Abstract
Downward movement and leaching of nitrate-N was studied following cattle urine application (60gN/m^2) to bahiagrass (Paspalum noraturn Flugge) dominated pasture on 8 August (Aug. Plot) and 15 October (Oct. Plot) 1991. Soil samples were taken at 1-2 week intervals by auger (2 cm diameter, six cores per sampling) at 15 cm increments to a depth of 150cm. Ammonium- and nitrate-N analysis were carried out on 10% KC1 soil extracts. Water potentials were recorded by mercury manometer tensiometers installed at 10cm intervals to 150cm. Daily rainfall was recorded throughout the experiment (1 Aug. 1991-31 July 1992). Most of the urine-N was transformed to nitrate-N within two and five weeks after application in the Aug. and Oct. plots respectively. Downward movement of nitrate through the soil was influenced greatly by soil water conditions. Seasonal changes in the top boundary phase of downward water movement in the soil were estimated by the hydraulic gradients of water potential and located at about 60 cm throughout most of the season. Nitrate-N loss by leaching was estimated by measuring the amount of nitrate contained in the soil from top boundary to a depth of 150 cm. By this method, nitrate-N leached amounted to 21.5 gN/m^2 (35.8% of applied urine-N)and 34.9 gN/m^2 (58.2% of applied urine-N) in the Aug. and Oct. plots respectively.
