Dr Romy Zyngier outlines the Australian debate on the role that soil carbon capture and storage can and should play in mitigating climate change and in offsetting emissions. This is a current and hot debate among farmers, scientists, environmental groups and policy makers with many seeing the opportunity to enhance soil health (and productivity). However, the overall potential of Australian soils to capture carbon is limited, and soils are under increasing risk of becoming an emissions source rather than a carbon sink under rising temperatures.

Soil organic carbon content is a key indicator of soil health. It supports ecosystem processes such as water storage capacity and availability to plants, nutrient and waste cycling, soil structure and biodiversity. Healthy soil underpins food and fibre production. Soil carbon is highly complex, and depending on how land is managed, it can be a significant source or sink for greenhouse gases.

Australian soils are some of the oldest in the world – through natural processes over time they have been depleted of carbon and are poor in nutrients. Land clearing for agriculture across Australia has been the primary driver of soil carbon loss since European settlement. Typically, the conversion of native vegetation to agriculture reduces soil organic carbon by 20–70 per cent. It has been estimated that approximately half of the original soil carbon has been lost in Australian agricultural soils. Uncertainty remains, especially within Australia, over the potential of agricultural soils to store carbon, the rate of carbon accumulation in soil and the permanence of captured carbon. 

Under the hotter drier conditions predicted in various global warming scenarios, Australian soils are at risk of becoming a carbon emitter rather than a sink, with this risk multiplied by the impacts of land use change and land clearing. The sixth assessment report from the IPCC highlighted that land based carbon sequestration will be significantly less effective as temperatures continue to rise and rainfall decreases in many areas of the globe. A recent Australian study has modelled the response of soil organic carbon to future climate change across Australia. Although results are highly nuanced, the general modelled trend across native and managed land indicates greater loss of soil organic carbon with rising temperatures (1.5–5°C). 

To support a fuller understanding of the potential for soil carbon sequestration, ClimateWorks’ Land Use Futures program is undertaking high spatial resolution modelling of carbon held in trees and soil under different agricultural land management practices and reforestation over time. The modelling will provide insights on carbon capture and storage potential for the food and land use system, and support recommendations for transitioning to a sustainable net zero emissions sector.

There’s a lot of focus on soil carbon right now

Soil carbon or ‘carbon farming’ is frequently proposed as a nature-based climate solution to support countries in meeting their commitments under the Paris Agreement. Soil carbon and the role of agriculture in climate change mitigation and emission reductions was highlighted in the Land Use Futures ‘briefing room’ webinar hosted by ClimateWorks Australia in July 2021. Soil carbon has been in the spotlight since the Federal government’s Technology Investment Roadmap identified soil carbon as one of the five tools to reduce emissions from agriculture and offset emissions from other sectors. It has also featured strongly in the Federal government’s net zero by 2050 announcement, suggesting that soil carbon can support Australia getting 40 per cent of the way to net zero

The Emissions Reduction Fund (ERF) offers Australia farmers and land managers incentives for emissions reduction and carbon sequestration (capture and storage of carbon) projects, and to earn Australian Carbon Credit Units (ACCUs) for emissions reductions. The National Soils Strategy supports the prioritisation of soil health including the improvement and maintenance of soil organic carbon. However, the measurement of soil carbon is labour and time intensive, and as such, costly. This makes soil carbon offsets out of reach for many farmers under the ERF. However, cost is not the only barrier: reliable, adoptable and profitable carbon sequestration methods on farms have also been identified as limiting factors for Australian farming systems. The role of agriculture and the responsibility of farmers to offset emissions from other sectors continues to spark debate. Many farmers and researchers are calling for a fair approach to carbon offsetting schemes and valuing carbon already captured on properties. 

Soil carbon can be viewed as part of a greater suite of nature-based climate solutions that can simultaneously deliver natural capital benefits, environmental outcomes and diversify livelihoods. ClimateWorks Australia’s Natural Capital Investment Initiative is working to deliver a resource that supports farmers and land managers to measure and monitor natural capital, with soil carbon as one of the many measures. Building soil carbon is also a key tenet of agro-ecology and regenerative agricultural practices, which are increasingly part of the discussion around sustainable farming practices.

There is both opportunity and uncertainty in the role of soil to sequester and store carbon for Australia in the near and long term. Rather than focusing on silver bullet solutions, we need a rapid transformation driven by a range of interventions from across the economy to achieve net zero.

Stay in touch: the next Food and Land Use systems article in this series will be published in two weeks. We hope you find it informative and useful – please share, discuss and offer your feedback. Visit our Food, Land and Oceans page to read the latest articles

The Land Use Futures program partners with Deakin University and the CSIRO.