The main form of fertilizer applied to lowland rice (Oryza sativa L.) is urea, but very little is known about its movement and transformations under flooded soil conditions. 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−1, equivalent to 150 kg N ha−1) of sterilized and nonsterilized Crowley silt loam soil (fine, montmorillonitic, thermic Typic Albaqualf) columns and urea diffusion, urea hydrolysis, and subsequent NH+4‐N diffusion were measured during a period of 30 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−1. The diffusion coefficient for both N forms (urea and NH+4) was estimated to be 3.5 × 10−10 m2 s−1 in the flooded soil columns. The basic urea data generated in this study should help provide a data base to model urea‐N behavior in submerged soils.
Ammonia volatilization from rice (Oryza sativa L.) paddies following urea application reduces the effectiveness of the urea‐N. The objectives of this study were to: (i) derive, for flooded soils, a model describing hydrolysis and diffusion of urea and diffusion of ammoniacal N; (ii) derive the necessary parameters from experimental data, and (iii) use the model to study implications of hydrolysis/transport parameters and management variables to NH3 volatilization. The parameters are diffusion coefficients of urea and NH4 in soil, hydrolysis coefficients of urea in the soil and overlying floodwater, and the sorption coefficient for NH4 on the soil. The effects on potential for NH3 volatilization of variation in these parameters and the management factors of depth of flood water and depth of fertilizer incorporation were calculated. The results illustrate that urea‐hydrolysis rates in the floodwater and soil are the most important factors influencing the potential for NH3 volatilization. If urea‐hydrolysis rates are very high, incorporation and shallow floodwater are the indicated management while, with low hydrolysis rates, deep floodwater and no incorporation are the indicated management.
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