Ammonium (NH[subscript]4[superscript]+) fixation is the trapping of NH[subscript]4[superscript]+ ions within clay lattices. This reaction may temporarily render fertilizer N unavailable to plants and nitrifiers. Although NH[subscript]4[superscript]+ fixation has been studied under laboratory conditions, there is a lack of information on NH[subscript]4[superscript]+ fixation and its importance under field conditions on timescales of interest for crop production. This dissertation describes results from several studies using nitrogen-15-labeled urea and anhydrous ammonia (NH[subscript]3) fertilizers applied to field soils. The objectives were to determine amounts of fertilizer N found as nonexchangeable (fixed) NH[subscript]4[superscript]+ throughout the growing season and to acquire a better understanding of NH[subscript]4[superscript]+ fixation and release under field conditions. Associated studies report the effects of long-term soil management practices on NH[subscript]4[superscript]+ quantity/intensity (Q/I) relationships in Iowa soils;The amounts of fertilizer N found as nonexchangeable NH[subscript]4[superscript]+ were highly variable, even without adjacent soils in a toposequence. Environmental factors such as soil moisture content and rainfall amounts greatly affected amounts of fixation. Concentrations of fertilizer-derived exchangeable and nonexchangeable NH[subscript]4[superscript]+ were highly correlated in NH[subscript]3-fertilized soils at 10 days after application of NH[subscript]3. Concentrations of fertilizer-derived exchangeable and nonexchangeable NH[subscript]4[superscript]+ in urea-fertilized soils were also highly correlated within and across soils. These results suggested a period of net fixation when exchangeable NH[subscript]4[superscript]+ concentrations were high, followed by a period of net release after depletion of exchangeable NH[subscript]4[superscript]+ by nitrification or other processes;Studies of NH[subscript]4[superscript]+ Q/I relationships in long-term rotation plots showed that indexes of NH[subscript]4[superscript]+ availability based on Q/I relationships were similar to those based on measurements of exchangeable NH[subscript]4[superscript]+. Furthermore, long-term soil management practices did not significantly affect the affinity of the soil exchange complex for NH[subscript]4[superscript]+;Overall, the results indicated that NH[subscript]4[superscript]+ fixation is an important transformation of N fertilizers in some Iowa soils. The nonexchangeable NH[subscript]4[superscript]+ is transitory and concentrations decrease throughout the growing season. Amounts of fixation are highly variable and depend upon environmental conditions following fertilizer application. Ammonium fixation is a reaction that deserves more attention in studies of fates and transformations of N fertilizers.