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 (ε_i) and converts that intercepted energy into biomass (ε_c). Elevated carbon dioxide (CO₂) increases photosynthesis and leaf area index (LAI) of soybeans and thus may increase ε_i and ε_c; elevated O₃ may have the opposite effect. Knowing if elevated CO₂ and O₃ 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 O₃ 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 (∼550 μmol mol^-1) and elevated O₃ (1.2 × ambient) in a factorial design. Aboveground biomass, LAI and light interception were measured during the growing seasons of 2002, 2003 and 2004 to calculate ε_i and ε_c. A 15%; increase in yield (averaged over 3 years) under elevated CO₂ was caused primarily by a 12% stimulation in ε_c, as ε_i increased by only 3%. Though accelerated canopy senescence under elevated O₃ caused a 3% decrease in ε_i, the primary effect of O₃ on biomass was through an 11% reduction in ε_c. When CO₂ and O₃ were elevated in combination, CO₂ partially reduced the negative effects of elevated O₃. Knowing that changes in productivity in elevated CO₂ and O₃ 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 ε_c for calculating regional and global plant productivity will need to accommodate the effects of global change on this important ecosystem attribute.