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

Abstract

AbstractThe 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−/SO42− 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 SO42− 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 SO42− that was adsorbed. The relative NO3−/SO42− 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 S42−. Some practical implications are also discussed.