ES&T FEATURE Air pollutants and forest decline There are growing indications that widespread dieback and decline of forests in both Europe and North America are caused by short- and long-range transport of air pollutants. This article is the first of two on the subject.
George H. Tomlinson II Domtar, Inc. Montreal H3C 3M1 Canada Forests play an important role in many facets of life on this planet. They play a significant part in weather control by helping to preserve the hydrological balance between the atmosphere and the earth. Trees absorb, store, and, by transpiration from their foliage, slowly release water to the atmosphere. In the process, they reduce the deleterious effects of heavy rains and help to sustain atmospheric humidity in areas distant from the sea. In addition, forests contribute to the ecological balance between carbon dioxide and oxygen and store the sun's energy. Through the photosynthetic reaction that converts carbon dioxide and water to organic matter and oxygen, they provide part of the oxygen needed by most living organisms. Commercially, the production and use of forest products are vital for the economies of many countries, particularly those with large areas of land not well adapted to agriculture. Aesthetically, the loss of forest over any substantial area, as in eastern Asia, has a devastating effect on the character of the landscape and consequently on the quality of life. Therefore, forest loss, whatever the cause, should be a matter of great concern. Widespread forest dieback A serious dieback of forests is now occurring in many areas of both Eu246A
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rope and North America. In the past, the death of trees near major sources of SO2 emissions was well documented (1-4). In recent years, a similar dieback has been occurring at a slower rate in forests at great distances from major sources of pollution (5-7). A large body of circumstantial evidence suggests that air pollutants are involved. Forest dieback and decline are particularly acute in West Germany. A comprehensive report recently issued by the Federal Ministry of Food, Agriculture and Forestry reports that forests covering 560 000 hectares are damaged t o . varying degrees. This represents approximately 7.7% of the total forest area (8). SO2 emissions and their reaction products are implicated as the primary cause. A summary of this study is contained in the box on p. 249A, and a map showing the widely distributed locations of the affected areas is also included (9) (Figure 1). The purpose of this article is to review studies that suggest a link between industrial air pollution and effects on forest ecosystems. It is hoped that this will be useful in the design of research to determine both the cause of this damage in Europe and North America and the steps that may be necessary to reduce it in the future. Early studies near point sources From the early 1940s to the early 1970s, many studies were made of damage to forests near point sources of sulfur dioxide. Since the first symp-
toms of tree injury were discolored foliage with early leaf fall and a consequent reduced rate of growth, scientists assumed that these resulted from direct phytotoxic action of gaseous sulfur dioxide on the foliage (3). Consequently, they believed that a "no-damage threshold" concentration could be established by measuring the ambient SO2 concentration just beyond the area of light damage. Not until some years later did certain studies suggest that another mechanism involving changes in the soil might also be responsible. In 1963, Gordon and Gorham described the serious damage to vegetation that occurred in a previously forested area downwind from an iron sintering plant at Wawa, Ontario ( / ) . This plant, with SO2 emissions amounting to 100 000 tons of sulfur per year, had begun operating in 1939. They categorized the different zones of injury that extended from the source as: • very serious—6.5 km, barren; • serious—6.5-19 km, most of the trees dead; • considerable—19-27 km, the canopy essentially destroyed with only a few live spruce and birch remaining; and • moderate—27-38 km, reddened needles on conifers and crown thinning on hardwood trees. Beyond this distance, tree damage was not apparent. Ten years after Gordon and Gorham made their observations, Murtha reported that regions of "total kill,"
0013-936X/83/0916-0246A$01.50/0
© 1983 American Chemical Society
In the past 18 years, nearly half the spruces have died on Camel's Hump in Vermont's Green Mountains.
"heavy damage," and "light damage" around Wawa had extended to 29,40, and 64 km, respectively (4). Compar ing these separate observations of the forests around Wawa suggests that not only the SO2 concentration, which decreases with distance from the source, but also the period of exposure determines the degree of forest dam age. It is also apparent that at a dis tance of 40 to 64 km from the source, 20 y or more of SO2 exposure were required before any damage first ap peared. Damage similar to that at Wawa was also seen downwind of Sudbury, Ontario (2). In 1966, conifers had died in a rel atively small area of the Ruhr district in West Germany, where the SO2 concentration had exceeded 240 Mg/m3 annually. Forest owners were compensated if they replanted with hardwoods, which then showed no
damage (5, 10). By 1970, however, trees exhibited signs of stress outside this area, in a zone where the average concentrations exceeded 80 μg/m 3 during the growing season. No forest damage was seen at that time in an area in which the S 0 2 annual average concentration was 60 Mg/m3 or less. Accordingly, in 1975 the German Engineers for Air Pollution Control set this value as "the maximum concen tration of SO2 that allows sufficient protection for vegetation" (5), and the same limit was set in Canada, where similar observations had been made near Sudbury ( / / ) . In the U.S. an annual average of 80 Mg/m3 and a 24-h standard of 365 Mg/m3 were set to protect public health. To meet these various standards, industry in North America and Europe constructed tall smokestacks so that the emitted flue gases would be diluted by mixing with
air before reaching ground level. This method was much less expensive than removing the S 0 2 prior to discharge. Failure of the threshold concept Unfortunately tree damage soon became apparent in areas where the annual average SO2 concentration was lower than 60 μg/m 3 . As early as 1973, Materna reported SO2 levels in forested areas in a triangle formed by Sokolov, Karlovy Vary, and Mariânské Lâzné in northwest Czechoslovakia (12). He observed that ambient concentrations of S 0 2 were higher near industrial centers and dropped off with distance, and that the intensity of forest stand damage corresponded to the intensity of the SO2 concentration. He found extensive tree damage at average concentrations in the range of 50 to 70 ^g/m 3 , while at higher elevations in the Krusné Hory (Ore Environ. Sci. Technol., Vol. 17, No. 6, 1983
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Mountains) tree injury was apparent at 3 0 ^ g / m 3 . On a 1979 map of the North Rhine-Westphalia state of Germany, Knabe showed that tree injury was prevalent well beyond the Ruhr area (13). The tree damage syndrome ob served at relatively remote sites with low ambient concentrations of SO2 appeared to be essentially identical to that seen earlier near point sources. At its meeting in 1978, the International Union of Forest Research Organiza tions ( I U F R O ) concluded that, ac cording to the state of research at that time, a limiting annual ambient con centration of S 0 2 of 50 μ g / m 3 would protect forest trees on most sites, al though for mountain sites a limiting value of 25 Mg/m 3 was required (14). Again this matter was reviewed by the I U F R O Air Pollution Section, at its 1982 meeting in Oulu, Finland. Be cause tree damage had appeared at even lower concentrations, it was de cided that no safe limit could be set (75). The map of West Germany, Figure 1, shows not only the major areas of forest damage, as established in the Forestry Department Survey, but also ambient SO2 concentrations and soil pH values (16). Concentrations of SO2 in or adjacent to these areas (as shown by Stienen and Rademacher) are in the range of 7 to 21 μ g / m 3 , well below any values at which direct damage to foliage has been observed, thus indi cating that an indirect effect through the soil may be involved. In all the large areas showing forest damage, the soil is characterized by low pH. Effects on soils and root systems When the concept of a no-damage threshold level was first considered, it was assumed that damage was limited to the direct action of gaseous SO2 on the foliage. Little consideration was given to the degrading effect of wet and dry deposition of acid and acidi fying materials on the soil, or to the critical nature of the time factor—that is, the cumulative effect over a number of years from a deposition rate that produces no visible effect over short time periods. In a detailed series of experiments spanning more than a decade in the Soiling district of West Germany, Ulrich observed that sulfuric acid and other airborne pollutants are deposited not only in rain and snow, but also di rectly on the canopy from fog, aerosols, and gases in the atmosphere (17). These dry- and interception-deposited pollutants and their reaction products 248A
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are periodically washed to the forest floor by wet deposition. From his studies on acidic soils, Ulrich advanced the hypothesis that acid deposited in these several ways has a continuing adverse effect on the chemical com position and buffering capacity of the soil. When acid seeps through the soil, calcium and other basic cations es sential to the tree's nutrition (both those chemically combined with min erals and those adsorbed on clay) are displaced from these sites by incoming hydrogen ions and are then carried from the rooting zone by an equivalent quantity of sulfate and other anions. As soils become more acidic because of the depletion of basic ions, hydrogen ions react with aluminum hydroxide or other aluminum-containing minerals in the soil, bringing Al 3 + into solution.
Ulrich found that the A l 3 + ion be comes toxic to the fine feeder roots of spruce as the ratio C a 2 + : A 1 3 + in the soil solution decreases with time. Consequently, nutrient assimilation in the roots can be interfered with either from a low C a 2 + : A 1 3 + ratio or from a complete loss of Ca 2 + . Ulrich also noted that typical aboveground changes in the tree occur as a result of the damaged root system (18). These changes involve a premature loss of needles or leaves (normally preceded by discoloration), followed by necrosis (localized death of living tissue) of the bark, twigs, and branches, particularly in the crown. The moisture content of the tree becomes abnormally low even though the soil may contain adequate moisture, thus indicating a defective root system. The growth rate slows,
Forest damage from air pollution— The German perception A report, published in December 1982, "Forest Damage from Air Pollution" (Waldschaden Durch Luftverunreinigung) describes the extent and nature of forest damage in the Federal Republic of Germany. The report concludes that although forest damage may be traced to several causes such as air pollutants, climatic extremes, and infestation by insects, fungi or bacteria, there are indications that air pollutants are the fundamental cause; among these pollu tants, sulfur dioxide and its reaction products are probably the most important. In a foreword to the report, Josef Ertl, Federal Minister of Food, Agriculture and Forestry, points out that even though the chain of causes has not been sci entifically explained in all detail, efforts must be intensified to combat air pollution by limiting emissions. The report was issued by the Federal Ministry of Food, Agriculture and Forestry, the Federal Ministry of the Interior, and the Committee of the Provinces for Protection from Emissions. The work group involved in its preparation consisted of 40 scientists and other experts from for estry-related institutes, from universities, and from federal and provincial government ministries. Although it has been known for a long time that forest damage is traceable to high concentrations of ambient air pollution, primarily in industrial areas, the occurrence of similar damage remote from industry was seen for the first time in the mid-seventies. Widespread symptoms of dam age, found in silver fir (Abies alba) and spruce were followed by reports of similar symptoms in Douglas fir and pine. Deciduous trees now showing damage include beech, red oak, oak, maple, ash, and mountain ash. To establish the scope of damage throughout West Germany, Federal Minister Ertl initiated a survey of the degree and nature of the damage through the provincial forestry offices. It was estimated from the survey that 560 000 ha, or approximately 7.7 % of the forest area, was affected. Degrees of damage were as follows:
Slightly damaged—stage 1 (loss of 10-25% needles or leaves, crown thinned) Damaged—stage 2 (loss of 25-50% needles or leaves, crown strongly thinned) Severely damaged—stage 3 (loss of over 50% needles or leaves, greatly damaged or dead)
Area (ha)
% of total damaged area
419 000
75
107 400
19
35 400
6
Industrialized areas where forest damage from high levels of gaseous air pollutants has been known to occur for decades were not included in this survey. Also, dead and dying trees normally removed in thinning operations were omitted. Furthermore, for individual species only the re duced damage area was included. For example, for a mixed
stand with an area of 10 ha where fir accounts for 3 3 % and shows signs of damage, only 3.3 ha of damaged trees were recorded. If the firs in a stand are damaged, however, the other species are probably also affected even though they do not yet show signs of stress. Although damage was widespread throughout almost all of Germany, the most severe damage was on the ridges and the windward (west) slopes of mountains. These are most exposed to various forms of acid deposition—dry deposition of SO2 and aerosols, "interception deposition" of acidic fog and cloud droplets, and heavy acidic rainfall. Observations in Lower Saxony showed that the trees were less damaged on the lee side of the mountains. SO2 emissions in West Germany were 3.9 million tons in 1970 and 3.5 million tons in 1980. In the damaged forest areas, the annual average S 0 2 concentrations, although in some locations lower, are usually between 20 and 50 μ g / m 3 , previously considered a relatively low level. In contrast, the entire country has a high total sulfur deposition rate (wet and dry) ranging from 100 to 150 kg/ha/y in the Ruhr area to 40 to 60 kg/ha/y throughout the rest of the country. The report explains that acid deposition reaching the forest floor leaches nutrient basic cations—calcium, magnesium, potassium and other trace nutrients—from the soil and solubilizes aluminum and other acidic metals such as manganese and iron. The temporary concentrations of these latter metals in the soil solution sometimes become high enough to damage the fine roots of trees. Also, accu mulated dry and wet acid deposition can result in a low pH film on needles and leaves. Unless buffered by cations transferred from the roots to the foliage, damage can occur. This can involve erosion of cuticular wax with resultant dessication and/or necrosis. Although damage to fir has occurred in some areas where the soil is alkaline, soils with low buffering capacity are considered "key areas" in the survey. For instance, in Bavaria, damage was much greater in the unbuffered eastern area with its acidic soils than in the better buffered soils in the northern Alps, where only isolated damage has occurred. The report points out that other air pollutants in addition to acid deposition may be causing forest damage as well. Airborne heavy metals and photo-oxidants are listed as possible causes; these have not been as well investigated. In addition, the report states that the combined effects of various air pollutants and their compound effects with other nonpollutant factors may be involved. Nevertheless, most of the evidence seems to point to S 0 2 and its reaction products as the primary cause. The question of whether the primary agent is another factor such as unusual climatic conditions, pathogenic or ganisms, or soil changes unrelated to ambient air pollution is also investigated. The report concludes that none of these could be considered the cause in the absence of air pollu tion, although in various regions they have had an important secondary role in causing damage to the stressed trees.
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and following an extended period often lasting many years, the tree may die. Ulrich's theory that a decreasing C a 2 + : A 1 3 + ratio in the soil damages the trees' fine root systems is a matter of controversy. According to Ulrich and a number of other scientists, there is some possibility that heavy metals may play some part in the tree decline now observed in parts of Europe and North America. Natural effects Ulrich called attention to the fact that natural acidification processes in the soil also play a role and can become increasingly damaging as the soil's composition changes and the tree becomes stressed as a result of acid deposition (19). When protein in humus and other organic matter in the soil decomposes, nitric acid is formed and the soil becomes more acidic: R—NH2 + 202 — R — O H + N O i + H+ When the nitrate ion is picked up by the fine feeder roots to produce protein in the tree, however, this reaction proceeds in the opposite direction and hydrogen ions are consumed: R — O H + N O i + H+ -* R — N H 2 + 2 0 2 These acid-forming and -consuming reactions can proceed at different times and in different locations in the soil. For instance, periods of humus decomposition and root growth do not necessarily coincide. Also, cyclical weather patterns can result in changes in the acidity of the soil. Very large amounts of nitrogen as N O j accumulate in humus and other organic matter in the soil, sufficient for several years' supply (20). Humus decomposition is a biotically controlled reaction, and high soil temperatures, such as occur during hot dry years, can result in the production of large quantities of nitric acid in a single year (19). At the lower soil temperatures in the cooler years that normally follow warm years, a deacidification phase occurs in which nitric acid, including that from atmospheric inputs, is consumed at a higher rate than it is formed, thus reducing the acidity. High soil temperatures may also result from tree harvesting, as observed by Likens et al. (21), or from tree dieback, because these events open up the canopy to allow the sun's rays to reach the forest floor. Fortunately, the rapid growth of early successional vegetation provides shade that gener250A
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ally causes the soil to grow cooler again within a few years after tree harvesting or dieback. Trees that have lost their fine root systems, not only as a result of aluminum toxicity but also as a result of extreme drought or oxygen depletion from flooding, can often reestablish these systems under subsequent more favorable soil conditions, and partial recovery can occur. But when the buffering capacity of the soil has been reduced by acid deposition and its nutrient supply has been depleted, trees with defective root systems are less able to cope with the stress resulting from transitory acidic imbalances and may never recover. Modes of deposition If the hypothesis discussed above (or some modification of it) proves correct, an understanding of the routes by which forest soil is modified by shortand long-range transport of air pollutants becomes central to solving the problem of forest damage. The SO2 discharged from stacks follows many separate routes as a result of a series of reactions with other substances in the atmosphere, including water, mineral dust, and ammonia, and forms an aerosol consisting principally of H2SO4, C a H S 0 4 , and NH4HSO4 in varying proportions (22). It is this aerosol that is largely responsible for atmospheric haze. Because of its hygroscopic nature, it absorbs water at high humidities and fog droplets are formed; if the aerosol is carried in an updraft, cloud droplets are formed. Unreacted SO2 that reaches cloud level undergoes similar reactions or is dissolved and oxidized in existing cloud droplets. Unreacted SO2 carried in the wind at lower elevations is dry-deposited by adsorption on surfaces and converted through oxidation to H2SO4. Tree foliage with its large surface area forms an important sink for this gas, particularly at the high concentrations near a source (23, 24). In addition to adsorption on leaf surfaces, S 0 2 enters the leaves through the stomata, the normal entry location for the carbon dioxide utilized in photosynthesis. A portion of the leaf acidity is neutralized by calcium stored in the leaves, which is replenished by calcium carried in solution from the fine roots as bicarbonate. The calcium ions enter the roots from the soil by exchange with hydrogen ions formed from carbonic acid that has been formed from water and CO2 generated by root respiration. Thus, a portion of the acidity is transferred indirectly from the leaves to the soil in the im-
mediate vicinity of the fine roots (25). N O j , primarily generated by combustion in industralized areas, and to a lesser extent by thunderstorms, other natural sources, and agricultural practices, also undergoes a series of atmospheric transformations. This results in the formation of nitrate in aerosols and nitric acid in gas and aerosol form (24). These are deposited on the foliage and other surfaces in gaseous, particulate, and liquid form (24). From the foliage they are washed in precipitation to the forest floor along with the sulfate and other deposited materials. In a healthy forest with an adequate supply of nutrient cations, these nitrogen compounds act as fertilizer. However, any air-deposited NO3" together with N O ^ formed by mineralization of humus, in excess of that taken up by vegetation, passes from the rooting zone with the sulfate, carrying equivalents of calcium and magnesium ions. This can occur in a slow-growing stressed forest and, as previously noted, is particularly serious following harvesting (21). Aerosol, fog droplets, and at high elevations, cloud droplets carried in the wind are also intercepted throughout the year by the trees' canopies. Nonmarine fog and cloud droplets form on mineral dust nuclei and, in locations such as Montana and northwest Alberta upwind of an SO2 source, are nonacidic. On the other hand, at Whiteface Mountain, N.Y., which is east of major sources of SO2, cloudwater has a pH of about 3.5 and is even more acidic than the rain at that location (26). Fog in certain parts of California has been found to have a pH of 2.5-3.0 (27). All the S 0 2 and N O , that goes up ultimately comes down in one chemical form or another, either over land or over the ocean. The quantity reaching the forest floor is greater than that reaching open fields because of the interception of pollutants by the trees, with their large surface areas. Most of the chemical substances deposited on the foliage eventually wash to the forest floor in rain, snow, or fog water, either as throughfall or stem flow, at concentrations higher than those in the precipitation that enters the canopy. The effect of the forest canopy The significance of tree canopies as receptors for dry deposition of S O 4 and also as receptors of S 0 4 ~ during such special events as fog and dew episodes is illustrated in Table 1 (28-31). This shows that substantially more sulfate is deposited by throughfall in a
forest than by precipitation in an open area. The wide difference in deposition rates between nonpolluted and pol luted areas illustrates the importance of the forest and the forest soil as a sink for sulfate dry-deposited from the at mosphere (and also wet deposited from fog) in addition to that deposited in precipitation. The table also indicates that the total amount of sulfate deposited de pends on how close the site is to a strong source of industrial pollution. From line 1 of the table it can be seen that during the summer months in the relatively unpolluted area of northern Norway, the sulfate deposited in open areas and in throughfall is relatively small, with values of 0.23 and 0.28 kg S/ha-30 d, respectively (28). In con trast, the corresponding sulfate depo sition values are about 10 and 30 times greater, respectively, for Miilheim in West Germany (29). (Miilheim is in the Ruhr district, where the forest owners were subsidized because of tree damage (3) and where the ambient SO2 concentration was measured in excess of 60 μ g / m 3 during the growing season.) The values for "control" and "affected" zones near a large S 0 2 source, a tar sands plant at Fort McMurray, Alberta, Canada (30), illustrate a similar situation, although the total quantities deposited near Fort McMurray are not as great as those reported for the Ruhr. This table also shows that the Huntington Forest in the Adirondacks receives almost half as much S deposition as an area (Fort McMurray) very near a strong S 0 2 source (31). The data in Table 1 indicate how the
quantity of acid deposited on the soil may be affected by the location of any forest area in relation to a source of SO2. They also suggest that if changes in the soil occur near a large point source of SO2 as a result of accumu lated deposition, similar changes can be expected to occur at a substantial distance downwind at a later time. The quantity of sulfate deposited per unit time under the canopy of a tree depends on a number of factors, including tree species, season, surface area of the foliage, amount of rain and its composition, concentration of am bient SO2 and of sulfate in the aerosol, frequency of acidic fog or clouds, and location and exposure of the tree. The area presented by the foliage is de pendent on the tree's height and, with conifers, the number of years' needles that remain on the tree. Trees at the edge of a forest or on a relatively steep slope are subject to much greater wind motion and turbulence than are trees in the interior of the forest. Because of the absence of a stagnant gas layer at the leaf surfaces, the former trees are more subject to dry deposition of gas. Also, trees on the windward side of a mountain are particularly subject to contact with clouds. Table 2 shows averaged data ob tained by Gunther and Knabe (29) and Knabe (3) in the Ruhr district, and by Mayer and Ulrich (23) in the Soiling, approximately 235 km east of the central Ruhr. The ambient S 0 2 concentration in the Ruhr area was stated to be > 8 0 Mg/m 3 (3), but the actual values during the test were not reported. Mayer and Ulrich wrote that the S 0 2 concentration in the Soiling
ranged from 5 to 10 Mg/m 3 in the summer and 10 to 20 yug/m 3 in the winter. The precipitation there is ap proximately 1 m / y , and at elevations greater than 500 m the Soiling is subject to considerable fog. The trees are relatively large in these forests, ~ 2 5 m high. It should be noted that in reporting the amount deposited, Mayer and Ulrich combined the SO|~ deposition from stemflow with that from throughfall, while Gunther and Knabe did not include stemflow. Table 2 indicates that a substan tially greater amount of SC4~ deposited (wet and dry) in an open area and the SO|~ deposited (wet and dry) under a canopy shows that measurements of throughfall are very important. To properly assess the ef fects of SO2 emissions on any given forest or watershed, it is particularly important to measure ambient SO2 and aerosol as well. Grennfelt et al. estimated the amount of sulfur and nitrogen depos ited in a coniferous forest ecosystem in a rural area of southern Sweden (24). Sulfur, as deposited, was in the form of SO2 and S O l - while nitrogen was in the form of N O z , HNO3, N O j , and NH4 . Their data, expressed as S and N , are summarized in Table 3. Al though the pH of rain is an important consideration, it is obvious from the above discussion that many other fac tors are involved in assessing acidic impact on forests.
TABLE 1
Variation of SO|~ deposition in throughfall from spruce in prist ne, downwind, and heavily polluted areas Open area bulk precipitation kg S/ha-30 d pH
Europe 1. Malsev, northern Norway, July-October 1973 (28) 2. Birkenes, southern Norway, June-November 1973 (28) 3. Mulheim, Ruhr district (29) April-October 1973-74 November-March 1973-75 North America Control area for tar sands plant (30) 4. 56 km southwest of Fort McMurray, July-September 1974 5. 24 km southwest of Fort McMurray Affected area from tar sands plant (30) 6. 8 km northwest of Fort McMurray, July-September 1974 7. 8 km south of Fort McMurray 8. Huntington Forest, Adirondacks (3 /), August-November 1979
Throughfall pH kg S/ha-30 d
Net removal of S 0 4 by canopy kg S/ha-30 d
0.23 0.82
5.2 4.3
0.28 1.65
5.9 4.4
0.05 0.83
2.19 2.56
— —
8.52 11.60
— —
6.33 9.04
0.11
—
0.22
—
0.11
0.11
6.4
0.33
6.4
0.22
0.55
6.5
3.2
4.7
2.65
1.2 1.01
—
4.1 1.71
—
4.2
4.2
2.9 0.70
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of the soil. These investigators tested the ability of soil near the smelter to support growth of indigenous and The amount of sulfate deposition collected by a tree cultivated plants. Unless the soil was depends on the species and the season pretreated with lime, the roots became Sulfate deposition deformed with blackened tips and the expressed as S District No. of plants died within a few weeks, despite in kg/ha-30 d in Stemflow years West Germany averaged included Summer Winter removal to a greenhouse to eliminate direct effects of SO2. Soil extracts Gunther and Knabe (29) added to a nutrient solution had a Open area Ruhr 2a 2.19 2.56 — similar effect, as did a nutrient solution Throughfall, beech Ruhr 2a No 2.94 2.61 with only aluminum added. (The latter Throughfall, oak Ruhr 2a No 6.14 6.50 solution was used to differentiate the Ruhr Throughfall, pine 2a No 6.17 7.23 2a Throughfall, spruce Ruhr effect of aluminum from that of nickel No 8.52 11.60 Mayer and Ulrich (23) and other heavy metals originating Open area Soiling 6" 1.18 c 1.13 from the smelter.) At concentrations — Throughfall, beech Soiling 6 Yes 4.20 4.25 of 2 and 4 m g / L , A l 3 + reduced the Soiling Throughfall, spruce 6 Yes 6.36 8.03 length of tomato roots by 80% and 90% a Summers 1973 and 1974, winters 1 9 7 3 - 7 4 and 1 9 7 4 - 7 5 respectively, relative to controls. Re " Summers 1 9 6 9 - 7 4 , winters 1 9 6 9 - 7 4 ferring to their 1970-71 observations, ° Wet fall and dry fall these authors stated, "Aluminum toxicity is apparently caused by highly TABLE 3 acidic rainfall mobilizing Al from clay minerals in the Sudbury soils. This Sulfur and nitrogen deposition in a rural coniferous forest of effect was unforeseen and may well be southern Sweden a salutary warning for those concerned Ν with acidic rainfall problems on eco s Mode of deposition kg/ha-y kg/ha-y systems, both aquatic and terrestrial in Scandinavia and in the eastern United Dry deposition of gas 10.2 4.5 States." Aerosol and particulate 3.0 2.6 The Coniston smelter was shut Fog and mist 0.8 0.4 down in 1972, and a high stack was Rain and snow 9.6 5.6 installed at Sudbury to dilute SO2 23.6 Ϊ37Γ from that source. In 1981, WinSource: Grennfelt et al. (24) terhalder reported that adding ground limestone to the soil in a semibarren area resulted in the regeneration of TABLE 4 birch from seeds that had apparently The acidity of the deposition, the acidity of the soil, and the remained dormant in the degraded soil 3 AI ' concentration in the soi were found to decrease with (52). He found that while adding increasing distance from the Coniston smelter C a C 0 3 to the soil would allow vege tative growth, using CaCl 2 to produce Distance Wet and dry Soil extract characteristics from deposition the same calcium concentration as the 3+ pH A l , mg/L smelter kg/ha* C a C 0 3 produced or Na2CC>3 to give Surface 5 cm 10 cm Surface 5 cm 10 cm (km) pH 28 d the same pH as that resulting from CaCC«3 would not. He concluded that 3.4 77 52 3.6 3.7 23 1.6 2.85 21.1 the CaCC>3 supplied the missing cal 15.4 99 3.3 3.5 3.9 31 14 1.9 3.40 cium required for nutrition and at the 3.4 7 7 4 17.6 3.5 3.7 7.4 3.25 same time precipitated the A l 3 + ion as 15 6 10.4 — 3.9 4.1 4.2 6 — Al(OH)3, thus eliminating the toxicity 2 13.5 3.72 3.3 3.9 4.3 4.7 nd nd of the soil. 4.4 nd nd 4.5 4.7 nd 19.3 — — Studies on regeneration of beech are Source: Hutchinson and Whitby (2). The data were assembled from Tables 1 and 10 being carried out by Gehrmann and in this article. Ulrich in North Rhine-Westphalia and Lower Saxony at four locations that have been subject to varying de grees of acid deposition (33). At each Soil effects near major SO2 sources of these sites, plots have been planted SO|~ in the deposited sulfuric acid had leached calcium from the soil ( / ) . In with beech in a) undisturbed soil, b) Even though it was initially assumed soil in which humus and mineral soil 1972, Hutchinson and Whitby mea that damage to trees near a source of sured the combined dry and wet de have been mixed by cultivation, c) soil SO2 emissions resulted exclusively to which lime has been added during position downwind from the Coniston from direct damage by gaseous SO2 to cultivation, and d) soil that has been smelter 14.4 km east of Sudbury and the foliage, certain early studies also replaced with a "standard" soil from observed that deposition was highest suggested that the soil was seriously a different area. The results of the first close to the source and fell off with damaged as well. In 1963, Gordon and year have been reported (33). Al distance ( 2 ) . Their data, shown in Gorham noted the high sulfate con though the beech grew well in the Table 4, show the effect the deposited centrations observed in ponds down limed and "standard" soils (c and d) at acid had on the pH and A l 3 + content wind from Wawa and the fact that TABLE 2
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all locations, distorted roots and poor survival were observed in the undis turbed and cultivated soils (a and b) at two locations, Haard and Burgholz. These experiments indicate that lack of regrowth is not the result of current atmospheric deposition, but rather of an accumulated degrading effect on the soil that has occurred over many years. Thus, the question of whether damage in most polluted areas is ini tiated by direct action on the foliage or indirectly through the roots is of less final importance than the fact that soil properties are altered. The rate at which the nutrient cat ions are leached from the rooting zone of a soil is dependent on the mobility and rate at which the anions, N O J and SO|~, pass through the soil, since they will take with them an equivalent quantity of cations. The major cations present in a soil include C a 2 + , M g 2 + , K+, N a + , AP+, F e 3 + , M n 2 + , N H 4 + , and H + , and the rate at which they are removed depends on the relative quantities of these ions present in the soil solution in exchangeable form. This in turn depends on the relative quantities of cations entering the soil from throughfall, from mineralization of humus and organic matter, and from weathering of minerals in the soil, minus those taken up by the fine feeder roots of living vegetation. The pro portions may be further modified by selective adsorption of specific cations by clay and other ion-adsorbing ma terials in the soil. Nitrate, other than that assimilated by the roots and used by vegetation, moves directly with water through the soil. On the other hand, the sulfate ion may be immobilized by adsorption on hydroxy aluminum compounds in the soil, and as a result, sulfate accumu
tain lates—as has been observed in certain ssee soils in North Carolina and Tennessee :idic (34). This is not the case in the acidic spodosols (an acidic type of soil)) of lern northern New England and northern fate New York state, where more sulfate »vith leaves than enters the soil, taking with it nutrients required by the trees. This ated suggests that previously accumulated S O ^ - is released while hydroxy alualu minum compounds are dissolved. An example of this phenomenon can be seen in studies performed in the lack Huntington Forest of the Adirondack iere Mountains of New York state. Here imethe A horizon (the layer of soil imme diately beneath the organic layer) was 1, il ilfound to have a pH of 3.6. Table 4, lustrating the release of sulfate in this soil, shows data of Raynal et al. [31 (31)) and of Molitor and Raynal (35) onl the the quantity of water and ions entering', the soil in bulk precipitation and and throughfall, and the loss throughι the imesoil as measured by means of lysime'ater ters. The Al 3 + in the mineral soil-water results from the action of acid on prepre roxy viously insoluble aluminum hydroxy tion compounds (19), and its mobilization could account for the fact that the loss ater of S O l " in the A horizon is greater than the input. This loss is twice: the Vom quantity entering the forest floor from alcithroughfall. The sulfate carries calci ntial um with it, resulting in a substantial loss of this nutrient. >0|~ In New Hampshire, the loss of SO|~ was also greater than the input. Likens kens et al. measured atmospheric inputs ts of S O l " , together with outputs toο a stream, over a 10-y period at Hubbard bard Brook, N . H . (36). Each year the loss of sulfate was consistently larger than the input. During the period 1963-66, -66, this differential amounted to 5.3 kg/ha-y. During the years ears
1971 -74, the corresponding value was 22.1 kg/ha-y, suggesting that the soil was becoming increasingly susceptible to loss of accumulated SOI", with accompanying nutrients. The net loss of C a 2 + also increased, averaging 7.4 kg/ha-y during the first 3-y period and 15.1 kg/ha-y in the last 3-y period, The data of Raynal et al. (Table 5) indicate that acid spodosols are highly sensitive to acid deposition (31). This sensitivity was not predicted in a hypothesis developed by Wiklander, who stated that acidic soils with a pH less than 5.0 are not greatly affected by acid rain and that the rainwater would pass through acidic soils with little change (37, 38). He also postulated that noncalcareous soils having a pH greater than 6.0 would be most sensitive to acid deposition. To explain the discrepancy between Raynal's data and Wiklander's theory, the chemical changes occurring when acid enters soils that exhibit pHs in different ranges must be considered, The pH range in which a soil is buffered depends on its mineral constitu ents, and as acidification proceeds and each active buffering chemical be comes sequentially exhausted, the pH drops to the next buffering range (19, 39). Although there is some overlap, incoming hydrogen ions are buffered for each of a series of approximate pH ranges as indicated: · at a pH above 6.5 by calcium carbonates (calcite, dolomite, etc.), · at a pH of 5.0 to 6.5 by calcium silicate and other silicates, · at a pH of 4.2 to 5.0 by cation exchange on clay, · at a pH of 3.0 to 4.2 by hydroxy aluminum compounds, and · at a pH below 3.0 by hydroxy iron compounds.
TABLE 5
Water and ion flux for a conifer site in the Huntington Forest (Adirondack Mountains, New York state) a 105-d flux Water
sq2
H+
Ca2+ eq/ha
A|3+
cm
L/ha
eq/ha
eq/ha
kg/ha
kg/ha
eq/ha
kg/ha
Bulk precipitation
46
4.6 X 10 6
267
219
10.5
Throughfall
33
3.3 X 10 6
166
371
17.8
25
0.5
130
2.6
— —
— —
Ο Horizon
23
2.3 X 10 6
563
281
13.5
262
5.2
—
—
A Horizon
23
2.3 X 10 6
615
772
37.1
423
8.5
922.3"
8.3
Β Horizon
23
2.3 X 10 6
151
548
26.3
342
6.8
706.1 "
6.4
Inputs to soil
Losses through soil
a
These measurements were made from Aug. 8 to Nov. 12, 1979. b The values shown for aluminum concentration in the soil solution of the conifer site in column 6, Table 5 of Molitor et al. (35), namely 401 and 307 eq/L, were multiplied by 2.3 X 106 L/ha, the volume of solution moving through the horizon. All other data in this table are from Raynal et al. (31), Table 3-13, which also provides data for the flux of NOJ, K + , Mg 2+ , and Na + . The nature and quantity of organic acids were not determined.
Environ. Sci. Technol., Vol. 17, No. 6, 1983
253A
In soils that no longer contain carbonates and silicates and that have a pH below 5.0, the incoming hydrogen ions in the soil solution, together with the trivalent aluminum ions mobilized by acidification of previously insoluble compounds, displace the previously adsorbed divalent calcium ions, which are flushed through the soil with the mobile sulfate ions. The relationship between base saturation and pH is shown in Figure 2, indicating that the capacity of soils to adsorb bases is essentially exhausted at pH 4.0 and lower (40). In his experiments, Wiklander demonstrated that when acid is added to the top of a laboratory column filled with kaolinite or other clay minerals on which C a 2 + had been adsorbed, the ratio C a 2 + : H + in the clay and effluent is lowered as the base saturation decreases, becoming essentially zero when the base saturation reaches zero, which occurs at a pH of about 4.0 (38). Based only on laboratory experiments, Wiklander concluded that noncalcareous soils having a pH > 6 . 0 are the most sensitive to acid deposition and that very acidic soils are far less sensitive. Although this might be readily assumed from the abiotic laboratory system with which Wiklander worked, precisely the opposite is true in a forest ecosystem. Wiklander ignored the fact that mineralization of humus and dead roots produces nutrient cations that are not immediately assimilated by active root systems. In soils with a pH of 5.0 or higher, these cations can be held in reserve by adsorption on clay until needed for vegetative regrowth. At pH values below 4.0, the sites on the clay are already occupied by H + and A l 3 + , and the important reserves of nutrient cations, as they become available from mineralization, cannot be adsorbed and are flushed from the system. The A horizon, which constitutes the top 15 cm of the mineral soil in the Huntington Forest, has a pH of 3.6. As can be seen from Table 5, the losses of C a 2 + from the A horizon are substantial. It can be calculated that (on an equivalent basis) the C a 2 + loss is 69% of that of H + , compared with a value of essentially zero percent as predicted by Wiklander's theory. The calcium concentration of the A horizon in 1979 was found to be only 0.08 meq/100 g, and the bulk density as reported by Raynal et al. was 1.1 g / c m 3 ( } / ) . Therefore, the 15-cm depth of this horizon contained 1650 t/ha of soil, and the total amount of calcium present in the soil was only 26.4 kg/ha. As can be calculated from 254A
Environ. Sci. Technol., Vol. 17, No. 6, 1983
FIGURE 2
Relationship between soil pH (H2O) and percent base saturation
5.55.0§4.5X
Q.
4.0-
}•
