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• In grasslands, the loss of structural carbon (C) from nonharvested plant parts is a primary C source for the soil. The amount of input depends not only on the size of structural C pools but also on their loss rates. • In the field, we examined the effects of elevated atmospheric partial pressures of CO2 (pCO2) and nitrogen (N) supply on pool size and rates of structural C loss in stubble and roots of perennial ryegrass ( Lolium perenne) by using multiple-pulse labelling and steady-state labelling. • Stubble retained structural C for roughly half the time it was retained in roots. Elevated pCO2 combined with low N supply enlarged the pools of roots and stubble. These conditions also stimulated the rate of structural C loss from stubble and, thus, the amounts available for further transformation. • The potential of multiple-pulse labelling as a field technique is highlighted. The stimulation of structural C loss from stubble by elevated pCO2 at low N provides a missing link between increased C assimilation and low yield response and indicates a potentially higher input of structural C into the soil. [ABSTRACT FROM AUTHOR]