Adsorbed Sulfate: Indicator of Anion Exchange Capacity or Competitor against Nitrate Adsorption?.

Variable-charge soils having sizable anion exchange capacity often contain a large amount of adsorbed sulfate. However, its effects on the transport of weakly-adsorbing anions like chloride and nitrate have not been fully elucidated. A traditional view is that the presence of sulfate, a strong competitor for the anion adsorption sites, substantially suppresses the adsorption of the monovalent anions. The presence of a large amount of adsorbed sulfate would thus discourage nitrate adsorption and favor its transport downward in the soil profile. To the contrary, when compared among different soils, the adsorption of chloride and nitrate increases with an increase in the amount of adsorbed sulfate. In this view, not only is the adsorbed sulfate indicative of the soil's positive electrical charge, but it also serves as proxy for the monovalent anion adsorption capacity in the presence of native sulfate.

In the present study, we review the adsorption and transport of weakly-adsorbing anions in the presence of adsorbed sulfates. Attention will be paid to the conditions in which adsorbed sulfate may be used as an indicator of soil's propensity to adsorb electrolyte anions, or simply acts as an competitor against their adsorption. We emphasize that adsorption of the monovalent anions is mostly due to the increase in the total anion adsorption rather than anion exchange with the native sulfate, but the adsorbed sulfate in heavily fertilized soils has a larger tendency for desorption via anion exchange. The need for developing adsorption models, which are applicable in the presence of adsorbed sulfate and thus more realistically mimic the field conditions, will also be discussed.