ELEVATED ATMOSPHERIC CO2 DOES NOT AFFECT PER SE THE PREFERENCE FOR SYMBIOTIC NITROGEN AS OPPOSED TO MINERAL NITROGEN OF TRIFOLIUM-REPENS L

The objective of this investigation was to examine the effect of an el evated atmospheric CO2 partial pressure (pCO(2)) on the N-sink strengt h and performance of symbiotic N-2 fixation in Trifolium repens L. cv. Milkanova, After initial growth under ambient pCO(2) in a nitrogen-fr ee nutrient solution, T. repens in the exponential growth stage was ex posed to ambient and elevated pCO(2) (35 and 60 Pa) and two levels of mineral N (N-free and 7.5 mol m(-3) N) for 36 d in single pots filled with silica sand in growth chambers. Elevated pCO(2) evoked a signific ant increase in biomass production from day 12 after the start of CO2 enrichment. For plants supplied with 7.5 mol m(-3) N, the relative con tribution of symbiotically fixed N (%N-sym) as opposed to N assimilate d from mineral sources (N-15-isotope-dilution method), dropped to 40%, However, in the presence of this high level of mineral N, %N-sym was unaffected by atmospheric pCO(2) over the entire experimental period. In plants fully dependent on N-2 fixation, the increase in N yield ref lects a stimulation of symbiotic N-2 fixation that was the result of t he formation of more nodules rather than of higher specific N-2 fixati on. These results are discussed with regard to physiological processes governing symbiotic N-2 fixation and to the response of symbiotic N-2 fixation to elevated pCO(2) in field-grown T. repens.