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Among energy crops, short-rotation coppices (SRC) are recommended to provide renewable energy. Compared to annual crops, willows and poplars are regarded as plants with low requirements for nutrients, herbicides, pesticides, and soil maintenance. However, only little is known about N-fertilizer effects on SRC and the few studies are even inconsistent. Therefore, we studied the effects of N on yields of willows and poplars in a field experiment. The effects of N fertilization on nitrate leaching and nitrous oxide emissions from the loamy-sand soil were also measured. Cuttings of willows (Salix viminalis clone Inger) and poplars (Populus maximovizcii × P. nigra clone max 4) were planted on farmland in 2008. The experiment was arranged in a random block design with three levels of N fertilizer (0, 50, and 75 kg N ha–1 y–1). After 2 y, the trees were harvested for the first time. Fertilization did not affect the yields of willows or poplars. However, the application of 75 kg N ha–1 y–1 caused an average increase of N leaching in the willow and poplar plots of 25 kg N ha–1 y–1 and 40 kg N ha–1 y–1, respectively. Emissions of N2O were increased by a maximum of only 0.2 kg N ha–1 y–1. Further, the N fertilizer stimulated the growth of the weed biomass in case of the willow plots by 46% and of the weed N content by 52% (r = 0.53). In conclusion, in the first 2 y, SRC could be produced in a more effective and environmentally friendly manner without mineral fertilizer.