Effects of CO2 enrichment and intraspecific competition on biomass partitioning, nitrogen content and microbial biomass carbon in soil of perennial ryegrass and white clover

Seedlings of perennial ryegrass (Lolium perenne L. cv. Parcour) and white clover (Trifolium repens L. cv. Karina) grown at five different plant densities were exposed to ambient (390 ppm) and elevated (690 ppm) CO2 concentrations. After 43 d the effects of CO2 enrichment and plant density on growth of shoot and root, nitrogen concentration of tissue, and microbial biomass carbon (Cmic) in soil were determined. CO2 enrichment of Lolium perenne increased shoot growth on average by 17% independent of plant density, while effects on root biomass ranged between -4% and + 107% due to an interaction with plant density. Since tiler number per plant was unaffected by elevated CO2, the small response of shoot growth to CO2 enrichment was atributed to low sink strength. A significant correlation between nitrogen concentration of total plant biomass and root fraction of total plant dry matter, which was not changed by CO2 enrichment, indicates that nitrogen status of the plant controls biomass partitioning and the effect of CO2 enrichment on root growth. Effects of elevated CO2 and plant density on shoot and root growth of Trifolium repens were not significantly interacting and mean CO2-related increase amounted to 29% and 66%, respectively. However, growth enhancement due to elevated CO2 was strongest when leaf area index was lowest. Total amounts of nitrogen in shoots and roots were bigger at 690 ppm than at 390 ppm CO2. There was a significant increase in Cmic in experiments with both species whereas plant density had no substantial effect.