Smaller than predicted increase in aboveground net primary production and yield of field-grown soybean under fully open-air [CO₂] elevation

The Intergovernmental Panel on Climate Change projects that atmospheric [CO₂] will reach 550 ppm by 2050. Numerous assessments of plant response to elevated [CO₂] have been conducted in chambers and enclosures, with only a few studies reporting responses in fully open-air, field conditions. Reported yields for the world's two major grain crops, wheat and rice, are substantially lower in free-air CO₂ enrichment (FACE) than predicted from similar elevated [CO₂] experiments within chambers. This discrepancy has major implications for forecasting future global food supply. Globally, the leguminous-crop soybean (Glycine max (L.) Merr.) is planted on more land than any other dicotyledonous crop. Previous studies have shown that total dry mass production increased on average 37% in response to increasing [CO₂] to approximately 700 ppm, but harvestable yield will increase only 24%. Is this representative of soybean responses under open-air field conditions? The effects of elevation of [CO₂] to 550 ppm on total production, partitioning and yield of soybean over 3 years are reported. This is the first FACE study of soybean (www.soyface.uiuc.edu) and the first on crops in the Midwest of North America, one of the major food production regions of the globe. Although increases in both aboveground net primary production (17-18%) and yield (15%) were consistent across three growing seasons and two cultivars, the relative stimulation was less than projected from previous chamber experiments. As in previous studies, partitioning to seed dry mass decreased; however, net production during vegetative growth did not increase and crop maturation was delayed, not accelerated as previously reported. These results suggest that chamber studies may have over-estimated the stimulatory effect of rising [CO₂], with important implications on global food supply forecasts.