Soil Cation Leaching by “Acid Rain” with Varying Nitrate‐to‐Sulfate Ratios

Abstract

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 H 2 SO 4 and HNO 3 , all with pH 3.5, but with varying NO 3 − /SO 4 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 SO 4 2− adsorption capacity because of its high Fe 2 O 3 content of 12 g kg −1 and high point‐of‐zero charge (PZC) of 6.0. The Oxisol, although strongly weathered, had a lower Fe 2 O 3 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 Fe 2 O 3 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 NO 3 − content of the leaching input due to the higher mobility of NO 3 − compared with SO 4 2− that was adsorbed. The relative NO 3 − /SO 4 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 S 4 2− . Some practical implications are also discussed.