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. Per ennial ryegrass plants (L. perenne L.) were labelled in a continuously labe lled C-14-CO2 atmosphere to follow carbon flow through the plant and all so il compartments. After 115 days, root biomass was 41% greater at elevated C O2 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. Aft er incubation for 230 days at 14 degrees C, roots grown at elevated CO2 dec omposed slower (14%) than roots grown at ambient CO2. Increasing the incuba tion temperature of the roots grown at elevated CO2 by 2 degrees C could no t compensate for this delay in decomposition. In addition,'elevated CO2' ro ot-derived material (C-14-labelled soil microorganisms plus C-14-labelled s oil residue) decomposed significantly slower (29%) than 'ambient CO2' root- derived material. At the end of the incubation experiment, the ratio betwee n C-14-labelled microorganisms and total (CO2)-C-14 evolved showed no diffe rence 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 increas e in atmospheric CO2 concentrations. Therefore, the lower decomposition rat e at elevated CO2 is not due to a change in the internal metabolism of micr oorganisms. (C) 2000 Published by Elsevier Science Ltd. all rights reserved .