Environ. Sci. Technol. 1996, 30, 1794-1796
Evidence of a Novel Mechanism of Semivolatile Organic Compound Deposition in Coniferous Forests MICHAEL HORSTMANN AND MICHAEL S. MCLACHLAN* Ecological Chemistry and Geochemistry, University of Bayreuth, 95440 Bayreuth, Germany
Introduction Atmospheric transport and subsequent deposition of semivolatile organic compounds (SOCs) like PAHs, PCBs, and PCDD/Fs are the processes primarily responsible for the ubiquitous presence of these compounds in the biosphere. While vegetation is known to accumulate these compounds (1, 2) and to be the most important vector for atmospheric SOCs into agricultural food chains (3), the broader role of vegetation in scavenging SOCs from the atmosphere and the consequences for accumulation in soil have not been extensively investigated (4). Soils contain the vast majority of persistent SOCs present in the terrestrial environment (5, 6). Furthermore, forest soils have been found to contain higher levels of SOC than agricultural or grassland soils sampled in the same region (5, 7). For example, Hagenmaier and Krauss found that forest soils in southwestern Germany contain on average three times more PCDD/Fs [expressed as toxicity equivalents (TE)] per square meter than agricultural soils. Because of the absence of other sources, atmospheric deposition to forest soils must be much larger than to agricultural soils. Hagenmaier and Krauss reported that two-thirds of the total environmental inventory of PCDD/Fs in southwestern Germany is present in forest soils. This implies that the processes responsible for the higher rates of deposition to forest soils are of critical importance to the environmental fate of SOCs. In this paper, we report on an experiment designed to explore the role of deposition processes in the accumulation of PCDD/Fs in forest soils.
Methods Bulk deposition and litter fall samples were collected in a mature 80-90-year-old spruce (Picea abies) stand and in an adjacent clearing from April 1994 to March 1995. In April 1995, sampling was extended to a third site, an 80year-old deciduous stand of beech and oak. All three sites are located 2 km south of Bayreuth and are less than 3000 m apart. The samplers in the clearing were at least 150 m from the edge of the forest, which had a height of 20-250 m. The bulk deposition samples were collected using the German VDI 2119 method (Bergerhoff) (8). Three sets of * Corresponding author telephone: 49-921-552254; fax: 49-921552334; e-mail address:
[email protected].
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10 glass jars (9 cm i.d.) were exposed at a height of 2 m in the clearing and in the spruce forest during the first year. In April 1995, some samplers were relocated to give two sets of 10 glass jars each at the clearing, the spruce forest, and the new deciduous forest sites. Litter fall was collected in metal sieves (2 mm mesh) mounted at a height of 70 cm at the forest sites. All samples were collected in 1-month intervals. Note that bulk deposition and litter fall are defined by the sampling procedure. The use of sieves in this study to collect litter fall means that this matrix consisted primarily of fallen needles/leaves. The contents of the 10 jars comprising one bulk deposition sample were filtered through glass wool into a single glass bottle. The jars were subsequently rinsed with 50 mL of acetone, toluene, and acetone again . After the toluene rinse, the internal surface of the glass jars was wiped with a glass fiber filter. The solvents were filtered through the same glass wool as the water sample into a second glass bottle. The filter, glass wool, and litter fall samples were packed in aluminum foil and stored at -16 °C. PCDD/Fs from the water subsamples were enriched on solid-phase extraction cartridges (SPE) packed with C18bonded silane and subsequently eluted from the cartridges with 50 mL of toluene. The bottles that had contained the water samples were rinsed with toluene. All organic solvents from the deposition samples were combined, concentrated almost to dryness, and taken up in toluene. This was then used to Soxhlet extract the glass wool and filter. The litter fall samples were freeze-dried and Soxhlet-extracted in toluene. For both the deposition and litter fall samples, a mixture of 12 isotope-labeled PCDD/F congeners was added to the solvent before the Soxhlet extraction. The initial sample cleanup was done on H2SO4-silica and NaOHsilica. The PCDD/Fs were then isolated from other compounds on an Al2O3 column. The HRGC/HRMS analyses were performed on a VG Autospec Ultima at a resolution of 10 000 in the selected ion mode. A more detailed description of the sample cleanup and the PCDD/F analysis is given elsewhere (9).
Results and Discussion Good agreement was found between the three parallel deposition and litter fall samples. The standard deviation of the TE values was on average 10% for the bulk deposition in the clearing, 18% for the bulk deposition in the forest, and 24% for the litter fall. The higher variability in the forest samples likely reflects the larger spatial heterogeneity of the deposition. The deposition fluxes measured in the clearing ranged from 0.7 to 6.4 pg TE m-2 d-1. These fluxes lie at the lower end of the range that has been reported for other European sites (10, 11). The annual deposition flux of the Cl7-Cl8DD/F was approximately equal in the clearing and in the spruce forest (see Figure 1). However, the deposition flux of the lower chlorinated congeners was up to five times higher in the forest than in the clearing during the same period (April 1994-March 1995). This is in agreement with the observations of Hagenmaier and Krauss, who reported that forest soils in a nearby region of southern Germany contain a much higher proportion of lower chlorinated PCDD/Fs than
0013-936X/96/0930-1794$12.00/0
1996 American Chemical Society
FIGURE 1. Annual bulk PCDD/F deposition in a spruce forest and an adjacent clearing (April 1994-March 1995).
agricultural soils (5). Thus, the higher deposition flux of the lower chlorinated congeners is further evidence for the link between atmospheric deposition and soil contamination. A large portion of the deposition in the spruce forest was associated with litter fall, ranging from 16% for Cl8DF to 48% for Cl4DD (see Figure 1). This can be explained by the well-documented accumulation of PCDD/Fs and other SOCs on needles (12-15) and the subsequent transport of this material to the forest floor with needle fall. The accumulation of PCDD/Fs in the spruce needles also provides an explanation for the differences in the homolog pattern measured in the bulk deposition from the spruce forest and the clearing (see Figure 1). It has been shown that dry gaseous deposition is the primary pathway of both PCBs and several organochlorine pesticides to spruce needles (16) as well as Cl4-Cl6DD/F to ryegrass (17), and it is thus likely that this form of deposition was primarily responsible for the accumulation of the lower chlorinated PCDD/Fs in the needles. On the other hand, the glass jars used to sample bulk deposition in the clearing have a low affinity for gaseous hydrophobic compounds and capture primarily wet and dry particle-bound depositions. Since the lower chlorinated PCDD/Fs partition more strongly to the gas phase, they show a stronger tendency for dry gaseous deposition than do the fully chlorinated congeners, which are virtually completely particle bound. This results in a PCDD/F homolog pattern in the bulk deposition from the spruce forest that has a much higher contribution from the lower chlorinated congeners than the profile in the bulk deposition collected in the clearing. While litter fall was an important deposition mechanism, it accounted for only 16-48% of the deposition flux in the spruce forest, indicating that there are more important deposition mechanisms. This non-litter fall deposition displayed strong seasonal variability, with particularly high levels during one or two summer months. Figure 2 illustrates the deposition fluxes of the different congeners measured in July 1994. During this month, the bulk deposition flux of some PCDD/F homologs in the spruce forest was up to 16 times higher than in the clearing. Litter fall accounted for at most 22% of the deposition flux of the PCDD/F homologs in the forest. The non-litter fall deposition accounted for 78-95% of the homolog fluxes in the forest. It was up to 12 times higher than the bulk deposition flux measured in the clearing, and hence the non-litter fall deposition cannot be attributed to canopy throughfall of wet or dry deposition. A similar behavior
FIGURE 2. Bulk PCDD/F deposition in a spruce forest and an adjacent clearing in July 1994 (mean of three parallel samples).
FIGURE 3. Bulk PCDD/F deposition in a spruce forest, an adjacent clearing, and a deciduous forest in July 1995 (mean of two parallel samples).
was observed 1 year later in July 1995 (see Figure 3). Both months were characterized by hot dry weather with occasional intensive thundershowers. Interestingly, the results from June 1994 were similar to those from July 1994, with a large non-litter fall deposition flux, while in June 1995 the non-litter fall deposition flux was small. The weather in June 1994 was also predominantly hot and dry, while June 1995 was cool and rainy. An indication of the possible source of the non-litter fall deposition flux can be found in its homolog pattern. As can be seen in Figures 2 and 3, it is characterized by high levels of the lower chlorinated PCDD/Fs and is similar to the pattern measured in litter fall. This leads us to believe that this phenomenon is related to dry gaseous deposition, and furthermore, that PCDD/Fs originate from the needles in the forest canopy. We propose that some portion of the needle surface is eroded and/or shed, especially during periods of hot summer weather. This eroded/shed material carries PCDD/Fs and other organic compounds that have accumulated in the surface material of the needles to the forest floor. There is evidence in the literature that the surface waxes of needles are eroded by wind, rain, and/or rubbing of the needles against each other (18, 19). In this case, the seasonal dependence of the deposition flux might be related to higher temperatures affecting the structure and erodability of the waxes. It has also been reported that strong electrical fields similar to those generated in thunderstorms draw waxes to the tips of the needles where they form droplets that can be easily eroded (20). A third possible explanation is that the high metabolic activity in the needles during hot summer weather is utilized for a partial renewal of the
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cuticular waxes, and this is accompanied by the shedding of old waxes (21). Finally, some plant products found on the surface of leaves such as triterpenes are known to have a detergent effect and may contribute to the solubilization of lipophilic compounds in cuticular waxes (22). The possible role of sampling artifacts in explaining these observations must be considered. The potential artifacts of primary concern in sampling bulk deposition of PCDD/ Fs are volatilization and photodegradation. Losses caused by one of these mechanisms would likely be larger in the clearing due to the higher temperatures and exposure to sunlight, and this could lead to an underestimation of deposition fluxes at this site. This possibility was examined in a separate experiment in which bulk deposition was collected on the university campus with glass funnels/ bottles. Two samplers were allowed to collect deposition undisturbed for 30 days, while two further samplers were rinsed daily with solvent to carry the deposited PCDD/Fs into the bottles, hence conserving the samples. Twelve sequential sets of samples were collected between February 1994 and January 1995. It was found that the PCDD/F concentrations in the samples that had been conserved daily were very similar to the concentrations in the samples that had been exposed on the funnel for the 30-day sampling period. The artifact arising from volatilization/photodegradation of PCDD/Fs was estimated to be at most 30% for Cl4DF, which cannot explain the differences of a factor of nearly 20 measured between the deposition fluxes in the clearing and the spruce forest. The fact that the deposition fluxes in the deciduous forest in July 1995 were also much lower than in the spruce forest (see Figure 3) is further evidence that the deposition fluxes in the spruce forest were elevated during this month. The non-litter fall deposition flux of Cl4DF was 12 times higher in July 1994 and 17 times higher in July 1995 than the bulk deposition to the clearing. The mechanism responsible for the non-litter fall deposition may play an even more important role for the deposition of more volatile SOCs such as polychlorinated biphenyls (PCBs). This is suggested by the increasing importance of the non-litter fall deposition with increasing volatility of the PCDD/F homologs. Given the large areas of the earth’s surface that are covered with coniferous forest, this phenomenon may be an important mechanism for removing SOCs from the air and transferring them to soil. At this point, there is indirect evidence as to the possible nature of this mechanism. We
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are continuing our work to further understand and quantify the processes involved.
Acknowledgments This work was supported by the German Federal Environment Office (UBA Berlin). We thank Markus Scholz for working up the samples and for assisting in the care of the sampling sites.
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Received for review December 13, 1995. Revised manuscript received February 26, 1996. Accepted February 28, 1996. ES950931O
