Soil organic matter in rainfed cropping systems of the Australian cereal belt

The Australian cereal belt stretches as an arc from north-eastern Australia to south-western Australia (24 degreesS-40 degreesS and 125 degreesE-147 d egreesE), with mean annual temperatures from 14 degreesC (temperate) to 26 degreesC (subtropical), and with annual rainfall ranging from 250 mm to 150 0 mm. The predominant soil types of the cereal belt include Chromosols, Kan dosols, Sodosols, and Vertosols, with significant areas of Ferrosols, Kuros ols, Podosols, and Dermosols, covering approximately 20 Mha of arable cropp ing and 21 Mha of ley pastures. Cultivation and cropping has led to a substantial loss of soil organic matt er (SOM) from the Australian cereal belt; the long-term SOM loss often exce eds 60% from the top 0-0.1 m depth after 50 years of cereal cropping. Loss of labile components of SOM such as sand-size or particulate SOM, microbial biomass, and mineralisable nitrogen has been even higher, thus resulting i n greater loss in soil productivity than that assessed from the loss of tot al SOM alone. Since SOM is heterogeneous in nature, the significance and fu nctions of its various components are ambiguous. It is essential that the r elationship between levels of total SOM or its identifiable components and the most affected soil properties be established and then quantified before the concentrations or amounts of SOM and/or its components can be used as a performance indicator. There is also a need for experimentally verifiable soil organic C pools in modelling the dynamics and management of SOM. Furt hermore, the interaction of environmental pollutants added to soil, soil mi crobial biodiversity, and SOM is poorly understood and therefore requires f urther study. Biophysically appropriate and cost-effective management pract ices for cereal cropping lands are required for restoring and maintaining o rganic matter for sustainable agriculture and restoration of degraded lands . The additional benefit of SOM restoration will be an increase in the long -term greenhouse C sink, which has the potential to reduce greenhouse emiss ions by about 50 Mt CO2 equivalents/year over a 20-year period, although cu rrent improved agricultural practices can only sequester an estimated 23% o f the potential soil C sink.