In this chapter, we use the hierarchical stratification of Australia's landforms from the Australian Soil Resource Information System (ASRIS).a The ASRIS mapping hierarchy divides Australia into three physiographic divisions, which are further subdivided into 23 provinces and 220 regions. These broadscale mapping units have similar geological origins and a characteristic suite of soils and landforms. Even then, a diversity of soils and land management systems often occurs within each region. It is therefore only possible to reach general conclusions about the state of the soil for each region—there are always local exceptions.
Many physical, chemical and biological processes occur in soils, and they operate at different rates across the landscape according to the climate, land use and soil type. The following processes are key indicators of soil condition:
- carbon dynamics
- soil erosion.
These processes have major environmental and economic consequences for Australia. Other aspects of soil change are also important, but they are dwarfed by the significance of these three.
A fourth key process affecting soil condition in Australia is dryland salinity. A large proportion of Australia's agriculture is undertaken in areas with a rainfall of 450–800 millimetres per year. In their natural condition, these landscapes had minimal deep drainage (generally less than 20 millimetres per year), and natural stores of salt brought in by rain and dust had accumulated in the soil in many regions. The removal of native vegetation changes the hydrological cycle, because trees and shrubs intercept significant quantities of rain—often 10–20% of rainfall fails to reach the soil surface. When vegetation is removed, more water either infiltrates or runs off the surface. If the original vegetation has been replaced by more shallow-rooted species that use less water (e.g. annual crops and pastures), even more water passes through the soil. This may lead to rising groundwater levels and, in some cases, dryland salinity.
Dryland salinity has been one of Australia's most costly forms of land degradation. The assessment by the National Land & Water Resources Audit (NLWRA)20 provides a comprehensive overview. Assuming no changes in water imbalance, the NLWRA expected dryland salinity to increase from 5.7 million hectares to 17 million hectares by 2050. However, the millennium drought appears to have halted the spread of dryland salinity in most of the worst affected regions, especially in south-west Western Australia. Chapter 4: Inland water, Box 4.3 summarises the current status of salinity and its impact on inland waters.
The outlook outlined by the NLWRA20 will need further moderation if current projections are correct for a drying of southern Australia. However, the long-term outlook for more recently cleared land in the northern Murray–Darling Basin and central Queensland is unclear. Large areas are yet to reach a new hydrological equilibrium after clearing.
Given the effects of drought over the reporting period, this report does not provide an update on previous SoE reports or the NLWRA assessment20 regarding salinity. However, close surveillance of groundwater systems is essential, particularly in regions that returned to wetter conditions in 2010–11. A key requirement for understanding the state of dryland salinity in Australia will be to maintain the groundwater monitoring network established under the National Action Plan for Salinity and Water Quality.
The management and monitoring of soil carbon has become a matter of national and international importance. Soil carbon can be a significant source or sink for GHGs depending on how land is managed. The management of soil carbon is also central to maintaining soil health and ensuring global food security.21 However, there are complex trade-offs between reducing GHG emissions and producing food.
At the global scale, the amount of carbon contained in terrestrial vegetation (550±100 petagrams [1 billion tonnes]—Pg) is of a similar order to that in the atmosphere (800 Pg). However, the organic matter in soils is two to three times this amount. Approximately 1500–2000 Pg of carbon is in the top metre, and as much as 2300 Pg in the top three metres.22
The carbon content of soil is a key indicator of its health, and is a master variable that controls many processes (e.g. nutrient cycling, development of soil structure, water storage). The carbon derives primarily from plant materials created through the capture of atmospheric carbon dioxide via the process of photosynthesis. These organic materials are cycled through the soil and used by organisms as a source of energy and nutrients. A significant amount of carbon dioxide is returned to the atmosphere as a result of respiration. Increasing soil organic carbon leads to an increase in:
- energy supply for biological processes
- direct nutrient supply to plants (particularly nitrogen, phosphorus and sulfur)
- capacity to retain and exchange nutrients
- aggregation of soil particles and stability of soil structure
- water storage and availability to plants
- beneficial thermal properties
- pH buffering (helping to maintain acidity at a constant level).
The maximum equilibrium carbon content for a soil at a given location is determined by environmental factors such as rainfall, evaporation, solar radiation and temperature. For example, soil carbon content is generally larger in cool, wet environments. A lack of nutrients and a limited capacity to store and supply water in a soil can reduce this potential maximum, as can other constraints to plant growth (e.g. toxicities). Within these constraints, the actual amount of organic carbon contained in a soil will be determined by the balance between carbon inputs and losses, which are strongly influenced by land management and soil type. Agricultural practices that alter rates of carbon input (e.g. plant residues, compost, mulch) or loss (e.g. removal of crops, cultivation) change the stock of soil organic carbon.
Soil carbon stocks in Australia
Soil carbon stocks are low in many Australian agricultural systems. Conversion from native vegetation to agriculture typically reduces soil carbon by 20–70%. 23-24 This reduction is often associated with declining soil health and significant emissions of GHGs. It is generally acknowledged that more conservative forms of land management can increase soil carbon stocks, and have a significant impact on national and global emissions. This opportunity is the motivation behind many schemes around the world that aim to restore soil carbon stocks, including the Australian Government's Carbon Farming Initiative. 15,25
Carbon resilience and land management
There are different types of carbon in soils. It is useful to recognise three primary fractions:26
- particulate organic carbon (POC): organic carbon associated with particles larger than 0.05 millimetres (excluding charcoal carbon)
- humus organic carbon (HUM): organic carbon associated with particles smaller than 0.05 millimetres (excluding charcoal carbon)
- resistant organic carbon (ROC): organic carbon found in the soil particles smaller than 2 millimetres having a chemical structure similar to charcoal.
The three primary fractions have contrasting dynamics. POC can be readily increased in a soil, but it also breaks down quickly. In contrast, ROC takes much longer to increase unless it is added via an amendment such as biochar.
The relative amount of each fraction in a soil determines the resilience of the soil's carbon stocks. Figure 5.3 shows a typical pattern of soil carbon loss for each fraction after conversion from native vegetation to a cropping system in southern Australia. The system is converted back to pasture after 33 years of cropping, to restore soil carbon. This results in a quick increase in POC, but only a small increase in HUM and ROC. After 10 years, the pasture is converted back to a cropping system (year 43), and it experiences a quick decline. It only takes 9 years to return to the pre-pasture low point. In the first period, this amount of decline took 18 years (years 15 to 33) because the soil had proportionally more HUM. The key message is that, even under improved systems of land management, carbon stocks can be less resilient than those developed over long periods under native vegetation.