392-9, Messenger Graphics, Phoenix, Ariz., Las Vegas, Nev., 1973. (6) Moghissi, A. A., Bretthauer, E. W., Compton, E. H., AnaLChern., 45,1565-6 (1973). (7) Lieberman, R., Moghissi, A. A., J.Appl. Rad. Isotopes, 21,3A-327 (1970). ( 8 ) McFarlane, J. C., Beckert, W. F., Brown, K. W., “Tritium in
Plants and Soil”, Ecological Research Ser. EPA-600/3-76-052, 1976. (9) Ehhalt, D. H., Heidt, L. E., Lueb, R. H., Roper, N., “Vertical Profiles of CH4, Hz, CO, NzO, and COz in the Stratosphere”, Third Conf. on CIAP, US.Dept. of Transportation, Feb. 1974.
Receiued for reuieu, June 3,1977. Accepted December 5, 1977.
NOTES
Lead Accumulation in a Northern Hardwood Forest Thomas G. Slccama’ and William H. Smith School of Forestry and Environmental Studies, Yale University, New Haven, Conn. 0651 1 Lead from the atmosphere is accumulating in the soil of a remote northern hardwood forest ecosystem at a rate of 305 g ha-’ yr-l. At the current input rate the doubling time of the lead concentration in the humus is approximately 50 years. Lead is naturally present in small amounts in biota, soil, rocks, surface waters, and the atmosphere. Since the beginning of the industrial revolution and especially since its use as a gasoline additive, lead has been introduced into the atmosphere in ever increasing amounts (1,2). The behavior of this potentially toxic metal in biogeochemical cycles of terrestrial ecosystems has long-term implications with respect to ecosystem functions such as productivity, decomposition, nutrient cycling, and insect and microbial population dynamics. Lead deposited from the atmosphere has a mean residence time of 5000 years ( 3 )in the surface organic soil horizons, and long-term concentration increases can be predicted as long as inputs exceed outputs. The deposition of lead on the forests of New England several hundred kilometers from major population centers is relatively higher than on many other rural regions of North America due to patterns of air movement and location of sources ( 4 , 5 ) . We have determined the lead budget for a forested watershed ecosystem at the Hubbard Brook Experimental Forest in central New Hampshire. The Experimental Forest ranges in altitude from 229 to 1006 m and covers 3076 ha. It is typified by an unbroken canopy of second-growth northern hardwoods with patches of spruce-fir, particularly at higher elevations. Major species of overstory trees are Acer saccharum (sugar maple), Fugus grandifolia (beech), Betula alleghaniensis (yellow birch), Picea rubens (red spruce), Betula p a p y r i f e r a (white birch), and Abies balsamea (balsam fir). A number of small watersheds within the experimental forest are under intensive hydrologic study by the U.S. Forest Service. Six of these have been under intensive biogeochemical study since 1962. The lead budget for watershed six (WS-6) was determined for 1975. Watershed 6 is the control area for study of the biota and flow of nutrients and energy through an undisturbed forested watershed ecosystem. Watershed 6 has an area of 13.23 ha and ranges in altitude from 546 to 791 m above sea level. Slope inclination averages about 12-13’, and aspect is generally toward the southeast. The watershed is covered by a mantle of bouldery glacial till with occasional outcrops of gneissic bedrock. The predominant soil is a sandy loam podzol of the Hermon series with a thick H layer and a discontinuous but often well-developed A2 horizon. Locally the soil surface has been disturbed by windthrows, and pits and mounds are extensive. Large 0013-936X/78/0912-0593$01 .OO/O
rocks are frequent. The soil surface is very permeable; however, a compact hardpan occurs at about 60 cm. Overland flow of water is minimal (6-8). Experimental
Precipitation was collected in forest openings in 1-L plastic bottles connected by vapor-lock loop-tubes to 27-cm-diameter plastic funnels mounted on wooden frames. Samples were collected monthly. Two collectors were used, and the annual lead input was calculated as the sum of the monthly mean concentration times monthly total precipitation. Winter snow collectors (two) were open 120-L plastic barrels. Stream water was similarly collected monthly in 1-L plastic bottles according to procedures developed and described by Likens et al. (8).All water samples were acidified with 1 mL of Ultrex nitric acid and sent to the Environmental Trace Substances Research Center a t the University of Missouri for analysis. Analyses of plant materials (including leaves, twigs, branches, bark and wood, but no roots) were done on all major woody species. Analyses were done independently at both Yale and Missouri. Sixty 15 X 15cm samples of the forest floor from WS-6 were obtained extending down to the mineral soil. Samples were ground in a Wiley mill to pass a 20 mesh sieve. A 2-g aliquot was ashed at 500 “C in a muffle furnace, and the ash was eluted with 6 N nitric acid. Lead was determined by atomic absorption spectrophotometry. R e s u l t s a n d Discussion
The mean and SE of the monthly lead precipitation input was 0.264 f 0.026 pg cm-2, Weighted lead concentration in bulk precipitation averaged 23.0 pg L-l, with a total lead input to the ecosystem of 317 g ha-l yr-l. This estimate of annual lead input is consistent with those obtained by others (9,lO). A distinctive seasonal pattern occurred with summer (June, July, August) input (42 g ha-l mo-l) approximately double winter (December, January, February) input, (19 g ha-’ mo-l). Since precipitation is evenly distributed throughout the year, this difference was due to increased concentration of lead in the summer rain. The seasonal pattern is probably due to the combination of summer air masses coming from the more urban regions southwest of the study area relative to northwest winter winds from Canada and to increased regional motor vehicle use during the summer. Concentration of lead in stream water draining from the ecosystem was approximately 1 pg L-l (mean and SE of 1 2 monthly determinations was 1.21 f 0.57). In addition to the 11.4 g ha-’ yr-l leaving in the dissolved form, 0.7 g ha-l yr-l left as coarse particulate matter. Use of the term dissolued does not imply that the lead in the stream water samples was
0 1978 American Chemical Society
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in the free ionic form or true solution. Since the stream water sample was unfiltered, it could have included lead in the micro particulate fraction. Macro particulate matter eroded from the ecosystem was trapped in the ponding basin (settling and retention pond) of the WS-6 wier (stream gauging station dam). This material was removed a t irregular intervals during the year. It consisted of all types and sizes of organic and inorganic debris. The lead content of 19 subsamples of this macro particulate matter was obtained over a 4-year period. The total macro particulate matter output is estimated a t 25 kg ha-‘ yr-l (11).No correlation of lead concentration with stream flow rate was discernible, but strong seasonal patterns in output of lead occurred due to the seasonality of runoff related to the spring snow melt and low summer flows. Based on input-output budgets (assuming that the lead leaving the system is the same lead that entered the system in precipitation), the ecosystem is retaining 97% of the lead input. However, PbZ1O studies have shown that less than one-tenth of 1%of the input lead actually leaves ( 3 ) ,and probably most of the lead in the output stream water and particulate matter originally comes from natural weathering processes of the resident lead containing minerals within the glacial till and soil of the ecosystem. T o determine the localization of the lead within the ecosystem, we have intensively examined the soil and vegetative components. Lead concentration in the vegetation is very low, especially in the wood which makes up approximately 86% of the total living above ground biomass. Estimates of total lead in the living biomass are less than 1kg ha-’. Calculating the lead content of the biomass using the Missouri data gave 0.91 kg ha-’ and the Yale data 0.70 kg ha-’. Biomass estimates of the various components were taken from Whittaker et al. (12). The vegetation does not appear to be functioning to accumulate lead nor serving as a major sink for it. The soil, especially the forest floor humus, is the primary sink for lead. This observation is consistent with those of others who have found similar lead concentration in surface soil horizons (13-15). Soils a t Hubbard Brook are strong acid podzols with a 7-10-cm-thick accumulation of unincorporated humus over a strongly leached A2 horizon. Lead concentration in the humus was 167 pg g-’ of organic matter, giving 14.6 kg ha-l of lead in the forest floor. Lead concentration is based on the dry weight of the forest floor organic matter rather than the total dry weight of the forest floor because most of the lead input via precipitation is thought to become complexed with the humus ( 3 ) .Lead concentration in the forest floor on the upper portion of the watershed (750 m) was slightly greater than on the lower slope (550 m). This may have been due to the increased input of lead impacted on the vegetation from the wind stream on the more exposed ridge and its subsequent washing to the forest floor (16). Although awareness of the potential deleterious effect of lead to biological systems has resulted in efforts to reduce the amount of lead in the atmosphere, forest ecosystems, even those remote from anthropogenic lead sources, continue to accumulate this element. The current net input of 317 g ha-l yr-1 to the 89 500 kg ha-’ forest floor humus results in a current annual increase in concentration of about 3.5 pg g-l of forest floor. Continuation of current levels of input will result in increments of about 100 pg gP1 per 30-year interval. The forest is functioning to remove lead from the atmosphere-hydrologic system and add it to the soil system. The
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result is to effectively filter the element from the water discharged downstream and to slowly increase the potential toxic effects of this element in the soil. The importance of this might be emphasized by comparing lead levels in streams if the lead was not retained by the humus. Since evapotranspiration is about 38% of the total precipitation in this region, if no retention occurred, lead levels would be elevated from 23 pg L-l in bulk precipitation to 35 pg L-’ in stream water by concentration alone. This is 35 times the actual stream water concentration. Assuming lead has a mean residence time of 5000 years in the forest humus, it would be necessary to reduce the lead concentration in the precipitation from 23 to 0.0023 p g L-l to effect equilibrium in the forest humus horizon and even more to bring about a reduction in present concentrations. Since such a reduction is unlikely in the near future, it is essential to determine the acute and subtle significance of lead input to forest soil ecosystem structure and function. Initial requirements include a determination of lead chemistry and distribution in the soil coupled with an evaluation of movement and influence on the soil biota. Acknowledgment
We acknowledge the cooperation of the USDA Forest Service. Analysis of precipitation and stream water samples was carried out by the Environmental Trace Substances Research Center, University of Missouri, Columbia, Mo. 65201. Duplicate samples of soil and plant material were analyzed a t both Yale and Missouri. Literature Cited (1) Ewing, B. B., Pearson, J. E., Adu. Enuiron. Sci. Technol., 3, 1 (1974). (2) Nat. Acad. Sci., “Airborne Lead in Perspective”, 1972. (3) Benninger, L. K., Lewis, D. M., Turekian, K. K., in “Marine Chemistry and the Coastal Environment”,T. M. Church, Ed., Am. Chem. Soc., Symp. Ser., No. 18, pp 201-210 (1975). (4) Lazrus, A. L., Lorange, E.,Lodge, J.P., Jr., Enuiron. SCL.Technol., 4.55 (1970). ( 5 ) ’Schlesinger, W. H., Reiners, W. A., Knopman, D. S., Enuiron. Pollut., 6,39 (1974). (6) Bormann,F. H.,Siccama,T.G.,Likens,G. E., Whittaker,R. H., Ecol. Monogr., 40,373 (1970). (7) Likens. G. E.. Bormann. F. H.. Johnson. N. M.. Pierce. R. S.. Ecology, 48,772 (1967). (8) Likens, G. E., Bormann. F. H.. Johnson, N. M., Fisher, D. W., Pierce, R. S., Ecol. Monogr., 40, 23 (1970). (9) Lazrus, A. L., Lorange, E., Lodge, J. P., Jr., Enuiron. Sci. Technol., 4,55 (1970). (10) Reiners, W. A,, Marks, R. H., Vitousek, P. M., Oikos, 26, 264 (1975). (11) Likens, G. E., Bormann, F. H., Pierce, R. S., Eaton, J . S., Johnson, N. M., “Biogeochemistry of a Forest Ecosystem”, SpringerVerlag, New York, N.Y., 1977. (12) Whittaker, R. H., Bormann, F. H., Likens, G. E., Siccama, T. G., Ecol. Monogr., 44,233 (1974). (13) Buchauer, M. J., Enuiron. Sci. Technol. 7, 131 (1973). (14) Shults, W. D., Van Hook, R. I., Eds., “Ecology and Analysis of Trace Contaminants”, Oak Ridge Nat. Lab., ORNL-NSF-EATC-6, 1974. (15) Reiners, W. A,, Marks, R. H., Vitousek, P. M., Oikos, 26, 264 (1975). (16) Schlesinger, W. H., Reiners, W. A., Ecology, 5 5 , 378 (1974).
Received for reuiew June 27, 1977. Accepted November 21, 1977. Conttibution of the Hubbard Brook Ecosystem Study funded by a National Science Foundation grant to F. H. Bormann and G. E. Likens.
