The Influence of Nitrogen on the Elevated CO₂ Response in Field-Grown Rice

Abstract

Rice (Oryza sativa L. cv. IR72) was grown in the tropics at ambient (345 μL L^-1) or twice ambient (elevated, 700 μL L^-1) CO₂, concentration at three levels of supplemental nitrogen (N) (no additional N (N0), 90 kg ha^-1 (N₁) and 200 kg ha^-1 (N₂)) 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_o 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 above-ground N did not change for the N₁ and N₂ 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.