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 l ysimeters (800 mm diameter x 1200 mm deep). Over a period of 1 year, t he pasture plants recovered the largest fraction of the applied N-15 ( 43%) and approximately 20% of the applied N-15 still remained in the s oil, the majority of which was found in the topsoil (0-20 cm). Althoug h the experiment was conducted under relatively intense rainfall and i rrigation conditions, only 8% of the applied N-15 was found to have le ached below 1200 mm after 1 year. During this time, the average peak c oncentration of nitrate in the leachate reached 42 mg NO3--N dm(-3). T he amount of nitrogen (N) lost by the process of denitrification was c alculated as 28% of the applied N-15. This large loss of N to the atmo sphere was attributed to the wet soil conditions which prevailed.