Summary The relative use of new photosynthate compared to stored carbon (C) for the production and maintenance of fine roots, and the rate of C turnover in heterogeneous fine‐root populations, are poorly understood. We followed the relaxation of a 13C tracer in fine roots in a Liquidambar styraciflua plantation at the conclusion of a free‐air CO2 enrichment experiment. Goals included quantifying the relative fractions of new photosynthate vs stored C used in root growth and root respiration, as well as the turnover rate of fine‐root C fixed during [CO2] fumigation. New fine‐root growth was largely from recent photosynthate, while nearly one‐quarter of respired C was from a storage pool. Changes in the isotopic composition of the fine‐root population over two full growing seasons indicated heterogeneous C pools; < 10% of root C had a residence time < 3 months, while a majority of root C had a residence time > 2 yr. Compared to a one‐pool model, a two‐pool model for C turnover in fine roots (with 5 and 0.37 yr−1 turnover times) doubles the fine‐root contribution to forest NPP (9–13%) and supports the 50% root‐to‐soil transfer rate often used in models.