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Forest Soils on Acid

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Forest Soils on Acid

Forest ecosystems are important both ecologically and economically. It is arguable that the most fundamental dynamic of the forest ecosystem is the forest soil. The acidity of forest soils can alter the chemistry, biota, and hydraulics of the soil, and thus, alter the soil formation characteristics and the soil composition. It follows that the acidification of forest soils demands a great deal of research and attention.

Forest soils are commonly found to have pH readings of 4-6, even in areas of moderate to low acid deposition (Binkley et al, p. 4). In fact, an abundance of forest vegetation thrives on and stabilizes most forest soils at relatively low pH levels. It seems as though forest ecosystems generally thrive upon strongly acid soils. Though forest soils naturally are acidic, problems can occur when the acidity levels are raised artificially through processes such as acid rain. This paper will investigate the effects of higher than normal acidity and acid deposition in forest soils to gain a greater understanding of current and potential problems to forest soils and ecosystems.

It is important to remember when discussing the implications of high acid in forest soils that there are several general factors that will alter acidic effects on soil chemistry, hydrology, biota, and weathering. These factors include soil type, soil sensitivity, and the quantity of precipitation. Texture, structure, grain size, and consistence are all crucial to defining the soil type or series and also to the amount of time soil is exposed to acid deposition. In a particular study on humus degradation based on simulated "acid rains" conducted by Greszta et al. (1991) revealed the extent to which soil type influenced soil acidity.

It results from the performed investigations that the influence of simulated aqueous solutions on soil depended, to a large extent, on a kind of soil. The pH of sandy soils showed the decrease by 1.5 value, between the soils treated with aqueous solutions at pH 2.0 and soils treated with aqueous solutions at pH 7.8. The greatest differences occurred in peat soils, where the decrease of the pH value was by 2.7 when treating the soils with aqueous solutions at pH 2.0 (p. 59).

Clay rich soil types that would hold water better than course sandy soils increased acidity by more than ten fold in this experiment. A similar study investigating a soil series in a contaminated area referred to as the S-area (pH 2.8 - 3.0) and an area unaffected by acidic deposition referred to as the R-area (pH 5.0 - 6.0) reached conclusions in accordance with Greszta. The study showed that the S-area had clay contents 20% greater than the R-area due to dispersion and illuviation (Rampazzo and Blum, 1992, p. 211).

Soil sensitivity is another important factor which influences the effects of high acidity; moreover, soil sensitivity may also be a factor of soil type.

One possible interpretation of this result is that both base saturation percentage and buffering capacity are determined by soil type. From this, the generalization can be made that clay plus silt percentage gives a reasonable indication of how sensitive a soil would be to acidification. This contention is supported by other authors. Brady (1974) reported that soils higher in silt plus clay content will have more binding sites for ionic exchange in the soil matrix. Jones and Suarez (1980) have shown that poorly buffered soils usually have very low exchangeable bases and sandy texture, and Reuss (1975) found that coarse sandy soils underwent a natural acidification resulting in lower soil pH and base saturation. Mikhail's results (1936) for Latrobe Valley support this contention. He found that the sandy soils were generally more acidic than the loamy soils in the region (Lau and Manwaring, 1984, p. 460-461).

Soil sensitivity to acidic deposition seems almost entirely based upon the contents of the soil. This makes sense because a soil's cation exchange capacity (CEC) is largely based upon soil composition. Mcfee (1980, in Lau and Manwaring, 1984, p. 451) suggested that CEC should be the primary tool to assess soil sensitivity to acidic deposition. This contention was supported by Binkley et al. (1989, p. 15) Although, "the use of a single soil property to describe the buffer capacity may be over-simplified. Donahue et al. (1977) pointed out that the physical properties such as soil texture, structure and permeability can be as important as the chemical properties such as cation exchange capacity, base saturation percentage, exchangeable base content and pH in influencing the buffering capacity of a soil" (Lau and Manwaring, 1984, p. 452). A computer simulation model was developed to describe the sensitivity of Pennsylvanian soil to acid deposition by Levine and Ciolkosz (1988). The model for Pennsylvania revealed that exchangeable bases, base saturation and pH were very important in determining soil sensitivity, but CEC "which has been used as a major determining factor of soil sensitivity to acid deposition showed the lowest correlation to soil sensitivity to acid deposition" (p. 214). Forest soils specifically are usually relatively insensitive to acid deposition, as shown by the Pennsylvanian model, which had areas of very low sensitivity in the heaviest forests.

The final major factor that can significantly alter the effects of acid deposition on forest soils is the amount of precipitation which will directly affect the concentration of H+ ions.

Over longer time scales, the concentration of H+ depends on total quantities of ions entering and leaving the soil. For example, a forest receiving 1000 mm of precipitation at a pH of 5.6 would receive 25 mol of H+/ha annually, whereas a forest receiving 2000 mm of precipitation at the same pH would receive twice as many H+ (50 mol/ha). Although each forest received rain of the same pH, the one receiving more precipitation might acidify more quickly because it received twice as much H+ (Binkley et al., 1989, p. 13).

Precipitation can also affect forest soil acidity by altering the amounts of water available in the system to promote chemical processes.

The level of forest soil acidification is largely based, in addition to pH, upon the factors of soil type, soil sensitivity, and precipitation. The major affects such acidification now need to be addressed to be able to predict the possibilities of higher than normal acidification in forest soils. There are four major soil properties that acidity has a major role in developing: chemical, hydrological, biological, and erosional. Chemically, acid can catalyze or hinder many important chemical soil processes. Acid forest soils often develop high levels of soluble aluminum. In fact, the more acidified a soil is, the more aluminum rich clay particles will release Al into solution (Singer and Munns, 1996, p. 270). This can be seen from field research as demonstrated by Mulder et al. (1987).

Compared with relatively unpolluted regions (Ugolini et al., 1977), soil solutions in areas with high atmospheric acid input (i) have increased total Al concentrations, (ii) contain mainly inorganic monomeric Al forms (Mulder and van Breemen, 1987; David and Driscoll, 1984; Driscoll et al., 1985) and (iii) have increased concentrations of SO42- and sometimes NO3- (Johnson et al., 1981; Ulrich et al., 1979).

Increased dissolved Al concentrations and fluxes in acidic soils are due to atmospheric acid deposition not only indicate a change in pedogenic processes, but in addition could have adverse effects on living organisms in soil and surface water (Anderson and Kelly, 1984) (p. 1640).

As stated by Mulder, not only does acidity increase soluble Al, the Al acts as a toxicant to organisms in the soil and alters the soil formation processes. Al can also reduce Mg, which is an essential root nutrient, causing severe root damage (Matzner et al., 1986, p. 274). Hutchinson (1983 in Matzner, 1986, p. 274) showed that Al-ions damage plant roots, and are often toxic to the root. It follows that aluminum toxicity can alter the properties of a single forest soil by altering or killing the vegetation and by changing the soil properties (Binkley et al., 1989). Binkley et al. (1989) using the "MAGIC" mathematical computer model showed that acid deposition that changes the pH of a soil 0.3, can radically alter the amounts of aluminum in the system. These models suggest that under current conditions a change in soil pH of 0.3 can take place in a matter of decades.

High acidity in soil also hinders the nitrification process, whereas, alkalization stimulated nitrification (Greszta et al., 1992, p. 56). Nitrogen fixation is a process that is very hard to accomplish in an acidic forest soil because of the sensitivity to acid that the nitrogen fixers have, which creates an environment where only a few types of plants are able to thrive. As free nutrient nitrogen becomes less and less available, it seems likely that more and more forests will experience die off.

The final major chemical process due to high acidity in soils is leaching. Binkley et al. (1989, p.8) states that leaching of nutrients is probably the biggest source of tree damage in acid soils. Leaching of S, Ca, K, NO3, PO4, and Mg are major problems for acidic forest soil development (Lau and Manwaring, 1984, p. 451). Sharpe et al. (1993, p. 130-132) found that Ca leaching significantly hindered tree growth, and the addition of bone meal treatment allowed for regeneration of the soil and tree growth. Binkley et al. (1989, p. 129) showed that sulfur was leached from the soil by acid forest soils, but also used the MAGIC computer model to show that artificial sulfur additions slowed down the acidification of forest soils by over 20 years.

Hydrologically, a soil can be modified by acidity, by changing the porosity and composition of the soil particles and by changing the overall cohesiveness and structure of the soil which in turn would alter the hydrodynamics of the groundwater. Rampazzo and Blum (1992, p. 211) illustrated the change in soil particles and clay minerals based upon acidity. They showed that the polluted soils had a higher aggredation grade, the stability and consistence of the soil was strongly reduced, and the soil pores continuity and sharpness were strongly reduced. As well they found that in the normal acidity area the predominant clay minerals were kaolinite, illite, and "secondary" chlorite; in the acid area the clay minerals were predominantly 2:1 vermicullite interstratified with illite, no "secondary" chlorite was seen (p. 218). This change illustrated the loss of Si compounds and the gain of Fe compounds.

Organically, the effects of forest soil acidity are more dramatic. As stated earlier, roots can suffer a large amount of damage from aluminum toxicity and the hindering of the nitrification process. As well, Greszta et al. (1992, p. 54) noted that forest soils saw bacteria content drop 50% after the addition of simulated strong acid rain. Hindrance of plant respiration and cellulose decomposition were also noted as affects of high acid deposition on forest soils (p. 65). Binkley et al. (1989, p. 8) found that leaves can be directly affected by acid in soils through destruction of the cell wall and erosion of the leaf cuticle. Finally, it seems likely that much of the forest die off, or Waldsterben forests, in northern Europe is a result of heavy acidification of the forest soil (Binkley et al., 1989, p. 3).

Finally, soil erosion and erosion of parent material can be affected by higher levels of acidity (Binkley et al., 1989). This erosion can lead to an increase in soluble toxicants or valuable nutrients to the soil. High acidity can weather the soil itself, by dissolving chemical particles or by changing the hydrodynamics as mentioned before which would change the path of groundwater erosion.

According to Binkley et al. (1989, p. 129) and the MAGIC model they created, it seems likely that forest soil acidification in the southeast of the United States will reach a point in about 50-100 years where many forests begin to die off. It also seems likely that this die off will not be an isolated event, as signs of it can already be seen in Germany, Norway, and Sweden (Binkley et al., 1989, p. 3). It has also been shown that it is possible to slow the effects of acidification on some forest soils through nutrient replacement and liming; moreover, it has been shown that a drop in the depositional acidification rates will allow for forest soil recovery to begin to take place. Thus, understanding the effects of higher than normal acidic deposition on forest soil becomes crucial to gaining a perspective on the future of forest ecosystems, and to finding solutions to a possible disastrous situation.


Binkley, D., C.T. Driscoll , H.L. Allen, P. Schoeneberger, and D. McAvoy. 1989. Acidic Deposition and Forest Soils: Context and Case Studies in the Southwestern United States. Edwards Brothers Publications. Ann Arbor, Michigan.

Greszta, J., A. Gruszka, and T. Wachalewski. 1991. Humus degradation under the influence of simulated 'acid rain'. Water, Air, and Soil Pollution 63: pp 51-66.

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Mitchell, M.J., M.B. David, I.J. Fernandez, R.D. Fuiler, K. Nadelhoffer, L.E. Rustad, and A.C. Stam. 1994. Response of buried mineral soil bags to experimental acidification of forest ecosystem. Soil Science Society of America Journal 58: pp. 556-563.

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Sharpe, W.E., B.R. Swistock, and D.R. Dewalle. 1992. A greenhouse study of northern red oak seedling growth on two forest soils at different stages of acidification. Water, Air, and Soil Pollution 66: pp. 121-133.

Singer, M.J., and D.N. Munns. 1996. Soils: An Introduction. Prentice-Hall, Inc. New Jersey.

Tamm, C.O., and L. Hallbacken. 1986. Changes in soil pH over a 50-year period under different forest canopies in SW Sweden. Water, Air, and Soil Pollution 31: pp. 337 341.

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