Forest Ecosystem Science Laboratory
NYSERDA

Assessing the Sensitivity of New York Forests to Cation Depletion from Acid Rain

Understanding mineral sources of calcium in soils of the northeastern USA is important because plant-available Ca has been leached from the soil at accelerated rates for several decades (Lawrence et al., 1999; Likens et al., 1998) possibly resulting in Ca limitation in some forest ecosystems (Horsley et al., 2000; Huntington, 2005; Long et al., 1997).  And, up to this point, geologically sensitive areas of New York State have been defined as those in which surface waters have been acidified (e.g., April et al., 1986).   This definition is incomplete in terms of protecting and managing forest health (Nezat et al. 2008).  Areas with few carbonate rocks, such as the Adirondacks and Hudson Highlands, may indeed be most susceptible to acidification of surface waters, and the low buffering capacity of soils may explain the phenomenon of dead lakes (Reuss and Johnson, 1986).  But the susceptibility of forest soils to Ca depletion may also depend on the presence of trace minerals such as apatite and the omission of non-silicate minerals as a Ca source may have seriously overestimated the threat to forest health from acid rain.  Apatite is already known as the dominant, P-bearing primary mineral in soil.

Predictions of Ca depletion have been based on the assumption that only the salt-exchangeable Ca pool is available to plants; the weathering of Ca from parent materials has been believed to be too slow to play a role in mediating the acidifying effects of air pollution on soils. Because of these assumptions, the role of readily weathered Ca-bearing trace minerals, such as apatite and calcite, have been overlooked in assessments of Ca depletion from acid rain. Information about the distribution of Ca sources and the ability of tree species to obtain Ca is essential to predicting the sensitivity of forests across New York State to Ca depletion, with implications for sustainable forest management and air pollution policy.  

With these in mind, we asked: “How important are Ca-bearing minerals in parent materials across New York State, and are various tree species able to use this calcium?”

Table 1.  Sampling locations in the northeastern USA. The bedrock at each site includes bedrock found up to 10 km northward. The surficial deposits are glacial till at all sites except for Brasher Falls (NW), Fort Jackson, and Southville which have proglacial lake deposits.

 

I. Soil source of calcium, and tree responses to same

The study began with the selection of 31 New York sites based on bedrock geology and the distribution of soil types, and the sampling of soils at each site (Figure 1, Table 1).   A new “sequential extraction” procedure was used to quantify the amount of calcium that is exchangeable, readily weathered (such as apatite) and resistant to weathering (such as silicate minerals).  Leaf litter was also collected from these sites and was used to determine the influence on contrasting parent materials on plant tissue chemistry.  

We used a nitric acid extract of samples from these sites to determine which soil parent materials in the northeastern USA contained apatite (Figure 2).  We identified apatite in most of these soil parent materials based on the similarity between the P:Ca ratio in the extract and that in apatite (3:5) (Figure 2).  Some of the soils derived from crystalline silicate rocks (Osborn, Stissing Mountain, Sabbaday Falls, and Lafayettville) had P:Ca>3:5 suggesting the presence of other P sources in addition to apatite.

We also looked at thin sections of soil parent materials from the Adirondack Mountains. We found apatite either as individual grains (Figure 3a), partially included in silicate minerals (Figure 3b), or completely included in silicate minerals (Figure 3c). Apatite grains ranged from 50 to 300 mm in diameter.  The concentration of apatite estimated from the extract may be less than the total because some apatite may occur as inclusions in, and may be completely armored by, weathering-resistant minerals.

We next compared the Ca concentrations in leaves to those we measured in the soils.  Birch species, red maple, and other hardwoods had the highest Ca in leaf litter in the carbonate sites and showed the most response to soil type (Figure 4).  Sugar maple had higher Ca in leaf litter than these species, on average, in the non-carbonate sedimentary soils, but increased little as a function of soil Ca.  Spruce and fir had low Ca across all sites.  Oak actually had higher Ca in crystalline silicate than carbonate sites. It is not clear from this analysis that any soil horizon or extract is a better predictor of leaf Ca.

Because of the range of properties in parent materials, the effect of parent material on foliar concentrations should be analyzed with soil as a continuous variable (Ca availability as defined by the sequential extraction procedure) in multivariate regression. The analysis by classes of sites is a good start and represents the final product of the funded project.  Future work will use strontium isotopes and the ratio of calcium to strontium to determine the relative importance of atmospheric deposition, silicate weathering, and trace minerals as Ca sources to different tree species or forest types.

II. Carbon and Nitrogen storage in soils

Four additional sites were evaluated where calcium had been purposely added during previous experiments (Table 2).   Samples were collected at sites where Ca was added in liming experiments 14 to 41 years ago to determine whether soil acidification might lead to changes in soil C and N storage.  We addressed one aspect of soil acidification by comparing forest soils that have been undisturbed but receiving ambient acidic deposition with soils that had been treated with lime to reverse the acidification process.  We found more organic matter and nitrogen in soils that were not treated.  This comparison suggests that soils may be storing more C and N in organic matter over time as they acidify.

Table 2.   Description of Liming sites and treatments.

 

III. Ramifications of research

Our examination of parent materials in forest soils across New York State demonstrates that the pool of readily weathered Ca can be substantial; this pool has not traditionally been included in assessment of the Ca available to plants.  In sites with carbonates or apatite in the parent material, the threat of Ca depletion is probably not as great as previously supposed.  It is possible that some tree species or forest types are better than others at accessing nutrients through weathering.  More research is needed to determine the degree to which forest management could be used to improve Ca availability to forests via weathering of native soil minerals.

IV. Acknowledgments

We wish to thank our industry collaborators who provided site locations: Robert O'Brien, Cotton-Hanlon, Inc.; Roger Dziengaleski and Dave Osterberg, Finch Pruyn & Company; Tom Hall, Pennsylvania Department of Conservation and Natural Resources; and Phil Malerba and Cathy Irwin, International Paper.  Samples were collected by Amber Knowlden, Byung Bae Park, Megan Rose Newhouse, Ryan Maher, Jackie Borza, Adrienne Graham, and Amy Smith.  Chemical analyses were performed under the direction of Joel Blum, University of Michigan.  Carmen Nezat, Byung Bae Park, Dustin Wood, and Melissa Lucash analyzed data.  Heather Engelman compiled this report.  Additional financial support was provided by the Agenda 2020 collaboration between forest industry and the USDA Forest Service and the National Science Foundation (DEB 0235650 and 0423259).  This work is a contribution to the Hubbard Brook Ecosystem Study, which is maintained by the USDA Forest Service and participates in the NSF Long-Term Ecological Research program

V. References

April, R., R. Newton, and L.T. Coles. 1986.  Chemical weathering in two Adirondack watersheds: Past and present-day rates. Geol. Soc. Am. Bull. 97, 1232-1238.

Horsley, S.B., R.P. Long, S.W. Bailey, R.A. Hallet, and T.J. Hall. 2000. Factors associated with the decline disease of sugar maple on the Allegheny Plateau. Can. J. For. Res. 30:1365-1378.

Huntington, T.G. 2005. Assessment of calcium status in Maine forests: Review and future projection. Can. J. For. Res. 35:1109-1121.

Lawrence, G.B., M.B. David, G.M. Lovett, P.S. Murdoch, D.A. Burns, J.L. Stoddard, B.P. Baldigo, J.H. Porter, and A.W. Thompson. 1999. Soil calcium status and the response of stream chemistry to changing acidic deposition rates. Ecol. Appl. 9:1059-1072.

Likens, G.E., C.T. Driscoll, D.C. Buso, T.G. Siccama, C.E. Johnson, G.M. Lovett, T.J. Fahey, W.A. Reiners, D.F. Ryan, C.W. Martin, and S.W. Bailey. 1998. The biogeochemistry of calcium at Hubbard Brook. Biogeochemistry 41:89-173.

Long, R.P., S.B. Horsley, and P.R. Lilja. 1997. Impact of forest liming on growth and crown vigor of sugar maple and associated hardwoods. Can. J. For. Res. 27:1560-1573.

Nezat, C.A., J.D. Blum, R.D. Yanai, and B.B. Park. 2008.  Mineral sources of calcium and phosphorus in soils of the northeastern USA.  Soil Science Society of America Journal 72(6): 1786–1794

Reuss, J.O. and D.W. Johnson.  1986.  Acid deposition and the acidification of soils and waters.  Springer-Verlag, New York.  117 pp.