Quantifying root lateral distribution and turnover using pine trees with a distinct stable carbon isotope signature

Summary1 In order to help assess spatial competition for below‐ground resources, we quantified the effects of fertilization on root biomass quantity and lateral root distribution of mid‐rotation Pinus taeda trees. Open‐top chambers exposed trees to ambient or ambient plus 200 µmol mol −1 atmospheric CO 2 for 31 months. 2 Tank CO 2 was depleted in atmospheric 13 C; foliage of elevated CO 2 trees had δ 13 C of −42·9, compared with −29·1 for ambient CO 2 trees. 3 Roots 1 m from the base of elevated CO 2 ‐grown trees had more negative δ 13 C relative to control trees, and this difference was detected, on average, up to 5·8, 3·7 and 3·7 m away from the trees for 0–2, 2–5 and >5 mm root‐size classes, respectively. Non‐fertilized tree roots extended as far as fertilized trees despite the fact that their above‐ground biomass was less than half that of fertilized trees. 4 These results are informative with respect to root sampling intensity and protocol, and the distances required between experimental manipulations to evaluate below‐ground processes of independent treatments. 5 Fine‐root turnover has usually been estimated to range from weeks to 3 years, representing a major avenue of carbon flux. Using a mixing model we calculated that 0–2 mm roots had a mean residence time of 4·5 years indicating relatively slow fine‐root turnover, a result that has major implications in modelling C cycling.