WA Mallee Sandplain Regional Summary

The region

The WA Mallee Sandplain agroecological zone combines the WA Mallee and WA Sandplain AEZ. The eastern extent of the combined region is Forrest near the SA border, the zone extends west to near Norseman and south to Esperance. Rainfall in the zone ranges from > 700 mm annum^-1 near the coast to < 400 mm annum^-1 for inland areas of the region. Rainfall seasonality is winter-dominant. Boundaries for the zone are shown in Figure 1.

Modelling regional practices

Producing models that reflect all farming practices throughout a region is difficult so “common practice” models for wheat, canola and field peas for the region were produced. A wheat crop with a 2.2 t ha^-1 yield and 53 kg of fertiliser N applied ha ^-1, a canola crop with a yield of 1.2 t ha^-1 with 60 kg fertiliser N ha ^-1 applied and a field peas crop with a yield of 0.9 t ha^-1 and 0 kg of fertiliser N ha ^-1 applied were modelled. The key assumptions of these models were that;

• Only one pass was made at sowing
• Split fertiliser applications were used for wheat and canola crops
• It was assumed that wheat had three fallow herbicide applications, that canola had two and lupins had four. Wheat also had one in-crop herbicide application while canola and lupins had two in-crop herbicide applications. Canola and lupins also had one pesticide application.
• Stubble was retained
• Lime was applied at the equivalent rate of 2 t ha^-1 every decade and incorporated through scarification

Impact indicators

Hydrogen ion impacts estimate the release of hydrogen ions to the soil associated with crop production. A negative value indicates a reduction in soil acidity whereas a positive value indicates an increase in soil acidity. Soil erosion is an estimate of soil loss that occurs during the production of the crop. The depth of soil lost will depend on many things but an approximate conversion is that 1.5 t of soil loss equals 1 mm of soil. Global warming impacts are the release of greenhouse gases to the atmosphere expressed as carbon dioxide equivalents (CO₂-e), Eutrophication impacts are the release of phosphorous to the environment and are expressed as phosphate equivalents and Particulate Matter impacts are the release of fine particles less than 2.3 micrometres in diameter.

Benchmark results

Results below in Table 1 show the environmental impacts of producing a t of wheat, canola or field peas in the region. Lime use in the region resulted in a reduction in free hydrogen ions in the soil for all crops and soil erosion was estimated at between 5.6 and 16.3 t soil loss t product^-1. Analysis suggests that Global warming impacts associated with the production of these crops ranged from between 226 and 753 kg CO₂-e, Eutrophication impacts ranged between 1.0 and 2.9 kg PO₄-e and Particulate Matter impacts ranged between 0.21 and 0.84 kg of < 2.3 µm particulate matter t product^-1.

Table 1: Hydrogen ion changes, soil erosion and Global Warming, Eutrophication and Particulate matter impacts associated with the production of wheat, canola and field peas in the WA Mallee Sandplain agroecological zone.

Greenhouse gas emissions

The emissions profile of all three crops are shown below in figure 2. For all crops, the greatest sources of emissions were the production and use of fertiliser. Emissions associated with the breakdown of residues were the next greatest emissions source.

Greenhouse gas mitigation strategies

Mitigation strategies tested for the region were;

• Sustainable intensification
• Additional applications of lime
• Implementation of variable rate fertiliser technology
• Changing a wheat-wheat rotation to a legume-wheat rotation

More information on the assumptions used to test these strategies and how they might reflect individual enterprises are available on the Mitigation strategies page.

Results (figure 3 below) indicate that emissions of a t of wheat can be reduced by;

• 66% through the implementation of sustainable intensification
• 18% by additional lime applications
• 14% by implementation of variable rate fertiliser technology

Results also indicate the replacing a wheat crop with a legume crop in a two-crop rotation can increase greenhouse gas emissions intensity by 103%.

More information

Dr Aaron Simmons
Orange Agricultural Institute
1447 Forest Road
Orange NSW 2800
P: 02 63913894
E: aaron.simmons@dpi.nsw.gov.au