INFLUENCE OF NITROGEN ON THE ELEVATED CO2 RESPONSE IN FIELD-GROWN RICE

Rice (Oryza sativa L. cv. IR72) was grown in the tropics at ambient (3 45 mu L L(-1)) or twice ambient (elevated, 700 mu L L(-1)) CO2 concent ration at three levels of supplemental nitrogen (N) (no additional N ( N-0), 90 kg ha(-1) (N-1) and 200 kg ha(-1) (N-2)) in open-top chambers under irrigated field conditions from seeding until flowering. The pr imary objective of the study was to determine if N supply alters the s ensitivity of growth and photosynthesis of field-grown rice to enriche d CO2. A second objective was to determine the influence of elevated C O2 on N uptake and tissue concentrations. Although photosynthesis was initially stimulated at the leaf and canopy level with elevated CO2 re gardless of supplemental N supply, with time the photosynthetic respon se became highly dependent on the level of supplemental N, increasing proportionally as N availability increased. Similarly, a synergistic e ffect was noted between CO2 and N with respect to above-ground biomass with no effect of elevated CO2 observed for the N-0 treatment. Most o f the increase in above-ground biomass with increasing CO2 and N was a ssociated with increased tiller and, to a lesser extent, root producti on. The concentration of above-ground N decreased at elevated CO2 rega rdless of N treatment; however, total aboveground N did not change for the N-1 and N-2 treatments because of the greater amount of biomass a ssociated with elevated CO2. For rice, the photosynthetic and growth r esponse to elevated CO2 may be highly dependent on the supply of N. If additional CO2 is given and N is not available, lack of sinks for exc ess carbon (e.g. tillers) may limit the photosynthetic and growth resp onse.