Abstract
The availability of nitrogen to crop plants is a universally important aspect of soil quality, and often nitrogen represents the immediate limitation to crop productivity in modern agriculture. Nitrogen is decisive for the nutritive value of plant products and plays a key role in the environmental impact of agricultural production. The fundamental doctrine of nitrogen management is to optimize the nitrogen use efficiency of both introduced and native soil nitrogen by increasing the temporal and the spatial coincidence between availability and root uptake of mineral nitrogen. Natural ecosystems have evolved to produce a high degree of coincidence and maintain a relatively tight nitrogen cycle. The management needed to assure the vitality of crop cultivars with a high production potential causes agroecosystems to be relatively open with respect to nitrogen. By shifting the focus from sheer productivity to a balance between productivity, product quality and environmental impact, the management of agroecosystems can be redesigned to allow for a higher nitrogen use efficiency. The important management measures to improve the nitrogen aspect of soil quality are crop sequences that incorporate cover crops, judicious use of soil tillage, improved timing and use of animal manures, crop residues and mineral fertilizers, and a sustainable balance between plant production potential and animal stocking density. This chapter addresses the characteristics of nitrogen cycling in plant production and the premises upon which an improved nitrogen use has to be based, and discusses management principles that can improve nitrogen use in cropping systems.