The aim of this investigation was to provide quantitative measures of gross nitrogen (N) mineralization-immobilization-turnover (MIT) in soil, and of the parameters controlling MIT. This is needed to predict adequately mineral N availability in agricultural land. Respiration, MIT, soil microbial biomass (SMB)-C, soil organic matter (SOM)-C and dissolved organic matter (DOM)-C were determined in soil collected over three depths at three stages in a continuous crop rotation. MIT was strongly correlated to the size and activity of SMB-C and the size of SOM-C but was only marginally affected by the stage in the crop rotation. The strength of the correlation of mineralization, immobilization and respiration with SOM-C increased when analysed by curvilinear regression compared with linear regression because SOM bioavailability varied with depth. It is argued that the limiting step in the decomposition was the breakdown of SOM into DOM. The major pathway for degradation of organic-N in this soil system appeared to be the MIT route, while only a small amount of N was likely to be assimilated in the organic form (direct route). Our findings suggest that modellers of C and N turnover in zero tillage cropping systems need to incorporate a curvilinear relationship between SOM pool size and MIT into their computer model(s).