Fixed NH4+ has been shown to occupy a significant portion of the total N of some tropical soils, which may alter and influence the N availability and dynamics in these soils. A series of laboratory investigations and a green house study were conducted to determine the extent and dynamics of fixation of fertiliser N in a range of agricultural soils using 15N techniques. Existing micro-diffusion procedures as a necessary step in the pre-concentration of exchangeable NH4+ and NO3--15N were validated, and new procedures for measuring fixed NH4+-15N fractions (total, strongly and weakly fixed) were developed. Laboratory incubations showed fixation to increase linearly with increasing N application rate. In green house studies cropping resulted in a slow release of a small portion (13.3 %) of the "recently" fixed NH4+ at 42 days after cropping. Potassium (K+) added after N fertilization resulted in the release of "recently" fixed NH4+ being blocked. These findings should provide agronomist and soil fertility researchers with pertinent information regarding alterations to N management tools and systems.