Journal of Conference Abstracts

Soil Calcium Status and the Response of Stream Chemistry to Declining Acid Rain

Gregory B. Lawrence¹ (glawrenc@usgs.gov), Mark B. David² (m-david@uiuc.edu),
Gary M. Lovett³ (72677.3177@compuserve.com), Peter S. Murdoch¹ (pmurdoch@usgs.gov), Douglas A. Burns¹ (daburns@usgs.gov), Barry P. Baldigo¹ (bbaldigo@usgs.gov),
Andrew W. Thompson³ (914-677-5976), James H. Porter⁴ (jporter@pppmail.appliedtheory.com) & John L. Stoddard⁵ (stoddard@heart.cor.epa.gov)

¹ U.S. Geological Survey, 425 Jordan Road, Troy, New York 12180, U.S.A.

² University of Illinois, W-503 Turner Hall, 1102 South Goodwin Avenue, Urbana, Illinois 61801, U.S.A.

³ Institute of Ecosystem Studies, Box AB, Millbrook, New York 12545, U.S.A.

⁴ New York City Department of Environmental Protection, P.O. Box 358, Grahamsville, New York 12740, U.S.A.

⁵ Dynamac International Inc., c/o U.S. Environmental Protection Agency,
200 S.W. 35th Street, Corvallis, Oregon 97333, U.S.A.

Calcium is a macronutrient for trees and plays a major role in neutralising acid rain. In forest soils of Europe and North America, however, concentrations of exchangeable Ca (soil Ca available for root uptake or leaching) have decreased over the past several decades (Rodhe et al., 1995). In regions with severe air pollution, this decrease is the result of leaching by acid rain (Wesselink et al., 1995, Cerny´, 1995), whereas in regions such as the northeastern United States, that receive moderate levels of acid rain, the decrease has also been attributed to net forest growth and lower atmospheric deposition of Ca (Johnson and Fernandez, 1992; Johnson et al., 1994a; Johnson et al., 1994b). Recent studies also have suggested that depletion of soil Ca is the reason that decreases in pollutant emissions have not yet reversed surface-water acidification (Kirchner and Lydersen, 1995; Likens et al., 1996). These studies, however, did not provide soil data that could be directly related to temporal trends in surface-water chemistry or acidic deposition.

An ongoing study of the Neversink River Basin, in the Catskill Mountains of New York, used in situ and laboratory soil leaching experiments in conjunction with measurements of acidic deposition and stream chemistry to directly test the hypothesis that acid rain has reduced soil base saturation, thereby limiting the recovery of surface-water chemistry from reductions in acidic deposition which have occurred over the past two decades.

Data from Winnisook Watershed, a headwater drainage in the Neversink Basin, showed a distinct increase in atmospheric deposition of SO₄^2- with increasing elevation that corresponded with an upslope trend of decreasing concentrations of exchangeable bases in mineral soil. The effect of atmospheric deposition on soil base leaching was further evaluated through emplacement of homogenized mineral soil (B horizon, 0-10 cm layer below the O horizon) in nylon mesh bags, directly beneath the O horizon at 34 locations of varying elevation. This soil had been collected lower in the Neversink basin, where exchangeable-base concentrations are higher than in Winnisook watershed. With one exception, all soil bags lost exchangeable bases in the first year and, most significantly, the spatial pattern of base loss was positively correlated with the elevational trend of acidic deposition (P < 0.01). Soil bags left in the ground for 2 years lost additional Ca and displayed the same relation to elevation as those collected after 1 year (P < 0.05).

The effect of decreased base saturation on stream chemistry was evaluated by leaching solutions of sulphuric and nitric acid through B horizon soil of three different base saturations, in varying concentrations similar to the range found in Oa horizon soil solutions in the Neversink watershed (pH 3.7 to 4.8). The soil used in the leaching experiment was the same as that used in the in situ soil bag experiment; soils with the highest base saturation (14.4%) were those that had not been leached in the field, whereas the others were obtained from soil bags that had been leached in situ for 1 year. Concentrations of Ca in leachates decreased as concentrations of SO₄^2- + NO₃^- decreased, and, as base saturation decreased, less Ca was released per equivalent of added acid anion (P < 0.01).

Concentrations of Ca in Winnisook stream water, under high flow conditions, decreased with increasing elevation, despite an upslope trend of increasing SO₄^2- concentrations in stream water (P < 0.01), an indication that the elevational trend of exchangeable-base concentrations in soil overrode the effect of the acid-anion concentrations of soil leachate. The ratio of base-cation concentrations to acid-anion concentrations in stream water also decreased with increasing elevation (P < 0.01), further evidence that Ca concentrations in stream-water are affected by the elevational trend of decreasing exchangeable-base concentrations in soil. This spatial pattern of soil Ca depletion explains the decrease in stream-water ANC with increasing elevation. Elevational gradients of precipitation and organic acid production did not occur in this watershed and therefore were ruled out as explanations for the elevational gradients of soil and stream chemistry.

In Winnisook watershed, net loss of exchangeable Ca in the mineral soil is probably continuing. The ANC of Winnisook stream water has decreased from June 1991 through May 1996 at a rate of 1.5 µeq L^-1 yr^-1 (p < 0.01, Kendall's Tau) despite a decrease in stream-water concentrations of acid anions (SO₄^2-, NO^3-, Cl^-) of 1.8 µeq L^-1 yr^-1 (p < 0.01, Kendall's Tau).

References

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