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 (http://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.