LYSIMETER STUDY OF THE FATE OF NITROGEN IN ANIMAL URINE RETURNS TO IRRIGATED PASTURE

The fate of N-15-labelled synthetic urine (50 g N m(-2)) applied to an irrigated pasture soil was studied using large undisturbed monolith lysimeters (800 mm diameter x 1200 mm deep). Over a period of 1 year, the pasture plants recovered the largest fraction of the applied N-15 (43%) and approximately 20% of the applied N-15 still remained in the soil, the majority of which was found in the topsoil (0-20 cm). Although the experiment was conducted under relatively intense rainfall and irrigation conditions, only 8% of the applied N-15 was found to have leached below 1200 mm after 1 year. During this time, the average peak concentration of nitrate in the leachate reached 42 mg NO3--N dm(-3). The amount of nitrogen (N) lost by the process of denitrification was calculated as 28% of the applied N-15. This large loss of N to the atmosphere was attributed to the wet soil conditions which prevailed.