Estimating symbiotic di-nitrogen (N-2) fixation is challenging, especially when working with woody N-2 fixers in field trials. Fortunately, isotope methods based on N-15 natural abundance or on N-15 artificial enrichment (dilution method) make it possible to estimate the proportion of nitrogen derived from the atmosphere (Ndfa) in N-2-fixing species. These methods have been extensively used in the field for herbaceous species, much less for tree species such as alder and acacia, and rarely for black locust (Robinia pseudoacacia). The objectives of this study were to characterize the fixation potential of black locust in a plantation by using the two N-15 isotope methods in the field, and to document values of isotope fractionation occurring during N-2 fixation (the B value). B values were estimated both by growing trees on an N-free medium in controlled conditions (B-lab) and by making Ndfa calculated with the natural abundance method converge with Ndfa calculated with the N-15 dilution method in the field (B-field). The two methods gave consistent estimates of the B value. B values ranging between -1.4 and -3.2 parts per thousand were found, varying with the age of the plant material. Up to 76% of the N in the black locust trees came from the atmosphere, representing more than 45 kg N ha(-1) over five years and confirming that black locust may be well adapted to N-poor soils.