Soil, carbon sequestration and the fight against climate change

It’s too late. There is now general agreement that the time has passed when we could have tackled climate change by only reducing the use of fossil fuels. Now we also have to look at mechanical ways to remove excess carbon from the air.

The current carbon-capture technologies still have quite some way to go, but there are existing mechanisms that deal with massive quantities of carbon every day, both capture and storage. They are forests, the ocean and soil. A global effort is now under way to understand the best ways to bring these ecosystems to the climate change fight.

Soil is receiving particular attention because of its carbon-storing credentials. It’s been estimated that there are 2500 billion tonnes of carbon in the world’s soil, compared with 800 billion tonnes in the atmosphere and 560 billion tonnes in plant and animal life. It is a powerful carbon machine, with one estimate saying it has the potential to store another 1 billion to 3 billion tonnes of carbon annually.

One of the leading international thinkers on soil is Professor Budiman Minasny, from the Faculty of Science. As a professor in soil-landscape modelling at the University and the theme leader of Carbon, Water and Soil at Sydney Institute of Agriculture, he has a deep understanding of soil and its potential to turn back the carbon clock.

“Compare the soil under Australia’s natural vegetation to the soil of croplands and you’ll see about half of the organic matter has been lost since European-style agriculture began here,” says Minasny who is softly spoken with an easy smile. “A lot of that lost organic matter has been exposed to oxygen by tilling the earth, and it’s decomposed into a lot of carbon.

“This has happened everywhere. But we don’t have to accept it. We can build the soil back up, we can put the carbon back in.”

The singularity of the word ‘soil’ downplays the fact that soil isn’t singular at all. It’s a vast, global collection of unique and complex environments that are ever changing. Understand this and you can identify parts of the landscape that can store more nutrients or moisture or has the potential to store more carbon.

The Carbon Farming Initiative 

That carbon potential caught the eye of the Australian Department of Agriculture, Water and the Environment which established the Carbon Farming Initiative (CFI) where farmers and land managers can earn carbon credits by storing carbon on the land. These credits can then be sold to people and businesses wishing to offset their emissions.

Many Australian farmers are already working to reduce emissions on their properties. In fact, they lead the world in a carbon storage approach called minimum tillage.

“That’s where you don’t remove harvest residue, including leaving the stubble and roots in the ground,” says Minasny. “About 80 percent of Australian farmland is reduced tillage now, which means a substantial amount of carbon not released.”

It also means more robust soil that is less likely to be washed away by rain, and more able to hold moisture during a drought.

Still, farmers haven’t much embraced the government’s Carbon Farming Initiative, partly because of rule complexity and partly because establishing the carbon content of soil is not straightforward. It involves a contractor taking core samples from multiple sites on a property, breaking up the cores by hand, putting them in plastic bags, sending them off to a testing laboratory and waiting perhaps a couple of months for the results to come back. All paid for by the farmer.

Finding light in the soil

Minasny and his team are working on a better idea: a system that allows tests to be done on site with a minimum number of samples needed. The key technology is infrared spectroscopy (IRS), which reads light reflected from a sample to identify the elements present. IRS is already used by museums to identify paint pigments, in the food industry to establish purity and in forensics.

“It will pick up the minerals in the soil and the composition of organic matter,” says Minasny. “But the challenge is to get rid of the external signals that we don’t want - the effect of moisture for example. That is done if you send it to a lab. We have to work out how to do it on the farm.”

This is happening under the umbrella of Minasny’s main research task. He is part of the effort to advance the relatively new field of digital soil mapping (DSM), which has seen him become one of the most cited soil researchers internationally; an unexpected achievement for someone who liked science and mathematics but didn’t intend a career in agriculture.

“My grandfather is a farmer, but I grew up in the city,” says Minasny, who was born and studied in North Sumatra, Indonesia, before coming to Sydney to do his Master of Agriculture. “When I was first thinking about what to study, I just found myself interested in soil science.”

The mathematical part of Minasny’s brain is particularly handy because DSM involves a lot of statistical algorithms, starting with existing knowledge and extrapolating it across larger areas.

Traditional soil mapping is still done but it’s a laborious process best suited to finding the boundaries between soil types. Digital soil mapping is more holistic, having grown out of the dramatic improvements in satellite photography, which became super-detailed, information-loaded and easy to access.

This gave soil scientists a view of vast areas that couldn’t possibly be soil mapped manually so DSM wasn’t so much about boundaries, but soil variations across a landscape, information that could inform larger scale planning; where to put crops, where to plant trees, where is best for grazing, what soil would most readily take up carbon?

Allowing that DSM is partly predictive, there is an ongoing focus on enhancing accuracy.  To that end, the team that Minasny works with has developed a more nuanced way of expressing the forces that create soil and change it over time. This approach can then be applied to the digital map calculations.

The acronym is SCORPAN: soil, climate, organisms (that are present), relief (the shape of the landscape), parent material (what the soil is made of), age (of the soil), and spatial location. How SCORPAN differs from the previous model, is that it adds the reality that the various elements don’t just change the soil, they can change each other. It also adds an allowance for error or uncertainty.

The starting point though, for any soil map, is a deep understanding of soil itself. The International Union of Soil Sciences lists 32 different soil reference groups with names like andosols and regosols, with 120 sub-groups, all affected by natural and human forces.

Healthy soil has two types of carbon: biomass carbon made up of living bacteria and fungi, and non-biomass carbon which is the degraded remains of dead plants, which had previously pulled carbon out of the air during the process of photosynthesis. Minasny has a particular respect for the bacteria and fungi; what he calls the ‘bugs’.

"Bugs are sort of the soil engineers,” he says. “They create a more habitable region for the plants and other things. They’re the soil’s fuel of life.”

Yet some modern farm practices work against the bugs: over tilling, over-reliance on chemical fertilisers, “Fewer bugs mean less fuel for the soil. So all the systems start to slow down. Some of the systems will break down.”

That said, Minasny clearly sees the practicalities. “Farming itself is not a natural system. It’s a human system created to feed humans,” he says. “So, it’s good to pull back on chemical fertiliser but we still need it.”

A great advantage of working with soil is that it’s a massive ecosystem. According to the UN’s Food and Agriculture Organisation, approximately five billion hectares, or 38 percent of the global land surface is devoted to agriculture. A small, positive change in such a huge area could bring real benefits but, as Minasny notes, there is a caveat.

“It’s not a substitute. We still have to stop using fossil fuels.”

Written by George Dodd for Sydney Alumni Magazine. Photography by Louise M Cooper.