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Soil Science Society of America Journal 64:2140-2148 (2000)
© 2000 Soil Science Society of America

DIVISION S-7-FOREST & RANGE SOILS

Persistent Organic Pollutants in Native Grassland Soils along a Climosequence in North America

Wolfgang Wilcke and Wulf Amelung

Institute of Soil Science and Soil Geography, University of Bayreuth, D-95440 Bayreuth, Germany

wolfgang.wilcke{at}uni-bayreuth.de


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Among the controls on the fate of harzardous persistent organic pollutants (POPs) in the environment, soil organic matter (SOM) and climate play an outstanding role. Thus, it may be possible to predict POP concentrations at background sites from SOM properties and climatic elements. We therefore related polycyclic aromatic hydrocarbon (PAH) and polychlorinated biphenyl (PCB) concentrations in 18 mollic epipedons under native grassland to SOM properties (lignin-derived phenols, polycarboxylic benzoic acids [PCBAs], aromaticity, and polarity of alkali-extractable SOM) and climatic elements. The sum of 20 PAH ({sum}20PAHs) concentrations ranged from 63 to 321 µg kg-1, and that of 14 PCB ({sum}14PCBs) concentrations ranged from 7.9 to 93 µg kg-1, except at one contaminated site (3136 µg kg-1). On average, phenanthrene (PHEN, 38% of the {sum}20PAHs concentrations) and naphthalene (NAPH, 28%) were the most abundant PAHs, congeners 28 (22% of the {sum}14PCB concentrations) and 101 (17%) were the most abundant PCBs. Soil organic C (SOC) concentrations correlated with the {sum}20PAHs concentrations; the C concentration in the sum of eight PCBAs, a marker for black C, correlated with the concentrations of higher molecular weight PAHs, except in soils with cyric temperature regime. The {sum}14PCBs concentrations was independent of any soil property. The contribution of NAPH to the {sum}20PAHs concentrations and that of the up to tetra-chlorinated PCBs to the {sum}14PCBs concentrations decreased with increasing mean annual temperature (MAT). The percentages of PCB 101 increased with increasing MAT. However, the temperature effect was not strong. Mean annual precipitation (MAP) neither effected PAH nor PCB patterns. Our results indicate that the easily measured SOC concentrations may be used to predict PAH concentrations in native grassland soils of the prairie. Including MAT improves the prediction of NAPH concentrations. The influence of MAT on PCB concentrations is obvious, but the correlation is too weak to be used for reliable predictions.

Abbreviations: ACEN, acenaphthene • ACENY, acenaphthylene • ANTH, anthracene • B(A)A, benz(a)anthracene • B(A)P, benzo(a) pyrene • B(BJK), benzo(b+j+k)fluoranthenes • B(E)P, benzo(e)pyrene • B(GHI), benzo(ghi)perylene • CHRY, chrysene+triphenylene • DIBE, dibenz(a,h)anthracene • DOM, dissolved organic matter • FLUA, fluoranthene • FLUO, fluorene • IND, indeno(1,2,3-cd)pyrene • KOC, soil solution–soil solid phase partitioning coefficient normalized to the fraction of organic C • KOW, octanol–water partitioning coefficient • MAP, mean annual precipitation • MAT, mean annual temperature • NAPH, naphthalene • NMR, nuclear magnetic resonance • PAH, polycyclic aromatic hydrocarbon • PCBA, polycarboxylic benzoic acid • PCB, polychlorinated biphenyl • PERY, perylene • PHEN, phenanthrene • POP, persistent organic pollutant • PYR, pyrene • SOC, soil organic C • SOM, soil organic matter • VSC-lignin, sum of the concentrations of vanyllil, syringyl, and cinnamyl phenolic CuO oxidation products • {sum}20PAHs, sum of 20 PAHs • {sum}14PCBs, sum of 14 PCBs

Abbreviations: *Significant at the 0.05 level of probability


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
THE GREAT PLAINS are a unique grassland environment extending across a wide range of temperature and moisture regimes. Their soils, mainly Mollisols, are among the most fertile in the world (Aandahl, 1982). Since most of the Great Plains region is remote from major industrial areas, contamination of soils with organic pollutants should be low. Large parts of the Great Plains may therefore be considered as a typical background area with respect to soil pollution via atmospheric deposition.

The most important sorbent of POPs such as the PAHs and PCBs in soils is organic matter (Strek and Weber, 1982; Sims and Overcash, 1983). Furthermore, Gustafsson et al. (1997) found that the black C concentration plays an important role in PAH sorption. Black C results from charring of organic material, for example, during vegetation fires, and is ubiquitous in soil (Goldberg, 1985). The sorption of PAHs and PCBs to soil is furthermore influenced by the concentration and quality of dissolved organic matter (DOM) in soil solution. Increasing DOM concentrations reduce the extent of POP sorption to soil solid phase. The association of PAHs with DOM is favored by increasing aromaticity of the DOM (Chiou et al., 1983; Gauthier et al., 1987; McCarthy et al., 1989). Recent work has shown that solid-state SOM quality also influences PAH and PCB sorption. Chiou et al. (1998) found a more pronounced sorption of PAHs to sediments than to soils and assumed that the polarity of organic matter influences PAH sorption. Kile et al. (1999) observed that the C-normalized partitioning coefficient between soil or sediment and water (KOC) of carbon tetrachloride correlated negatively with the polarity of soil and sediment samples. In their work, polarity was defined as the sum of the contribution of O-alkyl and carboxyl C signals to the total signal in solid-state 13C nuclear magnetic resonance (NMR) spectra. Schoone et al. (1997) found that the partition coefficient of pyrene between soil and water correlated negatively with the polarity of soil (defined as the ratio of the sum of O-alkyl and carboxyl C signals to the sum of aryl and alkyl C signals in solid-state 13C NMR spectra), while no relationship between the partition coefficient of phenanthrene and polarity was observed.

Early work indicated that the octanol–water partition coefficient (KOW) may be used to predict organic contaminant sorption in soils irrespectively of the compound composition (Chiou et al., 1979). Recently, substantial differences in the sorption of PAHs and PCBs to soil have been reported (McGroddy et al., 1996; Gustafsson et al., 1997; Chiou et al., 1998). Polycyclic aromatic hydrocarbons exhibit higher partitioning coefficients to soil and sediment than PCBs with similar KOW values, which is attributed to a preferred interaction of PAHs with aromatic structures in soil organic matter (Chiou et al., 1998) or in black C (Gustafsson et al., 1997). Thus, SOM quality and quantity may be more important for PAH than for PCB sorption to soils.

Theoretical considerations on the global distribution of POPs suggest that climate influences the composition of the PAH and PCB mixture (pattern) in soil (Wania and Mackay, 1996). Warm temperatures favor volatilization of POPs; cold temperatures favor deposition. As a result, there should be a global fractionation of PAHs and PCBs with increasing contributions of the more volatile compounds to total concentrations from warm to cold climates. However, the model of Wania and Mackay (1986) is restricted to a single pulse of POPs to the atmosphere in the tropics. The prediction of PAH and PCB patterns is complicated, among other reasons, by the fact that POPs are not produced by a single source but by many sources at various locations of the world, and that they are degraded during atmospheric transport. Transformation processes result in similar PAH profiles in the atmosphere all over the temperate zone (Jones et al., 1989; Jacob et al., 1993; Wild and Jones, 1995). Similar technical PCB mixtures have been used at many localities of the temperate zones from where they are dispersed and transformed, also resulting in a rather homogenous background mixture (Hutzinger et al., 1988). Thus, it may be hypothesized that the patterns of PAH and PCB deposited to soils at background sites (i.e., those far from industrial activities, urban centers, and heavily frequented roads) are similar throughout the temperate zone. In soil, PAH and PCB patterns should be modified by climatic conditions that control volatilization and degradation processes.

Because the measurement of POP concentrations in soils is labor- and cost-intensive, it would be promising to predict POP concentrations at background sites from easily measured or already available data. This would be possible if strong relationships existed between soil properties or climatic conditions and POP concentrations. The objective of our work was therefore (i) to detect possible relationships between SOM concentrations and quality and PAH and PCB concentrations and (ii) to assess climatic effects on PAH and PCB patterns in native North American grassland soils along a climosequence from Canada to Texas.


   Materials and methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Study Sites and Soils
Eighteen sites with moderate clay content (170–350 g kg-1) were selected for sampling in late spring 1994 along temperature and precipitation transects across the native North American prairie (Table 1) . More details about the sampling sites may be found in Amelung et al. (1998).


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  		Table 1 Description of the soils under study

 
We used a core sampler to collect composite samples (0–10 cm) from five subsites (200 cm3 core volume at each subsite) in a radial sampling scheme across each site; the distance between the subsites ranged from 10 m (for small site areas) to 100 m. All samples were air-dried immediately after sampling in air-conditioned storage rooms (20–25°C, unknown but comparable air humidity) for 2 d and sieved (<2 mm) prior to chemical analyses. To minimize contamination after air-drying, samples were kept in closed plastic bags that were only opened to remove aliquots for analyses. Minor sorption of PAHs and PCBs to the plastic may have occurred but are considered as negligibly small. To assess possible sample contamination or volatilization losses during air-drying, Krauss et al. (2000) compared in our laboratory PAH and PCB concentrations in the extract of one subsample of a field-fresh soil from the surroundings of the city of Bayreuth, Germany, with those in the extract of another subsample of the same soil air-dried in a storage room for 8 d. Differences were within the range of analytical error for most PAHs and PCBs except for NAPH and PCB 8. For these two compounds a loss of {approx}45% of the initial concentrations during drying, probably due to volatilization, occurred. Thus, the concentrations of NAPH and PCB 8 reported here are likely to be too low. Since air-drying also occurs in the field, we consider our sample preparation as normalization to a comparable soil moisture content, although the higher surface area of the broken sample may have resulted in somewhat higher volatilization losses or sample contamination as would occur in the field. There is no reason to expect a systematic change of the NAPH and PCB 8 concentrations along the climosequence due to our drying procedure.

Soil Characterization
The pH (0.1 M CaCl2, soil/solution ratio 1:2.5) and the effective cation-exchange capacities (1 M NH4-acetate, pH 7) were taken from USDA-SCS (1994) for the US soils. In the Canadian soils we determined pH and cation-exchange capacity with the same methods as used for the US soils in our laboratory. Total C of all samples was determined by C/H/N/S-Analyzer (Elementar vario EL, Elementar Analysensysteme GmbH, Darmstadt, Germany).

Soil Organic Matter Properties
Amount and degree of oxidative decomposition of lignin were estimated using alkaline CuO oxidation at 170°C for 2 h (modified from Hedges and Ertel, 1982). We replaced liquid–liquid extraction by a solid-phase extraction of the phenols using C-18 (Mallinckrodt Baker, Phillipsburg, NJ) as suggested by Kögel (1986). Phenolic oxidation products were dissolved and derivatized with a 1:1 mixture of pyridine and N,O-bis(trimethylsilyl)trifluoroacetamide (Fluka Chemie, Deisenhofen, Germany), separated by capillary gas chromatography (Ultra 2 fused silica column, 25 m, Hewlett Packard, Palo Alto, CA) and detected by a flame ionization detector (data from Amelung et al., 1999). Alkaline CuO oxidation releases phenols from reactive sites of the lignin macromolecule. Consequently, the sum of vanillyl, syringyl, and cinnamyl phenolic CuO oxidation products (VSC-lignin) gives a directly proportional, relative measure of the total lignin.

The sum of the polycarboxylic benzoic acids (PCBAs) hemimellitic, trimellitic, trimesic, pyromellitic, mellophanic, prehnitic, benzenepentacarboxylic, and mellitic acids, used as marker for charred residues (black C), was determined with the method of Glaser et al. (1998).

For liquid-state NMR measurements, selected samples were extracted three times with a 1:1 mixture of 0.1 M NaOH and 0.4 M NaF, the ratio of soil to extraction solution was 1:5 (v/v). The extracts were dialyzed, freeze-dried, redissolved in 0.24 M NaOD, and transferred to a 10-mm NMR tube ({approx}2.5 mL) for 13C NMR measurement. The method followed that of Schnitzer (1982), except that 0.1 M Na4P2O7 was replaced by 0.4 M NaF. For liquid-state 13C NMR spectra, we used a Bruker Advance DRX 500 spectrometer (Bruker Instruments, Manning Park, MA), spectrometer frequency, 125 MHz; inverse-gated decoupling; acquisition time, 0.33 s; delay time, 1.67 s; line-broadening factor, 100 Hz. Chemical shifts were given in ppm (Hz Mhz-1) relative to an external standard of TSP (3-(trimethylsilyl)propionic-2,2,3,3-d4 acid, sodium salt) in D2O. No internal standard was used. The extraction procedure yielded 21 ± 7% of the total C concentrations. Despite incomplete C extraction, Amelung et al. (1997) found in five of six cases that SOM composition of U.S. Great Plain topsoils as assessed by liquid-state NMR spectroscopy was similar to that assessed by solid-state NMR spectroscopy (using cross-polarization and magic angle spinning).

PAH and PCB Analysis
We determined 20 PAHs and 12 PCBs: naphthalene (NAPH), acenaphthylene (ACENY), acenaphthene (ACEN), fluorene (FLU), phenanthrene (PHEN), anthracene (ANTH), fluoranthene (FLUA), pyrene (PYR), benz(a)anthracene [B(A)A], chrysene+triphenylene (CHRY), benzo(b+j+k) fluoranthenes [B(BJK)], benzo(a)pyrene [B(A)P], benzo(e) pyrene [B(E)P], perylene (PERY), indeno(1,2,3-cd)pyrene (IND), dibenz(a,h)anthracene (DIBE), benzo(ghi)perylene [B(GHI)], PCB congeners 8, 20, 28, 52, 101, 118, 138, 153, 180, 199, 206, and 209 (numbers according to Ballschmiter and Zell, 1980).

The samples were extracted with hexane–acetone 2:1 (v/v) in an Accelerated Solvent Extractor (Dionex ASE 200, Dionex, Sunnyvale, CA). Between 3 and 25 g of the soil samples were transferred into 22-mL extraction cells. The cells were filled with solvent, pressurized to 14 MPa and heated to 120°C for 6 min. Pressure and temperature were held for a static extraction time of 5 min; afterwards the cells were rinsed with cold solvent (60% of cell volume) and purged with Ar for 150 s. This extraction cycle was performed twice for each sample and the extracts were combined.

All samples were purified with a column filled with 2 g of aluminum oxide (5% deactivated, upper part) and 2 g of silica (5% deactivated, lower part) and sequentially eluted with 15 mL hexane, 5 mL hexane–dichloromethane 9:1, and 20 mL hexane–dichloromethane 4:1. The eluates were combined and evaporated to {approx}0.1 mL before PAH measurement. For the determination of PCBs, the extracts were additionally purified with an acid–base silica column, that is, a column filled with (from top to bottom) 0.5 g 22% H2SO4-modified silica, 2.5 g 44% H2SO4-modified silica, 0.5 g activated silica, and 2.5 g 33% NaOH-modified silica. The PCBs were eluted from the column with 60 mL of hexane and evaporated to {approx}0.1 mL prior to analysis.

A Hewlett-Packard 5890 Series II gas chromatograph equipped with a Hewlett-Packard 5-MS fused silica capillary column (30 m by 0.25 mm by 0.25 µm) was used with He as carrier gas (constant pressure mode 80 kPA) and splitless injection (Hewlett Packard, Palo Alto, CA). Compounds were detected with a Hewlett Packard 5971 A mass selective detector with electron impact ionization in selected ion monitoring mode (Hewlett Packard). Details on the temperature program are given in Wilcke et al. (1999a) for the PAHs and in Wilcke et al. (1999b) for the PCBs.

Eight deuterated PAHs [NAPH-D8, ACEN-D10, FLUO-D10, ANTH-D10, PYR-D10, chrysene-D12, PERY-D12, B(GHI)-D12] and seven 13C-labeled PCBs (congeners 28, 52, 101, 138, 153, 180, 209) were used as internal standards and spiked to the soil samples prior to extraction. Fluoranthene-D10 was spiked as a recovery standard to the extracts prior to injection into the gas chromatograph. The principles of the quantification method are explained in detail in Kjeller (1998). The average recoveries of the internal standards ranged from 69 (NAPH-D8) to 94% (ANTH-D10) for the PAHs (n = 35) and from 84 (13C-PCB 28) to 91% (13C-PCB 138) for the PCBs (n = 33).

To eliminate the influence of a possible laboratory background contamination the PAH and PCB concentrations were corrected by subtracting the average of four analytical blanks. All analyses were performed duplicate or repeated until the coefficient of variation of the sum of PAH and PCB concentrations between two replicates was <10%.

Statistical Analysis
Statistical analysis was performed with STATISTICA for Windows 5.1 (StatSoft, Loll and Nielsen, Hamburg, Germany). The correlation analyses followed the least squares method. Significance was set at P < 0.05.


   Results and discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
Sum of PAH and PCB Concentrations
The {sum}20PAHs concentrations at all sites are similar to rural background concentrations in Europe (Table 2 , Jones et al., 1989; Berset and Holzer, 1995). The {sum}14PCBs concentrations are elevated at most sites when compared with background concentrations in European soils (Table 3 , Creaser et al., 1989; Harrad et al., 1994; Berset and Holzer, 1995). However, the {sum}14PCBs concentrations are less than those in urban soils (Weiss et al., 1994; Wilcke and Zech, 1998) and below the intervention value of the Dutch list (1000 µg kg-1) requiring remediation (Rosenkranz et al., 1995) except for the soil at Site XII. The Dutch list is frequently used as reference to assess the degree of soil contamination. Site XII, although located in a protected area (Konza prairie in Kansas), is heavily contaminated with PCBs. The soil at this site also exhibits the highest {sum}20PAHs concentration. While elevated PAH concentrations may be related to frequent fires, the elevated PCB concentrations indicate specific deposition of POP-contaminated materials.


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  		Table 2 Polycyclic aromatic hydrocarbon (PAH) concentrations in North American native grassland topsoils (0–10 cm).{dagger}

 

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  		Table 3 Polychlorinated biphenyl (PCB) concentrations in North American native grassland topsoils (0–10 cm)

 
PAH and PCB Patterns
The PAH patterns are clearly dominated by the relatively volatile representatives PHEN (on average 38% of {sum}20PAHs concentrations) and NAPH (28%, Table 2). All other PAHs, on average, individually account for <10% of the sum of PAH concentrations, with FLUA being the most abundant compound (9.6%). The result confirms that the study sites are located far from large industrial and vehicular PAH sources that frequently result in PAH patterns dominated by FLUA, CHRY, and B(BJK) as in urban soils (Bradley et al., 1994; Weiss et al., 1994; Berset and Holzer, 1995). As low molecular weight PAHs are more volatile, they remain in the atmosphere for longer periods and are thus more susceptible to long-distance transport compared with the higher molecular weight PAHs. The soil at Site XII is characterized by the highest percentage of NAPH (55.8%) of the {sum}20PAHs concentration, which may indicate either deposition of NAPH-containing materials or a recent fire next to or on the site producing mainly NAPH (Freeman and Cattell, 1990).

The most abundant PCBs are congeners 28 (on average 22% of the {sum}14PCBs concentrations) > 101 (16.8%) > 20 (16.5%). The hexa-chlorinated congeners 138 and 153, which dominate quantitatively in many urban soils (Weiss et al., 1994; Wilcke and Zech, 1998; Wilcke et al., 1999b), only account for 22%. The heavily contaminated soil at Site XII exhibits a different PCB pattern with congeners 138 and 153 accounting for 36 and 24% of the {sum}14PCBs concentration, respectively. The results indicate that this site has been specifically contaminated by deposition of one of the most common technical PCB mixtures (Hutzinger et al., 1988), while the other sites probably received their PCB burden by long-distance atmospheric transport shifting the PCB pattern to the more volatile representatives. Because of the high PCB contamination and the differences in PAH and PCB patterns from the other sites, we qualified the soil at Site XII as an outlier and omitted it in the further discussion.

Influence of SOM Concentration
The SOC concentrations correlate significantly with the {sum}20PAHs concentrations (Fig. 1) . The fact that the regression line does not pass through the origin of the coordinate system indicates that there is PAH sorption to soil constituents other than SOM. No correlation has been observed between the concentrations of SOC and those {sum}14PCBs or any individual PCB. A range of laboratory work has shown that the partition coefficient of POPs between soil water and soil solid phase increases with increasing organic matter concentrations (Sims and Overcash, 1983; Marschner, 1999). In the field, a weak correlation between SOC and PAH concentrations has been observed by Jones et al. (1989) for mainly rural Welsh soils. The correlation between SOC and PAH concentrations may be interpreted as result of the partitioning of PAHs from the atmosphere to SOM. In our study, the correlation between SOC and PAH concentrations, therefore, supports the hypothesis that most of the PAH burden of the grassland soils result from diffuse background concentrations in the atmosphere.



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  		Fig. 1 Relationship between soil organic C concentrations and the sum of 14 polychlorinated biphenyl (PCB) ({sum}14PCBs, open triangles) and 20 polycyclic aromatic hydrocarbon (PAH) concentrations ({sum}20PAHs, closed circles). Site XII was eliminated because it is an outlier

 
The correlation of the concentrations of SOC with those of {sum}20PAHs is mainly due to the correlation of the concentrations of the more volatile PAHs NAPH (r = 0.86*), ACENY (0.69*), FLUO (0.69*), PHEN (0.58*), and ANTH (0.73*) with SOC, while there is no correlation between the concentrations of SOC and higher molecular weight PAHs. The slopes of the regression lines decrease in the order: NAPH (1.50) > PHEN (0.50) > FLUO (0.09) > ACENY (0.03) > ANTH (0.02). Except for ACENY, the slope reflects the volatility of the compounds. The more volatile a PAH is, the larger is the gaseous portion in the atmosphere. The importance of SOM for PAH sorption should increase with increasing gaseous portions in the atmosphere, because only gaseous PAHs may be partitioned directly from the atmosphere to SOM. The lack of correlations between SOC and higher molecular weight PAHs confirms findings in the literature that these compounds are deposited in association with particles and therefore are not readily available for partitioning to SOM (Meharg et al., 1998).

The lack of correlation between SOC and PCB concentrations may indicate that PCB background concentrations in the atmosphere are not homogeneous. The sources of PCBs are less homogeneously distributed than those of PAHs, because PCBs are mainly used for well-defined technical purposes, while PAHs are produced by any combustion process (Sims and Overcash, 1983; Harrad et al., 1994). Thus, the distance of the study site to the nearest PCB source may be more important for PCB concentrations in soil than it is the case for the PAH concentrations.

Influence of SOM Composition
To obtain a quantitative measure of plant-derived aromatic structures we determined the concentrations of lignin-derived phenols in the bulk soil. The sum of eight PCBA concentrations was used as a marker of charred plant residues. To assess aromaticity and polarity of SOM, we characterized the alkali-extractable portion of SOM with 13C NMR spectroscopy. Polarity is defined as the ratio of the percentages of aryl + alkyl C to those of carboxyl + carbonyl C of the total 13C NMR signal intensity (Schoone et al., 1997).

The VSC-lignin concentrations correlated significantly with those of the {sum}20PAHs (r = 0.65*). However, the VSC-lignin and SOC concentrations were also significantly correlated (r = 0.88*) and the correlation between VSC-lignin and {sum}20PAHs concentrations was lower than that between SOC and {sum}20PAHs concentrations. This was also true for the significant correlations between VSC-lignin and NAPH (r = 0.69*), PHEN (0.54*), and ANTH (0.56*) concentrations when compared with the respective correlations of SOC and individual PAH concentrations. We therefore conclude that VSC-lignin is not promoting PAH sorption in the study soils.

When all soils with cryic temperature regimes (Sites I–III) and the soil at Site XII were eliminated from the correlation analysis, significant correlations between the C concentrations in the sum of eight PCBAs and the concentrations of all higher molecular weight PAHs (molecular weight > 200 g mol-1, r = 0.75*–0.89*, n = 11), except for PERY, were observed. The result indicated that the accumulation of charred plant residues goes along with the accumulation of highly condensed PAHs. Thus, higher molecular weight PAHs in the study soils may be derived from vegetation fires. The reason for the particularly high PCBA concentrations in some soils with cryic temperature regime is unknown because vegetation fires are common at all study sites (Table 1).

To assess a possible relationship of the PAH and PCB concentrations with SOM properties determined in the alkali-extracts with 13C NMR we normalized the PAH concentrations to those of SOC. Our intention was to reduce the strong influence of the SOC concentrations on those of the PAHs. We did not find any significant correlation between the SOM properties determined with 13C NMR and the SOC-normalized {sum}20PAHs and {sum}14PCBs concentrations. When the individual PAHs, except NAPH and ACEN, were considered, a negative relationship between aromaticity and SOC-normalized PAH concentrations was observed. However, the correlations were not strong (r = -0.36 to -0.74) and only significant for FLUO (r = -0.74*) and CHRY (r = -0.64*). Although the properties of alkali-extracted organic matter determined with liquid-state 13C NMR were similar to those of the total SOM determined with solid-state 13C NMR for some of our study soils (Amelung et al., 1997), this may not be true for all studied soils. Thus, the conclusion that SOM properties as determined with 13C NMR are not related to POP concentrations may not be generalized.

Influence of Climate
The percentage of NAPH of the {sum}20PAHs concentrations and the percentages of PCBs 20, 28, and 101 of the {sum}14PCBs concentrations tended to be related with MAT (Fig. 2a–2d) . The correlation for PCB 101 was significant, but not strong. While the percentages of NAPH, PCB 20, and PCB 28 decreased with increasing MAT, those of PCB 101 increased correspondingly. The strongest relationship was observed for the most volatile compound NAPH (vapor pressure: 101.05 Pa; Mackay et al., 1992b). When soils from Sites XII and XVII were omitted, the correlation became significant (r = 0.76*, n = 16). As NAPH is one of the major PAHs produced in vegetation fires (Freeman and Cattell, 1990) outliers with high NAPH percentages due to a recent vegetation fire near the study site should be expected.



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  		Fig. 2 Relationship between MAT and percentage of the sum of concentrations of (a) naphthalene (NAPH), (b) polychlorinated biphenyl (PCB) 20, (c) PCB 28, and (d) PCB 101. Site XII was eliminated because it is an outlier

 
Although PCBs with up to three Cl substitutions have vapor pressures (10-0.55–10-2.3 Pa; Mackay et al., 1992a) similar to those of ACENY, ACEN, FLUO, and PHEN (10-0.92–10-1.83; Mackay et al., 1992b), no influence of MAT on the percentages of these PAHs was observed. This may be the result of a stronger sorption of PAHs than of PCBs to SOM (Chiou et al., 1998) or to charred plant residues (Gustafsson et al., 1997). We did not observe any significant correlation between mean annual precipitation and PAH or PCB concentrations and patterns.

Naphthalene shows the fastest degradation rate of all PAHs (Sims and Overcash, 1983; Wild and Jones, 1995) and the degradation rates of PCBs decrease with increasing chloro-substitution (Abramowicz, 1990). Thus, the relationship of the percentages of NAPH of the {sum}20PAHs concentrations and the percentages with MAT of PCBs 20, 28, and 101 of the {sum}14PCBs concentrations may also partly be explained by enhanced degradation of the more easily degradable PAHs and PCBs due to increasing MAT.


   Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
The PAH concentrations of native grassland soils of the Great Plains are typical of remote background areas, while the PCB concentrations are higher than reported for background areas in literature. However, PCB concentrations are not critical except at the Konza prairie in Kansas where we detected a local contamination.

The correlation between SOC concentrations and con- centrations of the more volatile low molecular weight PAHs indicated that these compounds are partitioned from the atmosphere to SOM. No correlation between SOC and PCB concentrations existed, probably as a result of the less homogeneous distribution of PCB sources and therefore less homogeneous background concentration of PCBs in the atmosphere. The SOM quality as assessed by liquid-state 13C NMR, VSC-lignin, and PCBA analyses did not correlate with {sum}20PAHs and {sum}14PCB concentrations.

Mean annual temperature was correlated with percentages of the total concentration of POPs, while mean annual precipitation was not. The percentages of volatile and easily degradable PAHs (NAPH) and PCBs (congeners 20 and 28) of the total concentration decreased with increasing MAT, while the percentages of the less volatile PCB 101 increased. The result that climate had a more pronounced influence on the patterns of PCBs than of PAHs with similar vapor pressures further supports previous findings in literature that PAHs are more strongly sorbed to SOM than PCBs.

Our results indicated that the easily measured SOC concentrations may be used to predict PAH concentrations in native grassland soils of the prairie. Including MAT improves the prediction of NAPH concentrations. The influence of MAT on PCB concentrations is obvious, but the correlation is too weak to be used for reliable predictions.Soil Survey Staff 1997; USDA-SCS 1995


   ACKNOWLEDGMENTS
 
We are especially grateful to W. Zech and K.W. Flach for their important support. We thank V.O. Biederbeck, L. Brown, S. Brown, C. Campbell, E. Montemayor, G.R. Carlson, G. Creinwelge, R.F. Follett, D.W. Fryrear, J. Jochim, E. Knox, R. Molina, E.G. Pruessner, C. Richardson, G.E. Schuman, E. Skidmore, D. Sweeney, H. Tiessen, C. Thompson, D. Towns, F. Turner, M. Vorhees, R. Vredenburg, and R. Zink for help with site location and sampling, and Andrea Bergmann for contributing to the analyses. We are grateful to B. Glaser for providing the PCBA data and to M. Krauss for valuable discussions. The work has been supported by the Deutsche Forschungsgemeinschaft (DFG Ze 154/22-3).

Received for publication September 9, 1999.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
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