Carbon Storage – Albany Sand Plain

• 
  Actual organic carbon storage (0–30 cm) ranged from 29 t C/ha under cropping in the low rainfall zone to 93 t C/ha under pasture in the high rainfall zone.


• Soils had lower organic carbon storage than the long term attainable organic carbon storage (or upper limit) predicted by computer modelling.


• Cropping sites reached 40–55 % of the attainable organic carbon storage values while pasture sites were 55–89 % of the attainable organic carbon storage values.


• The average organic carbon storage under perennial pasture was greater than annual pasture.

 

Soil Carbon Research Program – Australia’s Farming Future

Sustainable management of soil, in particular organic carbon, is essential for the continued viability of Australian agriculture. Increasing the organic carbon retained in soil (also known as sequestration) improves soil quality and can also help to reduce atmospheric carbon dioxide. The Soil Carbon Research Program is working to identify land uses and management practices that growers can use to increase soil organic carbon storage and improve production in a changing climate.

 

Organic carbon storage in soil

The amount of organic carbon that soil can store varies. This is due to:

• Clay – As the clay content of soil increases, it increases the potential organic carbon storage of the soil.


• Climate – When the climate allows greater plant productivity, it increases the attainable organic carbon storage of the soil towards the potential organic carbon storage.


• Management – Growers can influence whether or not their actual organic carbon storage is as high as the attainable organic carbon storage. For more information see the fact sheet How much organic carbon can soil store?.

 

Albany sand plain

The Albany sand plain, located in the South-West agricultural production zone of Western Australia, comprises a level to very gently undulating landscape. The soils range from deep sands through to shallow, often gravelly, duplexes (sand over clay) (figure 1). Dominant land uses include continuous cropping, mixed cropping in rotation with annual pasture, permanent annual pasture (with the livestock enterprise being mainly sheep based), permanent perennial pasture and commercial forestry plantations. Closer to the coast, beef production dominates with most producers using a perennial pasture base (kikuyu) to cover the ‘summer to autumn feed gap’.

 

Figure 1: Dominant soil types: shallow duplex (left profile), representing 48 % of the land area and deep sand (right profile), representing 28 % of the land area. The scale is the same for both profiles.

Soil samples were collected from the mid-west area of the Albany sand plain over two rainfall zones (450–550 mm and 550–650 mm) from an area of approximately 137,000 hectares—a total of 261 sites (figure 2). Actual organic carbon storage was measured to a depth of 30 cm. The attainable organic carbon storage for the soil types and climate on the Albany sand plain was estimated using computer modelling. This was compared to actual organic carbon storage from the reference sites.

 

Figure 2: Satellite image of the Albany sand plain showing the soil organic carbon sample sites. The sample area covered two rainfall zones (450–550 mm in the north and 550– 650 mm in the south) and is approximately 50 km from east to west and 45 km from north to south.

 

Was the attainable organic carbon storage achieved?

Actual organic carbon storage in the 0–30 cm layer was less than attainable organic carbon storage predicted by modelling (figure 3, example for deep sand). Within the lower rainfall cropping zone the actual organic carbon storage on the deep sand was 29 t C/ha under continuous cropping (40 % of attainable organic carbon) and 39 t C/ha under mixed cropping (50 % of attainable organic carbon).

In the higher rainfall zone actual organic carbon storage on the deep sand was 61 t C/ha under annual pasture (55 % of attainable organic carbon) compared to 93 t C/ha under perennial pasture (77% of attainable organic carbon).

Perennial pastures have longer survival, deeper rooting and greater plant biomass during summer. As a result, it has been proposed that perennial pastures may store more soil organic carbon than annual pastures. This was the case for the Albany sand plain on the deep sand (figure 3); and to a lesser extent on the sandy duplex soil where the actual organic carbon storage was 83 t C/ha under annual pasture compared to 91 t C/ha under perennial pasture.

 

Figure 3: The actual organic carbon storage (orange bars) compared to the attainable organic carbon storage (orange + white bars) for the major land use categories on the deep sand in the Albany sand plain region.

Cropping occurs predominately in the lower rainfall zone (450–550 mm) in the north while the pastures are located closer to the coast in the south where the rainfall is higher (550–650 mm). The average clay content of the deep sands (0–30 cm) was 1.8 %. The attainable organic carbon storage limit was modelled using this clay content and maximal water use efficiency, growing period and plant residue return for each land use.

 

Achieving the attainable organic carbon storage requires maximum plant growth and maximum returns (plant residues and manures) in any farming system. Therefore, to increase organic carbon storage in soil, it is important that management practices remove any constraints to plant growth, where it is cost effective to do so. For example, claying is commonly practiced on the Albany sand plain to overcome water repellency. The aim is to increase the clay content in the surface soil to at least 5 %—applications of 100–300 t/ha will be needed. Because the clay physically protects organic material from microbial breakdown, claying water repellent sands has the potential to increase organic carbon storage through increased biomass production and increased organic material content. For more information, see the fact sheet Water repellence.

 

Authors: Daniel Murphy and Andrew Wherrett (The University of Western Australia), Tim Overheu and Karen Holmes (Department of Agriculture, Western Australia).

 

This project is supported by funding from the Australian Government Department of Agriculture, Fisheries and Forestry under its Climate Change Research Program and the Grains Research and Development Corporation (GRDC).

The participating organisations accept no liability whatsoever by reason of negligence or otherwise arising from the use or release of this information or any part of it.