Untapped potential of soil carbon

For National Agriculture Day this year we are highlighting the work of our world-class soil scientists who are rethinking how the Earth’s skin – our living soil – can be used in the fight to control climate change.
Cover crops help resink carbon into depleted topsoils.

Cover crops help resink carbon into depleted topsoils.

Soil organic carbon, the living and once living material in our soil, is a crucial component of terrestrial ecosystems and key to controlling nutrient and water cycling. In agricultural systems, it is considered one of the main indicators of soil health, increasing resilience to events such as erosion and drought.

For National Agriculture Day, we want to highlight how Australian agriculture can tap into this natural cycle to contribute to climate change mitigation and open new income streams for farmers.

Since the establishment of the Clean Energy Regulator, the opportunity has emerged for land managers to be awarded Australian Carbon Credit Units (ACCU) for each tonne of carbon sequestered on farmland. These credits may then be traded on an open market and bought by entities looking to offset their emissions (current spot price of $16.50/ACCU).

An additional push was provided in September by Angus Taylor, Federal Minister for Energy and Emissions Reduction, who identified soil carbon as one of five priority technologies of long-term strategic importance for the government to lower emissions.

New maps: keeping track of carbon

Carbon sequestration potential in Australian croplands. Source: Jose Padarian Campusano

Carbon sequestration potential in Australian croplands. Source: Jose Padarian Campusano

For this emission trading system to function efficiently, we need to know the current level of soil organic carbon (its condition) and potential for abatement (its capacity). This has recently become possible using maps based on a large database of soil analysis curated by Australian institutions such as CSIRO. Alongside this, universities have made a big effort to rescue legacy data (stored in documents and old repositories). This information has been used to train machine learning models to create the digital maps that allow us to understand how soil organic carbon content varies across the country and also to quantify the amount of carbon stored at any location within Australia.

Using soil carbon models that take into account soil and environmental variations we estimate the current carbon storage in the topsoil of Australian croplands to be 1605 million tonnes of carbon, with a sequestration potential of 541 million tonnes, which is equivalent to 18 years of annual CO2 emissions for the Australian agricultural sector (20 percent of total national emissions).

New sensors

For this emission trading system to function efficiently we need cheap and reliable methods to quantify local changes in soil carbon. Soil is highly heterogeneous; varying widely across a landscape and also with depth. To give the market confidence that we are accurately measuring changes in soil organic carbon this variation needs to be quantified. Existing lab methods are accurate but may prove cost prohibitive, a sentiment echoed by Minister Taylor who set a goal for soil carbon measurement at $3 a hectare annually.

To bring this cost down, scientists at the Sydney Institute of Agriculture are investigating methods to quantify soil carbon in-situ using new-generation miniaturised spectrometers, which could allow farmers to quantify soil organic carbon and other properties by simply using apps on their mobile phones.

Cover crops being terminated, Narrabri Plant Breeding Institute.

Cover crops are grown not for harvest but to shelter soil and promote above and below ground biological activity, build soil carbon through symbiotic relationships with soil fungi and microbes and once terminated through the direct addition of organic matter. (above) cover crops being ‘terminated’ at L’lara, Narrabri using a roller-crimper generously provided by Auscrimper Pty Ltd..

New sequestration methods

To improve sequestration, our researchers are investigating how to increase soil organic carbon while maintaining or improving agronomic performance. Trials are being conducted on L’lara, a 2000-hectare university-owned research property just north of Narrabri.

Although the land at L’lara is highly productive, decades of cropping have left the arable soil with only 0.8 percent organic carbon, which is less than half that of surrounding pasture. To address this deficiency, we are exploring the suitability and feasibility of cover cropping to replace carbon into the soils. A number of growers in the region have adopted this practice successfully and our researchers are working with them to quantify the changes in soil organic carbon among other properties in light of their impact on yields.

What we envisage is a virtuous cycle. Farmers can gain maximum yield from highly productive land and then we can return carbon to the soil, gaining sequestration credit.

The silver lining

Increasing the carbon in our agricultural soils has the potential to provide social, environmental and economic benefits. Soils will be able to absorb and store a larger proportion of rainfall, reduce runoff and erosion, and lead to more stable and less polluted water systems. With more moisture going into our soils, yields will become more consistent and predictable. In the face of an uncertain climatic future with sporadic and heavy rainfalls; predictability will go a long way to securing our national food supply and export markets.

What’s more, storing carbon in our soils takes it out of the atmosphere. With a long-term outlook, soil carbon sequestration can contribute to the fight against global warming if considered alongside an arsenal that includes reducing overall carbon emissions.

Taking the current ACCU spot price of $16.50 (which over the last year and a half has fluctuated between $14 and $17) and our estimated sequestration capacity of 541 million tonnes of soil organic carbon (equivalent to 1487.75 million tonnes of CO2), the potential value for sequestration in our cropping topsoils could theoretically be as much as $32.7 trillion. How much of this value reserve can be unlocked over time will depend on technology used, the type of soil and climatic conditions.

But even partially tapped, this could add a considerable additional income stream to farms and regional communities. However, this will not happen in isolation; supporting farmers who are storing carbon and developing management methods that sequester carbon more quickly than we emit is the key to attaining the untapped potential of soil organic carbon and securing our national water-food-soil security.

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