Science Show

As world temperatures rise, soils are releasing more carbon following increased micorbial activity and the breakdown of organic molecules. Alex McBratney from The Univerisity of Sydney suggests Australian farming methods will influence soils' carbon carrying capacity. He says in the cotton-growing regions of northern NSW, a shift from wheat pasture rotation to continuous arable farming under irrigation has seen carbon levels in soils drop by half in 25 years.

Transcript

This transcript was typed from a recording of the program. The ABC cannot guarantee its complete accuracy because of the possibility of mishearing and occasional difficulty in identifying speakers.

Claire Morgan: How can I stand on the ground everyday and not feel its power? How can I live my life stepping on this stuff and not wonder at it? A quote there from American writer William Bryant Logan. His subject: soil. When I think of climate change I see an image of the air loaded with ever increasing quantities of carbon dioxide but of course it is not just the atmosphere that contains carbon, the stuff is pretty much all around us including in the soil beneath our feet.

Understanding the carbon cycle whereby carbon dioxide is absorbed by plants which then decay in the soil and at some point release carbon back into the atmosphere is crucial for those trying to predict how the climate will change in coming years. As reported on The Science Show this September research carried out in the UK between 1978 and 2003 showed that far greater levels of carbon than expected were somehow being lost from the soil and released into the atmosphere. This was presented by the UK scientists at Cranfield University as being both a symptom of and a contributing factor to climate change.

Ground breaking research to be sure but that was the UK, what about soil carbon levels here in Australia? Time to ask an expert. Alex McBratney is Professor of Soil Science at the University of Sydney. He has carried out extensive research into soil health in Australia and thinks soils here have also lost large amounts of carbon in recent years albeit for different reasons.

Alex McBratney: Let me first of all say that I think the UK work is a fine piece of work but I think the jury's still out for a complete explanation of the figures that they presented. But I've got no doubt that climatic warming is a significant proportion of the affect. Now of course in Australia because mean annual temperature is much higher than the UK we would expect that we have less carbon in our soils and that's true, we have less carbon in our soils. We're in equilibrium with a warmer and a drier climate. Of course there are other issues about land use and clearing of land here is much more recent that it has been in the UK and much of the clearing in the UK was done before the Romans arrived and some a bit later.

Here it's all happened from post-European settlement and we do know that when forests are cleared for the first time they lose massive amounts of carbon very quickly and so I think the climatic affect here will be probably less, but the land use effect will probably be more.

Claire Morgan: Figures from the government's Australian Greenhouse Office support Professor McBratney's thesis. Since 1988 the AGO has measured the amount of CO₂ emitted from soil when forests are cleared for agricultural use and the results are thought-provoking. In 1990 the year land clearing rates peaked the AGO tells us that emissions from land use change constituted 20% of Australia's total CO₂ emissions at 126 million tonnes - that's a lot of carbon. And Alex McBratney's research goes further indicating that intensive farming practices also caused drops in carbon levels.

In the northern NSW cotton growing areas for example his work shows that with a shift from wheat pasture rotation to continuous arable farming under irrigation soil carbon levels dropped by half in 25 years. So climate change discussions aside, should farmers be worried about dwindling carbon content?

Alex McBratney: Carbon in the soil actually affects the formation and stabilisation of aggregates. Now these little crumbs or aggregates are water stable so when rainfall hits them they don't fall to pieces, they stick together and this prevents soil loss to water and wind erosion. It also gives the soil much better water holding capacities, a well-aggregated soil can hold more water and in Australian agriculture I guess the ability to hold water is the principle measure of how good a soil is for cropping. It's like how big is the bucket and if you've got a big bucket and you've got a good soil, if you've got more aggregates you've got a bigger bucket.

Claire Morgan: So if maintaining a good level of carbon makes for better soil all round what can farmers do to hang on to it?

Alex McBratney: So the only thing that a farmer can do, he can't control probably the rate of decomposition of the carbon very easily, that's really controlled by the climate. Where he can control the amount of carbon going back in so you really need to stubble mulch, retain stubble rather than burning it so any carbon that isn't harvested should be going straight back into the soil. The other thing is to add break crops which add carbon, sometimes called green manuring crops, so for instance in northern NSW people are looking at a crop called vetch which produces high amounts of carbon and nitrogen which can go back into the soil. And this is also why legumes are important in wheat sheep farming systems of Australia cause they're adding carbon back in as well as nitrogen. So systems that add carbon back in is really the key.

Still a bigger issue I think about whether the government, state of federal, should pay landholders for providing ecosystem services. To me this seems to be a very fair way of asking people to maintain their vegetation at the same time that through its increased biodiversity, through its ability to filter water, its ability to not release carbon dioxide into the atmosphere, these all provide a public good. This is for the good of all Australians.

Claire Morgan: Despite Professor McBratney's convincing argument I suspect public sector incentives may be a rather remote possibility. But carbon trading is up and running in Australia and internationally so there's clearly a demand for carbon sequestration services. While the majority of Australian offsetting operations are forestry based I wouldn't be surprised if farmers don't join in soon increasing their soil quality into the bargain.

Robyn Williams: That was Claire Morgan.

Guests

Claire Morgan
mailto:mail@clairemorgan.net

Alex McBratney
Porfessor of Soil Science University of Sydney