Management of cropping system takes into account the soil nitrogen's availability with its agronomical and environemental effects. An interactive model has been developped to calculate each nitrogenous flux that may modify the amount of available nitrogen for both plant uptake and leaching : modelN. The general equation and three typical applications are presented. Some experimental data have been established at a drainage site with wetting conditions (at Arrou, in the center of France) and others on a deep and well-drained soil (at Caulnes, in Brittany). Management of crop residues and pig slurry utilization have been studied under Winter wheat during Winter and the following growing season. Gaseous losses and net mineralization or immobilization have been estimated for various seasons. Moreover, nitrogen plant uptake has been compared with its availability in the rooting zone and its yield efficiency. / ModelN est construit autour d'un concept d'"azote disponible" et d'une représentation de ses liaisons avec les flux qui le composent ou en découlent. La disponibilité est finalisée sur un double devenir possible : le lessivage et l'absorption. Ce concept d'azote disponible sert d'interface avec l'élaboration de la production végétale.