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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.

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Gaius D. Eudoxie, PhD; Studied Soil Science at the University of the West Indies. Lecturer of Soil Science at the Department of Food Production, UWI, St. Augustine, Trinidad. Principle research focus is in the area of sustainable soil fertility management, using diagnostic tools and techniques. Laboratory management,ISO 17025 competent.

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