open hands made dirty from working in soil

Soil Carbon Sequestration: A Strong Case for Climate Mitigation?

Soil carbon sequestration is a good thing all around. But is it enough to hedge our emissions future and feed everyone well?

Soil carbon sequestration is critical to farmers, even though many might not know it or believe it to be true. Almost everywhere where farmers are farming for profit, agricultural soil is depleted. In the hundred or so years since the gasoline-powered tractor arrived, it has been more profitable to grow food with inputs than by managing the innate production capability of soil.

Humans are nothing if not chasers of profitable enterprise, and farming can be profitable.

Modern intensive agriculture uses machinery to clear large areas of native vegetation and till the soil, creating an even, uniform substrate to plant crops. Initially, crop production pulls on the nutrients available in the newly cleared soil. But as yield declines, the farmer uses his machinery, energy sources and financial means to add inputs such as fertilisers or pesticides and herbicides or irrigation to maintain and raise yields. He may also choose cultivars that respond best to this input-driven system. Production continues as long as the farmer can maintain inputs to subsidise the losses in soil capability and harvests. At least, that is the plan.

Agriculture differs for the 500 million smallholder farmers who grow food for themselves and their families. Many farmers, who grow roughly a third of the world’s food, will clear small forest areas with tools or fire and plant crops. This works well, but without resources for nutrient inputs, the crops deplete soil health, and yields decline, often rapidly, forcing the farmer to move and clear another parcel of forest. Nature is left to restore the abandoned farmland that can be returned to production a few decades later. This shifting cultivation—clearing, production, land abandonment, fallow period and reclearing—was common everywhere humans developed agriculture and is still prevalent in Asia, Africa and South America.

Here is what happens to the soil to explain why yields decline.

The natural carbon cycling from forest trees through dead leaves and roots is disrupted when the trees are cut down. Whatever soil carbon is in the upper layers of the soil is exposed to sun and wind. It dries and oxidises, removing the food source supporting soil biodiversity, which also declines. Carbon loss affects plant growth because it is an essential substrate for nutrient exchange between soil and plant roots, a process many soil organisms facilitate. Lose the soil carbon, and nutrient cycling declines with it.

Newly cleared land is productive for a few cropping cycles, but inevitably, production declines over time. How many crop cycles will depend on the soil type, climate, crop, cultivation practices, and how long the farmer can tolerate yield declines. 

Eventually, the effort needed to cultivate a crop outweighs the energy yield from the harvest. The farmer cannot continue and moves his cultivation by clearing the next small parcel of forest. He abandons his cultivated plot that regenerates into a secondary forest where the vegetation supports soil carbon sequestration—the return of carbon to the soil from the atmosphere through the soil biodiversity that uses the dead leaves, branches and roots of the plants as fuel, converting this detritus into soil organic matter. 

And all this has been going on for a long time.

In the 12,000 years of agriculture, farmers have progressively expanded the cleared land to its current 5 billion hectares—38% of the global land surface—and created a soil organic carbon debt. The expansion of agriculture was a boon for farmers and their sons, resulting in more food and more people, a boon for everyone else. As demand for food in cities and rural areas increased, so did farmers’ pressure to produce more food. They either couldn’t wait for nature to restore the soil or had insufficient means to replace the carbon and nutrients used by the crops and livestock. Inevitably, they pushed on, and the soil carbon became carbon dioxide in the atmosphere. 

Some call this a carbon debt. 

Soil carbon sequestration is an option to pay back the debt by pulling carbon dioxide from the air via plants and storing it in the soil as soil organic matter. This is good for climate mitigation but also helps create good soil.

What is good soil?

Good soil smells fertile, all musty and moist. 

Good soil doesn’t thoroughly crumble or stick to your fingers like glue but leaves a healthy stain, just like when peeling the potatoes.

Good soil is teeming with active life, from microbes to molluscs to mould, all interacting to transfer nutrients around and into plant roots.

Good soil has carbon in it.

Good soil is the stuff that farmers start with after vegetation is cleared. It varies from place to place, with ‘good’ being relative to the soil type and climate conditions, but all soils start with more carbon than they have after several cycles of agricultural production. Cultivated soil is usually depleted over time—not always, but it is a repeatable rule of thumb.

The aim should be to keep and build soil carbon.

Maintaining and building up soil carbon is the foundation of all the regenerative agricultural practices and farming pioneers.

Good soil happens with soil carbon sequestration

A few years ago, Australia’s premier science organisation, CSIRO, noted the potential to raise soil carbon levels and calculated that 35-90 million tCO2e could be stored annually in Australian soil via sound vegetation management and improved tillage practices. 

That’s 7-18% of the current annual greenhouse gas emissions of 490 million tCO2e and would significantly contribute to emission reduction targets.

Australia has primarily ancient soils that are low in carbon and nutrients. Millions of years of erosion, little uplift or tectonic activity, and a harsh climate depleted the carbon stocks before aboriginal peoples arrived and changed the vegetation with fire and long before Europeans arrived with their livestock. It is a long story and, for once, not initiated by agricultural practices.

Soil carbon sequestration is in the news again, with a report by AgriProve estimating there was potential for at least 103 million tonnes annually to be stored across the country—that’s over 21%, a potential lifesaver for a government serious about an agreement they signed in Paris.

Excellent news. 

However, here is what it says on the Agriprove website 

AgriProve is Australia’s leading soil carbon project developer, taking farmers on the journey from building soil carbon in the field to selling carbon credits to governments and companies with carbon liabilities. 

They are bound to be bullish.

We are too.

Good soil contains carbon.

I am a scientist, and so is Charlie, while Chris is a practitioner and educator. We have close to 100 years of experience in soil ecology—Chris is also a farmer—so we have been watching the science of soil carbon for a long time. Our considered view is that the case for soil carbon sequestration is strong.

First, the research and observational evidence that agricultural practices deplete soil carbon in most agricultural soils is unequivocal and is happening everywhere. Depletion to the point that biological activity, nutrient exchange and soil physical properties are compromised is defined as soil degradation—the FAO estimate that this might be true for 40% of global soils.

Soil carbon losses are real.

Scary as this should sound, there is good news too.

Second, there is research and real-world evidence that losses can be reversed. Actions that promote soil carbon retention—ground cover, minimum tillage, water retention—can sequester soil carbon and reverse degradation. Not everywhere and not always, but in many instances, soil carbon sequestration happens when regenerative practices are implemented. 

Good soil has carbon in it.

Ground cover, minimum tillage, and water retention actions are pivotal to future food, even without a sequestration benefit.

Suppose some actions can be taken. In that case, there is an opportunity for governments to encourage farmers to adopt land management practices that promote soil carbon sequestration and reduce soil carbon losses.  

A soil carbon sequestration policy

After over a decade of argument over climate action, the Australian government belatedly accepted that if good soil has carbon in it, then soil might be a place to put some of the excess carbon that society has emitted. 

Improve soil health and get a climate mitigation bonus.

Soil carbon sequestration became one of five priorities in the Australian government’s emission reduction technology roadmap, with over $200 million set aside in the 2021-22 Federal Budget to improve and protect Australia’s soils.

A policy that promotes soil carbon gains as an offset for carbon liabilities sounds like a win-win — sequestration and soil health benefits. Landholders win, and so do companies with a need to buy offsets. Heck, even the government succeeds by facilitating a market for offsets.

Enter the naysayers.

Soil carbon will not be enough to offset agricultural emissions, let alone the coal industry. The idea we can bail out the coal industry with soil carbon is just fanciful.

Richard Eckhard, Professor of Sustainable Agriculture, University of Melbourne.

Prof Eckhard also reminds us that soil carbon in Australia is 90% dependent on rainfall. This is true. Soil carbon and the soil biodiversity that promotes soil carbon sequestration is moisture driven, and it’s the biology responsible for moving and storing carbon. 

The snag is that rainfall is expected to decline due to climate change in many areas. 

Eckhard again…

“Why would we hedge our future climate change strategy on something that climate change itself is going to challenge?”

And so the polarised argument begins.

dried out soul with the occasional. weed trying to achieve soil carbon sequestration

Photo by Abhishek Pawar on Unsplash

What sFED suggests

When it comes to soil carbon sequestration, we are not surprised that 

  • it took a long time to accept the truth that agricultural practices deplete soil carbon stocks,
  • not everyone believes that losses can be reversed,
  • CSIRO hedged their bets with a wide range—35-90 million tCO2e—in their sequestration estimation,
  • governments dither and have a limited understanding of the science

We also accept that maintaining round cover, minimum tillage, and water retention actions that are necessary practices to recover soil carbon is not traditional. It will take time and encouragement for them to become a common practice among many farming communities.

And what practice combinations work best where, when and how are still open questions.

So should we hedge on soil carbon? Should governments promote actions by farmers for soil health?


And even though, on its own, soil carbon sequestration will never be enough for climate mitigation, we should do it nonetheless. 

Soil carbon should always be a core focus of land management because it helps plants grow. It is also worth remembering that ecological time matters, and patience is a virtue in anything to do with food production.

Postscript—Learn to love Variability

Nothing in nature is ever what it seems because nature varies in place and time.

Each arable field is different ecologically, no matter how hard we try to homogenise them for what we think is efficient production. No two patches, even teaspoons of soil, are the same.

There is Variability in nature, lots of it.

Start to think about variance rather than focusing on averages or the seduction of multiplication (that always ends up with a large number).

Ask questions about Variability, how much of it we understand, and what we can do to work with it.

Be comfortable that precision may not be possible, especially regarding soil.

Accept variation so long as the general is in the right direction. 

Sometimes soil carbon sequestration might be temporary, but that is ok if it is positive in an ecological time frame.

Hero image modified from a photo by Chris Yang on Unsplash


Mark is an ecology nerd who was cursed with an entrepreneurial gene and a big picture view making him a rare beast, uncomfortable in the ivory towers and the disconnected silos of the public service. Despite this he has made it through a 40+ year career as a scientist and for some unknown reason still likes to read scientific papers.

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