Modeling urea, ammonium, and nitrate transport and transformations in flooded soil columns

Understanding the fate of organic nitrogen (urea, ammonium, and nitrate) in troduced into wetlands is important for wetland restoration and environment al quality. However, the transformation of nitrogen in wetlands is complica ted by the coexistence of oxidized and reduced soil conditions. In this stu dy, we investigated the transport and transformations of N-15-urea, and its degradation products ((NH4)-N-15 and (NO3)-N-15) in laboratory columns pac ked with Crowley silt loam, a major rice soil in southwest Louisiana. A 2-c entimeter floodwater layer was maintained during the experiments to simulat e wetland conditions. Sterilized soil columns with the addition of urease i nhibitor [N-(n-butyl) thiophosphoric triamide] were used to measure urea di ffusion in the soil. Urea transformations were studied using N-15-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 N H, were determined using batch experiments under sterile conditions. A syst em of diffusion equations (DEs) was formulated to describe urea, NH4, and N O3 diffusion and transformation in soil. Urea hydrolysis was assumed to tak e place in the soil profile only, and nitrification in the floodwater and t he surface soil layer. Denitrification may take place in both the floodwate r and soil profile depending on oxygen depletion. Predicted urea and ammoni um distributions in the soil profile following urea application were highly correlated to their experimental values (r(2) > 0.91). Although nitrate co ncentrations at each sampling time were underpredicted, the model overpredi cted by 8.7% the amount of nitrate denitrified during the B-day experimenta l period.