Elevated atmospheric carbon dioxide concentration:: effects of increased carbon input in a Lolium perenne soil on microorganisms and decomposition

Effects of ambient and elevated atmospheric CO2 concentrations (350 and 700 mu l 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 C-14-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 C-14-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 degrees 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 degrees C could not compensate for this delay in decomposition. In addition,'elevated CO2' root-derived material (C-14-labelled soil microorganisms plus C-14-labelled soil residue) decomposed significantly slower (29%) than 'ambient CO2' root-derived material. At the end of the incubation experiment, the ratio between C-14-labelled microorganisms and total (CO2)-C-14 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. all rights reserved.