Effects of Sulfuric Acid Rain on Major Cation and Sulfate Concentrations of Water Percolating Through Two Model Hardwood Forests

Abstract

AbstractAcid precipitation falls on vast areas of forested land, including most of the eastern deciduous forest of the United States. Forest productivity, ground‐water quality, and surface waters might all be affected. To document and quantify ecosystem response to the onset of acid precipitation, simulated sulfuric acid (H2SO4) rain was applied to model forest plots of sugar maple (Acer saccharum Marsh.) and red alder (Alnus rubra Bong). One set of four plots (two alder and two maple) received a control rain consisting of a stock solution equilibrated with atmospheric CO2 to approximately pH 5.7. For three sets of four plots, sufficient H2SO4 was added to the stock solution to lower the pH to 4.0, 3.5, or 3.0. Nozzles were used to apply simulated acid rain at the rate of 2.8 mm/hour (maple) or 3.7 mm/hour (alder) for 3 hours/day, 3 days/week, throughout the year. Water samples were collected above and below the canopy, below the litter layer, and from 20‐cm and 1‐m depths below the surface of the soil.While constituents of the throughfall were similar to those of the simulated rain, the litter leachate (the actual input to the soil) had consistently higher concentrations of SO42−, Ca2+, and Mg2+, and higher pH, than the acid rain.For the first 6 months, SO42− adsorption by the soil prevented any apparent differences in the SO42−, Ca2+, or Mg2+ concentrations or the pH of samples taken from a depth of 20 cm from plots receiving either acid or control treatments. Then, SO42− concentrations at this depth on plots receiving the most acidic rain (pH 3.0) became increasingly higher than on the other plots until, after 3 years, they were approximately equal to SO42− concentrations in the rain. Soil solutions from the pH 3.5 and 4.0 treatments responded similarly starting 10 months and 1 year, respectively, after initiation of exposure to acid rain. Increased Ca2+, Mg2+, and, to a lesser extent, K+ concentrations and lowered pH in solutions from the 20‐cm depths occurred simultaneously with increased SO42− concentrations. At a depth of 1 m, no acid‐rain‐related effects were evident after 2.5 years exposure to pH 3.0 H2SO4 rain. Results showed that a hardwood canopy and litter layer can alter the input of chemicals to the soil from acid rain, that the relative sensitivity of ions to mobilization by acid rain is not the same for all components of an ecosystem, and that acid rain can alter the chemical composition of water within the root zone within a few years, even if the soil is a strong sulfate adsorber.