The cycles of several key nutrients have been substantially altered by agricultural activities over the past century. It is urgent to better manage nutrient cycling in agro-ecosystems. In developing countries, it will contribute to soil fertility recovery and food production increase, and in industrial countries it will participate in environmental impact mitigation. To better access the gains associated with nutrient management, this chapter proposes a multiscale view of the nitrogen (N) cycle in agro-ecosystems: (A) At animal-manure-soil-plant levels, the literature review indicates the large range of observed N loss rates, the complexity of processes implicated and the multiple specific technical options existing for limiting these losses. (B) At farm-household levels, two integrated simulation models are used to analyze N cycling within low and high-input systems. With a well chosen combination of technical options, N use efficiency can be substantially improved in farming systems of both developing and industrialized countries. (C) At a global level, this study underlines the central role played by livestock in the N cycle. Domestic animals globally excrete 102 MT N year⁻¹ (i.e., 102 × 10⁶ tons of N per year), from which only 32% is used to fertilize crops and, paradoxically, 78 MT N are annually generated to fertilize crops (via the energy costly Haber-Bosch process). A global view of the potential gains associated with better nutrient management is still lacking. This proposes for research to seriously investigate tropical contexts (1) to quantify more systematically nutrient losses occurring at the animal-manure-soil-plant levels, and (2) to develop widely, farm-household scale studies, based on the integrated models and indicators proposed in this study, in order to take better consideration of the diversity of world farming systems and to reduce uncertainty of analyses.