MODELING UREA, AMMONIUM, AND NITRATE TRANSPORT AND TRANSFORMATIONS IN FLOODED SOIL COLUMNS

Abstract

Understanding the fate of organic nitrogen (urea, ammonium, and nitrate) introduced into wetlands is important for wetland restoration and environmental quality. However, the transformation of nitrogen in wetlands is complicated by the coexistence of oxidized and reduced soil conditions. In this study, we investigated the transport and transformations of 15 N-urea, and its degradation products ( 15 NH 4 and 15 NO 3 ) in laboratory columns packed with Crowley silt loam, a major rice soil in southwest Louisiana. A 2-centimeter floodwater layer was maintained during the experiments to simulate wetland conditions. Sterilized soil columns with the addition of urease inhibitor [N-(n-butyl) thiophosphoric triamide] were used to measure urea diffusion in the soil. Urea transformations were studied using 15 N-urea, and the degradation products (ammonium and nitrate) were measured at 0.5, 1, 2, 4, and 6 days after urea application. Adsorption isotherms for urea and NH 4 were determined using batch experiments under sterile conditions. A system of diffusion equations (DEs) was formulated to describe urea, NH 4 , and NO 3 diffusion and transformation in soil. Urea hydrolysis was assumed to take place in the soil profile only, and nitrification in the floodwater and the surface soil layer. Denitrification may take place in both the floodwater and soil profile depending on oxygen depletion. Predicted urea and ammonium distributions in the soil profile following urea application were highly correlated to their experimental values (r 2 > 0.91). Although nitrate concentrations at each sampling time were underpredicted, the model overpredicted by 8.7% the amount of nitrate denitrified during the 6-day experimental period.