Anion Transport in Columns of Variable Charge Subsoils: Nitrate and Chloride

Acid variable charge subsoils exhibit a positive charge, which retards the passage of anions through the soil profile. The objective was to study the NO−3 adsorption isotherms and the effect of changing the subsoil positive charge on NO−3 and Cl− retention, estimate NO−3 and Cl− retardation in different subsoils, and quantify relationships between transport and adsorption parameters. Subsoils from different subtropical and tropical areas were used in column experiments. Four lime treatments and four leaching solutions were used to create different anion exchange capacities (AECs) in subsoils. In the Cecil subsoil (Fine, kaolinitic, thermic Typic Kanhapludult), NO−3 isotherms were linear between 5 and 30 mmol NO−3 L−1. When forced through the origin, an L‐curve NO−3 isotherm was observed at native and low soil pH. The NO−3 adsorption was largely affected by the changes in both pH and concentration of the leaching solution. Other variable charge subsoils have varying capacities to retard anions, which is reduced when either soil pH or leaching solution concentration is increased. Both parameters equally reduce the distribution coefficient Kd and the retardation coefficient R. The AEC correlates closely with R and Kd. As a result, AEC can be successfully used in the solute transport models to estimate Kd or R without measuring it to correct for the NO−3 retardation.