Soil carbon's role in reducing Australia's carbon dioxide emissions is critical to the federal government's plan for the nation to achieve net-zero emissions by 2050.
Estimates based on CSIRO data suggest that between 35 million and 90 million tonnes of CO2 equivalent could be stored annually through soil carbon sequestration - drawing carbon from the atmosphere and storing it in the earth. With Australia currently emitting around 500 million tonnes of CO2 a year, it means that the ground beneath us is predicted to take up to 20% of the burden of reducing the country's carbon load.
Is it a realistic expectation of our soil and the farmers who will be at the forefront of sequestration?
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"No, unless we actually do something about giving farmers those technologies to achieve that," says Dr Michael Crawford, chief executive officer of the Cooperative Research Centre for High Performance Soils (Soil CRC).
"Soils do reach an equilibrium beyond which they can't sequester or hold any more carbon. That's a thing we will reach in coming decades, and we have reached in some soils. There's plenty of opportunity in many soils, but to actually get to those limits, we probably also need some massive changes in land use, and they're not always changes that people want to make, and that's to be respected as well.
"If we are to go closer to achieving those numbers, we need to give farmers the tools and knowledge and confidence they can do it year in and year out across all our soil types, farming systems, climates etc.
"In [the government's] defence, they've also highlighted the assumptions in the caveats behind [those figures], and that sometimes gets lost in the headlines."
At the heart of carbon sequestration is the ability to increase the amount of CO2 naturally stored in the soil, which varies across soil types. Sandy soils contain little organic material, while clay-dominated soil is rich in organic material. Thus, soil with a higher clay content can naturally sequester more CO2 than soil with a higher sand content. Increased rainfall can also boost a soil's uptake of carbon, with rain boosting plant growth, which in turn means more biomass production and more inputs of carbon into the soil.
Carbon credits are issued only for increases above the natural levels of carbon in the land. Much of the cost and difficulty for farmers comes in proving that increases have occurred.
"When you're selling CO2, you're selling an odourless substance that you can't see or hold," says Louisa Kiely, director of Carbon Farmers of Australia.
"Therefore, the rigour of what you have to go through in order to prove that you've done it is very high.
"Testing of the soil has been one of the barriers to uptake of the method because it's very specific - you have to follow the method to the word. Your core sampling has to be X amount, as does the depth that you go to, and the OHS that you've got to do. There's a lot to do just getting a soil core."
And that, says Crawford, can mean prohibitive expense for farmers.
"Measuring soil carbon to give the markets confidence that what you say you've got in your paddock is actually there does take a bit of effort currently, because of the inherent variability and uncertainty associated with our measurements," he explains.
"To take an example, organic carbon in soil is typically 1% to 3% of the total soil. In one hectare, you've got 10,000 tonnes of soil, and you're trying to measure 100 tonnes of carbon per hectare. And the rate of change re: sequestration might be up to a tonne of carbon per hectare.
"How much of that soil do you need to sample to confidently estimate that? And how do you best sample, given carbon is not like water where it distributes evenly through the mass? It's very location specific. So you've got to take enough samples.
"It's not like a forestry situation where you can count the trees and you know how much wood is in each tree and how much carbon is in that wood and you can quickly calculate it. It's uncertain. That's why it costs so much."
It's a burden that researchers are racing to overcome.
"Satellite might be still a pipe dream, but there are technologies coming along that can detect the soil carbon without having to dig holes," says Kiely. "That's a really big change."
Crawford describes a future with infrared sensors that can cheaply, rapidly and accurately detect carbon levels in soil.
"There's a reflectance associated with a whole range of properties in the soil that can be detected either proximately or from a close distance or via satellite or remote sensing," he says. "The smart boffins in that area are really working hard with data to develop algorithms to be able to make estimates of what's in the organic carbon."
But even with the best measurement tools, farmers will need support to make possible the one thing that's vital in soil sequestration - drawing and storing the carbon.
"Just because you measure it doesn't mean we can actually make it or get it there," notes Crawford.
"You need to give farmers the tools and technologies they need to be able to confidently put in farming systems on soil types to sequester the soil carbon. With the emphasis from the government on technology - technology for gas and hydrogen and solar, etc. - we also need the technology for soil carbon to further make this work.
"The other role of government here is helping to support, fund and coordinate the development of the technology and research. A lot of the research that's important in this area is too costly for each individual farm business to fund by themselves."
And while state governments have been promoting carbon sequestration through the likes of land restoration funds - the Queensland government, for instance, has offered farmers a rebate of up to $10,000 towards advisers' fees - Kiely and Crawford both say that ultimately the responsibility belongs to the federal government.
"As we just saw demonstrated in Glasgow, this is part of Australia's commitment globally, and we also need to ensure that across Australia we have a consistent set of rules and terms and processes," says Crawford. "The worst thing that could happen in this potentially confusing space is if we had six, seven, eight different sets of rules and regulations for how organic material is measured, attributed, verified, recorded etc."
A point around which there is no confusion is the benefit that soil carbon sequestration can bring to farms, even at the individual level - carbon-rich earth is healthy earth. Farmers who take on the task of increasing the carbon pool in their paddocks are also investing in the future productivity and prosperity of the land.
"[CO2 sequestration] is a product that you can sell but keep the benefit on farm," says Kiely. "It is a very revolutionary product that way. You sell the CO2, but the CO2 remains in your soil and your trees.
"Not only have you sequestered that tonne of carbon, you've also improved the resilience of that farm, you've improved the way that that farmer can continue to deliver food and fibre."
Crawford agrees that looking after the carbon content in soil is a win-win situation for farmers.
"Ultimately, soil carbon is good for your soil through the impact it has on soil structure, soil fertility, nutrient availability, biological activity, water-holding capacity and watering filtration," he says. "Every backyard gardener knows that adding organic material to a veggie patch is good for it. So that's what we're trying to do on a broad-acre basis across hundreds of thousands of hectares across Australia, but it's not so easy to just shovel out a bag of chook manure. It's how we do that at scale."
It's no massive step for many Australian farmers, he says, who are already reducing tillage and cultivation and rotating livestock to reduce grazing pressure and allow plants to regrow and put carbon into the soil.
"These are practices that many farmers have been doing," Crawford says. "What we're looking for then is something else we do over and above both those good practices, and how we make that work in a farming business, such that the main game is still about agricultural and food production.
"It's about how we can make it work in that context and across a broad spectrum of soil types, farming systems, rainfalls and climates across Australia. That's where we need to have a bit more focus, as well as developing those technologies."
How that all looks years from now is an unknown, but when Kiely casts her mind forward, she sees soil carbon as an integrated and integral part of the rural landscape.
"We have a vision called the farm of the future, and what we want to see in 2030 is that a farmer understands and can benefit from a wide range of potential methods - offsets, stewardships, protection of native veg, wind farms, solar farms - all on their farm while maintaining their traditional farming enterprises of sheep, wheat and beef," says Kiely.
"We see that [farmers] are the stewards now of a biodiverse carbon resource and that, in 2030, that will be recognised and rewarded."
Crawford envisages a similar scenario, with soil carbon sequestration a part of the bigger picture of farm operations across Australia, but also cautions that despite its touted promise of clearing up so much of the country's carbon responsibility, it's no silver bullet.
"There are some great opportunities in soil carbon, which have many win-win situations," he says. "But at the same time, I'm probably also wanting to hose down some of the hype and over-expectation of the agricultural sector.
"We probably need to accept that we can't get rid of all emissions in our economy, and we need to purse those [sequestration] avenues to allow us to offset those emissions, but it doesn't negate the need to address other emissions.
"Soil carbon sequestration isn't a license to allow us to pollute elsewhere."
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