Levels of ozone are rising in the atmosphere, reducing yields of crops like soya. However, carbon dioxide levels, which increase yields, are rising too. Will that offset the negative effects? Alan Harman reports.

Higher atmospheric ozone levels are already suppressing soya yields by about 15%, according to Agricultural Research Service (ARS) and University of Illinois scientists. Yield reduction could easily reach 20%. However, this could be offset by higher carbon-dioxide levels.The team’s project, Soya Free Air Concentration Enrichment (soyFACE), measured how increases in carbon dioxide and ozone will affect soya production.Soya, wheat, and other crops grow better when carbon-dioxide levels are higher, because they capture more carbon during photosynthesis. But they’re stunted by elevated levels of ground-level ozone, a gas created when sunlight “cooks” pollutants from burning carbon-based fuels.
ARS plant physiologist Don Ort says there’s much to learn about how other factors affecting ozone uptake may come into play by mid-century. These studies will help breeders develop soya varieties better-adapted to the changing climate. The Intergovernmental Panel on Climate Change estimates daytime ozone in summer, now at about 50 to 55 parts per billion, may rise 20% by 2050.

The researchers also found higher carbon dioxide levels for the same year, rising from 380 parts per million to 550 parts per million, will only increase soya yields by about 12% – half of what previous studies estimated.Researchers found elevated carbon dioxide partially offsets the ozone damage, confirming the results of open-top chamber studies conducted at other ARS laboratories.
SoyFACE’s technology tests the effects of carbon dioxide and ozone in the open air, without the environmental modifications caused by the chambers. This reflects how plants respond in the real world. Horizontal pipes arranged in a 21m octagon surround each test plot. A computer measures wind direction and speed, then releases concentrated amounts of carbon dioxide and ozone, wind-dispersed across the soya crop.

The no-till factor
But the researchers are using open-topped chambers in a five-year project investigating no-till cropping. “We know no-till increases carbon sequestration in the soil,” says ARS plant physiologist Fitzgerald Booker. “But what will happen when atmospheric carbon dioxide levels rise, and how are changes in other atmospheric gases, particularly ozone, going to affect that?”Sixteen chambers are divided into four treatments: four with elevated ozone, four with elevated carbon dioxide, four with both gases elevated, and four controls. They determine the impact of ozone and carbon dioxide levels, in isolation, on plants.
The post-harvest plant residues are put back into the chambers, making them small, self-contained plots that mimic conditions found in no-till systems.

Researchers take soil samples twice a year to measure changes in carbon and nitrogen content. The carbon dioxide pumped into the chambers has a specific isotope marker so they can track it through the plants and into the soil. They also determine whether microorganism populations change.
After three years, preliminary results show a trend for more soil carbon in chambers with elevated carbon dioxide, but not in those with elevated ozone. Elevating carbon dioxide also reduced protein levels in wheat flour by 7% to 11%, but had no effect on soya seed protein.     |fw