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a Department of Soil Science, Institute of Ecology, Berlin University of Technology, Salzufer 11-12, D-10587 Berlin, Germany
b Synergy Resource Solutions, Inc., 1755 Hymer Ave., Sparks, NV 89431
* Corresponding author (wolfgang.wilcke{at}tu-berlin.de).
Received for publication July 8, 2003.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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20PAHs) concentrations ranged from 25 to 666 µg kg–1 in plant tissue, 7.4 to 32 µg kg–1 in litterfall, 206 to 287 µg kg–1 in organic soil, and 10 to 79 µg kg–1 in mineral soil. Among the living biomass compartments, the bark had the highest mean PAH concentrations and coarse roots the lowest, indicating that PAHs in the plants originated mainly from aboveground sources. Naphthalene and phenanthrene were the most abundant individual PAHs, together contributing 33 to 96% to the 20PAHs concentrations. The total storage of the 20PAHs in Cerrado was 7.5 mg m–2 to a 0.15-m soil depth and 49 mg m–2 to a 2-m soil depth. If extrapolated to the entire Brazilian Cerrado region, roughly estimated storages of naphthalene and phenanthrene correspond to 7300 and 400 yr of the published annual emissions in the United Kingdom, respectively. The storage of benzo[a]pyrene, a typical marker for fossil fuel combustion, in the Cerrado only corresponds to 0.19 yr of UK emissions. These results indicate that the Brazilian savanna comprises a huge reservoir of naphthalene and phenanthrene originating most likely from the aboveground parts of the vegetation or associated organisms. Thus, the Cerrado might be a globally important source of these PAHs.
Abbreviations: PAH, polycyclic aromatic hydrocarbon • 20PAHs, sum of 20 polycyclic aromatic hydrocarbons • POP, persistent organic pollutant
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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In the temperate zone, the majority of PAHs released into the environment originates from the combustion of fossil fuels. Minor sources include diagenetic processes, natural vegetation fires, and volcanic activities (Sims and Overcash, 1983; Baek et al., 1991; Neilson and Hynning, 1998; Kim et al., 2003). Most of the combustion-derived PAHs have a high molecular weight (at least four aromatic rings). However, little is known of the sources of PAHs outside the temperate zone. Recent research has indicated that biological sources of low molecular weight PAHs in the tropical environment may significantly contribute to the global PAH burden (Wilcke et al., 2000, 2002, 2003a). A quantification of the potential biological PAH production, however, is lacking.
Biological pathways of PAH production exist for perylene under anaerobic conditions (Venkatesan, 1988; Silliman et al., 2001). The depletion in 13C natural abundance of perylene extracted from termite nests and tropical soils from the Amazon basin compared with fossil-fuel-derived perylene indicated that biological perylene production may also occur in tropical anaerobic microenvironments (Wilcke et al., 2002). Phenanthrene is produced from plant-derived precursors in sediments (Wakeham et al., 1980; Wickström and Tolonen, 1987; Neilson and Hynning, 1998) and a biological production in other environments seems likely. Furthermore, there is increasing evidence of biological sources of naphthalene. Chen et al. (1998a)(1998b) found elevated naphthalene concentrations in subtropical termite (Coptotermes formosanus) nests. As these termites followed naphthalene-marked paths, the authors assumed that naphthalene has a function in their social organization. Wiltz et al. (1998) and Wright et al. (2000) reported that naphthalene inhibited the growth of insect pathogenic and saprophytic fungi. Naphthalene may form part of the termites' defensive system. This assumption is supported by the detection of elevated naphthalene concentrations in tropical termite nests from the Amazon basin by Wilcke et al. (2000). Naphthalene concentrations were particularly high in nests of the termite genus Nasutitermes relying exclusively on chemical colony defense (Martius, 1994). Wilcke et al. (2000) also found elevated naphthalene concentrations in woody plants. Besides, there are reports on the possible natural occurrence of naphthalene in biological materials such as magnolia (Magnolia spp.) flowers (Azuma et al., 1996), flower scents of different Annonaceaea species from the Amazon rain forest (Jürgens et al., 2000), or forehead hairs of white-tailed deer (Gassett et al., 1997). Daisy et al. (2002) have shown that naphthalene is produced by Muscodor vitigenus, an endophytic fungus of a liana growing in the Peruvian Amazon region. To assess the importance of this variety of biological PAH sources for the global PAH budget requires an ecosystem-based approach.
Despite the various indications of plant-related PAH sources, to our knowledge, no comprehensive survey of PAH concentrations in different plants has been undertaken. Particularly the knowledge of the PAH partitioning among different plant compartments would help in distinguishing PAH sources. It can be assumed that the leaves most closely represent the current atmospheric PAH concentrations. The PAH concentrations in leaves and needles are related to mean concentrations in the surrounding air and may, therefore, be used as a bioindicator of the airborne PAH concentrations (Kömp and McLachlan, 1997; Horstmann and McLachlan, 1998). The stem wood, in contrast, probably only receives PAHs from internal sources (i.e., plant metabolism or endophytic organisms).
As a result of their chemical recalcitrance and strong sorption to organic and mineral materials, PAHs accumulate in ecosystems (Wild and Jones, 1995). Therefore, the total storage of PAHs in an ecosystem may be used as indication of the size of PAH sources if emission rates are unknown, as is the case for the tropics.
Our main objective was to understand the importance of biological PAH sources in a typical tropical environment. We hypothesized that the comparison of the PAH storage in a Brazilian savanna ecosystem with that in other regions of the world for which emission rates are known, such as western industrialized countries, would be an indication of the size of PAH sources in this environment. We therefore quantified the PAH storage in a typical Brazilian Cerrado. The Brazilian savanna, with an area of 2 million km2, and its counterpart, the Llanos, north of the Amazon basin, is the ecosystem with the greatest areal extent in South America. The Cerrado vegetation varies from pure grassland to dense forests (Ribeiro and Walter, 1998). The most abundant typical Cerrado is open woodland with 15 to 40% tree canopy cover (Goodland, 1971; Archibold, 1995), and this type of Cerrado was selected for our study. To distinguish between external and internal PAH sources, we studied different plant compartments separately and evaluated the differences in the composition of the PAH mixture ("PAH patterns") among different ecosystem and plant compartments. Moreover, we estimated PAH turnover rates from the relation between PAH fluxes with litterfall and total PAH storage in the organic layer of the soil.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Within an area of about 100 km2, three plots of natural Cerrado were selected for fine litterfall and soil sampling and a 100-ha-large remnant of native old-growth Cerrado for biomass determination and harvest.
The soils were deeply (>2 m) weathered clayey (>755 g clay kg–1) Anionic Acrustoxes (Soil Survey Staff, 1997). The pH (KCl) of the upper 0.15 m was 4.0 and increased to 5.8 at a 1.2- to 2-m depth. The effective cation exchange capacity was 8.7 mmolc kg–1 in the upper 0.15 m.
The vegetation was a typical Cerrado according to the definition of Sarmiento (1984). It was characterized by an open grassland with a 15 to 40% cover of 3- to 5-m-high trees. Tree density was 6487 ha–1 with only 602 trees ha–1 taller than 2 m. Dominant tree species in the >2-m layer were Pouteria torta (Mart.) Radlk., Ouratea spectabilis (Mart.) Engl., Roupala montana Aubl., Byrsonima coccolobifolia H.B. et K., Dalbergia miscolobium Benth., Kielmeyera coriacea Mart., and Caryocar brasiliense Cambess., which together represented 70% of the biomass of the >2-m layer. In the 0.5- to 2-m-tall tree layer, many different species were found, of which Ouratea hexasperma (St.-Hil.) Baill., representing 33% of the biomass in the 0.5- to 2-m layer, was most abundant. The dominant shrub species were Miconia holosericea DC., Hortia brasiliana Vand. ex DC., Myrcia rostrata DC., Parinari obtusifolia Hook. f., and Campomanesia velutina Blume, contributing 93% to the total shrub biomass. Among the grass species we most frequently found Andropogon minarum Kunth, Axonopus barbigerus (Kunth) Hitchc., Tristachya chrysothrix Nees, and Echinolaena inflexa (Poir.) Chase of the family Poaceae, which comprised the highest number of species. Among the herbaceous species, members of the families Asteraceae, Rubiaceae, Fabaceae, and Mimosaceae were most abundant.
Sample Collection
Soils
We took one surface soil sample (0–0.15 m), consisting of five subsamples that were combined. At the 0.15- to 0.3-, 0.3- to 0.8-, 0.8- to 1.2-, and 1.2- to 2-m depths, we furthermore collected a sample in a representative way from the wall of a 2-m-long soil trench.
Fine Litterfall
On each plot, we installed five litter collectors with a surface of 0.25 m2. The litter collectors were placed on locations near to trees and further away from trees. The content of the five collectors was combined to a representative sample on each collection day. Litter was collected in 1- to 3-wk intervals between 25 Apr. 1997 and 28 Apr. 1999. For PAH analyses we bulked all samples of the 1997 dry season, the 1997–1998 rainy season, the 1998 dry season, and the 1998–1999 rainy season.
Biomass
For above- and belowground biomass harvest, we selected five large plots of 25 x 25 m at random in a primary, old-growth Cerrado area of approximately 100 ha representing the remaining native semideciduous Cerrado vegetation. Biomass determination is described in detail in Lilienfein et al. (2001).
Analyses
Soil bulk density was determined gravimetrically using 100-mL soil cores. Five replicates were collected per layer (Lilienfein et al., 1999).
The soil samples were air-dried under ambient conditions and the plant samples were dried at 40°C in a drying oven near the sampling site. Soil samples were sieved to <2 mm. The plant samples were ground.
Immediate drying could not be avoided. As the extraction yield of PAHs depends on the water content (Wilcke et al., 2003b), our sample treatment was a standardization to dry conditions. For the dead wood, litterfall, organic layer, and topsoil samples, complete drying commonly occurs during the dry season. Although we minimized the contact of the samples with the air in the drying room and with laboratory air, losses of volatile compounds or sample contamination from the indoor air may not be entirely ruled out. Wilcke et al. (2003b) tested the influence of air-drying on PAH concentrations in 36 soil samples varying in C concentrations between 14 and 477 g kg–1 and in sum of 21 PAH concentrations between 53 and 6870 µg kg–1 as compared with the extraction of field-fresh samples frozen immediately after collection. They found consistently lower concentrations of all studied PAHs in air-dried than in field-fresh extracted samples. The naphthalene concentrations in air-dried samples were, on average, 33% of those in the field-fresh extracted samples and all other PAH concentrations in the air-dried samples ranged between 61 and 90% of the field-fresh extracted samples. This was attributed to volatilization losses of naphthalene and reduction of the extractability for all other compounds. To check whether air-drying of the Brazilian samples resulted in similar changes in PAH concentrations compared with field-fresh extracted samples, we conducted additional drying tests with one bark and four soil samples. For these tests, the samples were dried in a storage room in our laboratory in Germany for 5 d.
The storage of the samples in plastic bags may have resulted in the loss of PAHs from the samples by sorption to the plastic. However, as the sample mass was in the range of several 100 g with only a small part attached to the bag, we considered these losses as negligible.
Total C concentrations were determined by dry combustion and gas chromatographic separation with a CNS analyzer (Elementar Vario EL; Elementar Analysensysteme GmbH, Hanau, Germany).
Total concentrations of 20 PAHs in soils were determined after pressurized solvent extraction with hexane and acetone (2:1) at 120°C and 14 MPa using an Accelerated Solvent Extraction (ASE) 200 device (Dionex, Sunnyvale, CA). Extracts were cleaned up by solid phase extraction with silica gel–aluminum oxide columns.
We identified and quantified PAHs in all extracts by gas chromatography (HP5890; Hewlett-Packard, Palo Alto, CA) using a Hewlett-Packard 5-MS fused silica capillary column (30 m x 0.25 mm x 0.25 µm) and mass spectrometry (HP 5971A). Twenty PAHs were quantified: naphthalene, acenaphthylene, acenaphthene, fluorene, phenanthrene, anthracene, fluoranthene, pyrene, benz[a]anthracene, chrysene + triphenylene, benzo[b + j + k]fluoranthenes, benzo[a]pyrene, benzo[e]pyrene, perylene, indeno[1,2,3-cd]pyrene, dibenz[a,h]anthracene, and benzo[ghi]perylene.
We used eight deuterated PAHs as internal standards that were spiked to the samples before extraction. Further details of the PAH analysis are given in Krauss et al. (2000). The recoveries of the internal standards ranged from 72 ± 26% (mean ± standard deviation) to 98 ± 25% relative to fluoranthene–D10 spiked to the extracts before injection.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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20PAHs) concentrations in the field-fresh sample and 49 to 130% for the individual PAHs. In the air-dried soil samples, we found 85 ± 18% (mean ± standard deviation, n = 4) of the 20PAHs concentrations in the field-fresh sample and 25 to 165% for the individual PAHs. When the extreme values of this range of 25% (acenaphtylene) and 165% (naphthalene) are removed, the range of the recovery of the remaining compounds in the air-dried compared with the field-fresh extracted samples (46–115%) is comparable with that in the bark sample. The lower extraction yield of most PAHs in air-dried samples is in agreement with the findings of Wilcke et al. (2003b) and probably is caused by a better access of the organic solvent to binding sites in the swollen organic matter of moist soils (Guggenberger et al., 1996). We consider it unlikely that high molecular weight PAHs were lost by volatilization because of their low vapor pressure (10–3.5 to 10–7.9 Pa for all PAHs with a molar mass of >200 g; Mackay et al., 1992). Furthermore, we did not observe a relationship between decrease in concentrations because of air-drying and volatility of the individual PAHs, which would have been the case if volatilization explained the different extraction yields. For the bark sample and for most individual PAHs in the four tested soils, the remaining differences between field-fresh and air-dry sample extraction may mainly be attributed to sample heterogeneity and the error of the analysis of 10 to 20%.
However, the four tested soil samples gained naphthalene and lost acenaphtylene during air-drying. Thus, naphthalene and acenaphtylene concentrations are associated with a large uncertainty. Consequently, differences among plant compartments may only be detected if they are larger than this error. Furthermore, the error has to be considered when naphthalene and acenaphthylene storages in ecosystem compartments are discussed.
Polycyclic Aromatic Hydrocarbon Concentrations
Mineral Soil
The 20PAHs concentrations in the 0- to 0.15-m layer of the mineral soil (42–65 µg kg–1) were at the lower end of the range of PAH concentrations in tropical topsoils of 6.5 to 397 µg kg–1 (for the sum of 16 PAHs; Wilcke, 2000). Tropical topsoils frequently have lower PAH concentrations than soils of the temperate zone (Wilcke, 2000). Wilcke (2000) reported in his review mean concentrations of 328, 284, 904, and 4420 µg kg–1 (sum of 16 PAHs) for a range of temperate arable, grassland, forest, and urban topsoils, respectively. The PAH concentrations in the Cerrado soils were also at the lower end of the range of PAH concentrations in soils of the remote subtropical Teide mountain in Tenerife of 1.9 to 6000 µg kg–1 (sum of 26 PAHs; Ribes et al., 2003).
The 20PAHs concentrations decreased with increasing soil depth to 10 to 34 µg kg–1 in the 1.2- to 2-m-depth layer. To a depth of 1.2 m, naphthalene was the most abundant individual PAH contributing 49 to 75% to the 20PAHs concentrations followed by phenanthrene (15–39%). In the 1.2- to 2-m-depth layer, phenanthrene was most abundant (52%) followed by naphthalene (34%). The PAH pattern in the Cerrado soils is typical of tropical climates (Wilcke, 2000; Wilcke et al., 2003a).
Plants
The 20PAHs concentrations in plant tissue of the Cerrado (Table 1) were lower than in vegetation of the UK of approximately 800 µg kg–1 (sum of 11 PAH concentrations including the four most abundant PAHs of our study) (Wild et al., 1992; Wild and Jones, 1995). The PAH concentrations in the Cerrado leaves, however, were similar or higher than in a mixed forest in the UK at a remote location of 28 to 72 µg kg–1 (sum of 23 PAH concentrations) (Howsam et al., 2000). The PAH concentrations in Cerrado leaves were well within the range reported for pine needles at various locations in Korea, Mexico, and the United States of 31 to 563 µg kg–1 that includes some urban and/or industrialized sites (Hwang et al., 2003). However, the latter authors did not include naphthalene in their analysis. We do not know of data on PAH concentrations in tropical plants.
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20PAHs concentrations varied by a factor of 2 to 3 among the sampled species. The highest PAH concentrations were found in the dominating understorey tree Ouratea hexasperma (Fig. 1) .
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20PAHs concentration (Table 1). In the living aboveground biomass, the bark had the highest mean 20PAHs concentration. A similar distribution of the 20PAHs among the plant compartments was observed for Ouratea hexasperma. In the shrub layer, again the dead wood had the highest 20PAHs concentration followed by the stems (wood and bark). The coarse roots of the tree and shrub layers had consistently the lowest 20PAHs concentrations of all living and dead biomass compartments. Thus, the PAH distribution among different plant compartments appears to reflect their exposure to the atmosphere. The bark contains more PAHs than the leaves because it is older and therefore had a longer time to accumulate airborne PAHs. Besides, elevated PAH concentrations in the bark may also indicate the production of PAHs by organisms residing in the bark or by the plant itself. Similar PAH concentrations in leaves and in stem wood point to such internal PAH sources because it is unlikely that PAHs are translocated in plants to a significant degree except perhaps naphthalene, acenaphtylene, and acenaphthene, which have an octanol–water partition coefficient smaller or close to 104, considered as the lowest water solubility that allows plants to draw persistent organic pollutants into the inner root or xylem (Simonich and Hites, 1995).
In all plant samples, either naphthalene or phenanthrene was the most abundant individual PAH in most cases followed by the other of these two compounds. There were only four exceptions in which pyrene (in Hortia brasiliana wood), the benzo[b + j + k]fluoranthenes (in Hortia brasiliana leaves), and acenaphthene (in Campomanesia velutina wood and leaves) were the second-most abundant compounds. Together, naphthalene and phenanthrene contributed 33 to 95% to the 20PAHs concentrations.
Similar to our findings, phenanthrene was also the most abundant PAH in oak and ash leaves of a mixed deciduous forest in the UK (Howsam et al., 2001) and in pine needles from Korea, Mexico, and the United States, except for heavily polluted locations (Hwang et al., 2003). Thus, phenanthrene seems to occur as a major PAH in plant tissue irrespective of climatic conditions. This supports the hypothesis that it may be synthesized by plants. However, Johansson and van Bavel (2003) also reported that phenanthrene was a major constituent of the PAH mixture in incineration ashes of various origins. Therefore, vegetation fires may also be phenanthrene sources for the Cerrado plants.
The prominent role of naphthalene in plant tissues, in contrast, seems to be typical of tropical environments. The naphthalene concentrations in plant tissue were at least one order of magnitude higher than in the leaves of a UK forest studied by Howsam et al. (2000). This difference is considerably larger than the uncertainty of our naphthalene measurement implying that the naphthalene concentrations in the Cerrado plants may be less than a factor of 2 higher or lower than we have measured them. Our finding may be explained by particularly high naphthalene concentrations in the atmosphere. The reason for such elevated naphthalene concentrations could be frequent vegetation fires or the common production of charcoal. From the high abundance of naphthalene in wood ash, it may be inferred that it is produced in wood fires (Bundt et al., 2001; Johansson and van Bavel, 2003). Alternatively, naphthalene may be produced by the plants or organisms associated with the plants.
The mean PAH pattern in the compartments of the upper tree layer varied systematically (Table 1). The contribution of naphthalene to the 20PAHs concentrations decreased in the order stem wood (57%) > dead wood (46%) > bark (36%) > twigs (33%) > coarse roots (32%) > leaves (21%). In the leaves and the coarse roots, phenanthrene was on average the most abundant individual PAH, contributing 27 and 40% to the 20PAHs concentrations, respectively. The mean contribution of acenaphthylene to the 20PAHs concentrations in the leaves (1.0%) was higher than in all other compartments (0.18–0.65%) and that of anthracene (0.75%) in the leaves was higher than in the coarse roots (0.21%). Furthermore, the contribution of fluoranthene to the 20PAHs concentrations in dead wood and in bark (5.4–7.0%) was higher than in stem wood (1.7%), and that of chrysene + triphenylene decreased in the order leaves (4.3%) > bark (1.6%) > dead wood (1.5%), twigs (1.4%) > stem wood (0.53%) > coarse roots (0.21%). Thus, the exterior plant compartments, particularly the leaves, had higher contributions of the more volatile low molecular weight PAHs except naphthalene, but also of some representatives of the high molecular weight compounds that are known markers of fossil fuel combustion (Baek et al., 1991). The fact that the contribution of presumably combustion-derived PAHs to the leaves did not include elevated naphthalene contributions suggests that naphthalene had a different source. This is further supported by the finding that the naphthalene contribution to the 20PAHs concentrations even increased in the older plant compartments although naphthalene should be lost relative to all other PAHs because of its high vapor pressure and biodegradability compared with other PAHs (Wild and Jones, 1995).
To better assess the possible contribution of vegetation fires to the PAH concentrations in the compartments of the Cerrado, we normalized selected individual PAH concentrations to the chrysene + triphenylene concentrations and compared these ratios with the means of the same ratios in aerosols produced by different kinds of vegetation fires obtained from Freeman and Cattell (1990). Most ratios in the emissions of vegetation fires (fluoranthene = 0.72–5.6, pyrene = 1.0–6.9, and benzo[a]anthracene = 0.14–0.93) were different from those in the plant compartments of the upper tree layer (1.3–14, 3.8–26, and 0.05–1.3, respectively) and of the shrub layer (5.6–6.1, 3.9–7.4, and 1.1–1.8, respectively), further supporting the assumption that there are other PAH sources than vegetation fires for the aboveground plant compartments. This again points at biological sources of naphthalene. In the organic layer, mineral topsoil, lower tree layer, and fine litterfall, the PAH to chrysene + triphenylene ratios were closer to those of the emissions of vegetation fires.
Litter
The 20PAHs concentrations in the fine litterfall were similar to those in the living leaves (Table 2). The PAH concentrations in fine litterfall showed a seasonal variation (Fig. 2)
. The PAH concentrations were higher in the dry than in the rainy season. The reason for this observation was probably the more frequent occurrence of vegetation fires during the dry than the rainy season. In most samples, phenanthrene was the most abundant PAH, contributing 25 to 48% to 20PAHs concentrations, followed by naphthalene (8.7–51%), which was most abundant in one sample. In one of the three litterfall samples of the 1998–1999 rainy season, fluorene was most abundant (contributing 35% to the 20PAHs concentrations) followed by phenanthrene (31%).
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20PAHs concentrations in the litter layer were one order of magnitude lower, on average, than in 95 temperate Oi horizons (Wilcke, 2000). However, they were higher than in the living biomass except for the understorey tree Ouratea hexasperma (Tables 1 and 2). This indicates that the PAHs were accumulated relative to the organic matter in the litter layer. Another explanation would be that there were additional PAH sources for the litter layer, such as the direct deposition from the atmosphere, that may be higher than the PAH flux with litterfall (Matzner, 1984; Howsam et al., 2001) or the biological production of PAHs by soil-living animals and fungi.
Polycyclic Aromatic Hydrocarbon Storage
The size of the PAH storage in the Brazilian savanna compared with that in the environment of fully industrialized countries such as the UK provides further clues to the role of the tropics as source and sink of PAHs. The total naphthalene storage per unit area of the studied typical Cerrado including the upper 15 cm of the soil is similar to the contemporary naphthalene burden per unit area of the UK environment (also including the upper 15 cm of the soil, strongly contaminated sites were not considered) (Wild and Jones, 1995) (Table 3). The total phenanthrene storage per unit area of the Cerrado including the upper 15 cm of the soil accounts for 10% of the mean phenanthrene storage per unit area of the environment in the UK, that of all other PAHs for less than 3%. The total PAH storage of the studied typical Cerrado to a 0.15-m soil depth represents 330 to 540 times and to a 2-m soil depth 2200 to 3600 times the annual deposition of the sum of 23 PAHs to remote mountain areas in Europe of 14 to 23 µg m–2 yr–1 (extrapolated from the monthly means obtained mainly for spring and summer months by Fernandez et al., 2003). These results demonstrate that PAH storage in the studied Cerrado cannot be explained by deposition from the atmosphere alone, again indicating that there must be additional sources such as the suggested biological ones.
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The soil (to a depth of 2 m) contains more than 90% of the total PAH storage in the Cerrado ecosystem except for the high molecular weight compounds benzo[a]pyrene (74%), indeno[1,2,3-cd]pyrene (80%), dibenz[ah]anthracene (47%), and benzo[ghi]perylene (63%). The high contribution of the aboveground biomass to the total storages of the mainly combustion-derived high molecular weight PAHs is probably related to the fact that these compounds are scavenged from the atmosphere by the vegetation canopy. The aboveground biomass contributed consistently more PAHs to the total PAH storage than the belowground biomass (except for similar contribution of above- and belowground biomass to the total perylene storage). Thus, the soil is the most important sink of PAHs and its current PAH burden still is not dominated by the markers of fossil fuel combustion, probably because increasing anthropogenic emissions are a recent phenomenon in the Brazilian savanna.
Polycyclic Aromatic Hydrocarbon Turnover
Turnover times of PAHs have to be known if the ecotoxicological meaning of PAHs in ecosystems and the strength of their sources are to be assessed. The annual fluxes of fluorene and phenanthrene with litterfall in our study forest were higher than in a rural forest in the UK but those of fluoranthene, pyrene, benzo[k]fluoranthene, and benzo[ghi]perylene were lower (Howsam et al., 2001; Table 2). The PAH fluxes with litterfall in the Cerrado were consistently lower by 1 to 2 orders of magnitude than 20 yr ago in German beech and spruce stands. The difference was much larger for high than for low molecular weight PAHs (Matzner, 1984). Thus, the PAH fluxes with litterfall in the Cerrado were more dominated by low molecular weight PAHs compared with those in European temperate forests, probably reflecting the much lower presence of high molecular weight PAH emissions in the tropics.
Higher PAH concentrations and higher litterfall in the dry (on average, 0.12 kg dry mass m–2 in 1997 and 0.16 kg m–2 in 1998) than in the rainy season (0.06 kg m–2 in 1997–1998 and 0.08 kg m–2 in 1998–1999) resulted in much higher PAH fluxes with litterfall in the dry than in the rainy season.
We calculated mean residence times of the PAHs in the organic layer as the ratio of total storage of a PAH in the organic layer to the flux of the same PAH with annual litterfall. The mean residence time of the 20PAHs concentrations (1.8 yr) was longer than the mean residence time of the organic matter of 1.2 yr (Wilcke and Lilienfein, 2002). At the first glance, this met the expectation that PAHs are less rapidly degraded than bulk organic matter. However, the residence times varied substantially between 0.2 and 7.8 yr (Table 2) and were independent of compound properties. Howsam et al. (2001) found residence times between 0.1 and 0.3 yr in a UK forest. They attributed this result to faster PAH losses by volatilization, degradation, and assimilation by decomposers than of the majority of the compounds in organic matter. Surprisingly, the low molecular weight PAHs had the longest residence times although they should be volatilized, leached, and degraded at the fastest rates. This contrasts the result of Howsam et al. (2001) who found the shortest residence time for fluorene and the longest for benzo[ghi]perylene. Thus, the deposition of the low molecular weight PAHs overcompensated the losses, which we again interpret as additional evidence for large biological PAH sources in the tropical environment.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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íková. 2003b. Persistent organic pollutant concentrations in air- and freeze-dried compared to field-fresh extracted soil samples of an eastern Slovak deposition gradient. J. Plant Nutr. Soil Sci. 166:93–101.Related articles in JEQ:
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