Enhancement of rice canopy carbon gain by elevated CO2 is sensitive to growth stage and leaf nitrogen concentration

Increasing our understanding of the factors regulating seasonal changes in rice canopy carbon gain (C-gain: daily net photosynthesis - night respiration) under elevated CO2 concentrations ([CO2]) will reduce our uncertainty in predicting future rice yields and assist in the development of adaptation strategies. In this study we measured CO2 exchange from rice (Oryza sativa) canopies grown at c. 360 and 690 mu mol mol(-1)[CO2] in growth chambers continuously over three growing seasons. Stimulation of C-gain by elevated [CO2] was 22-79% during vegetative growth, but decreased to between -12 and 5% after the grain-filling stage, resulting in a 7-22% net enhancement for the whole season. The decreased stimulation of C-gain resulted mainly from decreased canopy net photosynthesis and partially from increased respiration. A decrease in canopy photosynthetic capacity was noted where leaf nitrogen (N) decreased. The effect of elevated [CO2] on leaf area was generally small, but most dramatic under ample N conditions; this increased the stimulation of whole-season C-gain. These results suggest that a decrease in C-gain enhancement following elevated CO2 levels is difficult to avoid, but that careful management of nitrogen levels can alter the whole-season C-gain enhancement.