What is ‘soil carbon’, and why are we hoping it will mitigate climate change?

A major part of the federal government’s net zero plan is soil carbon sequestration. What is soil carbon, and can it really live up to the hype?

What is soil carbon?

Soil carbon is, in a sense, exactly what it says on the tin: the element carbon, stored in soils.

“Carbon is a critical part of soil organic matter,” says Dr Michael Crawford, CEO of the Cooperative Research Centre for High Performance Soils.

“Organic carbon exists in the context of the broader complex of organic material, along with nitrogen, phosphorus, sulphur, potassium, and a whole range of other nutrients.”

The carbon gets into the soil via photosynthesis: carbon dioxide (CO₂) molecules in the air are captured by plants, and turned into organic matter. The plants keep the C, and respire the O₂.

From there, some of the plant carbon gets below ground via the roots.

“As the plants die, they also secret and exude carbon into the soil,” says Crawford. This becomes carbon stored in “soil organic matter”.

So in a sense, the soil is an extra storage space for carbon that’s been removed from the atmosphere via trees and plants.

Can we increase the carbon sequestered in soils?

Since colonisation, the amount of carbon in Australian soils has decreased. On average, 20^th century agricultural practices tended to release more carbon from the soil than they produced.

“Most of the cropping soils have lost half of [their] carbon content since intensive agricultural systems were introduced,” says Professor Budiman Minasny, a researcher in soil-landscape modelling at the University of Sydney.

“As a result, many soils are degraded, which impacts their capacity to produce food.”

The good news is that it’s possible to increase the carbon content of soils again, with the right agricultural management. Changing grazing and irrigation patterns, using certain fertilisers and spreading clay can all increase soil carbon content, for instance.

In even better news: increasing the soil carbon content is better for the soil quality, and thus the output of a farm.

“The good thing fortunately is a lot of those management practices correlate with what we know is good farm management and good agricultural practices at the same time,” says Crawford.

“What’s good for building organic carbon is good for building organic matter in the soil, good for soil health, good for soil productivity, good for overall farm productivity and good for profitability at the end of the day.”

Does the carbon stay in the soil once it’s there?

The organic carbon in the soil doesn’t have the stability of a rock – it’s being eaten and exhaled by a range of other things. Microbes and plants in the soil both respire CO₂ back into the atmosphere, for instance.

Without agricultural or climatic interference, bodies of soil exist in equilibrium – with as much carbon dioxide being absorbed by the soil as is being released. But with management, it’s possible to manipulate the soil so that it’s absorbing more CO₂ than it’s producing.

“There are different types of carbon in soil – roughly speaking half of it short-lived, lasting 10-20 years, and the rest long-lasting of 1,000 years,” says Minasny.

So some of the carbon is locked up for a long time, but not forever.

How much CO₂ could we store?

In its 2020 and 2021 low emissions statements, the federal government has suggested that 35-90 million tonnes of CO₂ could be stored per year in soils, based on a calculation from analysis done by the CSIRO in 2010 and current agricultural land coverage. Is this feasible?

“That is assuming all soils used in cropping and pasture can sequester carbon or put carbon in the soil every year,” says Minasny.

“The estimate is at the high end.”

Crawford also thinks the estimate is optimistic, stating it’s impossible without a big investment in agricultural technology. “Using our current technologies and our current farming systems […] we’d have difficulties.”

Minasny says that his research has come up with a lower, but still very impressive, number.

“If implemented widely in cropping lands, our modelling indicated that it can offset about 70% emissions from the agricultural industry for the next 20 years. That is significant.”

He stresses, however, that “this is a short-term solution”.

Once soils reach equilibrium, they won’t be able to store any more carbon dioxide – so while we’ll have very high-quality soils at that point, we won’t be able to keep putting carbon dioxide in.

And, even at the most optimistic estimate (90 million tonnes), our soils can still only hold less than a fifth of Australia’s current annual emissions (roughly 500 million tonnes).

“Soil carbon sequestration is not meant to replace clean energy. It is part of the many climate solutions – reducing fossil fuel use being the main one,” says Minasny.

“Soil carbon sequestration isn’t a license to allow us to pollute elsewhere,” agrees Crawford. “It’s part of a bigger integrated picture.”

So while there’s massive potential for soil carbon, it’s not a silver bullet for the climate crisis.

Does storage potential change from place to place?

Part of the difficulties with widespread soil carbon sequestration is that effective techniques vary widely depending on the location.

“Soil carbon storage potential varies from region to region,” says Minasny. “The soil type dictates its capacity, eg, clay soils tend to hold more carbon than sandy soils.”

Beach sand has the lowest carbon content of all – it’s mostly silica (SiO₂), after all.

“This capacity is confounded by the climate as carbon inputs in the soil depend on rainfall (the more rain, more plant materials can be produced) and temperature (the hotter it is, the faster the carbon breakdown),” says Minasny.

What are we doing to store more?

Since 2011, farmers have been able to claim carbon credits for management practices that remove extra CO₂ from the atmosphere. A soil-explicit method for claiming these credits was released in 2018, meaning that farmers can now get an extra source of income for sequestering carbon in soils.

This is potentially a win-win for everyone: farmers can be compensated for removing CO₂, and improve the quality of their soil in the process.

Problems remain, however. The federal government is interested in the logistics and costs associated with measuring how much carbon has been stored. And researchers are still interested in ways that organic carbon can be increased further in soils.

“The government is focusing on reducing the price of measuring soil carbon,” says Minasny. “However, the fundamental research questions [are] on how to get soil to take up more carbon at a large scale.”

Soil carbon has great capacity to reduce our overall emissions, but it’s a bad idea to rely on it entirely. It still needs plenty of research to figure out the details, and it will only ever be part of the solution to the climate crisis.

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