A major land degradation process
Soil salinization and alkalinisation are major threats to the soil resource globally and are among the most common land degradation processes. This is especially true in drylands due to their erratic rainfall, high evapotranspiration rates and the wide presence of soluble salts. Accumulation of salts in soils is a natural process. In basins with no outlet, runoff from surrounding terrain and the salts it carries collects and evaporates, leaving salts behind which create saline and sodic soils that are harmful to plants. Salinization can also be human induced (secondary salinization) through irrigation that sustains crop production in much of the world’s drylands. Here, evaporation of irrigation water carrying dissolved solids may lead to accumulation of salts in the root zone. If sufficient water is applied, salts are carried beyond the root zone (leaching). When leaching is insufficient, salts accumulate in the root zone (salinization) and reduce yields by as much as 30 %. Poor irrigation practices (i.e. insufficient water application, use of saline water, limited provision for field drainage), combined with low rainfall, high evapotranspiration rates and limited inherent soil drainage characteristics accelerates salinization.
Saline soil management
Land productivity may be limited in salt-affected areas due to the toxicity of dissolved salts for plants or due to the destruction of soil structure that then restricts water infiltration capacity. Soil salinity is site-specific. Interventions may be prohibitively expensive and must be carefully managed. Saline soils may be treated by improving sub-surface drainage and applying sufficient water to move salts out of the root zone. Sodic or alkali soils can be treated through the application of chemicalbiological amendments, such as gypsum, along with organic manure. Adoption of crop-rotation systems using salt-tolerant crops (halophytes), followed by legumes and avoiding fallow periods may improve the soil resource. For example, Quinoa (Chenopodium quinoa Willd), a salt-tolerant crop, is widely used in saline environments.
A large proportion of salt-affected soils in irrigated areas occur on small farms with limited access to resources. Many affected farmers must supplement their low on-farm income with off-farm economic activities. This potentially limits their ability to invest in many salinity management opportunities. Although salt-affected soils are of low fertility and limited agricultural value, they can provide an alternative resource for growing biomass for renewable energy.
Salinization and climate change
Soil salinization plays a role in global biogeochemical cycles, but its significance is still not as well understood, particularly with regard to different potential management opportunities (e.g. small vs largeholder farms). Soil salinization is a key regulator of plant/soil nitrogen pools and, by altering soil electric conductivity and affecting the functioning of soil microorganisms, it impacts nutrient cycling and global fluxes. Moreover, soil salinity may increase N2O emissions, but the underlying regulatory mechanisms are complex. Soil salinity generally reduces plant productivity in croplands and, consequently, soil carbon storage. Furthermore, decomposition of soil organic carbon is limited because salinity reduces soil microbial function. Capturing the effects of salinity on carbon storage and emissions in climate change models is a challenge, but to ignore it would underestimate CO2 emissions and overestimate soil organic carbon stocks. On the other hand, through irrigation, dissolved inorganic carbon is leached from the soil and deposited in saline or alkaline aquifers. This storage appears to be underestimated. Climate change-induced sea surface rise will lead to saltwater intrusion in coastal areas. Moreover, when groundwater is overused, inland aquifers can also be affected, increasing risks of soil salinity. Under climate change scenarios, salinity in drylands can increase due to higher rates of evapotranspiration of shallow groundwater.
The extent of primary (natural) salinization is estimated to be slightly under 1 billion ha. Secondary salinization occurs on around 77 Mha, of which 58 % is in irrigated areas. Fully 20 % of all irrigated areas is estimated to be salt-affected, mostly in intensively cultivated areas of India, Pakistan, China, Iraq and Iran. Regions at risk of increasing salinization are the Mediterranean Basin, Australia, Central Asia, the Middle East and Northern Africa. As discussed above, soil salinization is an issue that is conditioned by local soil, terrain and hydrological situations which make comprehensive mapping and monitoring at global scale a challenge. Recent advances in satellite-based observations have supported regional mapping that is able to differentiate between salt-affected and non-salt-affected soils on the basis of biomass productivity indicators. Although the promise of high resolution satellite imagery is immense, field measurements remain indispensable.
Salt-induced land degradation facts
The global annual cost of salt-induced land degradation in irrigated areas is estimated to be US$ 27.3 billion related to lost crop production.
The most known regions where salt-induced land degradation takes place:
- the Aral Sea Basin (Amu-Darya and Syr-Darya River Basins) in Central Asia,
- the Indo-Gangetic Basin in India,
- the Indus Basin in Pakistan,
- the Yellow River Basin in China,
- the Euphrates Basin in Syria and Iraq,
- the Murray-Darling Basin in Australia,
- the San Joaquin Valley in the United States.
Salinity in Pakistan and India
In Pakistan, 13 % of all irrigated agricultural soils were classified as strongly saline, totalling some 6.8 Mha. In India, secondary salinisation has become a serious problem. The implementation of large irrigation schemes, many without complementary drainage plans, has changed and disturbed the hydrologic balance of the fertile lands in many parts of the Uttar Pradesh state and inside the Indo-Gangetic plains. In total, 5.26 Mha in India are affected by salinisation/alkanisation.
Increased soil salinity restricts most plant growth, due to the high osmotic pressure of the soil solution that makes water uptake by plants difficult. Moreover, the phytotoxicity induced by high concentrations of soluble salts, mostly of Cl-, Na+, Mg++ and compounds of boron. Saline and sodic soils are characterised by low organic matter content, very weak soil structure development and limited soil horizon differentiation. Saline soils Classified as Solonchaks by the The World Reference Base for Soil Resources (WRB), saline soils cover an estimated area of about 260 Mha worldwide but their most extensive areas are in the arid and semi-arid regions of North Africa, the Near East, the former Soviet Union and Central Asia. They are also widespread in India, Iran, Iraq, Australia and the Americas. Sodic (Alkali) soils Classified as Solonetz by the WRB sodic soils represent some of the “worst” soils of the world and cover an estimated global area of 135 Mha. Their typical characteristic is the dominance of Na+ in the soil exchange complex, and physical and chemical properties that are unfavourable for plant growth. Sodic soils occur in steppe climate regimes with total annual precipitation between 400-500 mm mostly in poorly drained flat lands. Their major distribution occurs in Ukraine, Russian Federation, Eastern Europe, China, India, USA, Canada, Southern and Eastern Africa and Australia.