Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/27/2007
Publication Date: 11/1/2007
Citation: Allred, B.J. 2007. Effects of concentration and ionic strength on nitrate anion exclusion under unsaturated flow conditions. Soil Science. 172(11):842-860. Interpretive Summary: Nitrogen is the nutrient that is needed in greatest quantity by plants, so large amounts of nitrogen are applied to agricultural fields. Nitrogen moves from the soil into the plants in a soluble form called nitrate. Because it is soluble and moves with the water, some of the nitrate moves out of the soil into streams and lakes where it is considered a pollutant. In an effort to better understand how to manage nitrogen use for crop production without losing a lot of it to streams, experiments were conducted to determine what controls nitrate mobility within the soil. The results indicate that nitrate mobility is affected by the total amount of chemical ions in the soil and not just the nitrate concentration. Use of these findings will have an environmental benefit by helping to better predict nitrate transport in soils.
Technical Abstract: A more complete understanding of the processes affecting nitrate (NO3-) mobility in the soil profile is needed to solve environmental problems associated with fertilizer application in agricultural areas. Transient unsaturated horizontal column experiments were therefore conducted to assess the impacts of NO3- concentration and solution ionic strength on anion exclusion, which in turn affects soil NO3- transport. A loam soil was used in all column experiments. Duplicate tests were conducted with seven different injection solutions containing either dissolved potassium nitrate (KNO3) alone or dissolved potassium nitrate and calcium chloride (KNO3 + CaCl2). These seven solutions allowed the anion exclusion effects of four different nitrate-N (NO3--N) concentrations (50, 200, 1000, 2690 mg/L) and four different solution ionic strengths (0.0036, 0.0142, 0.0714 and 0.1929 moles/L) to be evaluated. In each test, a one of the seven solutions was applied at the inlet of a relatively dry soil column, and the value of sorptivity kept constant at 0.0206 cm/s0.5. 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. For tests conducted with only KNO3 injection solutions, the magnitude of the NO3- anion exclusion effect, based on column inlet and wetting front conditions, was strongly correlated with the common logarithm of the injection solution NO3--N concentration. However, when results from tests conducted with KNO3 injection solutions and KNO3 + CaCl2 injection solutions are considered together, it is apparent that ionic strength rather than NO3--N concentration is what governs the magnitude of NO3- anion exclusion. Consequently, under unsaturated conditions, the anion exclusion process influencing NO3- mobility in the soil profile will be most affected by the total amount of anions and cations present in the soil solution and not just the NO3- concentration.