Cation Effects on Nitrate Mobility in an Unsaturated Soil

Abstract

Transient unsaturated horizontal column experiments were carried out with a computer-controlled syringe pump for the purpose of assessing accompanying cation effects on the anion exclusion/adsorption processes governing nitrate (NO3-) mobility in an unsaturated loam soil. Duplicate tests were conducted with six different injection solutions applied at the inlets of dry soil columns. All six injection solutions contained 0.0142 moles L-1 NO3- (200 mg L-1 NO3--N) but differed from one another with respect to the accompanying cation present, either K+, NH4+, Ca2+, Mg2+, Al3+, or Fe3+ Column inlet soil solution NO3--N concentration was used to quantify the accompanying cation effect on NO3- anion adsorption/exclusion. Average soil solution NO3--N concentrations at the column inlet were 165.3, 167.8, 175.7, 174.4, 190.2, and 214.2 mg L-1, respectively, for tests conducted with injection solutions containing K+, NH4+, Ca2+, Mg2+, Al3+, and Fe3+. Consequently, by taking into consideration the original 200 mg L-1 NO3--N injection solution concentration , the tests using injection solutions containing monovalent cations (K+ and NH4+) exhibited the largest inlet NO3- anion exclusion, while inlet NO3- anion exclusion was significantly less for tests using injection solution containing divalent cations (Ca2+ and Mg2+). Inlet NO3- anion exclusion was minimal for the tests with injections solutions containing the trivalent cation Al3+, and for the tests carried out with injection solutions containing the trivalent cation Fe3+, inlet NO3- anion adsorption actually occurred. Decreased inlet NO3- anion exclusion for the divalent cation tests compared to the monovalent cation tests was likely due to the divalent cation injection solutions having a greater ionic strength than the monovalent cation injection solutions. Soil pH reductions and aluminum/iron oxide/hydroxide precipitation at the column inlet probably account for the minimal NO3- anion exclusion or NO3- anion adsorption found with tests conducted using trivalent cation injection solutions. These experimental results provide valuable information that can be employed for better management of fertilizer applications and improved prediction of NO3- movement through the soil profile.