Effects of ambient and elevated atmospheric CO2 concentrations (350 and 700 μl l-1) on net carbon input into soil, the production of root-derived material and the subsequent microbial transformation were investigated. Perennial ryegrass plants (L. perenne L.) were labelled in a continuously labelled 14C-CO2 atmosphere to follow carbon flow through the plant and all soil compartments. After 115 days, root biomass was 41% greater at elevated CO2 than at ambient CO2 and this root biomass seemed to be the driving force for the increase of 14C-labelled carbon in all compartments examined, i.e. carbon in the soil solution, soil microbial biomass and soil residue. After incubation for 230 days at 14°C, roots grown at elevated CO2 decomposed slower (14%) than roots grown at ambient CO2. Increasing the incubation temperature of the roots grown at elevated CO2 by 2°C could not compensate for this delay in decomposition. In addition, 'elevated CO2' root-derived material (14C-labelled soil microorganisms plus 14C-labelled soil residue) decomposed significantly slower (29%) than 'ambient CO2' root-derived material. At the end of the incubation experiment, the ratio between 14C-labelled microorganisms and total 14CO2 evolved showed no difference among root incubation and incubation of root-derived material. Thus, the substrate use efficiency of microorganisms, involved with decomposition of roots and root-derived material, seems not to be affected by an increase in atmospheric CO2 concentrations. Therefore, the lower decomposition rate at elevated CO2 is not due to a change in the internal metabolism of microorganisms. (C) 2000 Published by Elsevier Science Ltd.