Transformation of urea and ammonium nitrate in an entisol and a spodosol under citrus production

Chemical transformations of ammonium nitrate (NH4NO3) and urea-nitrogen (N), at different rates of application, were studied in a Candler (Typic Quartzipsamment) and Wabasso (sandy, Alfic Haplaquod) sand by incubating fertilized surface soil (from 0 to 15 cm depth) samples at 10% moisture content (by weight) in the laboratory at 25 +/- 1 degrees C. During the 7 d incubation, the percentage of transformation of NH4-N into NO3-N was 33 to 41 and 37 to 41% in the Candler fine sand and Wabasso sand, respectively, at application rates of 1.00 g N kg(-1). In a parallel experiment, 85 to 96% of urea applied (equivalent to 0.25 to 1.00 g N kg(-1) soil) was hydrolyzed to NH4-N within 4 d in the Candler soil, whereas it required 7 d to hydrolyze 90 to 95% of the urea applied in the Wabasso soil. No nitrification was evident for 30 days in the Candler fine sand which received urea application equivalent to greater than or equal to 0.50 g N kg(-1). In the urea-amended Wabasso sand, the formation of NO, decreased as the rate of urea-N increased. Possible loss of N from NH3 volatilization or inhibition of activity of nitrifiers due to elevated soil pH (8.7 to 9.2) during the incubation of urea amended soils may have caused very low nitrification.