Environ. Sci. Technol. 1998, 32, 1573-1579
Sources of Organic Halogens in Spruce Forest Soil G. O ¨ B E R G * ,† A N D C . G R Ø N ‡,§ Department of Water and Environmental Studies, Linko¨ping University, S-581 83 Linko¨ping, Sweden, and Department of Geology and Geotechnical Engineering, Groundwater Research Centre, Technical University of Denmark, DK-2800 Lyngby, Denmark
It is known that large amounts of organic halogens are present in soil, but the relative contribution from different sources is unclear. The aim of the present study was to determine the storage of organic halogens in a spruce forest soil and the deposition by needle litterfall and to elucidate the relative contribution from needle litterfall in relation to other sources and sinks. Sampling was conducted in a small spruce forest area in Denmark. Soil samples were collected at six locations to a depth of 60 cm. Litterfall was collected in 15 sample collectors every third month during 1 year. Soil leachate was collected below the Bh horizon over two periods of 5 days at five locations distributed over the study site. Throughfall data was compiled from a previous study conducted at the same site and during the same period as the present study, and data concerning net formation in soil was estimated from previous studies at similar sites. The pool was 630 kg of Clorg ha-1, contribution from litterfall was 0.35 kg ha-1 yr-1, and loss by leaching was 0.63 kg of Clorg ha-1 yr-1 (mean values). The estimated contributions from throughfall and net formation within the soil were 0.38 and 0.36 kg of Clorg ha-1 yr-1, respectively. The results suggest that the pool in the investigated soil originates mainly from sources within the forest and that it increases with time.
Introduction The past years of research have shown that large amounts of organic halogens are present in the environment and that a considerable amount originates from natural sources (1). A major pool is found in soil, and there is evidence that halogens are natural constituents of soil organic matter, present in concentrations of 0.1-5 mg of Clorg g-1 dw (2, 3). It has been shown that inorganic halogens are incorporated into the organic matter during decomposition, which suggests that natural formation in soil is one possible source (4, 5). Other possible (natural or anthropogenic) sources are precipitation and litter, which contain organic halogens in typical concentrations of 2-10 µg of Clorg L-1 and ∼0.1 mg of Clorg g-1 dw, respectively (6-9). However, the relative contribution from these or other sources is not well known. * Corresponding author phone: +46-13-282279; fax: +46-13133630; e-mail:
[email protected]. † Linko ¨ ping University. ‡ Technical University of Denmark. § Present address: Department of Plant Biology and Biogeochemistry, Block PBK-124, Risø National Laboratory, P.O. Box 49, DK4000 Roskilde, Denmark. S0013-936X(97)00822-5 CCC: $15.00 Published on Web 04/24/1998
1998 American Chemical Society
The natural biogeochemical cycling of chlorine is practically a new field of research, which in itself makes it an interesting research area, especially as it seems like the formation of organic halogens in soil is related to the turnover of soil organic matter and to the carbon cycle (5, 10). In addition, many anthropogenic organohalogen compounds have undesirable environmental effects, and such compounds have, as a group, received considerable attention in the environmental debate for more than 30 years. As we now are learning that organic halogen compounds are formed and degraded naturally, it is becoming necessary to relate the occurrence of specific anthropogenic compounds to the natural biogeochemical cycle of such compounds. Therefore, the need to increase the understanding of the fate, effects, and cycling of anthropogenic organic halogen compounds demands an increase in the understanding of the natural background levels and processes. Hence, the biogeochemical cycle of organic halogens requires attention not only because it is a new field of research or because of its possible relationship to the turnover of organic matter but also because of its relationship to the cycling of anthropogenic organic halogen compounds. The aim of the present study was to determine the storage of organic halogens in a spruce forest soil, to estimate the deposition by litterfall, and to elucidate the relative contribution from these sources in relation to deposition by throughfall and an estimated net formation within the soil. Note on Terminology. Throughout the text, we use the term organic halogens, but the amount of organic halogens is quantified as the amount of chlorine for the following reason. When the total amount of organic halogens is determined, these elements are transferred as halide ions to a cell containing a solution of silver ions. The current (mA) used to re-establish the silver ion content of the cell after titration is proportional to the number of moles of halides added to the solution. Such a system detects chloride, bromide, and iodide but cannot distinguish between the different halides. Normally, chlorine is by far the most abundant halogen in environmental samples; hence, the total mass of halogens is, by tradition, calculated as the amount of chlorine (mg of Cl g-1 dw), as was done in the present study. Nevertheless, it should be kept in mind that since chlorine has a lower molecular mass than bromine and iodine, this methodology will result in an underestimation of the mass of organic halogens if the other halides are present in considerable amounts.
Sampling Procedures Litterfall. In order to collect samples representing the heterogeneity of the needle litterfall, 15 sample collectors were randomly placed in the area (Figure 1). Sampling was conducted every third month, and the samples were collected in plastic bags, deep-frozen, transported, and stored at -20 °C until further treatments were conducted. The litter was then dried at 105 °C for 24 h and weighed. Twigs, cones, and, on a few occasions, trash were removed since influence from such litter would bias the results due to the immense variability of such litterfall as compared to the attainable sampling devices. The weight of the remaining needle litter was determined. The needle litter was then ground and sifted through a 0.1-mm mesh sieve, and the total amount of organic halogen (TOX) was determined in the samples. Soil. Soil samples were collected on May 10, 1993, at six locations on an east-west line from the center to the edge of the forest (Figure 1). This was done in order to elucidate if the wind direction and distance from the forest edge would VOL. 32, NO. 11, 1998 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Key map showing the location of the Klosterhede Plantation on NW Jutland in Denmark (56°28′ N, 8°24′ E), including a map over the study site in the plantation. influence the organic halogen pool. At each location, the entire soil core was collected to a depth of 60 cm. The samples were divided into 10 cm long subsamples, collected in plastic bags, deep-frozen, transported, and stored at -20 °C until further treatments were conducted. The samples were then dried at 105 °C for 24 h, weighed, sieved (2 mm), ground, and sifted (0.1 mm), and the total amount of organic halogens (TOX) and organic carbon (TOC) was determined. Soil Leachate. Soil leachate was collected over two periods of 5 days each in May through June 1993 at five locations distributed over the study site. Sampling was done below the Bh horizon (45-50 cm depth), and three porous suction cups (PTFE) equipped with Teflon sampling lines were used at each location. As sampling was done after the onset of the dry season, several sampling cups yielded no or only low-volume samples on one or both occasions. Therefore, samples from the same location and same period were combined in order to obtain sufficient volume for AOX (adsorbable organic halogen) and DOC (dissolved organic carbon) analysis. Overall, seven combined samples from below the Bh horizon were analyzed. The samples were preserved by the addition of HNO3 (2 mL of concentrate) per liter of sample and stored cold (
