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Stephen P. Long,
Timothy A. Mies,
Patrick Morgan B. Morgan,
Randall L. Nelson,
Don R. Ort
Could Air Quality in Central Illinois be Depressing Soybean Yield Today and More So Tomorrow?
Surface ozone concentrations have risen steadily since the 1960s. Ozone is formed by the action of sunlight on nitrogen oxides in the atmosphere. Most nitrogen oxide results from combustion, and as our use of fossil fuels increases so does the concentration of ozone in the surface layer of the atmosphere. Ozone concentration is the most important factor determining the quality of air in rural areas, and is the major factor that annually triggers an official alert of “Air Unhealthy for Sensitive Groups” during our summer. Nitrogen oxides while highest around large cities may be transported thousands of miles in the atmosphere. In Central Illinois Daytime ozone concentrations probably less than 10 parts in a billion part of air (ppb) on a typical summer day in the 1950s, today they average 60 ppb. The Intergovernmental Panel on Climate Change predict that ozone will rise a further 20% (15 ppb) over the next 30 - 50 years. In the 1990s, USDA established that soybean yield is decreased once average ozone concentration exceeds 30 ppb, and that it would decline a half a percent for every 1 ppb increase above this threshold. These studies therefore predict that current average concentrations of ozone in Central Illinois will be lowering yields by 15%. However, mean ozone concentrations fluctuate from year to year, with weather. 2004 had the lowest ozone concentrations of the last 10 years, while 2002 was 50% higher. So yield losses will not be constant.
A criticism of the previous studies of ozone effects on soybean is that they were all conducted in chambers, mainly open-topped chambers. These are clear sided chambers open at the top to the atmosphere, through which air of a known ozone concentration is pumped. The environment inside these chambers is generally warmer, more humid and less windy than outside; all factors that could alter the response. There has been doubt as to whether the yield losses could occur in the field. The University of Illinois has developed the first replicated system to examine the effect of elevated ozone on soybean under fully open-air conditions. The facility, called “SoyFACE”, is part of the new South Farms. In 2002 the background daytime ozone concentration was 60 ppb, the current average for Central Illinois, and an increase to 72 ppb, the mean level expected for 2030 - 2050, decreased yield by 15%. This not only confirms the expectation from chamber studies of a yield loss, but is double the half percent per 1 ppb projected from chamber studies. In 2003 mean ozone concentration was low at 50 ppb. Increasing this by 20% raised the concentration to the 60 ppb typical of Central Illinois summers, and lowered yield by 25%. These two years suggest very substantial yield losses due to current and projected future levels of ozone for soybean. Fortuitously, 2003, as a low ozone year, showed the substantial yield loss that must occur today in an average year while 2002 showed the yield loss that will occur tomorrow if ozone continues to increase as projected. Although varieties vary in their response to ozone, all show some yield loss and there is no connection between release date and degree of yield loss. That is, varieties selected under the higher ozone levels of the last decade appear no more resistant than those of 100 years ago. National ozone standards are currently set based on human health, where 80 ppb is viewed as a threshold, but for soybean that threshold may be 30 ppb. Soybean will therefore have many more “bad air days” than you. In summary, air quality in Central Illinois is lowering