The message from high-profile scientists Dr Jeff Baldock and Professor Peter Grace was clear: soil carbon is intrinsically valuable, but on current understanding it seems unlikely to yield a meaningful return to farmers in a carbon trading scheme.
Dr Baldock, a leading CSIRO soil scientist, and Prof. Grace, a climate change specialist at the Queensland University of Technology, sailed against the prevailing mood of optimism at last week's Carbon Coalition's Carbon Farming Conference in Orange, NSW.
Prof. Grace observed that soil carbon will be traded under a scheme that also accounts for emissions—and right now, the farming ledger balances out with carbon inputs/outputs firmly in the red.
He showed modelling of emissions from a 400 hectare Darling Downs farm, with 300ha of crop, 12ha of trees, and some cattle, which collectively resulted in 416 tonnes of carbon dioxide equivalents (CO2e) per year.
As a rule of thumb, mainstream science considers soil carbon sequestration potential in the more fertile, high-rainfall parts of eastern Australia to be around 500 kilograms per hectare per year. The reality might be considerably less.
"You can't just sell the carbon," Prof. Grace said.
"You have to look at the whole farming system and your profitability. A whole farming systems approach is essential—all gases have to be taken into account."
Carbon isn't just carbon, Dr Baldock told the conference, and the type of carbon a soil contains determines whether the carbon has a role in a trading scheme.
At one end of the scale is the "labile" carbon pooled in plant residue and fragmented organic matter, which is quickly cycled and lost back to the atmosphere; at the other end is humus and charcoal, which lock away carbon and other nutrients.
"We can induce big variations in the carbon across various pools by changing farm management," Dr Baldock said.
The challenge for farmers looking to rebuild their carbon is ensuring that it is rebuilt in the right pools.
In an modelling example shown by Dr Baldock, 18 years of soil carbon rundown under one farming practice was rebuilt in 10 years by another farming practice—but the carbon lost was largely humus, and the carbon that was rebuilt was in more labile pools.
Dr Baldock also noted that building carbon requires nutrient, which comes at a cost.
While carbon has been run down on most Australian farms, in decomposing it released other nutrients like nitrogen and phosphorus, which masked the detrimental effects of carbon loss.
In an example, a soil that started with a carbon content of 3pc was progressively run down to 1pc carbon.
The nitrogen released as the carbon decomposed came to 2.8t/ha.
"I can turn this on its head," Dr Baldock said.
"If I want to build carbon from 1pc to 3pc, I have to find nitrogen."
Soil organic matter has a consistent carbon-to-nitrogen ratio, which depends on the parent material. As the amount of carbon grows, so must the amount of nitrogen to ensure the ratio is maintained.
"That nitrogen can come from legumes, it doesn't have to come from bag fertiliser.
"The important message to take away is that to build carbon, you have to supply nutrients. You can’t build one without the other."
Dr Baldock suggested that carbon trading would not be a natural fit for all farmers.
Deciding to build carbon, and keep it there under contract, would demand changes in production systems.
Before making the change, farmers would have to consider their profitability, and their willingness to incur the liability of contracted carbon that might compromise their flexibility to change production systems in response to new circumstances.
"There's potential there, but there's a lot of bits and pieces we need to put together before we can decide whether it's appropriate for a given landowner."
However, Dr Baldock and Prof. Grace agreed that increased soil carbon was a highly desirable objective in itself for any farming system.
"Soil carbon is the key to long-term profitability," Prof. Grace said.
"If you've got it, that's your superannuation."