Abstract
Over time, the stimulative effect of elevated CO 2 on the photosynthesis of rice crops is likely to be reduced with increasing duration of CO 2 exposure, but the resultant effects on crop productivity remain unclear. To investigate seasonal changes in the effect of elevated CO 2 on the growth of rice ( Oryza sativa L.) crops, a free air CO 2 enrichment (FACE) experiment was conducted at Shizukuishi, Iwate, Japan in 1998–2000. The target CO 2 concentration of the FACE plots was 200 µmol mol −1 above that of ambient. Three levels of nitrogen (N) were supplied: low (LN, 4 g N m −2 ), medium [MN, 8 (1998) and 9 (1999, 2000) g N m −2 ] and high N (HN, 12 and 15 g N m −2 ). For MN and HN but not for LN, elevated CO 2 increased tiller number at panicle initiation (PI) but this positive response decreased with crop development. As a result, the response of green leaf area index (GLAI) to elevated CO 2 greatly varied with development, showing positive responses during vegetative stages and negative responses after PI. Elevated CO 2 decreased leaf N concentration over the season, except during early stage of development. For MN crops, total biomass increased with elevated CO 2 , but the response declined linearly with development, with average increases of 32, 28, 21, 15 and 12% at tillering, PI, anthesis, mid‐ripening and grain maturity, respectively. This decline is likely to be due to decreases in the positive effects of elevated CO 2 on canopy photosynthesis because of reductions in both GLAI and leaf N. Up to PI, LN‐crops tended to have a lower response to elevated CO 2 than MN‐ and HN‐crops, though by final harvest the total biomass response was similar for all N levels. For MN‐ and HN‐crops, the positive response of grain yield (ca. 15%) to elevated CO 2 was slightly greater than the response of final total biomass while for LN‐crops it was less. We conclude that most of the seasonal changes in crop response to elevated CO 2 are directly or indirectly associated with N uptake.