Carbon partitioning and rhizosphere C-flow in Lolium perenne as affected by CO2 concentration, irradiance and below-ground conditions

Plant responses to increasing atmospheric CO2 concentrations have received considerable interest. However, major uncertainties in relation to interact ive effects of CO2 with above- and below-ground conditions remain. This mic rocosm study investigated the impacts of CO2 concentration on plant growth, dry matter partitioning and rhizodeposition as affected by: (i) photon flu x 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 mu mo l m(-2) s(-1) (high PFD) and two CO2 concentrations: 450 (low CO2) and 720 mu mol mol(-1) (high CO2). Partitioning of recent assimilate amongst plant and rhizosphere C-pools was determined by use of (CO2)-C-14 pulse-labelling . In treatments with high PFD and/or high CO2, significant (P< 0.05) increa ses in dry matter production were found in comparison with the low PFD/low CO2 treatment. In addition, significant (P < 0.05) reductions in shoot %N a nd SLA were found in treatments imposing high PFD and/or high CO2. Root wei ght ratio (RWR) was unaffected by CO2 concentration, however, partitioning of C-14 to below ground pools was significantly (P< 0.05) increased. In a s eparate study, L. perenne was grown for 28 d in microcosms percolated with nutrient solution, in either a sterile sand matrix or nonsterile soil, unde r high or low CO2. Dry matter production was significantly (P< 0.01) increa sed for both sand and soil grown seedlings. Dry matter partitioning was aff ected by matrix type. C-14-allocation below ground was increased for sand g rown plants. Rhizodeposition was affected by CO2 concentration for growth i n each matrix, but was increased for plants grown in the soil matrix, and d ecreased for those in sand. The results illustrate that plant responses to CO2 are potentially affected by (i) PFD, and (ii) by feedbacks from the gro wth matrix. Such feedbacks are discussed in relation to soil nutrient statu s and interactions with the rhizosphere microbial biomass.