<p>Nowadays, intensive cropping systems are increasingly dependent on mineral nitrogen (N) fertilizers, which contribute to global warming, hydrosphere pollution, and ecosystem eutrophication. Therefore, the traditional approach of optimizing crop N nutrition based on the crop's response to fertilizer applications must be reexamined. A new paradigm is proposed for crop N nutrition that merges recent advances in two complementary research areas. The availability of soil N to plants depends on interactions among different functional groups of plants and microbes, that contribute synchronizing the soil N supply with fluctuating plant N demand and favor also the closure of the N cycle and reduces environmental impacts. Moreover, the co-regulation of plant N uptake by soil N availability and plant growth enables feedback control of root N absorption by plant N demand. Therefore, soil N availability cannot be considered only a soil characteristic, but also a function of crop growth capacity. The allometry between crop N uptake and crop biomass accumulation enables determination of the crop's N nutrition status (Nitrogen Nutrition Index, NNI). Based on these recent advances, we propose a new approach to breeding crops and designing agroecosystems that integrate the ability of soil–plant systems to mobilize natural N sources and close the N cycle in various contexts. We also propose using NNI to phenotype the capacity of crop species and genotypes to acquire soil N resources and to evaluate cropping system managements in providing N resources to crops. This new paradigm represents a promising, groundbreaking approach to develop more resource-efficient agriculture.</p>