Effect of elevated atmospheric CO2 concentration on C-partitioning and rhizosphere C-flow for three plant species

The effects of elevated atmospheric CO2 concentration on the partitioning of dry matter and recent assimilate was investigated for three plant species (rye grass, wheat and Bermuda grass). This was evaluated in plant-soil microcosm systems maintained at specific growth conditions, under two CO2 regimes (450 and 720 μmol mol−1). The distribution of recent assimilate between plant, microbial and soil pools was determined by 14CO2 pulse chase, for each plant species at both CO2 concentrations. Growth of rye grass and wheat (both C3) was ca. doubled at the higher CO2 concentration. Dry matter partitioning was also significantly affected, with an increased root-to-shoot ratio for wheat (0.72–1.03), and a decreased root-to-shoot ratio for rye grass (0.68-0.47) at elevated CO2. For Bermuda grass (C4), growth and partitioning of dry matter and 14C were not affected by CO2 concentration. 14C-allocation to the rhizospheres of rye-grass and wheat was found to be increased by 62 and 19%, respectively, at the higher CO2 concentration. The partitioning of 14C within the rhizospheres of the two C3 species was also found to be affected by CO2 concentration. At the higher CO2 concentration, proportionately less 14C was present in the microbial fraction, relative to that in the soil. This indicates altered microbial utilisation of root-released compounds at the higher CO2 concentration, which may be a consequence of altered quantity or quality of rhizodeposits derived from recent assimilate.