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About FACE on CO2

A new study on the effect of carbon dioxide on crops has found that the anticipated fertilisation effect of this greenhouse gas has been greatly overestimated. Open-air simulations of the levels of atmospheric gases expected in 2050 found that CO2 fertilisation boosted production by only half the amount that had been previously expected. This was not sufficient to offset dramatic yield losses caused by the phytotoxic effects of ground-level ozone, the effect of which had not previously been studied.

"These results show that we need to seriously re-examine our predictions for future global food production as they are likely to be far lower than previously estimated," concluded Professor Steve Long of the University of Illinois, who led the study.

In the unfolding drama of climate change, carbon dioxide has been cast as both villain and hero. The broad consensus of scientists is that global warming forced by higher concentrations of the greenhouse gas will reduce crop yields in several ways, notably with high temperatures and more frequent drought. But many believe that augmented CO2 in the atmosphere will, at the same time, 'fertilise' plants and thereby negate the harm caused by warming. Some predict that CO2 fertilisation may even lead to bumper harvests.

Unfounded optimism

Field chamber used to test impact of increased levels of gases such as carbon dioxide (UIUC)
Field chamber used to test impact of increased levels of gases such as carbon dioxide

Professor Long considers such optimism unfounded. He argues that, whereas researchers can be fairly confident when predicting the effect on yields of changing temperature and precipitation, they can have very little confidence in predictions regarding heightened CO2. Previous experiments on CO2 fertilisation have been conducted in field chambers or greenhouses, which do not provide realistic simulations of open field conditions.

To test the effect of gas concentrations on crops more naturally, Long and colleagues at the University of Illinois at Urbana-Champaign and the Agricultural Research Service of the US Department of Agriculture developed open-air experiments using Free-Air CO2 Enrichment (FACE) technology. The field trials involved placing fumigation pipes in fields of soyabeans, arranged in circles measuring 20 metres in diameter. By using computerised wind sensors to control the release of CO2, concentrations over the plot could be maintained at around 550 parts per million, the projected CO2 level in 2050.

The results from the three-year field trails were dramatically different from earlier greenhouse studies. Whereas chamber experiments found CO2 accelerating crop maturation, Long's results found a delay in maturation of 2-7 days - depending on the germplasm. "This was a completely unexpected result," Long commented. The open field trials also contradicted chamber experiments regarding insect pests. The earlier experiments found heightened CO2 concentrations easing pest pressure, but Long and his colleagues found that Japanese beetles thrived in the future environment. They lived longer and produced more eggs. A further result was that mineral content of soyabean declined in high CO2 conditions.

From bad to worse

FACE trial in soyabean crop (UIUC)
FACE trial in soyabean crop

Greenhouse trials, as well as overestimating the fertilisation effect of increased levels of CO2, have not taken into consideration the impact of tropospheric (ground-level) level concentrations of ozone. Ozone is known to have a detrimental affect on most crops as it destroys rubisco, an enzyme crucial for photosynthesis. Ozone concentrations are anticipated to rise rapidly during this century, with concentrations predicted to expand across China to India, Arabia and North Africa. But so far there has been little research into the effect this will have on crops. Using the FACE apparatus for soya, Long and his colleagues tested ozone at 1.5 times its ambient concentration, both separately and in combination with heightened CO2. They found that ozone severely constrained photosynthesis and reduced yield by 20 per cent.

To determine the effect on different cultivars, 22 soya varieties were tested by Long. All showed a similar response, indicating that tolerance to ozone may not be an option in breeding improved cultivars. "Transgenics may be the quickest fix to solving this problem," said Long. He indicated that Monsanto and Syngenta are taking an interest in the results, particularly as soya yields have already begun to decline in the American Midwest, where high levels of tropospheric ozone are frequently recorded.

As yet, FACE trials for testing the combined effect of CO2 and ozone have only been conducted in Illinois. Other trials, conducted with wheat in Arizona, soya and maize in Illinois, and rice in Japan, have tested the effects of CO2 only. These results indicate that maize, a C4 plant, is less vulnerable to the effect of CO2 but that it conserves more water. No open field trials for carbon dioxide have so far been conducted in tropical or cold temperate regions. Long concedes that further studies on ozone are also required, particularly as tropospheric ozone levels vary regionally and day by day. Combined CO2 and ozone levels also impact on transpiration which has major regional implications for climate models.

Date published: July 2005


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