The effectiveness of N fertilizer depends on the ability of crops to compete with microorganisms for the available N in the soil. Adverse growth conditions (e.g. drought and waterlogging) diminish the capacity of the plants for N uptake, while N supply may be limited by transient assimilation by microorganisms, by longer-term immobilization into soil hum us and by losses through leaching and denitrification. Conversely, mineralization of soil organic matter contributes to the availability of N. Significant alteration of soil management methods (e.g. improvement of drainage, use of simplifed cultivation systems, or rotation of arable land with grass) may change the balance of processes within the nitrogen cycle. The results of field and lysimeter experiments (some with 15 N-labelled fertilizer) are used to discuss these aspects of the fate of applied N, with particular attention to the role of denitrification.