Interactive effects of elevated atmospheric CO # and arbuscular mycorrhizal (AM) fungi on biomass production and N # fixation were investigated using black locust (Robinia pseudoacacia). Seedlings were grown in growth chambers maintained at either 350 µmol mol −" or 710 µmol mol −" CO # . Seedlings were inoculated with Rhizobium spp. and were grown with or without AM fungi. The "&N isotope dilution method was used to determine N source partitioning between N # fixation and inorganic fertilizer uptake. Elevated atmospheric CO # significantly increased the percentage of fine roots that were colonized by AM fungi. Mycorrhizal seedlings grown under elevated CO # had the greatest overall plant biomass production, nodulation, N and P content, and root N absorption. Additionally, elevated CO # levels enhanced nodule and root mass production, as well as N # fixation rates, of nonmycorrhizal seedlings. However, the relative response of biomass production to CO # enrichment was greater in non-mycorrhizal seedlings than in mycorrhizal seedlings. This study provides strong evidence that arbuscular mycorrhizal fungi play an important role in the extent to which plant nutrition of symbiotic N # -fixing tree species is affected by enriched atmospheric CO # .