Description of the subject. This article presents the aboveground and belowground biomass balances of young non-fertilized short-rotation coppices, planted on marginal soils. Objectives. On two contrasting soils, the growth of willow, poplar and black locust was monitored firstly, to estimate their harvested biomass production, and the immobilization of root-derived carbon and nitrogen, and secondly, to quantify the restitutions occurring at the soil surface through leaf senescence. A comparison of two densities of black locust (2,500 vs 5,000 feet·ha-1) was carried out. Method. The devices used were instrumented to quantify the levels of carbon (C) and nitrogen (N) involved in: i) leaf senescence returning to the soil, ii) exports by the two harvests. Results. The total C immobilized in the second harvest oscillated from 7.8 to 16.1 t·ha-1, and the total N from 125 to 393 kg·ha-1. Leaf senescence corresponded to 15 to 22% of the total C on the soil surface, while 17 to 31% was immobilized in the roots. The N released into the soil by senescence varied from 48 to 79 kg·ha-1, i.e. 38 to 20% of the total respectively, and ranged and the one in the roots ranged from 22% to 48% of the total. We did not find an effect of planting density on different compartments of black locust. Conclusions. Willow and black locust have adapted better to the difficult conditions encountered than poplar. The symbiotic nitrogen fixation of black locust should be better exploited to increase the biomass production of other species, in mixed crops for example.