Submitted to: Environmental Pollution
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/1/2010
Publication Date: 3/1/2011
Citation: Bernacchi, C.J., Leakey, A., Kimball, B.A., Ort, D.R. 2011. Growth of soybean at midcentury tropospheric ozone concentrations decreases canopy evapotranspiration and soil water depletion. Environmental Pollution. 159:1464-1472. Interpretive Summary: Ozone that exists near the ground surface is a highly reactive gas that has negative impacts on the health of plants. Some species are more sensitive to high levels of ozone, including some of the major food crops grown around the world. Soybean is shown to be highly sensitive to ozone; as a result, the Soybean Free Air CO2 Enrichment (SoyFACE) experiment was established to determine how increases in ground-level ozone impacts the growth, development, and ecosystem functioning. In this study, we address the impacts of an increase in ozone to 25% higher than normal with the goal of determining how this might impact the canopy water use. Any large-scale changes in water use of this crop can have large implications on the local and regional hydrologic cycles where this species is grown, particularly the Midwestern U.S. We also measured the parameters used to model leaf level water use to determine whether any inherent changes in leaf physiology are linked with changes in water use at the canopy scale. Our results show that elevated ozone decreases canopy water use for all five years in which these measurements were made. Years with lower than normal ozone concentrations did not show statistically significant differences despite the similar direction of response. Also, there did not appear to be any acclimation responses at the leaf level as determined from the stomatal conductance model parameterization, suggesting the changes in canopy structure or water uptake by the roots drove the observed response. The implications of this study suggest that increases in ground-level ozone can have dramatic impacts on the hydrologic cycle including precipitation, stream flow, and atmospheric humidity.
Technical Abstract: Ground-level concentrations of ozone are increasing as a result of anthropogenic activities. This is having a negative impact on terrestrial ecosystems around the planet, including agricultural ecosystems. Critical questions surround the impact of rising ozone on soybean (Glycine max) since this species is one of the four major crops worldwide and the most economically important oilseed. Critical questions exist surrounding the impacts of elevated ozone on canopy water use for soybean. Leaf-level measurements from numerous previous experiments suggest that a decrease in stomatal conductance occurs, but whether this translates to decreases in canopy water use is unclear. The Soybean Free Air CO2 Enrichment (SoyFACE) experiment was established to address the impact on soybean of a changing atmospheric composition on physiology, development, and ecosystem functioning. In this study we present results from five years of micrometeorological measurements used to estimate evapotranspiration at the canopy-scale for soybean exposed to atmospheric and elevated (atmospheric + 25%) ozone concentrations. We also parameterized the Ball-Woodrow-Berry stomatal conductance model to determine whether changes in water use are driven by physiological changes at the leaf level. Our results show a decrease ranging from 17 to 8%, depending on year, in canopy-level water use for soybean grown in elevated ozone relative to the control for all years, although only statistically significant for three of these years. The years which did not show statistically significant differences are likely driven by low seasonal ozone concentration in the area. Despite the decreases in ET, there appeared to be no stomatal acclimation to growth in elevated ozone. These results suggest that rising tropospheric concentrations of ozone has the potential to alter the regional hydrologic cycle in areas where large-scale planting of soybean exists.