* Australia's fragile soils vulnerable to climate change
* Marginal cropping areas might become pasture lands
* Technological changes could offset global warming impacts
By Bruce Hextall
SYDNEY, Aug 3 (Reuters) - Land available for agriculture in Australia, one of the world's largest food exporters, is in danger of shrinking because of climate change, a leading scientist said on Tuesday.
More grain was also likely to be grown in the north as climate change cuts production in the drier south with more marginal areas turned over to pasture, threatening Australia's position as the world's fourth largest grain exporter.
The country's mostly infertile and fragile soils made the agriculture industry particularly vulnerable to climate change, said Andrew Ash, director of the Climate Adaptation Flagship project of the government-backed research organisation CSIRO.
"We've had a fairly strong drying trend over the past 12 years or so and we think now there is a climate change signal associated with that," Ash said in a telephone interview after an address to the World Congress of Soil Science in Brisbane.
He said major changes in land management were needed to keep the 60 percent of Australia's land mass now used for agriculture from shrinking.
"It is more likely that we will see some cropping areas at the margins shrink," he said, while other areas would expand in regions that now have high rainfall, such as parts of Victoria, but which are expected to become drier. This might make these areas more suitable for cereal crop production, he said.
Australia is also expected to become warmer in the coming decades, further limiting water availability to crops and run-off for rivers and dams, particularly in the south, as well as escalating the risk from bushfires.
This would also affect soils because they help in the cycling and storage of water, nutrients such as nitrogen and phosphorus as well as carbon dioxide, the main greenhouse gas.
Ash said technological changes such as the breeding of more heat-resistant crop varieties and the use of degradable plastic sheeting to protect crops in the early stage of development could offset the negative impacts of climate change.
"We will eventually get to the point in some farming regions, even with the new technologies, that there will have to be some larger land use changes," said Ash.
"Probably the clearest example of that at the moment, with the drying of southern Australia and the reduction of water availability, people are starting to look at northern Australia," he said.
That could mean more areas planted with wheat, cotton and other crops such as peanuts in the north, offsetting falling production further south.
But while the north has a distinct wet season, which could provide water for irrigation, Ash said there were constraints such as soil quality so only a relatively small amount of northern Australia would be suitable for cropping.
The country's agricultural production has long suffered severe swings in climate and scientists expected even greater extremes in the coming decades.
Drought in eastern Australia during most of the past decade cut grain production in half in some years.
Already, cotton production in Australia, the world's third largest exporter of the fibre, has been drastically cut by reduced water for irrigation.
Cotton production this year was 1.6 million bales, less than half the record 3.4 million bales produced in 2001, despite improved varieties that lifted yields and reduced disease risk. (There are 4.4 bales to every tonne).
Rice production, which relies on irrigation, has fallen from 1.0 million tonnes in 2005/06, a year when rain boosted water availability, to 205,000 tonnes in 2009/10.
"Overall, I think we probably won't see a great reduction in the land area used for agricultural production but we might see shifts in how it is used -- there might be more pasture in areas that are drying and can no longer sustain cropping," said Ash. (Editing by Ed Davies and David Fogarty)