The quantification of root turnover is necessary for a complete understanding of plant carbon (C) budgets, especially in terms of impacts of global climate change. To improve estimates of root turnover, we present a method to distinguish current- from prior-year allocation of carbon (C) to roots in global change experiments by using changes in 13C:12C ratios ( delta 13C values) resulting from application of 13C-depleted, tank-derived CO2.