Lysimeter and field studies on ¹⁵N in a tropical soil: I. Applied (NH₂)₂CO-¹⁵N and the movement of NO₃-¹⁵N in a loam soil: The effect of initial soil moisture content on infiltration rate

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)₂ CO fertilizer were applied at rates of 400 kg N ha⁻¹ and CaH₂PO₄ and KC1 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.