Abstract
Summary Plant responses to increasing atmospheric CO 2 concentrations have received considerable interest. However, major uncertainties in relation to interactive effects of CO 2 with above‐ and below‐ground conditions remain. This microcosm study investigated the impacts of CO 2 concentration on plant growth, dry matter partitioning and rhizodeposition as affected by: (i) photon flux density (PFD), and (ii) growth matrix. Plants were grown in a sandy loam soil for 28 d under two photon flux densities: 350 (low PFD) and 1000 μmol m –2 s –1 (high PFD) and two CO 2 concentrations: 450 (low CO 2 ) and 720 μmol mol –1 (high CO 2 ). Partitioning of recent assimilate amongst plant and rhizosphere C‐pools was determined by use of 14 CO 2 pulse‐labelling. In treatments with high PFD and/or high CO 2 , significant ( P 2 treatment. In addition, significant ( P 2 . Root weight ratio (RWR) was unaffected by CO 2 concentration, however, partitioning of 14 C to below ground pools was significantly ( P L. perenne was grown for 28 d in microcosms percolated with nutrient solution, in either a sterile sand matrix or nonsterile soil, under high or low CO 2 . Dry matter production was significantly ( P 14 C‐allocation below ground was increased for sand grown plants. Rhizodeposition was affected by CO 2 concentration for growth in each matrix, but was increased for plants grown in the soil matrix, and decreased for those in sand. The results illustrate that plant responses to CO 2 are potentially affected by (i) PFD, and (ii) by feedbacks from the growth matrix. Such feedbacks are discussed in relation to soil nutrient status and interactions with the rhizosphere microbial biomass.