Site-specific N management in corn ('Zea mays' L.) requires a better understanding on the spatial and temporal variability of processes occurring in a variable landscape under a given management practice (e.g. tillage and/or legume). The objectives of this thesis were to determine the spatio-temporal variability of N availability and N uptake by corn in a corn - red clover (cover crop) cropping system on a variable landscape in southern Ontario. Corn was planted on five landscape positions under three management practices that involved barley ('Hordeum vulgare' L.) or barley underseeded with red clover ('Trifolium pratense' L.) as the preceding crop, which was followed by corn with or without a conventional tillage operation in the spring. Plots received 0 (0N) or 140 kg N ha -1 (+N). Soil mineral N, aerial dry matter content (DM) and N concentration in DM were measured biweekly whereas the grain yield was recorded at maturity. The experiment was conducted over five seasons from 1997 to 2001. Field observations of the effect of conventional tillage on N availability were evaluated in a laboratory incubation study using soils that were disturbed by applying a known amount of energy. The increase in the plant available N content during the first two months after tillage was not consistent with the hypothesis of increased availability of protected organic matter in this soil. Spatial variation in N mineralization appears to be mainly controlled by the factors that influence potentially mineralizable N content rather than the factors that influence the rate at which organic N is mineralized. Spatial variability of N availability, N uptake, grain yield of corn and yield response to fertilizer N was significantly correlated with the spatial variation in soil organic carbon content (often in a quadratic manner) but the magnitudes varied from one season to the other. The red clover spring incorporation treatment that received 0N had a good synchrony between the N availability and crop requirement, and also produced yields similar to +N treatments across the range of organic carbon contents in this landscape, indicating the viability of this management practice in variable landscapes.