How much carbon can Western Australian soils sequester?

The potential for carbon sequestration in soils depends on several factors, including soil type, management practices, and climatic conditions. Generally, soils can sequester carbon through the process of incorporating plant residues and other organic matter into the soil. This carbon, derived from carbon dioxide absorbed by plants during photosynthesis, can be stored in the soil as organic carbon.

Research indicates that the capacity of soils to sequester carbon varies widely. For instance, according to the Australian Government’s Department of Agriculture, Water and the Environment, changes in land management can lead to an increase in soil carbon stocks. Techniques such as reduced tillage, cover cropping, and the application of organic amendments can significantly enhance this capacity.

Specifically, in the context of Western Australia, the sandy soils predominant in the region generally have lower organic carbon levels than heavier soils. However, with appropriate management practices, even these soils can potentially increase their carbon content.

Actively sequestering carbon in soils is a fairly new undertaking in Western Australia and there is very limited data available to help evaluate what the sequestration potential is in local climatic conditions.

Following is a summary of the data used by Carbon Sync to determine what carbon sequestration rates could be attained under the new management practices associated with the soil carbon projects to be developed.

What do we know?

  • Soils under native vegetation in the drier parts of South-West Western Australia are inherently low in soil organic carbon (SOC) (Sudmeyer et al, 2014)

  • Soil type (clay content, bulk density, depth, mineralogy) determines the potential for SOC storage capacity, while climate (rainfall, temperature, solar radiation) determines attainable organic carbon storage (Carson, nd; Sudmeyer et al., 2014)

  • Actual SOC stocks and sequestration rate are both highly influenced by land use and management practices (Banks, 2019; Sanjari, 2010; Sudmeyer et al., 2014; Carson, nd; Murphy et al., 2023)

  • Rapid and deep soil changes can be initiated simply through changing plant root architecture and thence water penetration to soil (Banks, 2019)

  • Regardless of soil type and climate, SOC is generally lowest (by more than 30%) in cropping systems compared to other land uses (Banks, 2019)

What has been measured in Western Australia?

  • Soil organic carbon stocks have been measured at more than 1000 sites across the Western Australian agricultural region, covering a wide range of land uses, agricultural practices, and soil types (mainly deep sands). These measurements show that SOC can vary enormously in similar climatic conditions according to land uses and management practices. For example, carbon stock at 0.3m depth ranged from 3.4 t C/ha to 231.3t C/ha in a Tenosol (Murphy et al., 2023). Considering an average bulk density of 1.3t/m3, this represents a soil organic carbon percentage ranging from 0.09% to 5.9%.

  • Measured annual rates of change are highly variable:

    • less than 0.1t C/ha/year (depth not stated) according to Minasny et al., 2017

    • 0.1 to 0.4t C/ha/year at 0.3m (0.0025% to 0.01% per year) according to Murphy et al., 2023

    • 0.44t C/ha/year at 0.125m (0.03% per year) on average according to Barrow, 1969

    • up to 0.6t C/ha/year at 0.3m (0.015% per year) according to Sanderman et al., 2010

    • 0.9t C/ha/year at 0.9m (0.01% per year) according to Wocheslander et al., 2016

Have ACCUs been generated in other parts of Australia?

Yes, several projects in the Eastern States have been issued with ACCUs. You can read more about those projects and their results here.

Discussion

The available data come with a number of limitations including:

  • Most research programs only measure carbon at 0.3m depth in the soil. However, there is strong evidence that new carbon is also sequestered deeper in the soil profile. For example, in a first at-scale soil carbon project in Queensland and New South Wales, 44% of new carbon was sequestered below 0.3m (Carbonlink, 2023). Note that carbon credits (ACCUs) can be issued for carbon sequestration measured down to 1m deep;

  • Most research programs compare a conventional system to a similar system with one changed management practice (e.g., conventional tillage vs reduced or no till in a cropping system), or two land use practices (e.g., cropping vs grazing); and

  • There is no known research program in Western Australia assessing the compounded benefits generated by multiple simultaneous and complementary changes to management practices such as those that could be induced through a Holistic Management approach.

Carbon Sync projects will use the Holistic Management framework and devise a suite of management and practice changes adapted to the context of each farm involved. The combination of all these changes is expected to result in higher carbon sequestration rates compared to those reported so far in Western Australia.

However, Carbon Sync exercises caution by using conservative figures in its projections. Therefore, taking into account the available data and the above limitations, the Return On Investment (ROI) to be presented to prospective soil carbon farmers is based on a low expectation of a 0.01% increase in soil organic carbon per year at 0.4m depth (although we intend to measure down to 1m).

Although it may sound like an insignificant change, a 0.01% increase in soil carbon is equivalent to 0.52t C/ha at 0.4m depth (assuming a soil bulk density of 1.3g/cm3). For the first 5 year term, this equates, after accounting for all applicable discounts, to 1.07t CO2e/ha/year or 1.07 ACCU/ha/year.

Note

It’s important to note that while soil carbon sequestration is a promising tool for mitigating climate change, the actual amount of carbon that can be sequestered will vary greatly depending on site-specific factors and must be estimated with detailed soil and management data.

Further Information

For a detailed understanding of how these practices can be applied to your specific situation, consulting with experts and using local soil data is recommended. Carbon Sync offers a free, no-obligation on-farm consultation to discuss the potential for your land.