What’s the dirt on soil carbon?

14 September 2022

Is capturing carbon in farm soil a practical way to help us reach net zero emissions?

Every farmer, ecologist and good gardener knows how important organic matter is for soil. High levels of plant litter, roots, soil fauna and microorganisms keep soils healthy with good structure and function.

Many farmers in Australia already do things to improve soil health, such as growing cover crops, applying compost or manure, and using notillage — a technique for leaving soil undisturbed which is used on about two-thirds of cropland in Australia.

Different organisations are now looking at practices such as these as a strategy to mitigate climate change. But their real contribution to achieving net-zero targets and reducing global warming is much less certain.

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WHAT IS SOIL CARBON?

Around 58 per cent of the organic matter in soils is carbon. Scientists estimate that 1500 to 2000 billion tonnes of carbon are stored in the top metre of the world’s soil. Two to three times more carbon is stored in soil than in the atmosphere and all vegetation combined.

Soil organic carbon begins as atmospheric carbon dioxide which plants take up through photosynthesis. The carbon then enters the soil in the form of decomposing plant material, compounds secreted by roots and soil organisms.

Building up organic carbon in soil is attracting increasing attention as a way to improve soil health while decreasing carbon dioxide in the atmosphere.

Australia’s Long-term 2050 Net Zero Plan includes storing more carbon in soil (for a nominal period of 100 years) to offset some of our greenhouse gas emissions. This plan aims to support our Paris Agreement commitment to emit, on net, no greenhouse gas by the middle of this century.

Increasingly, individual companies and industries setting their own netzero targets are also banking on a contribution from soil carbon offsets.

WHAT ARE SOIL CARBON OFFSETS?

Soil carbon offsets are measures of the amount of carbon from carbon dioxide locked away in the soil. They’re monitored under Australia’s Emissions Reduction Fund, which has outlined specific rules to measure how much more organic carbon is stored in a standard mass of soil.

Only carbon in organic matter less than 2mm in diameter is counted, so offsets don’t include larger roots and litter fragments. Offsets also exclude inorganic carbon, which is mostly found in carbonate minerals.

Organic carbon exists in different chemical and physical forms that decompose at different rates. The balance between inputs and outputs determines whether soil is gaining or losing carbon at any given time.

Micro-organisms readily decompose as much as 90 per cent of the organic residues added to soil and release the carbon as carbon dioxide. This process provides nutrients for plant growth and energy for the microorganisms. But while this is important for healthy soils and vegetation, it leaves only a small percentage of carbon available for long-term storage in stable forms, commonly called humus, and also some charcoal from historical burning.

This is the basis of sequestration. So, if a farmer adopts a new practice like adding more organic residues to cropping soil, sequestered soil carbon will probably only increase slowly, over years or even decades.

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Carbon is continuously added to and released from the soil through a range of complex processes. Changing land management practices can change the amount of carbon stored or released. (Source: ATSE Explainer: Australia’s Soil Carbon Opportunities and Risks)

HOW MUCH CARBON CAN WE STORE IN THE SOIL?

It isn’t easy to predict how much soil carbon we can sequester at a location like a farm, for multiple reasons.

Soil processes are complex, and the dynamics and persistence of organic carbon vary across the range of climate zones, soil characteristics and agricultural productivity.

Research also shows that global warming itself may impact storage. For example, modelling by the New South Wales Government suggests that climate change could cause significant declines in soil health and soil organic carbon in state forest lands as soon as 2040.

There also haven’t been enough longterm trials to give us sound evidence that the land management practices proposed to improve sequestration are effective and economically viable.

Other overarching barriers to better estimating farmland sequestration are that we don’t have enough good data on current levels of carbon across agricultural soils, and high costs limit our capacity to accurately measure changes due to management practices. Currently, reliable measurement requires sampling and laboratory analysis that are costly and time-consuming.

Recent investment and global cooperation to develop more costeffective methods with suitable accuracy are starting to show promising results. These include spectroscopic (the study of the absorption and emission of light) and modelling techniques. But unfortunately, they aren’t widely available for use yet.

HOW MUCH WILL SEQUESTRATION AFFECT CLIMATE CHANGE?

Predicting how much soil carbon we can sequester at a farm or project scale is already complex. But extrapolating local estimates to work out how much soil carbon is impacting our climate change mitigation targets at a national or even global level is even more challenging.

Research and experience show it’s possible to build soil carbon in certain crop and pasture lands. But significant uncertainties in the data and assumptions for scaling-up that sequestration limit how confidently we can estimate what soil carbon offsets could achieve for our net-zero targets.

Soil and climate characteristics and management systems vary enormously across Australia, and this influences how much carbon we can store and how quickly it will change. At any site, rainfall and temperature, and then soil factors, largely determine the maximum potential soil carbon content.

Moist, cool environments and clayrich soils support higher organic carbon levels. More arid regions with high seasonal variability and sandy or nutrient-poor soils, particularly with low nitrogen and phosphorus, can’t store as much carbon.

But it’s not just average rainfall that affects the rate and direction of change in soil carbon — the variability of rainfall does too. One Australian study estimated that 12 years of increase could be reversed in three years of drought, which of course slows net long-term gain.

Sequestration also generally slows over time, because after farmers implement a new practice, the system will eventually approach a new steady state.

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ARE THERE OTHER CHALLENGES?

Local conditions influence which practices will be most effective for improving soil health. But this may not necessarily equate to higher carbon sequestration or more carbon credit units. Generally, soils with a high clay content have a higher sequestration potential.

For example, farmers can overcome a nitrogen deficiency by using fertiliser or planting legumes. But this may increase emissions of nitrous oxide (a strong greenhouse gas) and reduce how much the increased carbon stored in soil reduces global warming.

There are few long-term field studies in Australia to help us understand the factors affecting soil carbon sequestration and improve our confidence in scaling up these processes to inform policy development.

A CSIRO review of data from multiple studies published in 2010 shows that many farmlands are unlikely to return high rates of sequestration over decadal periods.

For example, across extensive grazing land the average annual sequestration rate in the top 30cm of soil was estimated to be between 0.1 to 0.3 tonnes of carbon per hectare (equivalent to removing 0.37 to 1.2 tonnes of carbon dioxide from the atmosphere each year in each hectare).

Since colonisation, settlers converting natural environments to westernstyle farmland has reduced Australian soil carbon stocks by 20 to 70 per cent. Overcoming this deficit with practices that store more organic matter would theoretically greatly benefit our efforts to alleviate climate change.

However, economic and social constraints on adopting and implementing these practices, as well as biological and physical factors, mean that the realistic potential is much less than the technical potential; possibly only 10 to 30 per cent.

IS SOIL CARBON THE ANSWER?

Few people dispute that increasing the organic matter in farmlands soil creates more resilient, profitable agricultural systems and improves the health of ecosystems.

But there are many things a farmer needs to consider when deciding whether to implement a soil carbon sequestration project to sell carbon credits, such as:

  • opportunity costs for farm businesses
  • time and cost of measurement and reporting
  • project maintenance
  • permanence period obligations
  • carbon price.

Future developments will affect how economically viable and practical these projects are in reducing global warming. New innovations may make farmers more confident to participate in carbon markets, but these are difficult to predict.

Uncertainty in achievable sequestration at farm or project scale in turn affects the potential for soil carbon offsets in Australia’s long-term emissions reduction commitments and in corporate and industry net-zero targets.

We already understand the many benefits of organic carbon in soil. But to more confidently estimate how they impact emissions, we need more cost-effective, accurate measurement.

We also need evidence-based estimates of achievable long-term soil carbon sequestration for different management practices in Australia’s diverse and variable climate and soil systems.

Nevertheless, while soil carbon sequestration seems unlikely to substantially offset emissions from industrial sectors, it does have potential to help agricultural industries become carbon neutral in line with market and consumer expectations

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Emeritus Professor Alan Robson AO FTSE

Agricultural scientist

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Adjunct Associate Professor Beverley Henry

Agricultural and climate change scientist

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Professor Peter Grace

Agricultural and climate change scientist

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Honorary Professor Snow Barlow FTSE

Agricultural scientist


IMP213-spreads-1

IMPACT #213 — A tech powered, human driven future

This issue of IMPACT considers what a tech powered, human driven future for Australia might look like. With 60,000+ years of science, Australia is driven by a long culture of innovation and Professor Marcia Langton discusses how incorporating Indigenous knowledge can and should inform our society. Professor Lachlan Blackhall and Professor John Söderbaum examine Australia’s energy future. Professor Andrew Parfitt and Professor Genevieve Bell consider the next generation of custodians of innovation – technologists and systems thinkers. Professor Emma Johnston reflects on ‘riskiest’ stages of research commercialisation and the spirit of experimentation. Expert groups also consider the potential of both AI and soil carbon to shape Australia’s economic, social, and environmental future.

See the rest of the IMPACT #213