Ozone decreases soybean productivity and water use efficiency

The combination of population growth and climate change will increase pressure on agricultural and water resources throughout this century. An additional consequence of this growth is an increase in anthropogenic emissions that lead to the formation of tropospheric ozone (O₃), which in concert with climate change, poses a significant threat to human health and nutrition. In addition to being an important greenhouse gas, O₃ reduces plant productivity, an effect that has been particularly pronounced in soybean, which provides over half of the world's oilseed production. Plant productivity is linked to feedbacks in the climate system, indirectly through the carbon cycle, as well as directly through the partitioning of radiation into heat and moisture fluxes. Soybean, along with maize, comprises the largest ecosystem in the contiguous U.S. Therefore, changes in productivity and water use under increasing O₃ could impact human nutrition as well as the regional climate. Soybean response to increasing O₃ concentrations was tested under open-air agricultural conditions at the SoyFACE research site. During the 2009 growing season, eight 20 m diameter FACE plots were exposed to different O₃ concentrations, ranging from 40 to 200 ppb. Canopy growth (leaf area index) and physiological measurements of leaf photosynthesis and stomatal conductance were taken regularly throughout the growing season. Canopy fluxes of heat and moisture were measured using the residual energy balance micrometeorological technique. Our results indicate that as O₃ increased from 40 to 200 ppb, rates of photosynthesis and stomatal conductance decreased significantly. Further, the seed yield decreased by over 60%, while water use decreased by 30% and the water-use-efficiency (yield/water-use) declined by 50%. The growing season average canopy temperatures increased by 1°C and midday temperatures increased by 2°C compared to the control. Warmer and drier canopies may result in a positive feedback on O₃ as temperature and humidity affect the production/destruction rate of O₃. In order to broaden these experimental results, time series modeling is being used to evaluate the historical statistical relationship between soybean production and tropospheric O₃ variations at the regional level. A multiple linear regression approach is being taken to understand the effects that O₃ has had on soybean yields in Illinois, Iowa, and Indiana over the past 30 years. By combining county yield data with EPA and NASA O₃ data, and controlling for variation in temperature and moisture, the O₃-exposure relationship for the Midwestern U.S. will be determined. Together, these results indicate the critical need to understand the response of other crops to O₃ and to put in place policies that mitigate the emissions of O₃ precursors.