Mineralization and immobilization of N often occur simultaneously in soils, and this results in a turnover of N within pools of inorganic and organic N. Measurement of mineralization-immobilization turnover provides important information about N transformations in soil. Failure to consider the effects of this turnover, however, introduces errors in interpretations of [superscript]15N-tracer studies of transformations and movement of N in soils. Laboratory studies were conducted under controlled conditions to learn more about factors affecting amounts of turnover of N in soils used for corn production;Studies involving additions of various amounts of corn residue and [superscript]15N-labeled nitrate showed that mineralization-immobilization turnover was largely the result of sequential immobilization and mineralization rather than simultaneous processes. Moreover, availability of N in soil influenced rates of immobilization and microbial growth efficiencies occurring during the initial stages of decomposition, rates of mineralization occurring during the latter stages of decomposition, and the amount of turnover that occurred. These observations offer a rational explanation for existing confusion associated with phenomena often called priming effects and added N interactions. Studies showed that fixation and release of ammonium from clay minerals is another source of turnover of N in soils and that failure to recognize this turnover can result in an overestimation of mineralization-immobilization turnover;Studies of mineralization-immobilization turnover in soils having varied cropping histories showed that the period of immobilization during the early stages of decomposition varied in length and could extend into the next cropping season under field conditions. Observed differences in rates and amounts of immobilization during the decomposition of corn and soybean residues provide a reasonable explanation as to why corn grown after soybean requires less fertilizer than does corn grown after corn. They also provide a rational explanation as to why high rates of N fertilization promote loss of soil organic matter. Overall, the results demonstrate that a better understanding of the factors affecting mineralization-immobilization turnover in soils should help predict N availability within cropping seasons and the changes in concentrations of soil organic matter that occur across many cropping seasons.