Austrilia土地盐化背景文章

Surprisingly in such a dry continent as Australia, salinity occurs when there is too much water. To understand why salinisation occurs, you need to know about ground water.

Ground water rising
                

Ground water is, as the name implies, water in the ground. Usually, somewhere below the surface of the soil, the soil is saturated with water. It is not quite like an underground lake – the water is most commonly held within the soil profile rather than in some vast underground cavern. The top surface of the ground water layer is called the water table.

Ground water recharge is the amount of water being added to the ground water. If this is higher than discharge, which is the amount of water lost from the ground water, then the water table rises. As it does, the water dissolves salt held in the soil profile, and the salt becomes more and more concentrated as the water moves upwards. If the salty water keeps rising, it eventually reaches the surface and subsurface layers of the soil. The water evaporates, leaving the salt behind.

Why is the ground water rising?
                

In our quest to prepare Australian soils for agriculture, we cleared trees by the billion. Yet trees played a crucial role in maintaining the water balance in our ancient soil profiles. It was our success in clearing trees that has led to the development of dryland salinity. (Irrigated-land salinity is caused by a similar effect – the application of excess water to land causes the water table to rise. The problem is made worse if the irrigation water itself is also saline.)

Trees help control ground water levels in two ways: by decreasing recharge and by increasing discharge.

• Decreasing recharge. Most ground water recharge is supplied by rainfall (except in the case of irrigated-land salinity), and more of it reaches the ground water when trees are cleared. This is because trees develop extensive root systems to trap the water, which is then used for tree growth or returned to the atmosphere by evaporation and transpiration. Scientists estimate that the amount of water that percolates below the root zone of crops and pastures can be 10 to 100 times the amount percolating below trees.

• Increasing discharge. Trees also play a role in discharging ground water. The roots of many Australian tree species reach down deep into the ground, often making contact with the water table. During drought, such trees use this water in order to survive and keep growing. In contrast, annual crops that farmers plant on cleared land usually don't have deep roots. Moreover, many crops only grow during winter, spring and early summer while most Australian trees require water all year round.

Trees: Weapons against salt?
            

If salinisation is caused by the removal of trees from the landscape, it seems logical that putting them back will solve the problem. Farmers throughout the country, including those in zones most affected by salinisation, have embarked on a massive tree-planting campaign, giving hope that the rural landscape will recover from its many ailments, including salinisation. The ability of trees to reduce salinisation is still not fully known, although they have been shown to lower water tables in some areas. Australian scientists continue to investigate the potential of trees to reclaim saline areas and to prevent currently unaffected land from becoming salty.

In the meantime, programs such as the Joint Venture Agroforestry Program have produced guidelines to ensure that any trees planted have the maximum positive benefit. For example, healthy, highly productive trees will be more effective than less productive trees in lowering water tables because they will use more water. Some farmers may be tempted to plant trees on the salty areas, but unless these are specially adapted to saline soils they may not grow well and therefore not play much of a role in solving the salt problem. In some situations, trees planted higher up in the catchment, in areas of high recharge, may be more effective. And, in general, the more trees planted, the more impact they will have on water table levels.

Other weapons against salt
                

Tree-planting is just one of many strategies that show promise in the fight against salinity. For example, deep-rooted perennial crops such as lucerne lower water tables and may often be a viable alternative to trees. In places where soils are likely to remain saline for some time, salt-tolerant species such as saltbush – which can be eaten by sheep – have had some success. And scientists and agriculturalists are working to enhance the salt tolerance of other plant species through breeding programs.

Innovative farmers are experimenting with other possible solutions. For example, driven by the knowledge that salt is a potentially valuable product, some farmers are pumping their saline ground water into evaporation ponds. The salt harvested from these ponds can be sold as a raw material in the production of important chemicals such as sodium carbonate and sodium hydroxide, or as table salt.

Related sites
                

• Salinity – our silent disaster (The Slab, Australian Broadcasting Corporation)

• Australian Broadcasting Corporation (transcripts)

• The curse of the salt in Western Australia (Earthbeat, 27 October 2001)

• Hidden heart of Australia (Ockham's Razor, 23 July 2000)

• Overcoming salt (Ockham's Razor, 22 March 1998)

• Trees, water and salt (Rural Industries Research and Development Corporation, Australia)

• Soil salinity tolerance of plants for agriculture and revegetation (Department of Agriculture, Western Australia)

• Museum Victoria

• Saline solutions: Goulburn Valley, Victoria
                      

• Assault on salt: Loddon Plains, Victoria
                      

• Alley farming in Australia (Joint Venture Agroforestry Program, Rural Industries Research and Development Corporation)

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So where does the salt come from?

The sources of salt involved in dryland salinity are: rainfall (cyclic salt), rock weathering, aeolian deposits and ancient seas (connate salts).

Cyclic salts

Cyclic salts are those salts which are included in water evaporated from oceans and deposited over coastal areas. Rainwater generally contains 10 to 30mg/l of salts. Studies have shown that in Australia the rainfall input of salts is as great as 300kg/ha/yr near the coast, about 30kg/ha/yr 250km inland and about 15kg/ha/yr more than 600km inland.
                

Salts from rainfall only become a problem when there is not enough precipitation to flush the deposited salts from the soil profile via leaching. The salts will accumulate and if they are subsequently released then they will become an environmental issue.

Connate salts

Connate salt is salt incorporated in marine sediments at the time of deposition. The sediments were deposited by inland seas millions of years ago that naturally contained large quantities of salts. For example, the Wiannamatta Shale Group of the Sydney
                    Basin were deposited by a retreating ocean (marine regression) during the Tertiary. These shales contain salt from seawater trapped during the time of deposition. These shales provide the salts responsible for salinity outbreaks in Western Sydney.

Continued weathering of rocks will release salts. This is generally not a source for large quantities of salt because large amounts of water are required for weathering reactions to occur. This means that if there is a continual flux of water salts will be flushed through the system rather than accumulating.

The Australian continent is geologically very old and so has had a long time for salts to naturally accumulate in the landscape. During this time the landscape has been affected by climatic oscillations (change) between glacial and interglacial conditions. During periods of aridity, winds were capable of blowing salt laden dust from western New South Wales to the east. One such period was the Pleistocene, when the climate was significantly more arid than today and strong easterly winds carried significant quantities of this salt laden Aeolian dust from the Murray
                    Darling
                    Basin and deposited it on the Southern Tablelands of New South Wales. It is the mobilisation of these salts incorporated in debris flows that is today the major source of salt for the dryland salinity occurrences on the Southern Tablelands.