Submitted to: American Geophysical Union
Publication Type: Abstract only
Publication Acceptance Date: 5/15/2005
Publication Date: 5/23/2005
Citation: Allred, B.J. 2005. Nitrate mobility in unsaturated soil affected by the presence of different clay mineral types. American Geophysical Union. H43B-10. CD-ROM. Interpretive Summary:
Technical Abstract: Transient unsaturated soil column experiments were conducted to assess the magnitude of the anion adsorption/exclusion effects on nitrate mobility due to the type of clay mineral present. In all tests, a manufactured soil was employed that was comprised of 90 percent by weight medium sand and 10 percent by weight clay mineral (kaolin, illite, or montmorillonite). Prior to initiating an experiment, the soil within the column was completely dry. The Nitrate-N solution concentration injected at the soil column inlet was 200 mg/L. During testing, the moisture content at the soil column inlet was maintained at a constant value. All tests were replicated to ensure accuracy of results. For the 10 percent kaolin tests, soil solution Nitrate-N concentrations greater than 200 mg/L near the soil column inlet, and soil solution Nitrate-N concentrations less than 200 mg/L at the wetting front edge indicate that anion adsorption is an important process impacting nitrate mobility when kaolin is the dominant clay mineral type present. The 10 percent illite tests showed Nitrate-N concentrations less than 200 mg/L near the column inlet and Nitrate-N concentrations of 800 mg/L at the wetting front edge, thereby providing strong evidence that anion exclusion is an important transport process affecting nitrate mobility in soils having illite as the dominant clay mineral. The 10 percent montmorillonite test results also had Nitrate-N concentrations less than 200 mg/L near the column inlet, but with greater Nitrate-N concentrations of 1200 mg/L at the wetting front edge, thereby implying, that with montmorillonite the dominant clay mineral, there is an even stronger anion exclusion impact on nitrate mobility. Consequently, the overall test results suggest that either anion adsorption or anion exclusion can significantly impact nitrate mobility in unsaturated soil based on the clay mineralogy present. These findings may have important implications with regard to agricultural fertilizer application, particularly when initial soil conditions are relatively dry.