TRANSFORMATION AND MOVEMENT OF UREA IN SOIL AS INFLUENCED BY WATER APPLICATION RATE, MOISTURE MANAGEMENT REGIME, AND INITIAL MOISTURE-CONTENT

Laboratory experiments were conducted to study the influence of water application rate (2.5, 5.0, 75, and 10.0 cm), moisture management regime (i.e., frequency of water application), and initial soil moisture content (25, 50, 75, and 100% of field capacity) on the transformation and movement of urea in soil columns. In general, moisture content and the amounts of urea and NH4+-N decreased with depth and time at all water application rates. Whereas approximately 79 to 82 % of the added urea was hydrolyzed within 48 h, there was no nitrification of the urea-derived NH4+ within this time period. The application of 1.0, 2.5, 5.0, and 10. 0 cm of water resulted in the percolation of water to depths of 33, 36, 39, and 42 cm, respectively. Moisture management had a significant effect on the distribution of N-species in the soil columns. Increasing the frequency of water additions from one (10.0 cm added on day 1) to ten (additions of 1.0 cm day-1 for 10 days) resulted in an increase in the amount of urea-derived NH4+ in the surface layers of the soil, but a decrease in the amount of NO3--N in these layers. The initial moisture content of the soil had a considerable effect on the movement of urea. The movement of urea and urea-derived N into the soil increased with increasing initial soil moisture content, but was restricted to the upper 7 cm of the soil columns. At all soil moisture contents, 60 to 65 % of the urea-derived N was recovered in the top 2 cm of the soil column. In general, the recovery of urea-derived N from the soil ranged from about 78 to 85 %.