Estimates of rhizosphere C budgets for winter wheat and spring barley under conventional (CONV) or integrated (INT) management were obtained using a combination of crop growth measurements, C-14 pulse-labelling and a model rhizodeposition technique. In both crops the proportion of C-14 allocated to shoots 3 wk after labelling increased with the developmental stage, which resulted in maximum belowground C fluxes at tillering. At this stage C-14 released from the roots was relatively more in organic form in wheat and more as CO2 in barley, and this was reflected in the C fluxes accumulated over the growing season. In wheat no significant effect of management on C-14 allocation was found. In barley relatively more C-14 tended to be transferred to roots in CONV than in INT, and this was reflected in considerably higher calculated root growth, root respiration and rhizodeposition fluxes in CONV than in INT. Therefore, the hypothesis that plants invest more in roots and rhizodeposition in the system with lower nutrient input (INT) was rejected. Total rhizodeposition, including root decay, amounted to 450-990 kg C ha(-1) y(-1) (7-15% of net assimilation) which was twice the quantity of roots left at crop harvest. The relevance for microbe-mediated processes in relation to soil fertility is pointed out.