggregate surfaces might be expected). The XPS gave an estimate of elemental composition at the aggregate surface that was selectively removed by Ar+ bombarding.


   Material and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Material and Methods
Results and Discussion
 Conclusions
 REFERENCES
 
Samples
Composite surface samples were taken from the Mollic epipedon (0–10 cm) of a Typic Haplustoll (Soil Survey Staff, 1998) close to Kursk, Russia. The site was used as meadow for hay production. More details concerning site history and sampling are described by Rodionov et al. (1998). Aggregate fractions were obtained by wet-sieving and sedimentation (Rodionov et al., 2000). No ultrasonic power was applied. To increase flocculation of silt-sized aggregates after fractionation, we added traces of a 10% (wt./wt.) MgCl2 solution. After fractionation, all aggregate fractions were air dried. Since this treatment caused some reaggregation, and sedimentation relies on the assumption that aggregates are spherical, we dry-sieved the <20-µm fraction again for 53 µm. About 30% of the actually <20-µm material remained on the 53-µm sieve. Strong organic matter–mineral interactions probably caused these particles (<20 µm according to sedimentation; >53 µm after subsequent dry sieving) to reaggregate or to violate Stokes' law of sedimentation, that is these organo-mineral associations seemed ideal for XPS studies of the soil organic matter. X-ray photoelectron spectroscopy spectra were recorded on both untreated and Ar-gassed subfractions on this >53-µm (sieved) but <20-µm (indicated by sedimentation) material. The material contained 35 g kg-1 of organic C and 3 g kg-1 of total N.

An illuvial Bs horizon was sampled from a Typic Haplorthod (Soil Survey Staff, 1998) in the Fichtelgebirge (NE Bavaria, Germany). The soil was derived from a loamy granitic solifluction layer. Soil structure was medium subangular blocky. The site was forested with 80-yr-old Norway spruce (Picea abies (L.) Karst.) with few European larch (Larix decidua P. Mill.). More details on the site and the soils are given in Kaiser et al. (2000). Bulk samples were taken from a soil pit, air dried, and sieved to <2 mm. These particles represented mainly aggregates of silt and clay and little quartz, feldspar, and glimmer fragments. Prior to the XPS analyses, visible organic residues, mainly root fragments >0.1 mm, were removed with forceps. The sample contained 51 g kg-1 of organic C and 4.4 g kg-1 of total N.

X-ray Photoelectron Spectroscopy
A detailed description of the XPS apparatus is given in Kilo et al. (1996). Briefly, the XPS surface analysis system (Leybold–Heraeus, Germany) used was comprised of a concentric hemispherical electron energy analyzer, a twin-anode x-ray source, and an Ar-ion gun for sputtering (5 kV excitation energy). In the present study, monochromated Al K{alpha} radiation was used for excitation (Eexc = 1496.6 eV, high voltage 14 kV, emission current 10 mA). For measurement, samples were placed on either Ag or Au sample bowls, then flushed with Ar. Wide-scan XPS were recorded with the energy analyzer set to a pass energy of 202 eV. The total acquisition time was >=120 min. During the acquisition of the spectra, the main chambers of the system were kept at about <5 x 10-5 Pa (<5 x 10-7 mbar). After sputtering with Ar+ ions for 3.5 h, x-ray photoelectron spectra were recorded again. To avoid the formation of craters, the beam was over an area much greater than the sample geometry, that is 6 by 6 mm. Atomic force microscopy of model compounds (different alloys) with a density of 2.0 g cm-3 showed that the depth of the surface abrasion because of the 3.5-h sputtering is ~50 nm. The exact sputtering depth of soil particles remained unknown because of differences in the density and chemical composition of soil constituents. Problems arose from electrostatic charging of the samples during the sputtering. This led to an accumulation of the sample material at the wall of the sample bowls and so their bottoms were partly uncovered. Thus, some of the spectra showed signals because of Au and Ag.

Data analysis was done by the software package DS100 (Leybold-Heraeus, Germany). Identification of binding energies was done according to literature (Moulder et al., 1992). Peak areas were determined by electronic integration. Calculation of the elemental composition was done using atomic sensitivity factors. The sensitivity factors derive from estimates of the photo-ionization cross-sections of electrons, which differ for individual orbitals of the same element and for orbitals of different elements (Moulder et al., 1992). Elemental lines used for calculation of the elemental composition were O 1s, C 1s, N 1s, Si 2p, Al 2p, Fe 2p, Ti 2p, Ca 2p, Mg Auger KLL, K 2s, F 1s, and Cl 2p. The accuracy of the quantitative elemental analysis by XPS for spectra recorded under identical experimental conditions is ±10% relative error (Moulder et al., 1992; Vempati et al., 1996). A measure for the accuracy of XPS measurements of materials containing Si is the ratio of Si 2p to Si 2s close to unity (Yuan et al., 1998). For our samples, this ratio ranged from 0.95 to 1.04.


   Results and Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Material and Methods
 Results and Discussion
Conclusions
 REFERENCES
 
Gassing the sample with Ar for 24 h at room temperature had no effect on the line intensities of C (285 eV; data not shown). We therefore concluded that adsorption of air-borne CO2 did not affect the x-ray photoelectron spectra for soil material. Nevertheless, we continued to gas samples with Ar prior to analysis. In addition, spectra recorded of empty Au and Ag sample bowls showed exclusively signals because of either Au or Ag. This suggests that there was no contamination by adventitious C compounds in the vacuum chamber by, for example, vacuum pump oil. These results fit well to those tests Arnarson and Keil (2001) did with different compounds.

The x-ray photoelectron spectra (Fig. 1) of the microaggregate fractions from the Mollic epipedon exhibited pronounced line intensities of electrons in the 1s orbital of O (530 eV, not shown here), in the 2p orbital of Ca (349 eV), in the 1s orbital of C (287 eV), of electrons in the 2s and 2p orbitals of Cl (267 and 196 eV, respectively) as well as of Auger KLL electrons of Mg (304 eV). The position of the signal of C at 287 eV suggests that it is bonded to O (Moulder et al., 1992), presumably of organic compounds since the sample was free of carbonates. The signals of 1s electrons of N at 400 eV pointed also to organic compounds (Moulder et al., 1992; Yuan et al., 1998).



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  		Fig. 1. Wide-scan x-ray photoelectron spectra of the surface of small aggregates (<20 µm equivalent diameter, >53 µm maximum real diameter) of a Mollic epipedon (0–10 cm) of a Typic Haplustoll before (a) and after surface abrasion by bombarding with Ar ions (b).

 
Minor signals were assigned to the 2s and 2p electrons of Si (150 and 98 eV, respectively), 2s electrons of F (684 eV, not shown here), 2p electrons of Ti (461 eV), 2s and 2p electrons of Mg (88 and 50 eV, respectively), and of 2s electrons of O (23 eV). Signals of Al (69 and 115 eV) and Fe (707 eV, not shown here) were small. Although the signals for Si and Al were small these elements might have contributed significantly to the elemental composition of the probed sample since their line intensities were relatively weak. Signals of Na were not detected.

The elemental composition at the surface of aggregates from the Typic Haplustoll indicated a strong enrichment of organic matter (Table 1). The surface concentration of C exceeded that of the bulk sample (35 g kg-1) by a factor of approximately four. The C/N ratio of 9.6 was lower than that of the bulk sample (11.6). This finding likely reflected preferential accumulation of microbial residues or microbially altered organic materials, as found for larger aggregate size classes (Amelung and Zech, 1996). In addition, the surfaces were enriched in Mg and Cl. The mass ratio of Cl to Mg was 2.9 (Table 1), which is exactly the ratio of these two elements in MgCl2. We propose that the surface concentrations of both elements were entirely because of the salt addition. Considering the entrained MgCl2, the surface enrichment of C was even larger. The presence of Al and especially of Si suggests that organic matter did not cover all mineral surfaces.


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  		Table 1. Elemental composition of soil samples according to x-ray photoelectron spectroscopy (XPS) before and after surface abrasion by bombarding with Ar ions.

 
Bombarding the surface of the Haplustoll microaggregate fraction with Ar+ changed relative line intensities in the x-ray photoelectron spectrum. The signal of C vanished almost completely, whereas the signals for Al, Fe, and Si increased (Fig. 1, Table 1). This implies that C was located primarily at the surface of these aggregates covering an underlying matrix dominated by silicate minerals. Parallel to the decrease of the signals of C, the signals due to N disappeared entirely (Fig. 1). Also the signals of F disappeared suggesting it to be bound to outer surfaces of the soil secondary particles.

Magnesium chloride that was added to the silt-size fraction to improve its flocculation, was predominantly situated at the surface of the particles, indicated by decreasing line intensity for Cl after Ar+ bombarding. As not all of the signal intensities for Mg and especially Cl disappeared, a portion of the salt penetrated the aggregates to a depth >50 nm, possibly in course of particle reaggregation induced by Mg2+. This and Mg in the mineral matrix may be the reason why the signal intensities of Mg did not change upon Ar+ bombardment (Table 1).

We resume that the elemental composition of the Haplustoll sample after surface abrasion differed clearly from that of the untreated sample (Table 1). The concentration of C was at similar magnitude as that of the bulk material and the concentrations of elements typical for a mineral matrix such as Si, Al, Fe, Ti, Ca, and K increased.

The x-ray photoelectron spectra of the fine-earth fraction of the Bs horizon of the Haplorthod exhibited a different surface chemistry compared with the microaggregates in the Haplustoll (Fig. 2) . The slight shifts of signals compared with the spectra of the Haplustoll were within the usual uncertainty of the analytical method. Peaks of Ca were lacking and those of Al (74 and 119 eV) were more pronounced, reflecting the large contribution of Al oxides–hydroxides to the surface composition mineralogy of the investigated particles and the depletion of Ca because of the acidic weathering. Also the signals for Mg and Cl were lacking because the sample was not treated with MgCl2 for flocculation and because of the small concentrations of these elements in this soil. The signal of C was stronger as compared with that of the microaggregates of the Typic Haplustoll (Table 1), indicating a significant accumulation of organic matter at the surface of particles of the Bs horizon. The large surface accumulation of organic matter may be the reason for the relatively strong signals due to N (401 eV). The position of the signals of N was at the upper limit for N in an organic matrix (Moulder et al., 1992) and thus the presence of N in inorganic compounds is likely.



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  		Fig. 2. Wide-scan x-ray photoelectron spectra of the surface of soil particles (<2 mm) of the Bs horizon of a Typic Haplorthod before (a) and after surface abrasion by bombarding with Ar ions (b).

 
The elemental composition of the surface of the sample was strongly dominated by C (Table 1) which is in line with the XPS results on other podzolic B horizons (Yuan et al., 1998). The surface concentration of C exceeded that of the bulk sample (51 g kg-1) by a factor of ~5 while the surface C/N ratio of 9.3 (Table 1) was, similar to the findings for the Haplustoll, below that of the bulk sample (11.7).

Both the C and N signal decreased after sputtering the sample surface for 3.5 h with Ar+. This suggests that a significant portion of the organic matter associated with the investigated particles was exposed at the particle surfaces. This is in line with findings about the accumulation of organic matter on the surface of soil and sediment particles as indicated by the close relationship between surface area and C content (Mayer 1994; Keil et al., 1994). Because XPS offers no information about the lateral distribution of elements it is not possible to decide whether the C was in a continuous layer or accumulated in microsites. Recent studies showed that the surface accumulation of organic matter is more likely to occur in distinct patches than to result in the formation of a disperse coating (Ransom et al., 1997; Mayer, 1999; Mayer and Xing, 2001). Increased Ar+ bombarding time (up to 7 h) did not result in further losses of C and N signals, indicating that the remaining portion of organic matter was either homogeneously distributed between particles, or, more likely, comprised of particulate organic matter that may not be peeled off by Ar+ bombarding. In fact, despite the manual removal of particulate organic matter, the sample still contained a small amount of small root fragments (<0.1 mm).

Similar to the microaggregates of the Typic Haplustoll, the elemental composition of the sample changed drastically upon surface abrasion (Table 1). The concentrations of C and N strongly decreased and those of Si, Al, and Fe increased instead. The concentration of C approached that of the bulk sample which is in line with unchanged spectra after longer surface abrasion (see above). The C/N ratio of 11.4 after surface abrasion corresponded to that of the bulk sample (11.7).

In summary, the elements detected at the surfaces of soil particles presented here are in line with a previous XPS study on the distribution of elements at the surfaces of particles in acid soils (Yuan et al., 1998). The application of surface abrasion by Ar+ bombarding followed by repeated XPS measurements presents additional evidence of declining C concentrations from aggregate surfaces to their interior.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Material and Methods
 Results and Discussion
 Conclusions
REFERENCES
 
X-ray photoelectron spectroscopy coupled with Ar+ bombardment provides an additional opportunity to identify the accumulation of organic C at surfaces of small secondary soil particles from different soil types. The XPS spectra did not identify accumulations of microbial cell biomass locations within the microaggregate interiors. The absence of increasing C peaks along aggregate surfaces suggested either insignificant accumulations of organic matter within the microaggregates or these compounds were located beyond ~100 nm into the aggregate surfaces which is the presumed maximum etching depth after Ar+ bombarding for 7 h. Added salts such as MgCl2 and possibly organic compounds appear to be sequestered at surfaces rather than inside soil aggregates. In contrast, XPS from the Bs horizon confirmed the hypothesis that soluble organic matter entraining illuvial subsoils of Spodosols primarily attaches to exposed mineral surfaces.

Since only two different soil fractions were analyzed, the exact depth of surface abrasion by the Ar+ bombardment remains unknown. Calibration of abrasion depth, however, should be possible when applying the technique to model substances with known surface thickness and chemistry. Then XPS might be a powerful tool in investigations of the surface heterogeneity of soil particles, of the distribution of sorbed compounds between particle surface and interior, and of penetration velocity of tracers into soil aggregates at the microscale level.


   ACKNOWLEDGMENTS
 
We thank A. Rodionov for providing the aggregate samples and the Deutsche Forschungsgemeinschaft (DFG) for financial support. The manuscript profited from the profound suggestions of two anonymous reviewers.

Received for publication July 9, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Material and Methods
 Results and Discussion
 Conclusions
 REFERENCES
 




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