Urea Transformations in Flooded Soil Columns: I. Experimental Results

Abstract

AbstractThe 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.