Effects of nitrate concentration and ionic strength on nitrate anion exclusion under unsaturated flow conditions

Transient unsaturated horizontal column experiments were conducted with a computer-controlled syringe pump to assess the impacts of nitrate (NO3) concentration and solution ionic strength on anion exclusion processes that affect NO3 transport through soil. A loam soil was used in all the column experiments. Duplicate tests were conducted with seven different injection solutions applied at the inlets of relatively dry soil columns (initial volumetric water content averaged 0.018). The injection solutions contained either dissolved potassium nitrate (KNO3) alone or a combination of dissolved KNO3 and calcium chloride (KNO3 + CaCl2). These seven solutions allowed for the evaluation of the anion exclusion effects of four different NO3-N concentrations (50, 200, 1000, and 2690 mg L) and four different solution ionic strengths (0.0036, 0.0142, 0.0714, and 0.1929 M) applied at the column inlet to be evaluated. Soil water content and soil solution NO3-N concentration profiles were compared between tests with different injection solutions to quantify the effects of NO3 concentration and ionic strength on NO3 anion exclusion.Anion exclusion was exhibited in the NO3-N concentration profiles for all experiments carried out in this investigation. Specifically, NO3-N concentrations at the column inlet were 5% to 26% less than the injected NO3-N concentrations, and NO3-N concentrations at the wetting front were greater than injected NO3-N concentrations by factors of 1.1 to 2.6. Considering results from tests conducted with both the KNO3 and KNO3 + CaCl2 injection solutions, it is apparent that ionic strength governs the magnitude of the NO3 anion exclusion effect to a far greater extent than the NO3-N concentration. A strong logarithmic relationship (R ranged from 0.9474 to 0.9850) was found to exist between the injection solution ionic strength and various column inlet or wetting front parameters used to quantify the anion exclusion effect. Consequently, under unsaturated conditions, the anion exclusion process influencing NO3 mobility in the soil profile will be more affected by the valences and amounts of all anions and cations present in the soil solution and not so much by just the NO3 concentration.