AbstractNitrogen (N) cycling can be an important constraint on forest ecosystem response to elevated atmospheric CO2. Our objective was to trace the movement of 15N, injected into tree sap, to labile and stable forms of soil organic matter derived partly from the turnover of tree roots under elevated (545 ppm) and ambient (394 ppm) atmospheric CO2 concentrations at the Oak Ridge National Laboratory (ORNL) FACE (Free‐Air Carbon Dioxide Enrichment) Experiment. Twenty‐four sweetgum trees, divided equally between CO2 treatments, were injected with 3.2 g 15N‐ammonium sulfate (99 atom %), and soil samples were collected beneath the trees over a period of 89 weeks. For 16 cm deep soil samples collected beneath the study trees, there was 28% more fine root (less than or equal to 2 mm diameter) biomass under elevated CO2 (P = 0.001), but no significant treatment effect on the amounts of necromass, coarse root biomass, or on the N concentrations in tree roots and necromass. Nitrogen‐15 moved quickly into roots from the stem injection site and the 15N content of roots, necromass, and labile organic matter (i.e. particulate organic matter, POM) increased over time. At 89 weeks post‐injection, approximately 76% of the necromass 15N originated from fine root turnover. Nitrogen‐15 in POM had a relatively long turnover time (47 weeks) compared with 15N in roots (16 to 22 weeks). Over the 1.7 year period of the study, 15N moved from roots into slower cycling POM and the disparity in turnover times between root N and N in POM could impose progressive limitations on soil N availability with stand maturation irrespective of atmospheric CO2, especially if the release of N through the decomposition of POM is essential to sustain forest net primary production. Published in 2009 by John Wiley & Sons, Ltd.