How do elevated CO2 and O3 affect the interception and utilization of radiation by a soybean canopy?

Net productivity of vegetation is determined by the product of the efficiencies with which it intercepts light (epsilon(i)) and converts that intercepted energy into biomass (epsilon(c)). Elevated carbon dioxide (CO2) increases photosynthesis and leaf area index (LAI) of soybeans and thus may increase epsilon(i) and epsilon(c); elevated O-3 may have the opposite effect. Knowing if elevated CO2 and O-3 differentially affect physiological more than structural components of the ecosystem may reveal how these elements of global change will ultimately alter productivity. The effects of elevated CO2 and O-3 on an intact soybean ecosystem were examined with Soybean Free Air Concentration Enrichment (SoyFACE) technology where large field plots (20-m diameter) were exposed to elevated CO2 (similar to 550 mu mol mol(-1)) and elevated O-3 (1.2 x ambient) in a factorial design. Aboveground biomass, LAI and light interception were measured during the growing seasons of 2002, 2003 and 2004 to calculate epsilon(i) and epsilon(c). A 15% increase in yield (averaged over 3 years) under elevated CO2 was caused primarily by a 12% stimulation in epsilon(c) , as epsilon(i) increased by only 3%. Though accelerated canopy senescence under elevated O-3 caused a 3% decrease in epsilon(i), the primary effect of O-3 on biomass was through an 11% reduction in epsilon(c). When CO2 and O-3 were elevated in combination, CO2 partially reduced the negative effects of elevated O-3. Knowing that changes in productivity in elevated CO2 and O-3 were influenced strongly by the efficiency of conversion of light energy into energy in plant biomass will aid in optimizing soybean yields in the future. Future modeling efforts that rely on epsilon(c) for calculating regional and global plant productivity will need to accommodate the effects of global change on this important ecosystem attribute.