Minisymposium 9: Global change

Abs # 22005: Energy balance and evapotranspiration of soybean exposed to Free Air gas-Concentration Enrichment of carbon dioxide and ozone at SoyFACE

Presenter:	Bernacchi, Carl J, bernacch@uiuc.edu
Authors	Bernacchi, Carl J (A) (B)  Dermody, Orla C (D)   Long, Stephen P (C)   Ort, Donald R (B) (C)  Kimball, Bruce A (E)
Affiliations:	(A): Illinois State Water Survey/University of Illinois (B): Photosynthesis Research Unit, ARS, USDA (C): Departments of Crop Sciences and of Plant Biology, University of Illinois (D): Department of Plant Biology, University of Illinois (E): U.S. Water Conservation Laboratory, ARS, USDA

Atmospheric concentrations of carbon dioxide ([CO₂]) have risen over 40% since the dawn of the industrial revolution and tropospheric concentrations of ozone ([O₃]) have risen 0.5 to 2.5% per year over the same time period as a result of land-use changes and fossil fuel combustion.  These increases are expected to continue through the end of this century and are predicted to influence plant water use. While leaf-level responses to elevated [CO₂] and [O₃] are well documented, few studies have addressed canopy level responses to increases in these pollutants. Evapotransporation (ET) for soybean, one of the most important agricultural crop species, was measured using an energy balance approach. Open-air plots of soybean were exposed to either a 45% increase in [CO₂], a 21% increase in ambient [O₃], or increases in both [CO₂] and [O₃] through the use of Free-Air gas Concentration Enrichment (FACE) at SoyFACE. Data for the control, elevated [CO₂] and elevated [O₃] treatments were collected during the 2002 growing season and data for all three treatment were collected in 2003. Replicated micrometeorological measurements for each treatment were recorded in ten minutes averages over the growing season. ET was calculated as a residual in the energy balance based on measured net radiation, sensible heat flux, and soil heat flux. The results show a decrease in ET for all three treatments with the largest decrease observed for growth in elevated [O₃]. When integrated over the season, plants grown in elevated [CO₂] and elevated [O₃] used 10 and 18% less water, respectively. While the directional response of soybean exposed to increases in [CO₂] and in [O₃] were similar, the mechanisms for these responses differ. Growth in elevated [O₃] resulted in a decrease in leaf area compared with the control. It is likely that the [O₃]-induced damage to the plant canopy, responsible for the lower biomass and leaf area, is responsible for the lower ET. Alternatively, soybean grown in elevated [CO₂] demonstrated higher leaf area while showing a reduction in ET. These results suggest that a decrease in stomatal conductance, which lowers the transpiration component of ET, was sufficient to more than offset the increase in leaf area associated with growth in elevated [CO₂]. These results imply that future atmospheric change may influence soybeans response to drought conditions and may have feedback effects on atmospheric moisture, potentially altering regional precipitation patterns.