Urea Transformations in Flooded Soil.

Abstract

Laboratory incubation studies were conducted to measure adsorption, movement, and transformations of urea and hydrolyzed ammoniacal N in flooded soil columns. Urea was added to the floodwater (750 mg N L$\sp{-1}$, equivalent to 150 kg N ha$\sp{-1}$) of Crowley silt loam soil columns and urea diffusion and urea hydrolysis were measured. Subsequent NH$\sb4\sp+$-N and NO$\sb3\sp-$-N distribution after urea application and hydrolysis were measured over a 30 d period under aerobic and anaerobic conditions. Effects of percolation rates (0, 1, and 2 cm d$\sp{-1}$), and the use of urease inhibitor, N-(n-butyl) thiophosphoric triamide (NBPT), on movement and transformations of urea in flooded soil columns were also investigated over a period of 0.5, 1, and 2 d. Urea adsorption by the soil increased with increasing concentration of added urea-N and adsorption coefficients ranged from 0.037 to 0.064 but modeling found adsorption to be too small to be an important factor in urea movement and hydrolysis. Urea hydrolysis rates in the flooded soil columns increased with time and followed first-order reaction kinetics. Rate constants measured in the soil varied from 0.036 to 0.288 h$\sp{-1}$. The diffusion coefficient for both N forms (urea and NH$\sb4\sp+$) was estimated to be 3.5 $\times$ 10$\sp{-10}$ m$\sp2$ s$\sp{-1}$ in the flooded soil columns. The distribution of NH$\sb4\sp+$-N in soil columns under aerobic and anaerobic conditions did not show distinct or different patterns. Percolation rates affected the movement of urea into deeper soil layers and had little effect on the movement of NH$\sb4\sp+$-N (hydrolyzed from urea) in the soil except at the highest rate of percolation 2 d after urea application. Addition of NBPT with urea in the floodwater drastically retarded urea hydrolysis and enhanced the effect of percolation on the transportation of urea into soil layers. The average percentage inhibition of NBPT on urea hydrolysis was 81.5 and 56.7% at 1 d and 2 d after urea application, respectively.