The ups and downs of more CO2
Researchers at the Australian Grains Free-Air Carbon Dioxide Enrichment (AGFACE) facility at Horsham, Victoria, are measuring the effects of a changing climate through field experiments in which additional carbon dioxide (CO2) is released onto test crops, a technique known as free-air CO2 enrichment (FACE).
Horsham is the location of Australia’s main FACE experiment for grain crops. Over three years, CO2 levels equivalent to those expected by 2050 caused an increase in both biomass and grain yield of 25 per cent when averaged across a range of agronomic treatments including supplementary irrigation. However, across individual treatments and years, responses to elevated CO2 ranged from minus five per cent to plus 70 per cent for biomass at anthesis, and from –11 per cent to +70 per cent for final grain yield.
This large variation reflected responses to climate, particularly soil water variation within and across a range of seasons. The search for more enduring crop traits that can take advantage of beneficial CO2 is continuing. Such traits include higher stem reserves of carbohydrate, greater transpiration efficiency and drought tolerance. Field and controlled-environment experiments are being backed up by computer simulation studies that allow the science to be extrapolated to other locations. For example, if the known response to elevated CO2 at Horsham over a three-year period is 25 per cent, a model can examine what the response should be at a drier or wetter site that also experienced increased temperatures.
Consistent increases in growth and yield of dryland crops have been measured in elevated CO2 fertilisation experiments throughout the world. The responses in Australia are among the largest – for the cultivar Yitpi, there was about a 25 per cent yield increase from CO2 levels of 550 parts per million. Present day CO2 levels are about 400ppm.
However, while yields have increased in these studies, there is a decline in grain protein (six per cent) and an even greater decline in the volume of bread loaves (eight per cent) that can be produced from the grain.
It has been found that the potential yield gain from rising atmospheric CO2 concentrations can be wiped out by other factors, particularly reduced water supply to plants and high temperatures, including more extreme heatwaves. Future plant breeding will therefore need to compensate for these declines in protein and loaf volume if the yield potential from elevated CO2 is to be realised.
It brings into sharp focus the importance of ongoing lifts in water use efficiency, as this could be the key. Wheat grown under higher levels of CO2 uses water more efficiently (30 per cent more at 550ppm). If the water supply to plants can be increased with careful agronomic management (for example, stubble retention and weed control) the effects of rising temperatures and lower growing-season rainfall could be reduced.
More information:Simone Dalton,
03 5362 0762,
AGFACE project video
This work has been funded by the GRDC, the Australian Department of Agriculture and Water Resources, and Agriculture Victoria. Other significant collaboration has occurred with the University of Melbourne, CSIRO, the University of Florida, the US Department of Agriculture and NASA in a worldwide model comparison project (AgMIP).
GRDC Project Code DAV00137
Region Overseas, South