Carbon trading systems must be careful not to undervalue soil carbon, according to a leading soil scientist, because the true productivity value of soil carbon to farmers may be hundreds of dollars per tonne.
Dr Rattan Lal, a professor at Ohio State University, told last week's Carbon Farming Conference in Orange, NSW, that in order to commoditise carbon, a realistic value must be established that reflects its value to farmers and society.
Initial estimates of carbon's starting value under the Australian Carbon Pollution Reduction Scheme (CPRS) are around $20 per tonne.
When Dr Lal looked at humus, of which carbon is the main component, and teased out the nutrients and water typically held within a kilogram of humus, he arrived a value of US$250 a tonne on today’s prices.
Farmers, and society at large, also benefit from the fact that soils with high levels of organic carbon (humus) are resistant to erosion, deliver less pollution to waterways, biodegrade chemical pollutants and buffer climatic extremes.
"Whether the trading process can provide farmers with all of that value remains to be seen," Dr Lal told the conference via an internet link.
"Undervaluing a resource can lead to its abuse."
If soil carbon ultimately earns a high price, it raises questions about the value and use of crop residues that contribute to soil carbon formation.
Cellulosic ethanol plants that will draw on crop residues are being built in the United States, and the technology is under discussion in Australia.
However, Dr Lal observed that the world's estimated four billion tonnes of annual crop residues should play an important part on the farming process.
In Dr Lal's estimation, those global residues contain 30 million tonnes of nitrogen, 3.5mt of phosphorus and 47mt of potassium—and crop residue contains about 40pc carbon.
He studied the effects of crop residue in a Nigerian corn system. With residue left in the field rather than removed, soil carbon levels were 0.2pc higher, soil pH was 5.1 under residue and 4.6 without, and that corn yields on a field sown into residue were 2.7t per hectare compared to 1.5t/ha in a bare field.
"Soil quality is significantly influenced by residue retention," he said.
Dr Lal has also extrapolated how improving soil carbon might affect food security for the 854 million people currently considered "food insecure".
In 2000, the global food deficit was considered to be 13 million tonnes; by 2010, this will have risen to 22 mt, mostly in sub-Saharan Africa.
By increasing soil carbon levels in the 532 million hectares of agricultural soils in developing countries by a modest one tonne per hectare per year, Dr Lal said, an extra 30-50mt of food could be produced per year.
Dr Lal, the director of the Carbon Management & Sequestration Center at Ohio State, was a keynote speaker at the two-day Carbon Farming Conference hosted by the Carbon Coalition.
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