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Partitioning of 14C was assessed in sweet chestnut seedlings (Castanea sativa Mill.) grown in ambient and elevated atmospheric [CO2] environments during two vegetative cycles. The seedlings were exposed to 14CO2 atmosphere in both high and low [CO2] environments for a 6-day pulse period under controlled laboratory conditions. Six days after exposure to 14CO2, the plants were harvested, their dry mass and the radioactivity were evaluated. 14C concentration in plant tissues, root-soil system respiratory outputs and soil residues (rhizodeposition) were measured. Root production and rhizodeposition were increased in plants growing in elevated atmospheric [CO2]. When measuring total respiration, i.e. CO2 released from the root/soil system, it is difficult to separate CO2 originating from roots and that coming from the rhizospheric microflora. For this reason a model accounting for kinetics of exudate mineralization was used to estimate respiration of rhizospheric microflora and roots separately. Root activity (respiration and exudation) was increased at the higher atmospheric CO2 concentration. The proportion attributed to root respiration accounted for 70 to 90% of the total respiration. Microbial respiration was related to the amount of organic carbon available in the rhizosphere and showed a seasonal variation dependent upon the balance of root exudation and respiration. The increased carbon assimilated by plants grown under elevated atmospheric [CO2] stayed equally distributed between these increased root activities. ei]H Lambers