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The lack of an effect of lead and acidity on leaf decomposition in laboratory microcosms
Environmental Pollution (Series A) 38 (1985) 295-303
The Lack of an Effect of Lead and Acidity on Leaf Decomposition in Laboratory Microcosms Thomas O. Crist, Nathan R. Williams* Jeffrey S. Amthor & Thomas G. Siccama School of Forestry and EnvironmentalStudies, Yale University,New Haven, CT 06511, USA
ABSTRACT Green leaves JJ'om tree species representative oj the central hardwood Jorest oj eastern North America were collected and dried. The leaf mixture was then amended with Pb over the rangeJrom 0 to 1000 Izg g - 1 and incubated in laboratory microcosms. During the subsequent 18-week decomposition period, I-I2S0 4 solutions (pH 3.0-5.0) were applied weekly to the leaJ mixtures. The Pb and acid treatments had no eJJect on decomposition, as determined by weight loss at the end of the 18-week period. These results indicate that there is little influence of these pollutants on the early stages oJ deciduous leaf decomposition.
INTRODUCTION Leaf litter decomposition is a critical link in forest ecosystem nutrient cycles. Possible adverse effects of man-made inputs of Pb particulates and acid precipitation on the decomposition process are a growing concern (Johnson & Siccama, 1983). There is considerable evidence of Pb accumulation in the forest floor of the northeastern United States (Andresen et al., 1980; Smith & Siccama, 1981; Johnson et al., 1982; Friedland et al., 1984), although increased soil acidification due to atmospheric inputs has not been unequivocally demonstrated (Krug & * Presentiaddress: Bennington County Regional Planning Commission,Arlington, VT 05250, USA. 295 Environ. Pollut. Set. A. 0143-1471/85/$03.30 © ElsevierApplied SciencePublishers Ltd, England, 1985. Printed in Great Britain
296
Thomas O. Crist
et al.
Frink, 1983). However, as suggested by Tyler (1982), acid inputs may increase the mobility and biological availability of toxic heavy metals in soil and litter. Several studies have shown that Pb or other heavy metals inhibit microbial activity and decomposition rates. Both total numbers and relative species abundance of soil bacteria and fungi are affected by high Pb concentrations (Doelman & Haanstra, 1979b, c; Bisessar, 1982). Soil respiration and microbial enzyme production are also depressed in high Pb or Zn environments (Ruhling & Tyler, 1973; Tyler, 1974; Doelman & Haanstra, 1979a, b). Jackson & Watson (1977) further showed a disruption of nutrient turnover in a deciduous forest floor contaminated with high Pb levels. There is less evidence of such inhibition due to acidic inputs, although Killham & Wainwright (1984) recently documented reductions in populations of heterotrophic bacteria and soil chemistry changes directly adjacent to a coking plant where acid inputs were significantly increased. The studies cited above involved unusually stressed environments adjacent to smelters or processing plants where Pb concentrations or S-oxides are locally very high. Under these conditions, soil Pb concentrations may be well above 2000/~gg-1. Similarly, laboratory experiments often employ simulated acid rain with a pH less than 3. Few studies have investigated the effects of these pollutants on decomposition at the lower levels of Pb and acidity that are widespread in the forests of the northeastern United States and southeastern Canada. Johnson et al. (1982) and Friedland et al. (1984) extensively sampled Pb concentrations in the forest floors of the northeastern United States and found Pb levels ranging from 50-500/ag g- 1 in the litter layer. The upper end represents levels found within the Washington, DC-Boston transportation corridor. Additionally, the annual weighted mean pH of rain throughout the region is above 4, and only under rare and localized conditions is it more acidic. Our objective in this investigation was to determine the individual and combined effects of Pb particulates and acidity on deciduous leaf decomposition under controlled laboratory conditions, using amendments at levels generally observed in the field. MATERIALS AND METHODS Green leaves were collected from hillsides on the eastern edge of Lake Saltonstall 6 km east of New Haven, Connecticut, USA during September
Lead, acidity, and lea[' litter decomposition
297
1983. Leaves from species representative of the central hardwood forest of eastern North America were obtained" Acer saccharum, Acer rubrum, Quercus alba, Quercus rubra, Betula lenta, and small amounts of Liriodendron tulipiJera. The entire sample was mixed thoroughly and a small subsample was air-dried to serve as a microbial inoculant for each decomposition chamber. The remainder of the leaf mixture was ovendried at 80°C to constant weight. To increase the surface area and homogeneity of the leaf mixture, both samples were then macerated in a food processor so that the average particle diameter was approximately 3 mm. Each decomposition chamber/'eceived a mixture of 1.0 g of the airdried inoculum and 99.0 g of the oven-dried leaf material. The experiment was conducted in a walk-in environmental chamber in the dark with temperature control (26 _+ 1 °C). The apparatus consisted of 55 glass chambers (cylinders) 10.2cm in diameter and 16.5cm tall. Nylon screens were placed in the bottom of the chambers to prevent particle loss while permitting air flow through the leaf mixture. Laboratory air, which was filtered through distilled water to ensure maximum chamber humidity, entered each chamber in parallel through a port in the bottom cover and exited through a port in the top cover. Tubing clamps below each bottom cover regulated air flow through each chamber, which was measured with hand-held flow meters and maintained between 200 ml m i n - ~ and 500 ml m i n - 1. The air flow varied from week to week but not from chamber to chamber within a given week. Lead sulphate was amended to the leaf chambers with five replications of each of the following concentrations: 0, 25, 50, 75,100, 200, 300, 400, 500, 750, and 1000 pg Pb g - 1 leaf. An appropriate amount of KESO 4 was used so that all chambers contained 1000 pg g - 1 added S O l - to eliminate a possible anion effect. Lead sulphate was chosen as the Pb source since it is thought to be the predominant inorganic Pb species in soil and atmospheric particulates (Olson & Skogerboe 1975; Biggins & Harrison, 1980). Simulated acid rain was prepared analytically from distilled water by serial dilutions of concentrated HzSO 4 (technical grade). Acid treatments represented HESO 4 concentrations which corresponded to the following pH values based on dissociation constants of 1.0 (K1) and 1-2 x 10 .2 (K2): 5.0, 4-4, 3.8, 3.4, and 3.0. Each pH treatment was applied to one of the five Pb concentration replicates. The experimental design was therefore a matrix of 11 Pb concentrations by five rain pH values for a total of 55 treatments. During the 18-week decomposition period rains were administered to
298
Thomas O. Cr&t et al.
each chamber once a week at a rate of 40 ml week- 1 with a hand-held atomizer. This quantity was completely retained within the chamber yet thoroughly wet the contents. The leaf mixtures were gently stirred three times during the experiment to ensure even distributions of air flow and moisture, although such stirring may not be representative of events in the natural environment. At the end of the decomposition period, the leaf samples were carefully removed from the chambers and dried to constant weight in a forced-air oven at 80°C. Weight loss was then calculated. To determine background Pb concentrations, 1.0g untreated leaf mixture samples were dry-ashed 12 h at 475 °C. The Pb was eluted with 10ml of 6 N H N O 3, heated on a hot plate for 20min, filtered through Whatman no. 41 filter paper, and diluted to 50 ml with distilled water. Lead analysis was then carried out by flame atomic absorption spectrophotometry.
RESULTS A N D DISCUSSION Leaf weight loss as a function of Pb concentration when data from all rain treatments were pooled is shown in Fig. 1. Linear regression analysis revealed that the slope of the line relating Pb concentration to weight loss
60 50-
MEAN
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(.3 bJ
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30-
.....
!- ...........
20100
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|
loo
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200
I
300
,;o
s;o
600 '
7;0
800 '
9;0
IOO0
Pb CONCENTRATION (~g g-') Fig. 1. Leaf weight loss after 18 weeks of incubation as a function of Pb concentration with the data from all rain pH treatments pooled. The dotted line is the mean weight loss over all treatments (32.5 g). Each chamber contained 100 g of leaf mixture at the initiation of the study.
Lead, acidity, and leaf litter decomposition
299
was not different from zero (P=0.88). The linear equation best describing these data is given by: WL -- 32.4 + 0.000 159. Pb where WL is weight loss in g and Pb is Pb concentration in #g g-1. The observation that Pb had no effect on decomposition is consistent with the results of Doelman & Haanstra (1979b). They found an inhibition of respiration and dehydrogenase activity in clay and peat soils only with Pb concentrations greater than 1500 pg g- 1. However, in sandy soils inhibition was reported at Pb concentrations of 375 #g g- 1. Other studies have also demonstrated the importance of clay or organic matter content in promoting the complexation and adsorption of heavy metals (Stevenson, 1979; Miller et al., 1983; Friedland et al., 1984). Another factor, especially relevant to this study, is the low solubility of PbSO 4 which has a solubility product of 1.8 x 10- 8. It is therefore possible that a significant portion of the added Pb, particularly at the higher concentrations used, remained inaccessible to microorganisms. Although complexation, insolubility, and adsorption may mitigate toxic effects on decomposers, nutrient turnover may be affected. Stevenson (1979) found that added Pb did not adversely affect decomposition of plant residue in a mineral soil, but did cause slight increases in the preservation of N and C in the soil humus. Leaf weight loss as a function of rain pH when data from all Pb treatments were pooled is shown in Fig. 2. Linear regression analysis 6O 50MEAN
4.0 ¸ O__j 3 0 "1-
......
,
.......
....
.....
..,
......
20-
ILl 10-
o
s.s
4:5
3'.5 RAIN
Fig. 2.
2.5
pH
Leaf weight loss as a function of simulated rain pH with the data from all Pb treatments pooled.
300
Thomas O. Crist et al.
demonstrated that there was no significant influence of pH on decomposition (P -- 0.48). The linear equation best describing these data is given by: W L - - 33.7-0.321 .pH where pH is the pH of the simulated rain. Hovland (1981) concluded from his work that acid rain would have at most a minor impact on coniferous needle litter decomposition under natural conditions. Killham et al. (1983) reported that simulated rain of pH 2.0 inhibited both respiration and enzymatic activity in a forest soil, while pH 3.0 and 4.0 treatments actually stimulated microbial activity. As with Pb, the impact of rain acidity is probably influenced by the chemical and physical properties of the litter or leaf material. The presence of organic matter or clay is important in determining the influence of added acidity on bulk pH (Bewley & Stotzky, 1983). Since the microcosms in this experiment contained only leaf material, the added acidity may have been inconsequential due to the high CEC of organic material and the production of organic acids in the decomposition process (Krug & Frink, 1983). When both Pb concentration and rain pH were included as independent variables in models of leaf weight loss in the present study, the surfaces generated were not statistically distinguishable from the plane normal to the weight loss axis. Thus acidity did not appear to have a significant influence on Pb activity during the 18-week decomposition period. The study by Tyler (1978) indicated that acid rain did not increase heavy metal mobilization except under prolonged leaching with rain of pH less than 3. We employed a laboratory microcosm, containing leaf material but not a soil fraction. However, it is clear that decomposers were present since about one third of the dry weight was lost during the experiment. The loss of dry weight was quite uniform across the treatment matrix, which of course indicated that the Pb and acidity had little if any influence on weight loss, but also indicated that the environmental parameters regulating decomposition were similar from chamber to chamber. If Pb and acidity are having significant effects on decomposition in natural systems, we would expect to detect similar responses in the laboratory. In addition, the use of a thoroughly mixed sample of fresh leaves had the experimentally desirable characteristics of homogeneity and negligible background Pb levels ( < 5/~g g- 1).
Lead, acidity, and leaf litter decomposition
301
Simple systems similar to the one used here can yield important information regarding the influence of pollutants on litter decomposition. In addition to leaf material alone, this system could be used with intact forest floor samples (Chaney et al., 1978), although the path of air flow might need to be altered. By monitoring CO 2 exchange by the chambers, estimates of 'instantaneous' decomposition rates can be obtained (Doelman & Haanstra, 1979b; Hovland, 1981; Leetham et al., 1983). However, the expression of CO 2 evolution on a cross-sectional area basis may yield results different from those based on dry weight due to varying ratios of area to dry weight (Chaney et al., 1978). Our data clearly demonstrate that Pb and acidity did not affect the quantitative aspects of leaf weight loss under the conditions of this study. Based on this study and available literature, we conclude that these pollutants are having a minimal effect in the early stages of deciduous leaf decomposition at levels generally observed in natural systems. The possible qualitative impacts that Pb and pH may have on decomposition processes were not studied, nor were longer term aspects of nutrient cycling. Further study of temporal changes in mineralization, microbial species composition, respiration and enzymatic activity, and soil chemistry are necessary to better understand the impacts of Pb and acidity on litter decomposition and the related nutrient fluxes.
ACKNOWLEDGEMENT This research was supported by a grant from the Andrew W. Mellon Foundation.
REFERENCES Andresen, A. M., Johnson, A. H. & Siccama, T. G. (1980). Levels of lead, copper and zinc in the forest floor in the northeastern United States. J. environ. Qual., 9, 293-6. Bewley, R. J. F. & Stotzky, G. (1983). Simulated acid rain (H2SO4)and microbial activity in soil. Soil Biol. Biochem., 1G, 425-9. Biggins, P. D. E. & Harrison, R. M. (1980). Chemical speciation of lead compounds in street dust. Environ. Sci. Technol., 14, 336-9.
302
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Bisessar, S. (1982). Effect of heavy metals on microorganisms in soils near a secondary lead smelter. Water, Air, Soil Pollut., 17, 305-8. Chaney, W. R., Kelly, J. M. & Strickland, R. C. (1978). Influence of cadmium and zinc on carbon dioxide evolution from litter and soil from a black oak forest. J. environ. Qual., 7, 115-9. Doelman, P. & Haanstra, L. (1979a). Effect of lead on soil respiration and dehydrogenase activity. Soil Biol. Biochem., 11. 475-9. Doelman, P. & Haanstra, L. (1979b). Effects of lead on the decomposition of organic matter. Soil Biol. Biochem., 11, 481-5. Doelman, P. & Haanstra, L. (1979c). Effects of lead on the soil bacterial microflora. Soil Biol. Biochem., 11,487-91. Friedland, A. J., Johnson, A. H., Siccama, T. G. & Mader, D. L. (1984). Trace metal profiles in the forest floor of New England. Soil Sci. Soc. Am. J., 48, 422-5. Hovland, J. (1981). The effect of artificial acid rain on respiration and cellulase activity in Norway spruce needle litter. Soil Biol. Biochem., 13, 23-6. Jackson, D. R. & Watson, A. P. (1977). Disruption of nutrient pools and transport of heavy metals in a forested watershed near a lead smelter. J. environ. Qual., 6, 331-8. Johnson, A. H. & Siccama, T. G. (1983). Acid deposition and forest decline. Environ. Sci. Technol., 17, 294A-305A. Johnson, A. H., Siccama, T. G. & Friedland, A. J. (1982). Spatial and temporal patterns of lead accumulation in the forest floor in the northeastern United States. J. environ. Qual., 11,577-80. Killham, K. & Wainwright, M. (1984). Chemical and microbiological changes in soil following exposure to heavy atmospheric pollution. Era,iron. Pollut. (Set. A), 33, 121-31. Killham, K., Firestone, M. K. & McColl, J. G. (1983). Acid rain and soil microbial activity: effects and their mechanisms. J. environ. Qual., 12, 133-7. Krug, E. C. & Frink, C. R. (1983). Acid rain on acid soil: a new perspective. Science, N.Y., 221, 520-5. Leetham, J. W., Dodd, J. L. & Lauenroth, W. K. (1983). Effects of low-level sulfur dioxide exposure on decomposition of Agropyron smithii litter under laboratory conditions. Water, Air, Soil Pollut., 19, 247-50. Miller, W. P., McFee, W. W. & Kelly, J. M. (1983). Mobility and retention of heavy metals in sandy soils. J. environ. Qual., 12, 579-84. Olson, K. W. & Skogerboe, R. K. (1975). Identification of soil lead compounds from automotive sources. Environ. Sci. Technol., 9, 227-30. Ruhling, A. & Tyler, G. (1973). Heavy metal pollution and decomposition of spruce needle litter. Oikos, 24, 402-16. Smith, W. H. & Siccama, T. G. (1981). The Hubbard Brook ecosystem study: biogeochemistry of lead in the northern hardwood forest. J. environ. Qual., 10, 323-32. Stevenson, F. J. (1979). Lead organic matter interactions in a mollisol. Soil Biol. Biochem., 11,493-9.
Lead, acidity, and leaf litter decomposition
303
Tyler, G. (1974). Heavy metal pollution and soil enzyme activity. PI. Soil, 41, 303-11. Tyler, G. (1978). Leaching rates of heavy metal ions in forest soil. Water, Air, Soil Pollut., 9, 137-48. Tyler, G. (1982). Does acidification increase metal availability and thereby inhibit decomposition and mineralization processes in forest soils? In Swedish Ministry Acidification of the Environment Conj,, 245. Stockholm.
The Lack of an Effect of Lead and Acidity on Leaf Decomposition in Laboratory Microcosms Thomas O. Crist, Nathan R. Williams* Jeffrey S. Amthor & Thomas G. Siccama School of Forestry and EnvironmentalStudies, Yale University,New Haven, CT 06511, USA
ABSTRACT Green leaves JJ'om tree species representative oj the central hardwood Jorest oj eastern North America were collected and dried. The leaf mixture was then amended with Pb over the rangeJrom 0 to 1000 Izg g - 1 and incubated in laboratory microcosms. During the subsequent 18-week decomposition period, I-I2S0 4 solutions (pH 3.0-5.0) were applied weekly to the leaJ mixtures. The Pb and acid treatments had no eJJect on decomposition, as determined by weight loss at the end of the 18-week period. These results indicate that there is little influence of these pollutants on the early stages oJ deciduous leaf decomposition.
INTRODUCTION Leaf litter decomposition is a critical link in forest ecosystem nutrient cycles. Possible adverse effects of man-made inputs of Pb particulates and acid precipitation on the decomposition process are a growing concern (Johnson & Siccama, 1983). There is considerable evidence of Pb accumulation in the forest floor of the northeastern United States (Andresen et al., 1980; Smith & Siccama, 1981; Johnson et al., 1982; Friedland et al., 1984), although increased soil acidification due to atmospheric inputs has not been unequivocally demonstrated (Krug & * Presentiaddress: Bennington County Regional Planning Commission,Arlington, VT 05250, USA. 295 Environ. Pollut. Set. A. 0143-1471/85/$03.30 © ElsevierApplied SciencePublishers Ltd, England, 1985. Printed in Great Britain
296
Thomas O. Crist
et al.
Frink, 1983). However, as suggested by Tyler (1982), acid inputs may increase the mobility and biological availability of toxic heavy metals in soil and litter. Several studies have shown that Pb or other heavy metals inhibit microbial activity and decomposition rates. Both total numbers and relative species abundance of soil bacteria and fungi are affected by high Pb concentrations (Doelman & Haanstra, 1979b, c; Bisessar, 1982). Soil respiration and microbial enzyme production are also depressed in high Pb or Zn environments (Ruhling & Tyler, 1973; Tyler, 1974; Doelman & Haanstra, 1979a, b). Jackson & Watson (1977) further showed a disruption of nutrient turnover in a deciduous forest floor contaminated with high Pb levels. There is less evidence of such inhibition due to acidic inputs, although Killham & Wainwright (1984) recently documented reductions in populations of heterotrophic bacteria and soil chemistry changes directly adjacent to a coking plant where acid inputs were significantly increased. The studies cited above involved unusually stressed environments adjacent to smelters or processing plants where Pb concentrations or S-oxides are locally very high. Under these conditions, soil Pb concentrations may be well above 2000/~gg-1. Similarly, laboratory experiments often employ simulated acid rain with a pH less than 3. Few studies have investigated the effects of these pollutants on decomposition at the lower levels of Pb and acidity that are widespread in the forests of the northeastern United States and southeastern Canada. Johnson et al. (1982) and Friedland et al. (1984) extensively sampled Pb concentrations in the forest floors of the northeastern United States and found Pb levels ranging from 50-500/ag g- 1 in the litter layer. The upper end represents levels found within the Washington, DC-Boston transportation corridor. Additionally, the annual weighted mean pH of rain throughout the region is above 4, and only under rare and localized conditions is it more acidic. Our objective in this investigation was to determine the individual and combined effects of Pb particulates and acidity on deciduous leaf decomposition under controlled laboratory conditions, using amendments at levels generally observed in the field. MATERIALS AND METHODS Green leaves were collected from hillsides on the eastern edge of Lake Saltonstall 6 km east of New Haven, Connecticut, USA during September
Lead, acidity, and lea[' litter decomposition
297
1983. Leaves from species representative of the central hardwood forest of eastern North America were obtained" Acer saccharum, Acer rubrum, Quercus alba, Quercus rubra, Betula lenta, and small amounts of Liriodendron tulipiJera. The entire sample was mixed thoroughly and a small subsample was air-dried to serve as a microbial inoculant for each decomposition chamber. The remainder of the leaf mixture was ovendried at 80°C to constant weight. To increase the surface area and homogeneity of the leaf mixture, both samples were then macerated in a food processor so that the average particle diameter was approximately 3 mm. Each decomposition chamber/'eceived a mixture of 1.0 g of the airdried inoculum and 99.0 g of the oven-dried leaf material. The experiment was conducted in a walk-in environmental chamber in the dark with temperature control (26 _+ 1 °C). The apparatus consisted of 55 glass chambers (cylinders) 10.2cm in diameter and 16.5cm tall. Nylon screens were placed in the bottom of the chambers to prevent particle loss while permitting air flow through the leaf mixture. Laboratory air, which was filtered through distilled water to ensure maximum chamber humidity, entered each chamber in parallel through a port in the bottom cover and exited through a port in the top cover. Tubing clamps below each bottom cover regulated air flow through each chamber, which was measured with hand-held flow meters and maintained between 200 ml m i n - ~ and 500 ml m i n - 1. The air flow varied from week to week but not from chamber to chamber within a given week. Lead sulphate was amended to the leaf chambers with five replications of each of the following concentrations: 0, 25, 50, 75,100, 200, 300, 400, 500, 750, and 1000 pg Pb g - 1 leaf. An appropriate amount of KESO 4 was used so that all chambers contained 1000 pg g - 1 added S O l - to eliminate a possible anion effect. Lead sulphate was chosen as the Pb source since it is thought to be the predominant inorganic Pb species in soil and atmospheric particulates (Olson & Skogerboe 1975; Biggins & Harrison, 1980). Simulated acid rain was prepared analytically from distilled water by serial dilutions of concentrated HzSO 4 (technical grade). Acid treatments represented HESO 4 concentrations which corresponded to the following pH values based on dissociation constants of 1.0 (K1) and 1-2 x 10 .2 (K2): 5.0, 4-4, 3.8, 3.4, and 3.0. Each pH treatment was applied to one of the five Pb concentration replicates. The experimental design was therefore a matrix of 11 Pb concentrations by five rain pH values for a total of 55 treatments. During the 18-week decomposition period rains were administered to
298
Thomas O. Cr&t et al.
each chamber once a week at a rate of 40 ml week- 1 with a hand-held atomizer. This quantity was completely retained within the chamber yet thoroughly wet the contents. The leaf mixtures were gently stirred three times during the experiment to ensure even distributions of air flow and moisture, although such stirring may not be representative of events in the natural environment. At the end of the decomposition period, the leaf samples were carefully removed from the chambers and dried to constant weight in a forced-air oven at 80°C. Weight loss was then calculated. To determine background Pb concentrations, 1.0g untreated leaf mixture samples were dry-ashed 12 h at 475 °C. The Pb was eluted with 10ml of 6 N H N O 3, heated on a hot plate for 20min, filtered through Whatman no. 41 filter paper, and diluted to 50 ml with distilled water. Lead analysis was then carried out by flame atomic absorption spectrophotometry.
RESULTS A N D DISCUSSION Leaf weight loss as a function of Pb concentration when data from all rain treatments were pooled is shown in Fig. 1. Linear regression analysis revealed that the slope of the line relating Pb concentration to weight loss
60 50-
MEAN
4-0O, I--1-
(.3 bJ
-i .... i ....
30-
.....
!- ...........
20100
o
|
loo
I
200
I
300
,;o
s;o
600 '
7;0
800 '
9;0
IOO0
Pb CONCENTRATION (~g g-') Fig. 1. Leaf weight loss after 18 weeks of incubation as a function of Pb concentration with the data from all rain pH treatments pooled. The dotted line is the mean weight loss over all treatments (32.5 g). Each chamber contained 100 g of leaf mixture at the initiation of the study.
Lead, acidity, and leaf litter decomposition
299
was not different from zero (P=0.88). The linear equation best describing these data is given by: WL -- 32.4 + 0.000 159. Pb where WL is weight loss in g and Pb is Pb concentration in #g g-1. The observation that Pb had no effect on decomposition is consistent with the results of Doelman & Haanstra (1979b). They found an inhibition of respiration and dehydrogenase activity in clay and peat soils only with Pb concentrations greater than 1500 pg g- 1. However, in sandy soils inhibition was reported at Pb concentrations of 375 #g g- 1. Other studies have also demonstrated the importance of clay or organic matter content in promoting the complexation and adsorption of heavy metals (Stevenson, 1979; Miller et al., 1983; Friedland et al., 1984). Another factor, especially relevant to this study, is the low solubility of PbSO 4 which has a solubility product of 1.8 x 10- 8. It is therefore possible that a significant portion of the added Pb, particularly at the higher concentrations used, remained inaccessible to microorganisms. Although complexation, insolubility, and adsorption may mitigate toxic effects on decomposers, nutrient turnover may be affected. Stevenson (1979) found that added Pb did not adversely affect decomposition of plant residue in a mineral soil, but did cause slight increases in the preservation of N and C in the soil humus. Leaf weight loss as a function of rain pH when data from all Pb treatments were pooled is shown in Fig. 2. Linear regression analysis 6O 50MEAN
4.0 ¸ O__j 3 0 "1-
......
,
.......
....
.....
..,
......
20-
ILl 10-
o
s.s
4:5
3'.5 RAIN
Fig. 2.
2.5
pH
Leaf weight loss as a function of simulated rain pH with the data from all Pb treatments pooled.
300
Thomas O. Crist et al.
demonstrated that there was no significant influence of pH on decomposition (P -- 0.48). The linear equation best describing these data is given by: W L - - 33.7-0.321 .pH where pH is the pH of the simulated rain. Hovland (1981) concluded from his work that acid rain would have at most a minor impact on coniferous needle litter decomposition under natural conditions. Killham et al. (1983) reported that simulated rain of pH 2.0 inhibited both respiration and enzymatic activity in a forest soil, while pH 3.0 and 4.0 treatments actually stimulated microbial activity. As with Pb, the impact of rain acidity is probably influenced by the chemical and physical properties of the litter or leaf material. The presence of organic matter or clay is important in determining the influence of added acidity on bulk pH (Bewley & Stotzky, 1983). Since the microcosms in this experiment contained only leaf material, the added acidity may have been inconsequential due to the high CEC of organic material and the production of organic acids in the decomposition process (Krug & Frink, 1983). When both Pb concentration and rain pH were included as independent variables in models of leaf weight loss in the present study, the surfaces generated were not statistically distinguishable from the plane normal to the weight loss axis. Thus acidity did not appear to have a significant influence on Pb activity during the 18-week decomposition period. The study by Tyler (1978) indicated that acid rain did not increase heavy metal mobilization except under prolonged leaching with rain of pH less than 3. We employed a laboratory microcosm, containing leaf material but not a soil fraction. However, it is clear that decomposers were present since about one third of the dry weight was lost during the experiment. The loss of dry weight was quite uniform across the treatment matrix, which of course indicated that the Pb and acidity had little if any influence on weight loss, but also indicated that the environmental parameters regulating decomposition were similar from chamber to chamber. If Pb and acidity are having significant effects on decomposition in natural systems, we would expect to detect similar responses in the laboratory. In addition, the use of a thoroughly mixed sample of fresh leaves had the experimentally desirable characteristics of homogeneity and negligible background Pb levels ( < 5/~g g- 1).
Lead, acidity, and leaf litter decomposition
301
Simple systems similar to the one used here can yield important information regarding the influence of pollutants on litter decomposition. In addition to leaf material alone, this system could be used with intact forest floor samples (Chaney et al., 1978), although the path of air flow might need to be altered. By monitoring CO 2 exchange by the chambers, estimates of 'instantaneous' decomposition rates can be obtained (Doelman & Haanstra, 1979b; Hovland, 1981; Leetham et al., 1983). However, the expression of CO 2 evolution on a cross-sectional area basis may yield results different from those based on dry weight due to varying ratios of area to dry weight (Chaney et al., 1978). Our data clearly demonstrate that Pb and acidity did not affect the quantitative aspects of leaf weight loss under the conditions of this study. Based on this study and available literature, we conclude that these pollutants are having a minimal effect in the early stages of deciduous leaf decomposition at levels generally observed in natural systems. The possible qualitative impacts that Pb and pH may have on decomposition processes were not studied, nor were longer term aspects of nutrient cycling. Further study of temporal changes in mineralization, microbial species composition, respiration and enzymatic activity, and soil chemistry are necessary to better understand the impacts of Pb and acidity on litter decomposition and the related nutrient fluxes.
ACKNOWLEDGEMENT This research was supported by a grant from the Andrew W. Mellon Foundation.
REFERENCES Andresen, A. M., Johnson, A. H. & Siccama, T. G. (1980). Levels of lead, copper and zinc in the forest floor in the northeastern United States. J. environ. Qual., 9, 293-6. Bewley, R. J. F. & Stotzky, G. (1983). Simulated acid rain (H2SO4)and microbial activity in soil. Soil Biol. Biochem., 1G, 425-9. Biggins, P. D. E. & Harrison, R. M. (1980). Chemical speciation of lead compounds in street dust. Environ. Sci. Technol., 14, 336-9.
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