NITRATE MOBILITY UNDER UNSATURATED FLOW CONDITIONS IN FOUR INITIALLY DRY SOILS

Abstract

Solving environmental problems associated with nitrate (NO3−) requires a better understanding of how NO3− moves through the soil profile. Transient unsaturated horizontal column experiments were conducted to assess processes affecting soil NO3− transport. Duplicate tests were conducted on four soils having different physicochemical and mineralogical properties. In each test, a 200 mg/L NO3−-nitrogen (NO3−-N) solution was applied at the inlet of the relatively dry soil columns, and the value of sorptivity kept constant at 0.0073 cm/sec0.5. Comparison of corresponding soil water content and soil solution NO3−-N concentration profiles from the column tests clearly indicated anion exclusion to be an important process impacting NO3− mobility under unsaturated flow conditions. Evidence of anion exclusion for all four soils included soil solution NO3−-N concentrations near the inlet that were 13% to 21% less than the concentration (200 mg/L NO3−-N) injected at the inlet. Further evidence of anion exclusion included peak soil solution NO3−-N concentrations up to twice the injected concentration near the wetting front for three of the four soils. The fourth soil, possibly because of a combination of dispersion processes, low pH, and the mixture of clay minerals present, behaved somewhat differently than the other soils by having a peak soil solution NO3−-N concentration above 200 mg/L located approximately halfway between the column inlet and wetting front. Overall, this research indicated that anion exclusion can be a key process affecting NO3− mobility in a variety of soil environments.