Charge properties and nitrate adsorption of some acid Southeastern soils

Numerous studies have been conducted examining nitrate (NO3) leaching losses from agricultural land. Simulation models have been developed that allow one to predict the potential of NO3 to leach to groundwater. However, many of these models treat NO3 as a conservative tracer and do not evaluate surface chemistry. This study evaluated the surface charge properties and NO3 adsorption rapacity of four acid southeastern subsoils. Significant anion exchange capacity and NO3 retention was found for two of the soils. Point of zero net charge (PZNC) was determined using an ion exchange method. Values of 3.1 and 3.6 were determined for two of the soils while PZNC values were not quantifiable for the other two soils in the pH range of 3 to 7. Nitrate adsorption isotherms were measured on untreated and chloride-saturated soils. Nitrate adsorption maxima determined from the linearized form of the Langmuir equation ranged from 1.40 to 2.13 cmol(c) kg(-1). Coefficients of determination (R-2) and adsorption maxima increased after chloride saturation. This was attributed to competition from anions such as sulfate, fluoride, and phosphate. Net positive charge and NO3 retention were found to depend on the type and quantity of both variable and permanent charged minerals present in the soil and the composition of the exchange complex. These results demonstrated that acid subsoils high in variable charge minerals may have the potential to retard NO, movement to groundwater. There fore, simulation models may need to account for NO3 adsorption when modeling NO3 movement in acid soils dominated by variable charge minerals.