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 (345 mu L L(-1)) or twice ambient (elevated, 700 mu L L(-1)) CO2 concentration 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 primary objective of the study was to determine if N supply alters the sensitivity of growth and photosynthesis of field-grown rice to enriched CO2. A second objective was to determine the influence of elevated CO2 on N uptake and tissue concentrations. Although photosynthesis was initially stimulated at the leaf and canopy level with elevated CO2 regardless of supplemental N supply, with time the photosynthetic response became highly dependent on the level of supplemental N, increasing proportionally as N availability increased. Similarly, a synergistic effect 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 of the increase in above-ground biomass with increasing CO2 and N was associated with increased tiller and, to a lesser extent, root production. The concentration of above-ground N decreased at elevated CO2 regardless 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 associated with elevated CO2. For rice, the photosynthetic and growth response 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 excess carbon (e.g. tillers) may limit the photosynthetic and growth response.