Estimating symbiotic di‐nitrogen (N2) fixation is challenging, especially when working with woody N2 fixers in field trials. Fortunately, isotope methods based on 15N natural abundance or on 15N artificial enrichment (dilution method) make it possible to estimate the proportion of nitrogen derived from the atmosphere (Ndfa) in N2‐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 15N isotope methods in the field, and to document values of isotope fractionation occurring during N2 fixation (the B value). B values were estimated both by growing trees on an N‐free medium in controlled conditions (Blab) and by making Ndfa calculated with the natural abundance method converge with Ndfa calculated with the 15N dilution method in the field (Bfield). The two methods gave consistent estimates of the B value. B values ranging between –1.4 and –3.2‰ 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.

& Key message In a mixed poplar/black locust plantation in central France, adverse conditions have led to a prevalence of interspecific competition, resulting in a poorer performance than monocultures. & Context In mixed tree plantations, the presence of woody N 2 -fixing species is thought to reduce N needs by fertilization. However, benefits associated to soil nitrogen enrichment have to outweigh the negative effects of interspecific competition. To do so, co-occurring tree species have to be chosen carefully to promote niche sharing between species and reduce competition. Black locust and poplar mixtures therefore seem promising since both species are fast growing and have potentially complementary crown shapes. & Aims Our objective was to evaluate the impact of the poplar/black locust mixture on the growth, above-and belowground biomass production, and nitrogen allocation of the two species, as compared to their respective monocultures. & Methods An experimental plantation mixing poplar and black locust was set up in central France. For five growing seasons, growth, nitrogen allocation, and carbon allocation were monitored for the two species growing either in mixture or in monoculture. & Results After a couple of promising growing seasons, black locust growth and survival slowly declined, mainly in the mixture. At the stand level, biomass production in the mixed plots was nearly 50% below the most productive monoculture (poplar) by age 5 years. & Conclusion Under adverse conditions, interspecific competition in the mixture was the preponderant interaction, resulting in higher mortality and lower biomass production than the two monocultures.