TRANSPORT OF UREA IN FLOODED SOIL PROFILES AS AFFECTED BY WATER PERCOLATION RATES AND NBPT

Urea transformation was studied under a laboratory-simulated flooded s oil condition, where the soil columns were preincubated for 3 months w ith 2 cm of standing water. Urea distributions in the flooded soil col umns were determined under different water percolation rates and ureas e inhibitor treatments. The soil used in this study was a Crowley silt loam, a common rice paddy soil found in southwest Louisiana, and the urease inhibitor was N- (n-butyl) thiophosphoric triamide (NBPT). Thre e percolation rates (0, 0.5, and 1.0-cm d(-1)) and two NBPT treatments (0 and 10% of urea weight) were investigated. The experimental data s howed strong effects of NBPT and percolation rates on urea distributio n. More than 70% of the urea was hydrolyzed after 2 days of applicatio n without NBPT treatment, whereas only 40% was hydrolyzed with NBPT. M ore urea was distributed in the soil profile in NBPT treated columns. High percolation rates caused substantial urea movement downward into the soil when urea was not hydrolyzed. These results were successfully described by a convective-dispersive transport model with equilibrium -type adsorption and first-order hydrolysis. First-order hydrolysis ra te constants (mu(m)) for NBPT treated soil columns were 8 to 16 times lower than for soil without NBPT. However, the distribution coefficien ts (K-d) and diffusion coefficients (D) were similar for both NBPT tre atments.