Lysimeter and field studies on15N in a tropical soil

Twenty four plots, each 2.0 m² in area, were established on St. Augustine loam soil series as field plots and microplots (containing lysimeters) in a completely randomised block design of four treatments (mulched fertilized, unmulched fertilized microplots; mulched fertilized and unmulched fertilized field plots), replicated three times. Labelled (¹⁵N) and unlabelled (NH₂)₂CO fertilizer were applied at rates of 400 kg N ha^–1 and CaH₂PO₄ and KCl were applied at rates of 100 and 150 g ha^–1 respectively to the field plots and microplots. Mulch (bagasse) was maintained to a depth of two cm and the plots were kept bare with regular applications of gramoxone.The maximum depth of leaching as measured by diffusion of NO ₃ ^– –¹⁵N in both the dry and wet seasons was 30 cm. The potential for downward movement of water and NO ₃ ^– –¹⁵N was low in the wet season because high intensity rainfall followed high soil moisture contents. Effects of mulching, on the mobility of applied N fertilizers were inconclusive. Infiltration rates were significantly (P=0.25) inversely correlated with soil moisture content, supporting the hypothesis that high intensity rainfall on a saturated soil surface is more likely to result in NO ₃ ^– –¹⁵N dispersion than NO ₃ ^– –¹⁵N leaching.