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Comments and Responses

Comments on "Thirty Years of Change in Forest Soils of the Allegheny Plateau, Pennsylvania"

Natural Resources and Environmental Science, University of Nevada, Reno, Reno, NV 89557

dwj{at}cabnr.unr.edu

Bailey et al. (2005) found substantial declines in exchangeable base cations over a 30-yr period in soils of the Allegheny Plateau. They cannot attribute these changes to uptake and sequestration by vegetation and therefore ascribe the changes to "off-site leaching," and note that these changes are "consistent with studies that have suggested that acid deposition has induced significant losses of exchangeable base cation pools by hydrologic leaching" (p. 688). Close examination of Table 3 in their manuscript does not support this speculation, however. If the observed changes in exchangeable Ca²⁺ and Mg²⁺ are converted to kmol_c ha^–1, the changes in Ca²⁺ + Mg²⁺ (M²⁺) that cannot be ascribed to vegetation uptake are 581.3, 184.0, 19.8, and –9.0 kmol_c ha^–1 for the Dewdrop (DD), Fools creek (FC), Heart's Content (HC), and North Branch (NB) sites, respectively. On an annual basis (dividing by 30 yr), this amounts to 19.4, 6.1, 0.7, and –0.3 kmol_c ha^–1 yr^–1, respectively. The values for DD and FC appear to be well beyond what could be attributed to total S and N deposition, equivalent to 310 and 98 kg ha^–1 yr^–1 of S or 271 and 86 kg ha^–1 yr^–1 of N. According to NADP maps, wet deposition of S for this region appears to be on the order of 12 kg ha^–1 yr^–1 as S (35 kg S ha^–1 yr^–1 as SO₄^2–) and –N deposition is on the order of 8 kg ha^–1 yr^–1 as N (21 kg ha^–1 yr^–1 as NO₃^– and 4.1 kg ha^–1 yr^–1 as NH₄⁺) (NADP, 2003). Even if major N inputs occurred by dry deposition, it seems extremely unlikely that atmospheric deposition contributed substantially to the observed changes at DD and FC. Nor would natural leaching rates, typically on the order of 0.5 to 2 kmol_c ha^–1 yr^–1 (Johnson and Lindberg, 1992), contribute this much to base cation leaching, unless limestone parent material is being weathered. The value for HC is within the range of what might be expected due to leaching processes, but the value for NB is in fact negative, implying that weathering exceeded base cation loss because soils did not lose as much base cations as were sequestered by vegetation.

Thus, the changes in soils documented by Bailey et al. (2005) are not consistent with what could be expected due to leaching or to acid deposition. Given their careful and extensive quality assurance program, the observed changes have to be accepted as real until proven otherwise, and further investigation is definitely warranted to uncover the causes of these changes.

REFERENCES

• Bailey, S.W., S.B. Horsley, and R.P. Long. 2005. Thirty years of change in forest soils of the Allegheny Plateau, Pennsylvania. Soil Sci. Soc. Am. J. 69:681–690.[Abstract/Free Full Text]
• Johnson, D.W., and S.E. Lindberg (ed.). 1992. Atmospheric deposition and forest nutrient cycling: A synthesis of the integrated forest study. Ecol. Ser. 91. Springer-Verlag, New York.
• National Atmospheric Deposition Program. 2003. Isopleth maps. http://nadp.sws.uiuc.edu/isopleths/maps2003/.

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