To make intensive bioenergy poplar plantations attractive on an economic and environmental point of view, their sustainable resource management is mandatory. These plantations are indeed often installed on marginal lands to avoid competition with food agriculture and large amounts of nutrients are repeatedly exported in the young harvested biomass. Besides the application of organic residues to reclaim impoverished soils, the use of species or genotypes efficiently using nutrients, and especially nitrogen, is also a way to slow down fertility losses in short-rotation poplar plantations. Our goal in the present study was to document the genotypic variability available in terms of yield, nitrogen-use efficiency (NUE), and nitrogen export rate, the links between these traits, and their plasticity in response to contrasting site conditions and during two rotations in a wide set of Populus deltoides × Populus nigra genotypes. A wide range of variation has been highlighted for biomass production, tree dimensions, and nitrogen use and export at both sites, and particularly at the most productive one. Environmental and genotype by environment effects were very strong and the ranking of genotypes was not maintained between the two sites for most studied traits. Because of climatic conditions less contrasted between sites during the second rotation, the differences were less marked than during the first rotation. The most productive and efficient genotypes to use nitrogen were also those responsible for the highest nitrogen exports during harvest. Indeed, although the most productive genotypes had high NUE, their high biomass production resulted in a more important nitrogen export at harvest than the less productive genotypes.