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a Dep. of Crop and Soil Sciences, Cornell Univ., Bradfield and Emerson Hall, Ithaca, NY 14853
b Dep. of Crop and Soil Sciences, 418 ASI Building, The Pennsylvania State Univ., University Park, PA 16802
* Corresponding author (ds278{at}cornell.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CEC, cation-exchange capacity • eV, electron volt • PIPS, passivated implanted planar silicon detector • R-S/O-S, ratios of most reduced S to highly oxidized S • I-S/O-S, ratios of intermediate S to highly oxidized S • SOC, soil organic C • SOM, soil organic matter • XANES, X-ray absorption near-edge structure spectroscopy
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The soil environment is the primary component of the global biogeochemical S cycle, acting as a source and sink for various S species and mediating changes in oxidation states. Sulfur occurs in soil both in organic and inorganic forms. However, most of the soil S (>95%) in temperate and tropical ecosystems is present in organic forms (Janzen and Ellert, 1998; Saggar et al., 1998; Solomon et al., 2001a). Knowledge of the speciation of S in organic materials could, therefore, provide a clearer understanding of the amount, form, and biogeochemical transformations of S in the ecosystem. Our knowledge of the biogeochemical S cycling is, however, severely limited by relatively crude analytical techniques used in S measurements and speciation. Most of the present studies on soil organic S are based on a fractionation technique using HI reduction of organic S compounds in soil (Tabatabai, 1982; Kowalenko, 1993a,b). The biochemical characterization of organic S as ester-SO4–S (HI-reducible S) and C-bonded S has advanced the understanding of the organic S cycle in soil (e.g., McGill and Cole, 1981; Janzen and Ellert, 1998; Lehmann et al., 2001; Solomon et al., 2001a). However, this technique is only an indirect method involving differential reduction of organic S compounds to H2S and is, therefore, not an entirely satisfactory procedure to directly speciate S within complex organic functional groups. Moreover, this chemical reduction method cannot identify intermediate oxidation states of S in organic matter and consequently almost no information is available about their turnover rates. Other studies have attempted to speciate S into its functional groups using pyrolysis gas chromatography mass spectrometry in aquatic humic substances (Van Loon et al., 1993) and kerogens (Carmo et al., 1997) for characterization of S containing moieties, but this approach is limited to speciation of thermally stable forms of S (Hundal et al., 2000).
Synchrotron-based S K-edge XANES spectroscopy has been successfully used to speciate and quantify all oxidation states of S in a variety of geochemical samples ranging from petroleum (Waldo et al., 1991a, b), coal (Huffman et al., 1995), marine sediments (Vairavamurthy et al., 1993, 1994), biosolids (Hundal et al., 2000) to soil humic and fulvic acids (Morra et al., 1997; Xia et al., 1998; Szulczewski et al., 2001). X-ray absorption near-edge structure spectroscopy has also proven to be a valuable tool in the identification and quantification of S oxidation states in organic soils (Martínez et al., 2002). Sulfur XANES spectroscopy is a solid-state technique that circumvents the limitations of chemical reduction methods and provides a means to directly determine the various S species based on the energy required for core electron transitions to bound states or ejection into continuum. The proportions of S in each oxidation state could be influenced by soil degradation and may reflect labile and stable S pools. However, up to now the influence of land use changes on S oxidation states of soils could not be determined. Therefore, there is a need to conduct comparative studies that involve land use changes, which may help to describe important reactions of S to long-term management changes. This technique can be combined with physical fractionation of soil according to particle-size separates, which has been demonstrated to provide ecologically meaningful soil S pools (Solomon et al., 2001a). Thus, S K-edge XANES spectroscopy may offer a significant potential to evaluate the influence of anthropogenic changes on the nature and distribution of S and follow its dynamics in terrestrial ecosystems.
The objectives of this study were, therefore, to assess the potential of S K-edge XANES spectroscopy to characterize the impact of deforestation and subsequent land use changes on the amount, form, and distribution of S fractions in size separates of soils. The results from XANES spectroscopy were related to previous results of wet-chemical S fractionation from clay and silt separates (Solomon et al., 2001a) of the soils under investigation.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Extensive deforestation of native natural forests in Southern Ethiopian highlands and their subsequent conversion in to low-input crop land or plantations either for the production of tea or lumber has caused extensive soil degradation. In the present study, we investigated natural forest, tea plantations (Camellia sinensis O. Kuntz) (35 yr old) and fields cultivated for 25 yr at the Wushwush site while at the Munesa site Podocarpus dominated natural forest, Cupressus plantations (Cupressus lusitanica Mill.) (25 yr old) and fields cultivated for 30 yr were investigated. In the cultivated fields of both sites, maize (Zea mays L.) was grown without fertilizer inputs. However, during the intermittent dry periods, sorghum (Sorghum bicolor L. Moench) was grown at Munesa. The plowing depth both at the Wushwush and Munesa sites varies from 10 to 12 cm. Crop residues that remain on the fields are normally collected and used as animal feed at both sites.
We selected three representative fields from each land use system. After considering the depth of cultivation to minimize differences, which may arise due to the dilution of SOM from mixing of the surface soil with the subsoil through cultivation, we used a core sampler (200 cm3 core volume) and collected nine subsamples in a radial sampling scheme (Wilding, 1985) from the upper 10 cm of each field. A composite sample was then prepared from the subsamples. The samples were air-dried and sieved (<2 mm) before fractionation and chemical analysis.
Physical Fractionation
After removing visible root remnants, particle-size fractionation was done on <2-mm material using a probe type sonicator (Branson Sonifier W-450, Schwäbisch Gmünd, Germany) in a soil/water ratio of 1:5 (w/v). The detailed procedure of isolation of the different size separates is described elsewhere (Solomon et al., 2002a). Previous studies on the amount, form, and structures of organic C (Solomon et al., 2002a,c), P (Solomon et al., 2002b), and S (Solomon et al., 2001a) compounds on these samples indicated no evidence of organic matter redistribution following soil fractionation procedure used in the present study.
Chemical Analysis
Carbon, N, and S contents of clay and silt size separates were analyzed by dry combustion with a C/H/N/S-analyzer (Elementar Vario EL; Elementar Analysensysteme GmbH, Hanau, Germany). The pH-H2O and pH-KCl were determined in 1:2.5 soil/water (w/v) suspension. Cation-exchange capacity (CEC) was determined with 1 M NH4OAc (pH = 7.0) according to Avery and Bascomb (1974). Dithionite-citrate-bicarbonate-extractable Al and Fe (Ald, Fed) as well as oxalate-extractable Al and Fe (Alo, Feo) were determined using atomic absorption spectrometer (Varian AAS-400; Varian Techtron, Victoria, Australia) as described by Mehra and Jackson (1960) and Blume and Schwertmann (1969). The analysis of S fractions (inorganic SO4–S, ester-SO4–S, and C-bonded S) in the clay and silt size separates was done by HI reduction of SO2-4 to sulfide and subsequent determination of S as bismuth sulfide by spectroscopy (Kowalenko, 1993a,b). Selected soil physical and chemical characteristics are shown in Table 1.
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Solid-state characterization of S oxidation states in clay and silt size separates and their humic substance extracts were performed using S K-edge XANES spectroscopy at beamline X-19A, the National Synchrotron Light Source (NSLS), at Brookhaven National Laboratory. The measurements from the clay and silt separates and humic substance extracts were conducted under standard operating conditions, that is, after calibrating the X-ray energy to the K-edge of elemental S, the spectrum was assigned an energy value of 2472 eV and scans ranging from 50 eV below to 150 eV above the absorption edge of S were collected with a step size of 0.2 eV. Each XANES spectrum was composed of an average of three scans. A monochromator consisting of double crystal Si (111) with an entrance slit of 0.5 mm and minimum energy resolution of 2 x 10-4 (0.5 eV) at the S K-edge was used. The spectra were recorded in fluorescence mode using a passivated implanted planar silicon (PIPS) detector (Canberra Industries, CT). The beam path from incident ion chamber to the sample chamber was purged with He gas. The samples were pressed into thin films by using 0.5-mm thick acrylic holder and covered with a 2.5-µm thick Mylar film (Complex Industries, NY).
The S K-edge data collection and analysis was done as described by Xia et al. (1998), Hundal et al. (2000), and Martinez et al. (2002). Deconvolution of XANES spectra for each sample into pseudocomponents was done using the nonlinear least squares fitting routine Solver supplied by MS-Excel (Microsoft Inc., Redmond, WA). The XANES spectra were fitted using a series of Gaussian peaks (G1, G2, G3, G4, and G5) that represent the s 
p transitions (white line) and arctangent step functions (AT1 and AT2) that represent the transition of ejected photoelectrons to the continuum (step height or background). The energy positions (eV) of the Gaussian curves were used to identify the oxidation states of S present in the sample. The linear component of the spectral baseline was removed before fitting and the areas were corrected for the change in absorption cross-section with increasing oxidation state. The areas of the Gaussian curves were used to calculate the percentage of S present at that particular oxidation state. Because XANES reflects the distribution of electrons in the valence shell of S atoms in their actual bonding environment, the difference between electronic and formal oxidation states can be substantial, especially for reduced S species in complex organic materials, depending on whether S is bonded to S, H, C, or metals (Waldo et al., 1991a; Morra et al., 1997; Xia et al., 1998; Hundal et al., 2000). Due to the higher electronegativity of O, the differences are not significant for higher-valence (
+4) S species and S atoms bound to multiple oxygen atoms. Therefore, in this study, we have reported the electronic oxidation states rather than formal oxidation states, as they reflect the actual electron density in the valence shell of S. Moreover, because of the above reasons, integer values were used to report the electronic oxidation states of the high-valence S species, while noninteger values were used for the low-valence (
+4) S compounds.
Statistics
Correlation coefficients showing the relationship between the different S compounds measured by HI reduction technique and S K-edge XANES spectroscopy were run by Pearson product moment correlation using the software package STATISTICA 5.0 for Windows (StatSoft. Inc., 1995).
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In contrast to the XANES spectra taken directly from the size separates, the deconvoluted and fitted experimental spectra using a series of Gaussian curves and two arctangent step functions of the humic substance extracts resulted in a very good fit (Fig. 3) . Therefore, all qualitative and quantitative comparisons and subsequent discussions in the present study are based on the results from the humic substance extracts.
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Ester-SO4–S is predominantly generated through biochemical processes by the soil microflora, which metabolize organic residues; its abundance is controlled by the supply of S (Saggar et al., 1998). In contrast, C-bonded S (amino acid and sulfonate S) is directly derived from leaf litter and root inputs, as well as microbial protein synthesis. Stevenson (1986) and Brady and Weil (1999) pointed out that the major form C-bonded S is mainly in the form of proteins and amino acids such as cysteine [HS-CH2–CH(NH2)-COOH], cystine [(S-CH2–CH(NH2)-COOH)2], and methionine [CH3–S-(CH2)2–CH(NH2)-COOH]. In contrast to current understanding (Stevenson, 1986; Brady and Weil, 1999), however, our S K-edge XANES spectra (Fig. 4 and 5) and the relative proportions of the different S compounds (Table 3 and 4) indicated that the humic substances extracted from clay are generally enriched with sulfonates. These results compare positively with the results of Stanko-Golden and Fitzgerald (1991), who demonstrated using a wet-chemical degradation technique that sulfonates are the dominant components of the C-bonded S pool representing as much as 68% of the total S of forest-derived soils from Puerto Rico. Our results also indicate that the most reduced S compounds (sulfides, disulfides, thiols, and thiophenes) occur in larger amount in humic substances extracted from the clay than from the silt. This is also illustrated by the higher ratios of most reduced S to highly oxidized S (R-S/O-S) in the humic substances extracted from clay (0.40–2.2) than silt (0.20–0.44) and by the ratio of intermediate S to oxidized S (I-S/O-S), which lies between 0.89 to 3.3 in the clay and 0.59 to 1.1 in the silt size separates (Table 3 and 4). The results from the present experiment are supported by our previous studies on the composition of organic matter in the size separates of these (Solomon et al., 2002a) and other tropical soils (Solomon et al., 2000, 2001b), in which larger proportions of microbial metabolites such as carbohydrates (hexoses and deoxyhexoses) and amino sugars (hexoseamines and muramic acid) were found concentrated in clay than in silt. The higher prevalence of organic S at the intermediate and most reduced oxidation states in the humic substances extracted from the clay than from the silt may be attributed to physical and chemical stabilization of these products as a result of: (i) a higher association of Fe and Al oxides and hydroxides in clay, (ii) a higher reactivity and specific charge characteristics of clay minerals, and (iii) a higher active surface area that enhances the enrichment of these organic compounds in clay than in silt size separates.
Bettany et al. (1980) and Anderson et al. (1981) reported that clay separates are preferentially enriched in ester-SO4–S. In contrast to the results of these researchers and the patterns observed in the humic substances extracted from clay, the unique feature of S XANES spectra of the humic substances extracted from the silt (Fig. 4 and 5) is the presence of a large and distinctive white line in the positive high-valence region representing a highly oxidized S. According to Table 3 and 4, this highly oxidized S is the predominant form of S representing 40 to 56% of the total organic S in humic substance extracts from silt. Because inorganic S species are not present in significant proportions in the humic substances extracted from these soils (Solomon et al., 2001a), the large proportion of S found in the highly oxidized state is presumed to be ester-SO4–S. Sulfur in the intermediate oxidation state accounted for 33 to 46%, whereas the most reduced S oxidation states represented only 11 to 20% of total organic S in the humic substances extracted from silt. These results show that most reduced S compounds represent a small fraction of the total organic S pool in humic substances extracted from the silt size separates of these tropical soils. Moreover, we have also observed that the white lines of the S XANES spectra obtained from the humic substance extracted from silt resemble more like the spectra recorded from fulvic than those of humic acids as reported by Morra et al. (1997).
Comparison of Sulfur Speciation Obtained by Hydroiodic Acid Fractionation and XANES Spectroscopy
In our previous study using the traditional wet chemical analysis technique, we have found that most of the organic S in the clay and silt size separates was present in C-bonded S (78–94%), whereas ester-SO4–S constituted merely 6 to 22% of the organic S pool (Solomon et al., 2001a). These values were almost constant regardless of agricultural management and did not change much between the different size separates, despite the fact that total S decreased on the average by 46% on cultivation. With the help of S-XANES spectroscopy, however, we were able to successfully speciate, and quantify several S species according to their oxidation states. Moreover, the result of XANES spectroscopy demonstrated clear differences both in the pattern and proportion of organic S species between physically fractionated size separates, soils and land use making it a useful tool to assess S cycling in soil.
Hundal et al. (2000) showed that the relative proportions of reduced and oxidized S classes obtained from biosolid-derived fulvic acid agreed well with the estimates from HI-reducible S. We compared the relative abundance of the most reduced (G1 and G2), intermediate (G3 and G4), and highly oxidized S (G5) as revealed by XANES to our earlier determinations of C-bonded and ester-SO4–S using wet chemical analyses (Solomon et al., 2001a). Unlike the results of Hundal et al. (2000), correlation coefficients showed a poor relationship (e.g., ester-SO4–S as revealed by XANES vs. HI-fractionation, r = 0.23; P < 0.001) between the compounds extracted using the two approaches. The widely differing values for the distribution of the S species reported in our previous (Solomon et al., 2001a) and current studies on these soils can be attributed to the procedures used to extract and quantify S species. In the previous study, we used a biochemical characterization of organic S as ester-SO4–S (HI-reducible S) and C-bonded S using an indirect method involving differential reduction of organic S compounds to H2S. This procedure is not entirely effective to speciate S in complex organic functional groups compared with the direct and nondestructive technique used in the present study.
Based on the above results it is possible to suggest that, because of the sensitivity of XANES to the electron structure, oxidation state and geometry of the neighboring atom, S K-edge XANES spectroscopy coupled with physical fractionation provided a characteristic fingerprint information of the organic S compounds present in these tropical soils. These features could make S K-edge XANES spectroscopy a valuable method to assess the long-term impact of anthropogenic changes on the nature and distribution of S and to follow its dynamics in terrestrial ecosystems. However, S K-edge XANES spectroscopy was not sensitive enough to differentiate the reduced S species such as sulfides, disulfides, and thiols. Sulfur L-edge XANES spectroscopy has been shown to provide complementary information to identify these S moieties in materials that contain large amounts of S (Kasrai et al., 1996) and may complement such studies.
Effects of Land Use Changes on Sulfur Compounds in Size Separates
In mature and undisturbed tropical forest ecosystems where the supply of S from the atmosphere is low, SOM plays a major role in S nutrition of plants by affecting the release of S present as part of complex organic polymers. In these ecosystems, S cycling is essentially closed with minimal short-term losses or gains, as a balance exists between the input and output processes. The distribution of organic S across various oxidation states and functional forms thus reflects the history, present structure and functioning of the natural ecosystem. The equilibrium of SOM attained under undisturbed conditions and thereby the forms and dynamics of organic S, however, can be greatly affected by anthropogenic changes (McGill and Cole, 1981; McLaren and Swift, 1977; Solomon et al., 2001a), which often involve dramatic changes in vegetation cover, biomass production, and nutrient cycling in the ecosystem. Such changes also seem to be reflected in the S K-edge XANES spectroscopy of the humic substances extracted from these tropical soils. Following land use changes, the major shift in the composition of organic S compounds in the humic substances extracted from clay occurred in the most reduced S species (Fig. 4 and 5). The relative proportions of these S species (G1 and G2) decreased in the order: natural forests (38 and 35%) > plantations (25 and 34%) > cultivated fields (17 and 30%) at the Wushwush and Munesa sites, respectively. The proportions of intermediate S species in clay showed a relatively small but consistent decrease (from 46 and 50% in natural forests to 41 and 48% in plantations and 39 and 40% in the cultivated fields for Wushwush and Munesa, respectively) at both sites. The ratio of R-S/O-S in the humic substances extracted from clay decreased in the order: natural forests (2.2 and 2.3) > plantations (0.76 and 1.8) > continuous cultivation (0.40 and 0.80), while the ratio of I-S/O-S dropped from 2.7 and 3.3 at the natural forests, to 1.2 and 2.5 in the plantations and to 0.89 and 1.2 in the cultivated fields at Wushwush and Munesa, respectively (Table 3 and 4). On the contrary, both the stacked spectra (Fig. 4 and 5) and the relative proportions of S compounds (Table 3 and 4) indicate that the highly oxidized S increased considerably following deforestation and subsequent land use changes (from 17 and 15% in natural forests to 34 and 19% in plantations and 44 and 36% in continuously cultivated fields) at both sites.
The impact of land use changes on the different S compounds resolved by XANES spectroscopy was also visible in the humic substances extracted from silt size separates. Lower proportions of the most reduced and intermediate S species were generally found in size separates from the continuously cultivated fields and plantations than the corresponding natural forests (Table 3 and 4). Moreover, the R-S/O-S and I-S/O-S ratios in the humic substance extracts from the silt were generally lower than the ones from the natural forests. In contrast, the proportions of highly oxidized S forms increased in the order: natural forests (40 and 41%) < plantations (47 and 46%) < cultivated fields (50 and 56%) in humic substances extracted from the silt separates both at Wushwush and Munesa, respectively. The observed changes in the highly reduced and intermediate oxidation states following land use changes could be attributed to physical disruption of aggregates due to frequent exposure to rain drop impact, rapid wetting and drying, as well as through shearing by agricultural implements. The net effect of which is the loss of C-bonded S (S in the most reduced and intermediate oxidation states) through increased aeration, stimulation of oxidation and exposure of the originally inaccessible organic S to the attack by soil microorganisms.
Even though a number of S speciation studies involving differential reduction techniques have observed both qualitative and quantitative changes in soil organic S due to cultivation, no consistent trends with regard to the interchange between different organic forms of S is yet apparent (Saggar et al., 1998; Solomon et al., 2001a). There is still conflicting evidence on the globally dominant labile form of organic S fraction, which can be taken as a major source of mineralizable S. For example, in organic S mineralization studies on Canadian soils, Lowe (1964) considered C-bonded S to be of little value as a source of mineralizable S. In contrast, David et al. (1982) observed a decrease in C-bonded S and an increase in ester-SO4–S during an incubation experiment using forest soils. Freney et al. (1975), McLaren and Swift (1977), and Ghani et al. (1991) found that most of the inorganic SO2-4 generated from soil originated from C-bonded S although there was a change in ester-SO4–S in a number of laboratory and field experiments conducted on both tropical and temperate soils. In our previous study on these soils, we have also demonstrated that C-bonded S contributed from 75 to 88% of the total S depletion, while only 11 to 26% of this loss derived from the ester-SO4–S due to land use changes, supporting the results of Freney et al. (1975), McLaren and Swift (1977), and Ghani et al. (1991). Based on the results of the present experiment, it is possible to suggest that there seems to be a link between the oxidation states, composition and bioavailability of the organic S species and level of soil degradation manifested by the loss of soil organic matter induced by land use changes. Our results indicate that organic S in the most reduced and intermediate oxidation states plays the most important role in providing mineralizable organic S. Therefore, organic compounds in which S is directly linked to C (R-SO3–H or R-S) can be considered as the most labile form of organic S in these subhumid tropical highland soils. These results further support the opinion that ester-SO4–S may have a more transitory nature and its mineralization is controlled by the supply of the end product. On the other hand, since C-bonded S is mineralized as a result of C oxidation to provide energy for soil microorganisms, the increased aeration and thereby microbial activity associated with cultivation could result in an accelerated transformation of the C-bonded S, which may passes through ester-SO4–S before release as inorganic SO4–S. This mechanism would tend to maintain or increase the level of ester-SO4–S fraction observed in these soils, while continually diminishing C-bonded S (McGill and Cole, 1981 and Saggar et al., 1998; Solomon et al., 2001a). However, it is also possible that part of the organic S in the most reduced and intermediate states may have been directly mineralized to inorganic SO2-4 and either taken up by the plants or leached out of the system.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The white lines of the XANES spectra showed the presence of multiple oxidation states of organic S in the humic substances and provided unequivocal evidence that S in intermediate oxidation states (especially sulfonates) are the dominant species of organic S followed by highly oxidized and most reduced S compounds in organic matter associated with clay. In contrast, highly oxidized S (ester–SO4–S) is dominant in organic matter associated with silt.
Sulfur XANES provided information about the link between S oxidation states, bioavailability of organic S moieties and the shifts that occur following land use changes. The results may indicate that organic S in the most reduced and intermediate oxidation states plays a major role in providing mineralizable organic S in these tropical soils.
Based on our results, it is possible to conclude that S K-edge XANES spectroscopy offers a significant potential to evaluate the influence of anthropogenic changes on the nature and distribution of S and follow its dynamics in terrestrial ecosystems.
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ACKNOWLEDGMENTS |
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Received for publication November 18, 2002.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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