Effect of elevated CO2 on orchard grass and red clover grown in mixture at two levels of nitrogen or water supply

A mixture of orchard grass (Dactylis glomerata L.) and red clover (Trifolium pratense L.) was grown in microcosms at either ambient (40 Pa) or elevated CO2 (78 Pa) and supplied with two levels of nitrogen (N) or two levels of irrigation. The aim was to study how reduced N or water supply affect the CO2 response of shoot and root growth, in relationship to changes in the plant C/N ratio. Plant growth was monitored non-destructively, and shoot dry mass was determined after 41 days (first growth period) and after 67 days (second growth period). Stubble and root dry mass, and C/N ratios in roots and shoots were measured only after regrowth. Elevated CO2 continuously stimulated growth of the mixture, and increased the shoot biomass in the absence of N or water limitations without changing the shoot/root dry weight ratio, nor the C/N ratio. The CO2-effect on orchard grass tended to be stronger than the effect on red clover, and was more pronounced during the first as compared to the second growth period. At low N, yield of red clover showed the stronger CO2 response, whereas with reduced water supply the relative CO2-stimulation of shoot biomass in orchard grass was more pronounced. Both low-N and reduced water supply decreased shoot, root, and stubble biomass, decreased the shoot/root ratio, and increased the C/N ratio. Elevated CO2 reduced negative effects of limited N or water supply on shoot growth, but the positive CO2 effect at low N declined with time. The interaction between CO2 and N was most pronounced for stubble mass, whereas the interaction between CO2 and reduced water supply was only significant for root mass. It is concluded that changes in shoot/root ratio are mainly caused by low N and reduced water supply via changes in the N-status of the plant, and that elevated CO2 has little effect on the shoot/root ratio, but tends to reduce negative effects of limiting N and water on growth. (C) 1997 Published by Elsevier Science B.V.