Carbon sequestration from soils

Potential environmental benefits:

• Increased soil fertility, productivity and health – through increased soil organic matter (SOM) and soil organic carbon levels.
• Improved water retention from increased SOM and therefore improved crop water use efficiency.
• Support greater diversity and productivity of soil microbes.

Potential economic benefits:

• Additional income through the Australian Carbon Credits Unit (ACCU).
• Reduced input costs, due to improved soil fertility and water retention capacity.
• Increased crop yields from improved soil productivity.

The Australian Carbon Credit Unit (ACCU) scheme supports projects that reduce gas emissions and/or sequester carbon from the atmosphere. This includes project that:

• implement changes in vegetation management to increase carbon storage
• change land use management practices to reduce/avoid greenhouse gas emissions
• use new technology or upgrade equipment to reduce energy use or emissions.

Scheme participants can earn a credit for every tonne of carbon dioxide equivalent (tCO₂-e) emissions stored or reduced by their project.

Sequestering soil carbon can provide opportunities for land managers to participate in emissions reduction projects and receive financial remuneration.

Soil organic carbon is the form of carbon used to calculate sequestration. For carbon sequestration to occur, there needs to be transfer of carbon from the atmosphere (usually via plants) to storage in the soil. Conversely though, soil can also be a source of greenhouse gas emissions when exposed to soil disturbance such as through erosion, tillage or fire.

When soil organic carbon is sequestered from, or mineralised to, atmospheric CO₂, a conversion factor of 3.67 is used to determine the change in mass due to the loss or gain of oxygen molecules. This is based on the ratio of the molecular weights of carbon (12) and carbon dioxide (44), that is: 44/12 = 3.67

Therefore 1 tonne of soil carbon = 3.67 tonnes of CO₂ (sequestered or emitted).

Several carbon accounting calculators are available for use by farmers, agronomists and land managers in different sectors. These help identify sources of emissions, model different management scenarios for reducing emissions and optimising operations.

Some example calculators are presented below:

• Greenhouse Accounting Frameworks (GAF) for Australian Primary Industries – Primary Industries Climate Challenges Centre
• Cool Farm Tool – Cool Farm Alliance
• LOOC-C - CSIRO
• Australian Wine Carbon Calculator – Australian Wine Research Institute
• Full Carbon Accounting Model (FullCAM)

Grain Growers has prepared a report, which compares two commonly used calculators (Grains Greenhouse Gas Account Framework (G-GAF) and the Cool Farm Tool) and evaluates their suitability for supporting growers.

The inherent capacity of a soil to permanently store soil organic carbon can be increased with the addition of clay and sufficient organic matter, particularly legumes, manures and nutrients to support microbes to form soil organic carbon. However, soil organic matter and carbon can also be lost through erosion. There is a limit to the amount of organic matter and carbon that soils are able to retain, based on their mineralogy and annual rainfall. 

DEW have calculated soil organic carbon benchmarks for common topsoil textures in different rainfall zones, to assist land mangers in assessing the capacity for increasing soil carbon sequestration and carbon content in their soils.

• S.O.C.R.A.T.E.S - a simple model for predicting long-term changes in soil organic carbon in terrestrial ecosystems.
• PLANR – helps you explore options for biodiversity credits (nature repair market) as well as carbon credits.