An overlooked carbon source for grassland soils: loss of structural carbon from stubble in response to elevated pCO 2 and nitrogen supply

Summary•  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 CO 2 (pCO 2 ) 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 pCO 2 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 pCO 2 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.