Effect of elevated atmospheric CO 2 concentration on C-partitioning and rhizosphere C-flow for three plant species

Abstract

The effects of elevated atmospheric CO 2 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 CO 2 regimes (450 and 720 μmol mol −1). The distribution of recent assimilate between plant, microbial and soil pools was determined by 14CO 2 pulse chase, for each plant species at both CO 2 concentrations. Growth of rye grass and wheat (both C 3) was ca. doubled at the higher CO 2 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 CO 2. For Bermuda grass (C 4), growth and partitioning of dry matter and 14C were not affected by CO 2 concentration. 14C-allocation to the rhizospheres of rye-grass and wheat was found to be increased by 62 and 19%, respectively, at the higher CO 2 concentration. The partitioning of 14C within the rhizospheres of the two C 3 species was also found to be affected by CO 2 concentration. At the higher CO 2 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 CO 2 concentration, which may be a consequence of altered quantity or quality of rhizodeposits derived from recent assimilate.