Nitrate mobility under unsaturated flow conditions in four initially dry soils
2007
Allred, Barry J. | Brown, Glenn O. | Bigham, J. M.
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/sec. 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.
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