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Soil Cation Leaching by "Acid Rain" with Varying Nitrate-to-Sulfate Ratios¹

ABSTRACT

The influence of the anion composition of simulated "acid rain" on cation leaching of three soils with different surface-charge properties was examined. Four acid mixtures of H₂SO₄ and HNO₃, all with pH 3.5, but with varying NO₃^–/SO₄^2– mole ratios of 1.00:0.00, 0.75:0.25, 0.55:0.45, and 0.00:1.00, were used to leach an Ultic-Alfisol, an Oxisol, and an Entisol. The taxonomic names of these three soils are (i) Cornutt series: fine, mixed, mesic Ultic Haploxeralfs, (ii) unnamed Rhodustox, and (iii) Hanford series: coarse-loamy, mixed, nonacid, thermic Typic Xerorthents. The Alfisol had a high SO₄^2– adsorption capacity because of its high Fe₂O₃ content of 12 g kg^–1 and high point-of-zero charge (PZC) of 6.0. The Oxisol, although strongly weathered, had a lower Fe₂O₃ content of 5 g kg^–1 and PZC of 4.5. The Entisol was a relatively unweathered soil derived from silicaceous alluvium, with even less Fe₂O₃ of 3 g kg^–1 and a lower PZC of 3.5, and represented a soil of fixed charge. Cation leaching of the Alfisol varied directly with the NO₃^– content of the leaching input due to the higher mobility of NO₃^– compared with SO₄^2– that was adsorbed. The relative NO₃^–/SO₄^2– contents of inputs had no effects on cation leaching of the Entisol. Effects on leaching of the Oxisol were intermediate between those of the Alfisol and Entisol. It was clearly demonstrated that the anion composition of "acid rain" plays a significant role in the cation leaching of soils with amphoteric-charge properties, which are able to adsorb S₄^2–. Some practical implications are also discussed.

Key Words: amphoteric soils • anionic composition • atmospheric precipitation • soil leaching

¹ Contribution of the Dep. of Plant and Soil Biology, Univ. of California, Berkeley, CA 94720.

² Graduate Research Assistant and Professor, respectively. Present address of senior author: Dep. Soils, Water, and Engineering, Univ. of Arizona, Tucson, AZ 85721.

Received for publication June 7, 1983.