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

Abstract Eects of ambient and elevated atmospheric CO 2 concentrations (350 and 700 ml l À1 ) 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 14 C-CO 2 atmosphere to follow carbon¯ow through the plant and all soil compartments. After 115 days, root biomass was 41% greater at elevated CO 2 than at ambient CO 2 and this root biomass seemed to be the driving force for the increase of 14 C-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 148C, roots grown at elevated CO 2 decomposed slower (14%) than roots grown at ambient CO 2 . Increasing the incubation temperature of the roots grown at elevated CO 2 by 28C could not compensate for this delay in decomposition. In addition,`elevated CO 2 ' root-derived material ( 14 C-labelled soil microorganisms plus 14 C-labelled soil residue) decomposed signi®cantly slower (29%) than`ambient CO 2 ' rootderived material. At the end of the incubation experiment, the ratio between 14 C-labelled microorganisms and total 14 CO 2 evolved showed no dierence among root incubation and incubation of root-derived material. Thus, the substrate use eciency of microorganisms, involved with decomposition of roots and root-derived material, seems not to be aected by an increase in atmospheric CO 2 concentrations. Therefore, the lower decomposition rate at elevated CO 2 is not due to a change in the internal metabolism of microorganisms.