CARBON FLUXES IN THE RHIZOSPHERE OF SWEET CHESTNUT SEEDLINGS (CASTANEA-SATIVA) GROWN UNDER 2 ATMOSPHERIC CO2 CONCENTRATIONS - C-14 PARTITIONING AFTER PULSE LABELING

Partitioning of C-14 was assessed in sweet chestnut seedlings (Castane a sativa Mill.) grown in ambient and elevated atmospheric [CO2] enviro nments during two vegetative cycles. The seedlings were exposed to (CO 2)-C-14 atmosphere in both high and low [CO2] environments for a 6-day pulse period under controlled laboratory conditions. Six days after e xposure to (CO2)-C-14, the plants were harvested, their dry mass and t he radioactivity were evaluated. C-14 concentration in plant tissues, root-soil system respiratory outputs and soil residues (rhizodepositio n) were measured. Root production and rhizodeposition were increased i n plants growing in elevated atmospheric [CO2]. When measuring total r espiration, i.e. CO2 released from the root/soil system, it is difficu lt to separate CO2 originating from roots and that coming from the rhi zospheric microflora. For this reason a model accounting for kinetics of exudate mineralization was used to estimate respiration of rhizosph eric microflora and roots separately. Root activity (respiration and e xudation) was increased at the higher atmospheric CO2 concentration. T he proportion attributed to root respiration accounted for 70 to 90% o f the total respiration. Microbial respiration was related to the amou nt of organic carbon available in the rhizosphere and showed a seasona l variation dependent upon the balance of root exudation and respirati on. The increased carbon assimilated by plants grown under elevated at mospheric [CO2] stayed equally distributed between these increased roo t activities.