Submitted to: Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2006
Publication Date: 1/1/2007
Citation: Allred, B.J., Brown, G.O., Bigham, J.M. 2007. Nitrate mobility under unsaturated flow conditions in four initially dry soils. Soil Science. 172(1):27-41. Interpretive Summary: Growing environmental concerns about agriculture’s role in nitrate contamination of aquifers and the Gulf of Mexcio hypoxic zone, suggest the need for a better understanding of the mobility and fate of nitrate in agricultural soil. Experiments were conducted to determine how soil physical, chemical, and mineralogical properties affect the adsorption or exclusion of nitrate on exchange sites on soil particle surfaces and the consequent impact on nitrate movement with water entering dry soil. The results showed that exclusion is the key process that produced high concentration pulses of nitrate that move with the wetting front as water enters dry soil. This information is important to scientists working to understand the dynamics of nitrate movement in soil, and to the agrichemical supply industry.
Technical Abstract: Solving environmental problems such as nitrate (NO3-) contamination of shallow aquifers and the Gulf of Mexico hypoxic zone will require, in some part, a better understanding of anion exclusion and anion adsorption processes affecting soil profile NO3- mobility under unsaturated flow conditions. For this reason, transient unsaturated horizontal column experiments were conducted to assess electrostatic processes affecting NO3- transport in soil. Duplicate tests were conducted on four soils having different physicochemical and mineralogical properties. The four soils investigated were a Slaughterville sandy loam and a Teller loam from Oklahoma along with a Hoytville silty clay and a Paulding silty clay from Ohio. In each test, a 200 mg/L nitrate-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/s0.5. Comparison of corresponding volumetric water content and soil solution NO3--N concentration profiles clearly indicate anion exclusion to be an important process impacting NO3- mobility under unsaturated flow conditions. Evidence of anion exclusion for all four soils includes soil solution NO3--N concentrations near the inlet that were 13% to 21% less than the 200 mg/L NO3--N solution concentration originally injected at the inlet. Further strong anion exclusion evidence includes peak soil solution NO3--N concentrations that were greater than 200 mg/L (sometimes by a factor of 2) found at or very near the wetting front for the Slaughterville, Teller, and Hoytville soils. The Paulding soil, possibly because of its pH and soil mineralogy, 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 research results indicate anion exclusion is a key process affecting NO3- mobility in a variety of soil environments.