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Published online 9 August 2005
Published in J Environ Qual 34:1581-1590 (2005)
DOI: 10.2134/jeq2005.0005
© 2005 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America
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TECHNICAL REPORTS

Organic Compounds in the Environment

Polycyclic Aromatic Hydrocarbons (PAHs) in Soils of the Moscow Region— Concentrations, Temporal Trends, and Small-Scale Distribution

Wolfgang Wilckea,*, Martin Kraussa, Grigorij Safronova, Alexej D. Fokinb and Martin Kaupenjohanna

a Department of Soil Science, Institute of Ecology, Berlin University of Technology, Salzufer 11-12, D-10587 Berlin, Germany
b Department of Radioecology, Timiryazev Agricultural Academy, 49 Timiryazevskaya str., Moscow, 127550, Russian Federation

* Corresponding author (wolfgang.wilcke{at}tu-berlin.de)

Received for publication January 7, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The knowledge of the environmental fate of polycyclic aromatic hydrocarbons (PAHs) is restricted to few climatic regions of the world almost excluding the Taiga. Our objectives were to (i) separate anthropogenic from background contributions to PAH concentrations and (ii) determine temporal trends in PAH concentrations during the last century including the change in distribution of PAHs in interior and exterior portions of aggregates in soils of the Moscow region. Along a southeast-bound transect from Moscow (windward in winter) and at a background location northeast of Moscow (leeward in winter), seven topsoil samples were collected in 1910–1954 and 35 in 1998–2003. We fractionated the soils in interior and exterior portions of aggregates > 10 mm and remaining soil without aggregates. The sum of 21 PAHs ({Sigma}21PAHs) concentrations in recent bulk soil ranged from 59 to 1350 ng g–1. The concentrations of all PAHs were lower outside than in Moscow. The range of the concentrations of the {Sigma}21PAHs in archived soil samples (159–1280 ng g–1) was similar as in recent soils. In most recent and archived samples, naphthalene and phenanthrene, were most abundant. The concentrations of low-molecular-weight PAHs decreased during the last century at most sites; those of high-molecular-weight compounds increased. The {Sigma}21PAHs concentrations were accumulated in the exterior of aggregates (109%) and depleted in the interior (95%) relative to the concentration in bulk soil (defined as 100%), which was similar to that in the soil without aggregates (99%). The differences between aggregate interior and exterior did not change during the last century. The dominance of naphthalene and phenanthrene is typical of remote regions. The urban influence on PAH concentrations in the last century was small.

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 • COR, coronene • DIBE, dibenz(a,h)anthracene • FLUA, fluoranthene • FLUO, fluorene • IND, indeno(1,2,3-cd)pyrene • NAPH, naphthalene • PAH, polycyclic aromatic hydrocarbon • PERY, perylene • PHEN, phenanthrene • PYR, pyrene • SOC, soil organic carbon • {Sigma}21PAHs, sum of 21 polycyclic aromatic hydrocarbons


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
MOST OF OUR KNOWLEDGE of the persistent, partly toxic polycyclic aromatic hydrocarbons (PAHs) in soils is restricted to temperate areas of west and central Europe and North America (Sims and Overcash, 1983; Wilcke, 2000). As PAHs can travel long distances in the atmosphere and are therefore globally distributed (Wania and Mackay, 1996), it is crucial to determine the sources and fate of PAHs in all climatic zones of the world. 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 in soil 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, particularly in less anthropogenically affected regions, biological sources may be more important than previously assumed. There is some evidence that at least naphthalene, phenanthrene, and perylene are produced biologically (Venkatesan, 1988; Silliman et al., 2001; Neilson and Hynning, 1998; Thiele and Brümmer, 2002; Wilcke et al., 2003a).

It has been shown that the composition of the various PAH sources and the alteration in the atmosphere and in soils result in typical PAH spectra, which are specific for different, mainly climatically defined ecoregions (Jones et al., 1989c; Wilcke, 2000; Wilcke et al., 2003a). Such "fingerprints" of PAHs have up to now been identified for west and central Europe, the North American Prairie, and selected tropical regions (Wilcke, 2000; Wilcke and Amelung, 2000; Wilcke et al., 2003a; 2004).

For several locations in west and central Europe and North America, temporal trends in deposition rates of PAHs have been traced back with the help of archived soil samples, dated peat cores, or dated marine and lacustrine sediments (Jones et al., 1989a, 1989b, 1989c; Sanders et al., 1993, 1995). In general, the deposition rates of PAHs increased markedly at the end of the 19th century following industrialization, reached maximum concentrations around the mid-20th century and decreased thereafter. However, they are today still several times higher than before the start of industrialization. No information is available on the temporal trend in other regions of the world. At the Timiryazev Agricultural Academy in Moscow, undisturbed soil cores from the beginning of the 20th century are stored, which could be used to establish baseline concentrations of PAHs in the Moscow region at the beginning of the 20th century.

In most studies on the contamination of soils with PAHs, homogenized bulk soil samples are used. However, it has been shown that PAHs may accumulate preferentially on the surfaces of soil aggregates and preferential flow paths (Wilcke et al., 1996a, 1996b; Bundt et al., 2001a). As the biological activity (root growth and microbial activity) is higher in the aggregate exterior than in bulk soil (Whitely and Dexter, 1983; Bundt et al., 2001b), conventional analyses may underestimate the ecological risk of PAHs in soils. It has been hypothesized that the gradients of PAH concentrations in aggregates are caused by higher inputs of PAHs from the atmosphere or above-lying soil horizons, mainly transported along preferential flow paths in contact with aggregate surfaces, compared with losses by leaching, volatilization, and degradation (Wilcke et al., 1996a).

Our objectives were to (i) separate anthropogenic from background PAHs in soils along an urban–rural transect in the Moscow region, (ii) determine temporal trends of PAH concentrations by comparing archived samples from the beginning and the middle of the 20th century with recently taken samples, and (iii) examine the small-scale distribution of PAHs in soil aggregates in archived and recent samples.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Study Region
Samples were collected in the city of Moscow (Lesnaja Dacha, urban forest park near the Timiryazev Agricultural Academy), and along a southeast-bound transect following the main northwesterly wind direction in winter, when PAH emissions are highest, at distances from the city center of Moscow of 20 km (Vidnoe), 30 km (Schapovo), 40 km (Ramenskoe), and 50 km (Bronnitsi). Furthermore, we sampled a background site at a distance of 140 km to the northeast from Moscow (Nenashevo, Vladimir Region, Fig. 1) . We hypothesized that the influence of the emissions of Moscow decreased with increasing distance from the city. We assumed that the reference site showed the natural background PAH pattern because of its location leeward of Moscow during most of the time (exceptions are southwesterly winds in summer). The background site is located at a greater distance to Moscow than the farthest site along the main wind direction to the southeast. Therefore, we assumed that it would even not be significantly impacted by emissions from Moscow during southeasterly wind situations in summer. The study region belongs to the south Taiga with a mean annual precipitation of 650 mm and a mean annual temperature of 3 to 3.5°C. The annual amplitude of the temperature is 28°C and the number of days with frost ranges from 120 to 135. All soils are Typic Glossudalfs except at Nenashevo (Histic Humaquept; USDA Natural Resources Conservation Service, 2003). All soils derived from the loess-like mantle loam of Late Weichselian age except those in the urban forest park of Moscow, Lesnaja Dacha, where soils derived from moraine material.



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  		Fig. 1. Location of the study sites.

 
Sampling
Recent soil samples were collected between 1996 and 2003. At each distance from the city center of Moscow, we collected three samples of the A horizons from each of adjacent grassland and forest soils except at Nenashevo where only grassland was present. Sampling depth varied between 0.1 and 0.2 m depending on the depth of the A horizon. The three replicate samples of the same location and land use were taken at a distance of at least 200 m so that they could be considered as independent replications. One replicate at each site except Nenashevo was collected in 2000 exactly at the same location from which archived samples taken in 1910–1954 were available. The results of the analyses of these samples were used to assess changes in PAH concentrations and patterns during the last century to minimize the confounding effect of spatial heterogeneity. In 2003, two more samples were collected from each of the grassland and forest soils at each of the study sites south of Moscow to assess the recent spatial variation. At Nenashevo, two samples were taken in 1996 and three in 2003. All samples were taken from the walls of a soil pit. The pits were freshly dug on the sampling day. Sampling was done consistently in summer (July–September).

The forest sites were continuously under forest since the beginning of the 20th century. The grassland sites were to various extents used for agricultural purposes between the historic and recent sampling, but they were continuously under grassland since at least 1995. The study site at Schapovo (50 km from Moscow) was under arable use in 1954 and under grassland in 2000 and 2003.

Archived samples were taken from five intact soil monoliths collected between 1910 and 1922 and one soil monolith collected in 1954 at the same locations as the recent samples. The archived monoliths had been air-dried and stored in a wooden frame covered with paper in a storage room. No chemicals were added to stabilize and conserve the monoliths. The monoliths were stored in a room exclusively used for the storage of soil samples. They were not touched before we used them for our study. We may not rule out that the soils have been contaminated by deposition from the air and will consider the possible error in our discussion. The identity of the archived and recently collected samples at Lesnaja Dacha, Bronnitsi, and Nenashevo has been previously proven by detailed analysis of the mineralogical composition (Ilg et al., 2004). Although this early soil sampling was not designed to capture the spatial variation, we consider the opportunity to study these samples as unique because they contain important information on a climatic region, which is clearly underexplored with respect to the dynamics of persistent organic pollutants. The recent soil samples were air-dried to enable the comparison of archived and recent samples. This may have changed the composition of the PAH mixture. Wilcke et al. (2003b) found that the concentrations of PAHs in air-dried samples were consistently lower than in field-fresh extracted samples because of the volatilization of naphthalene and a decrease in extractability of all other compounds. The mean differences between field-fresh extracted and air-dried samples were <33% except for naphthalene, of which 67% was lost. Tables 1 and 2 summarize sampling year, location, land-use, soil type, sampling depth, and selected physical and chemical soil properties.


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  		Table 1. Selected properties of the recent soil samples of the Moscow region.

 

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  		Table 2. Selected properties of the paired archived and recent soil samples of the Moscow region (distance to Moscow, soil types, and texture as in Table 1).

 
Soil Fractionation
Soil aggregates > 10 mm in diameter were collected manually from the samples. The aggregate exterior was removed by peeling with a scalpel. We removed 30% of the aggregate's mass as aggregate exterior. The contribution of the aggregates > 10 mm to the bulk soil was 0.9 to 6.9% of the mass of the sampled soil layers at all sites except Nenashevo where the aggregates > 10 mm contributed 31% to the total mass. The remaining soil was sieved to <2 mm. The soils were free of gravel.

Chemical Analyses
The pH of the soil samples was determined in a suspension of 20 g of soil in 50 mL of 1 M KCl using a standard pH electrode after equilibration over night. Exchangeable cations were extracted with 1 M NH4–acetate (Sumner and Miller, 1996) and measured with atomic absorption spectrometry. The concentrations of soil organic carbon (SOC) and total N were determined with a C and N analyzer (Vario EL from Elementar Analysensysteme GmbH, Hanau, Germany or an instrument from Leco, St. Joseph, MI).

Total concentrations were determined for 21 PAHs, including naphthalene (NAPH), acenaphthylene (ACENY), acenaphthene (ACEN), fluorene (FLUO), 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)], and coronene (COR). The benzofluoranthenes and chrysene/triphenylene could not be separated from each other with the method used.

We extracted up to 30 g of soil with hexane and acetone (2:1) in an Accelerated Solvent Extractor (ASE 200; Dionex, Sunnyvale, CA). Extraction 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 and cells were rinsed with cold solvent (60% of cell volume) and purged with Ar for 150 s. The static extraction, rinse, and purge steps were performed twice for each sample and the extracts were combined.

The extracts were evaporated to 1 mL and purified by solid phase extraction with 2 g of aluminum oxide (5% deactivated, upper part) and 2 g of silica gel (5% deactivated, lower part). The PAHs were sequentially eluted with 15 mL of hexane, 5 mL of hexane and dichloromethane (9:1), and 20 mL of hexane and dichloromethane (4:1). The eluted fractions were combined and evaporated to approximately 500 µL before PAH measurement. We added eight deuterated PAHs (NAPH-D8, ACEN-D10, FLUO-D10, ANTH-D10, PYR-D10, chrysene-D12, PERy-D12, BGHI-D12) as internal standards to soils before extraction. To check the recovery of the internal standards, FLUA-D10 was added to the extracts before injection into the gas chromatograph. The mean recoveries of the internal standards ranged from 91 ± 13% (mean ± standard deviation) to 99 ± 9.4%.

We used 5890 Series II and 6890 gas chromatographs (Hewlett-Packard, Palo Alto, CA) equipped with a Hewlett-Packard 5-MS fused silica capillary column (30 m x 0.25 mm x 0.25 µm) with He as carrier gas (constant pressure mode 80 kPa) and splitless injection. Compounds were identified and quantified with Hewlett-Packard 5971 A and 5973 mass selective detectors with electron impact ionization at 70 eV in the selected ion monitoring mode. The inlet temperature was 280°C, and detector temperature was 320°C. Temperature program started at 80°C, held for 4 min, increased to 160°C at 14°C min–1, held for 1.5 min, and increased to 300°C at 4.8°C min–1, held for 3 min.

Calculations and Statistical Evaluation
Bulk soil concentrations were calculated as mass-weighted mean of the concentrations in the three fractions "soil without aggregates > 10 mm" and interior and exterior of aggregates > 10 mm according to their contributions to the total soil mass. For the contribution of the aggregates > 10 mm to the mass of the bulk soils at Schapovo the mean contribution of the aggregates > 10 mm of all the other Podzoluvisols sampled in the mantle-loam region was used because no measured value was available.

Differences in mean concentrations, percentages, and SOC-normalized concentrations among the study sites were tested with the Games Howell post hoc test (Sachs, 2002, p. 650–652). This test does not require homogeneity of variances. Concentrations of PAHs in aggregate fractions were pairwise compared using the "paired differences test" (Sachs, 2002, p. 408–410). Significance was set at p < 0.05.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Concentrations and Patterns of PAHs in Recent Bulk Soil
The concentrations of the sum of 21 PAHs ({Sigma}21PAHs) in the recent topsoils ranged from 59 to 1350 ng g–1 (Table 1). The concentrations of PAHs in the soils outside Moscow were similar to frequently reported background values for grassland and forest soils of the temperate zone (Wilcke, 2000) except in the forest soil at Vidnoe. They were within the range found in topsoils of remote sites in the North American Prairie (63–321 ng g–1 for the sum of 20 PAHs; Wilcke and Amelung, 2000), except the samples from Nenashevo collected in 2003. The much higher concentrations of PAHs in the samples collected at Nenashevo in 2003 than in those collected in 1996 may be attributable to high PAH inputs because of the burning of adjacent mires in 2002 (own observation). The highest concentrations of the {Sigma}21PAHs at the Moscow and Vidnoe sites were lower than the mean of urban soils of the temperate zone (4420 ng g–1 for the sum of 16 PAHs; Wilcke, 2000). The concentrations of B(A)P, frequently correlated with the sum of PAH concentrations (Wilcke, 2000), in our recent samples are lower than in roadside soils in the eastern part of Moscow (59–1544 ng g–1) and similar to urban background concentrations in Moscow of 7.2 to 58 ng g–1 (Nikiforova and Alekseeva, 2002). This indicated a comparatively low contamination of the soils with PAHs even at the studied urban sites in Moscow.

The forest soil samples had higher mean PAH concentrations than the corresponding grassland soil samples except the soils at Ramenskoe (Table 1). However, the differences in the mean {Sigma}21PAHs concentrations between grassland and forest soils were only significant at Schapovo. Higher PAH concentrations in forest than in grassland soils may be explained by the scavenging effect of the forest canopy, which increases the deposition of PAHs to the soil (Matzner, 1984; Horstmann and McLachlan, 1998).

The mean composition of the mixture of PAHs, the "pattern of PAHs," was quantitatively dominated by PHEN and NAPH (Fig. 2a) . This is similar to findings of Nikiforova and Alekseeva (2002) for remote background soils of the Moscow region where homologs of NAPH and PHEN (i.e., unsubstituted and alkylated compounds) dominated the PAH mixture. One of PHEN or NAPH was also most abundant in all individual samples except the Vidnoe forest soil where FLUA and B(BJK) dominated the pattern of PAHs quantitatively. Apart from the Vidnoe forest soil, this is different from patterns commonly reported for central and west Europe where the benzofluoranthenes, chrysene, and fluoranthene are most abundant (Wilcke, 2000). However, the mean pattern of PAHs in the studied Taiga soils resembled that of the remote North American Prairie (Wilcke and Amelung, 2000) except that the contribution of the high-molecular-weight PAHs was higher in Russia (and that of NAPH and PHEN consequently lower). High-molecular-weight PAHs typically characterize the emissions of fossil fuel combustion (Baek et al., 1991; Menichini, 1992). As the PAH patterns in soils of the Moscow region clearly deviated from those frequently observed in western Europe in that the contribution of high-molecular-weight compounds was lower but still higher than in the remote North American Prairie, we suggest that the Russian pattern of PAHs was the result of the combination of natural background and additionally some inputs of predominantly high-molecular-weight PAHs derived from combustion of fossil fuels or biomass. This would imply that NAPH and PHEN in soils of the south Taiga were produced biologically, by local vegetation fires, or imported by long-distance transport. Phenanthrene occurs in plants, which may synthesize it biologically (Laflamme and Hites, 1978; Wickström and Tolonen, 1987). We only know of one report on the presence of NAPH in plants outside the tropics. Azuma et al. (1996) detected NAPH in the floral scent of Magnolia species from Japan. The evidence from tropical locations suggests that possible biological sources include metabolic processes in plants (or plant-associated microorganisms; Jürgens et al., 2000; Daisy et al., 2002; Wilcke et al., 2000, 2003a, 2004; Krauss et al., 2005).



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  		Fig. 2. Mean pattern of polycyclic aromatic hydrocarbons (PAHs) (a) in recent Russian topsoils (collected 1996–2003) and North American Prairie topsoils (Wilcke and Amelung, 2000) and (b) in Russian topsoils archived between 1910 and 1922. In (b) additionally the pattern of PAHs in the Bronnitsi topsoil sample of 1922, an outlier, and in the Schapovo topsoil sample of 1954 is shown. 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; COR, coronene; DIBE, dibenz-(a,h)anthracene; FLUA, fluoranthene; FLUO, fluorene; IND, indeno-(1,2,3-cd)pyrene; NAPH, naphthalene; PERY, perylene; PHEN, phenanthrene; PYR, pyrene.

 
The concentrations of all PAHs in the topsoils tended to decline with increasing distance from Moscow (Table 3) identifying the city of Moscow as point source of PAHs. This was also the case when the concentrations of the PAHs were normalized to those of SOC and was true for both grassland and forest sites (Fig. 3) . However, not all differences in the mean SOC-normalized {Sigma}21PAHs concentrations among the study sites were significant. Thus, elevated PAH concentrations in soils because of the human activities in Moscow including its outskirts were restricted to a distance of <30 km. The contribution of the sum of the six low-molecular-weight PAHs (NAPH, ACENY, ACEN, FLUO, PHEN, and ANTH) to the concentration of the {Sigma}21PAHs increased significantly with increasing distance, if the samples collected in 2003 at Nenashevo were not considered (Fig. 4) . There are two possible explanations for this result. The low-molecular-weight compounds occur in the atmosphere to a large part in gaseous form and are therefore further distributed than the high-molecular-weight compounds (Yang et al., 1991; Meharg et al., 1998). The other explanation would be that the influence of the fossil fuel combustion–related emissions decreases with increasing distance from the city of Moscow and the background concentrations and patterns of PAHs prevail. The low contribution of low-molecular-weight PAHs to the PAH concentrations in the samples taken in 2003 at Nenashevo again indicates that there were recent inputs of a PAH mixture dominated by high-molecular-weight compounds probably emitted by the burning of mires.


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  		Table 3. Concentrations of 21 polycyclic aromatic hydrocarbons (PAHs){dagger} in the paired archived and recent bulk soil samples of the Moscow region (values calculated as mass-weighted mean of the concentrations in the soil without large aggregates and the interior and exterior fractions of the aggregates > 10 mm).

 


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  		Fig. 3. Relation between distance to the center of Moscow and the recent mean concentrations of the sum of 21 polycyclic aromatic hydrocarbons ({Sigma}21PAHs) normalized to the concentration of soil organic carbon (SOC) (n = 3 for each of the forest and grassland sites, except n = 2 for the grassland samples collected at Nenashevo in 1996). Error bars represent standard errors. Different lowercase letters indicate significant differences of the means at p < 0.05 (Games Howell post hoc test).

 


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  		Fig. 4. Relation between distance to the center of Moscow and the recent mean contributions of the low-molecular-weight polycyclic aromatic hydrocarbons (PAHs) (NAPH, ACENY, ACEN, FLUO, PHEN, and ANTH) to the concentrations of the sum of 21 PAHs (n = 3 for each of the forest and grassland sites, except n = 2 for the grassland samples collected at Nenashevo in 1996). Error bars represent standard errors. Different lowercase letters indicate significant differences of the means at p < 0.05 (Games Howell post hoc test). ACEN, acenaphthene; ACENY, acenaphthylene; ANTH, anthracene; FLUO, fluorene; NAPH, naphthalene; PHEN, phenanthrene.

 
Comparison between Archived and Recent Soil Samples
For this comparison only the paired samples shown in Table 2 were considered. The range of the {Sigma}21PAHs concentrations in the samples collected between 1910 and 1922 of 159 to 1280 ng g–1 was similar as in the recently collected samples (Table 3). The {Sigma}21PAHs concentration in the sample taken in 1954 at Schapovo was even markedly higher than in that taken in 2000. This was also true for the topsoil of the grassland site at Bronnitsi where the sample taken in 1922 was clearly contaminated whereas that taken in 2000 showed low background concentrations for grassland soils. In contrast, the topsoil samples at the Lesnaja Dacha sites had higher {Sigma}21PAHs concentrations in 2000 than in 1910. At the Nenashevo site, again all archived samples had higher concentrations than those taken in 1996. Thus, there was no consistent trend of the differences in PAH concentrations between archived and recent samples except the samples of the Lesnaja Dacha site showing clearly increasing contamination during the last century because of urban emissions of PAHs. There are several possible explanations for the lack of consistent temporal trends at the sites outside Moscow. First, in contrast to the observations in west and central Europe and North America, the deposition rates of PAHs in the Moscow region have not increased because of a lower growth of industrial production and traffic, a higher concentration of the emissions at few hot spots, and a system of district heating reducing the emissions of domestic heating. Second, the spatial heterogeneity of the contamination of soils with PAHs was large and the historic sampling did not adequately capture this variation. However, the present variation in PAH concentrations among the three recent replicated samples was comparatively small. Third, the archived samples may have been contaminated with PAHs during their storage. As all samples were stored in the same room, this would not explain why there were such large differences in the concentrations of PAHs among the different samples. Although we may not entirely rule out some contamination of the samples with PAHs during storage, we consider generally lower deposition rates than in west and central Europe as the most likely explanation that would also be in line with the comparatively low concentrations of PAHs in most study soils.

The patterns of PAHs in all samples archived at the beginning of the 20th century and of the Schapovo sample of 1954 were similar except for that of the soil at Bronnitsi (Fig. 2b). The Bronnitsi sample of 1922 contained considerably higher contributions of FLUA (27% of the sum of 21 PAH concentrations) and PYR (18%) than all other archived samples on average (FLUA: 5.3%, standard error 1.4%; PYR: 3.3%, standard error 0.8%). The identity of these two PAHs was confirmed by mass spectrometry in full scan mode (m/z = 40–550 u). The mean pattern of PAHs in the samples of the beginning of the 20th century except Bronnitsi was different from that of the recently collected samples in that the high-molecular-weight PAHs were almost completely absent (Fig. 2). The pattern of PAHs in the archived samples resembles that of the remote Prairie soils even more than the pattern of PAHs in the recent samples supporting the assumption that contamination of the samples during storage was not important. Furthermore, the ratio of the concentrations of PHEN/ANTH decreased consistently in all recent samples (mean 29, standard error 3.7) compared to the archived samples (mean 63, standard error 5.4). If it is assumed that PHEN is typical of natural background concentrations and ANTH representative of modern PAHs resulting from the enhanced combustion of fossil fuels, our results reflect the recent contamination of the study soils with modern PAHs. The contamination of the archived samples with these two similarly volatile PAHs should, in contrast, result in a similar PHEN/ANTH ratio as in recent soils because the latter reflect the recent mean pattern of PAHs in the atmosphere.

The Bronnitsi sample of 1922 seems to be contaminated with PAHs from another source than all other samples. Again, it is unlikely that this occurred in the storage room. Possibly, the Bronnitsi site was temporally affected by specific emissions containing elevated FLUA and PYR contributions (e.g., fuel oil; Masclet et al., 1986) around 1922. However, nowadays most of this FLUA and PYR has dissipated.

To calculate the change in amount of PAHs in the topsoils, we assumed a bulk density of 1 g cm–3. At the two Lesnaja Dacha sites, we observed a consistent increase in the amounts of all PAHs in the topsoil during the last approximately 90 yr (Table 4). The amount of the sum of the six low-molecular-weight PAHs (NAPH, ACENY, ACEN, FLUO, PHEN, ANTH) and FLUA and PYR increased approximately 3- to 8-fold whereas that of high-molecular-weight PAHs (the remaining 13 compounds) increased approximately 9- to 50-fold. At Bronnitsi, the amount of most PAHs decreased during the last 80 yr. However, the amounts of the high-molecular-weight PAHs B(A)P, PERY, IND, DIBE, B(GHI), and COR also increased slightly at Bronnitsi. At Nenashevo, the amounts of the low-molecular-weight PAHs and of FLUA and PYR decreased during the last approximately 90 yr whereas those of the high-molecular-weight PAHs increased. At Schapovo, again the amounts of most low-molecular-weight PAHs except ACEN and FLUO decreased in the last approximately 50 yr whereas those of the high-molecular-weight PAHs except IND increased. Thus, there was clearly a stronger accumulation of high- than of low-molecular-weight PAHs. As we already ruled out a significant influence of the deposition of PAHs from the atmosphere during sample storage we assume that this was the consequence of the higher persistence of the more condensed compounds (Wild and Jones, 1995). However, it may also be the result of a shift in the composition of the sources of PAHs from mainly low-molecular-weight to the high-molecular-weight compounds because of increasing combustion of fossil fuels during the last century. A similar result was reported for agricultural soils in England by Jones et al. (1989a)(1989b, 1989c).


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  		Table 4. Mean annual rates of accumulation (positive values) or depletion (negative values) of polycyclic aromatic hydrocarbons (PAHs){dagger} in topsoils of the Moscow region.

 
From the difference in amount of PAHs in the topsoils and the time between the two sampling dates we estimated mean annual rates of accumulation or depletion of PAHs (Table 4). These rates were the result of the deposition of PAHs from the atmosphere and the dissipation by volatilization, degradation, plant and animal uptake, and leaching to deeper soil horizons. At the Schapovo site, we assumed that the concentrations of PAHs of the 0- to 15-cm soil layer of the grassland soil sampled in 2000 used as recent reference soil were representative of the 0- to 20-cm soil layer of the arable soil sampled in 1954. The annual rates of accumulation of all PAHs were lower at all Russian sites except one of the Lesnaja Dacha sites than those reported for arable topsoils near Rothamsted, a rural site in England (Jones et al., 1989c). At the Lesnaja Dacha site, where the most pronounced increase in PAH amount was observed, only the annual rate of accumulation of FLUA was similar to that in England and that of PHEN and B(E)P even greater. At the Moscow sites, the mean annual rates of accumulation of FLUA, B(A)P, IND, and B(GHI) were greater than at a remote forest site in Germany (Matzner, 1984). The observed rates of accumulation of PAHs outside Moscow are in line with the comparatively small contemporary concentrations of PAHs and thus further support the above conclusion that the studied soils of the Moscow region are less heavily impacted by the deposition of PAHs than observed at similarly exposed sites in central and west Europe. This was, however, only partly true for the study sites in the city of Moscow (Lesnaja Dacha).

Small-Scale Distribution of PAHs
The PAHs were unevenly distributed among the three studied soil fractions. Both in topsoils archived at the beginning of the 20th century and recently collected, the mean concentrations of the {Sigma}21PAHs expressed in % of their concentrations in bulk soil (i.e., the PAH concentration in bulk soil is defined as 100%) increased in the order, aggregate interior < soil without large aggregates < aggregate exterior (Fig. 5) . The {Sigma}21PAHs concentration in the aggregate interior of the sample taken in 1954 at Schapovo represented 100% of the {Sigma}21PAHs concentrations in bulk soil (i.e., {Sigma}21PAHs concentrations were the same in aggregate interior as in bulk soil). The {Sigma}21PAHs concentration in the soil without large aggregates represented 99% and that in the aggregate exterior 168% of the {Sigma}21PAHs in bulk soil. The concentrations of ACEN, FLUO, PHEN, ANTH, FLUA, CHRY, COR, and {Sigma}21PAHs were significantly higher in the aggregate exterior than in the interior of the recent samples. In the archived samples only the concentrations of ACENY and FLUO were significantly higher in the exterior than in the interior.



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  		Fig. 5. Mean distribution of the concentrations of the sum of 21 polycyclic aromatic hydrocarbons ({Sigma}21PAHs) among the soil without large aggregates (<10 mm) and the interior and exterior of macroaggregates > 10 mm in soils archived between 1910 and 1922 (n = 6) and in soils sampled between 1996 and 2000 (n = 10) expressed in % of the concentrations of the {Sigma}21PAHs in bulk soil. Error bars represent standard errors.

 
The accumulation of PAHs on aggregate surfaces is in line with previous findings and may be attributed to preferential sorption of PAHs deposited from the atmosphere and mainly transported along preferential flow paths that are in contact with the aggregate surface (Wilcke et al., 1996a, 1996b; Bundt et al., 2001a). The particularly strong accumulation of PAHs at the aggregate surface in the 1954 sample coincides with the maximum of the deposition rates of PAHs in west and central Europe (Sanders et al., 1993, 1995). The mean pattern of PAHs in all three studied soil fractions was almost identical (Fig. 6) . This is different from previous findings where the low-molecular-weight PAHs were accumulated on aggregate surfaces (Wilcke et al., 1996a, 1996b).



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  		Fig. 6. Mean pattern of polycyclic aromatic hydrocarbons (PAHs) in the soil without large aggregates (<10 mm) and the interior and exterior of macroaggregates in soils sampled between 1996 and 2000 (n = 10–12). 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; COR, coronene; DIBE, dibenz(a,h)anthracene; FLUA, fluoranthene; FLUO, fluorene; IND, indeno(1,2,3-cd)pyrene; NAPH, naphthalene; PERY, perylene; PHEN, phenanthrene; PYR, pyrene.

 
Although the concentrations of PAHs in the topsoil samples decreased with increasing distance from Moscow indicating decreasing deposition rates along the urban–rural transect, there was no correlation between the ratio of PAH concentrations in the aggregate exterior to interior and the distance to Moscow (Fig. 7) . This indicated that the size of the gradients in the concentrations of PAHs on aggregate level did not only depend on the deposition rates as it has been postulated for shorter deposition gradients in the surrounding of a point source (Wilcke et al., 1996b). Possible other controls of the size of these gradients include different sorption properties, rates of degradation, volatilization, leaching, and turnover of aggregates.



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  		Fig. 7. Mean concentration ratio of the sum of 21 polycyclic aromatic hydrocarbons ({Sigma}21PAHs) in the aggregate exterior to the interior in soils archived between 1910 and 1922 (n = 6) and in soils sampled between 1996 and 2000 (n = 10). Error bars represent standard errors.

 
If the aggregates > 10 mm of the studied soils had been stable during the last century, the differences in concentrations of PAHs between aggregate exterior and interior should have grown, because of the persistence of PAHs and the likely increase in the rates of deposition of PAHs during the last century. This was not consistently the case for our samples (Fig. 7). As PAHs have limited mobility, we consider it unlikely that the PAHs have moved from the exterior to the interior. The transport in dissolved form would furthermore have caused a chromatographic fractionation of the individual PAHs resulting in a preferential accumulation of the more mobile low-molecular-weight PAHs in the aggregate interior which has not been observed. We conclude that only the turnover of the aggregates (e.g., by the burrowing activity of soil animals and swelling/shrinking of clay minerals) may explain our results. Thus, mean lifetimes of macroaggregates in the studied topsoils should be shorter than a century even in forest soils.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The studied soils along an urban–rural transect in the Moscow region are less contaminated with PAHs than similarly exposed soils in central and west Europe. The pattern of PAHs in the studied south Taiga soils resembles that at background locations of the North American Prairie with quantitatively dominating NAPH and PHEN. The urban influence is reflected by increased concentrations of high-molecular-weight PAHs in the Russian soils compared with the North American Prairie that decrease with increasing distance from Moscow.

Both in archived and recent soils, PAHs were unevenly distributed between the soil without large aggregates and the interior and exterior of aggregates > 10 mm in diameter. Whereas the aggregate interior and soil without large aggregates showed similar concentrations of PAHs, those of the aggregate exterior were clearly elevated. Thus, the concentrations of PAHs in biologically active soil zones were higher than usually determined in the conventional bulk soil samples. From the finding that the gradients in PAH concentrations between aggregate interior and exterior did not consistently increase during the last century, we conclude that aggregates were completely turned over at least once during this time period.


   ACKNOWLEDGMENTS
 
We thank Wolfgang Zech for providing access to his laboratory and Andrea Popp, Brigitte Burckhardt, Sabine Dumke, and Katrin Ilg for contributing part of the analyses. We are indebted to the German Research Foundation (DFG) for funding this project (Wi 1601/1-1). Wolfgang Wilcke acknowledges the Heisenberg scholarship of the DFG (Wi 1601/3-1).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 


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