USE OF N-15 FOR FERTILIZER N RECOVERY AND N MINERALIZATION STUDIES INSEMIARID KENYA

Maize and beans were grown on a ferralsol at Kiboko, Kenya, with up to 120 kg N ha(-1). Within the 10 kg N ha(-1) plots, N-15 labelled ferti lizer was applied in microplots. There was no significant response in yield to fertilizer N and labelled N recovery was low, being 7.5% or l ess in one season and 17.7% or less in the second season. Samples of K iboko soil at four different water contents were incubated and the rat e of gross N mineralization over 7 days was calculated, utilizing N-15 labelling of the mineral N. Gross N mineralization increased greatly with soil moisture and a fitted relationship between gross N mineraliz ation rate and soil water content was obtained. Using measurements of soil water content at the field site, daily values of the soil N suppl y by gross mineralization were calculated. On average, modelled gross soil N mineralized could supply much (> 69%) of the N removed from the plots. It is suggested that the lack of response to fertilizer N may be explained by the coincidence of a high rate of N mineralization, an d increased crop demand, caused by the onset of rain.