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Charge Properties and Nitrate Adsorption of Some Acid Southeastern Soils

Dep. of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061-0404;

Warren D. Brady

IT Corporation, Port Allen, LA 70767-3200;

Colleen K. Lynch

Dep. of Agronomy, Louisiana State Univ., Baton Rouge, LA 70803.

^* Corresponding author (eick{at}vt.edu).

ABSTRACT

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 capacity 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 (R²) 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 NO₃ movement to groundwater. Therefore, simulation models may need to account for NO₃ adsorption when modeling NO₃ movement in acid soils dominated by variable charge minerals.

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