Soil salinization is a serious and difficult to reverse form of soil degradation. Salinization occurs when dissolved salts in water tables rise to the soil surface and accumulate as water evaporates. Often rise in a water table is due to the replacement of deep-rooted vegetation, such as trees, with shallower rooted vegetation, such as grasses. Application of irrigation water or heavy rainfall can also cause water tables to rise. Topsoil salts can greatly reduce agricultural productivity, erode infrastructure, and impose long-term limitations on land productivity. Soils containing high levels of salts are much more likely to experience this regime shift.
Key direct drivers
- Vegetation conversion and habitat fragmentation
- Harvest and resource consumption
- External inputs (eg fertilizers)
- Environmental shocks (eg floods)
- Large-scale commercial crop cultivation
- Extensive livestock production (rangelands)
Key Ecosystem Processes
- Soil formation
- Primary production
- Nutrient cycling
- Water cycling
- Food crops
- Wild animal and plant products
- Fuel and fiber crops
- Wild animal and plant foods
- Water purification
- Water regulation
- Regulation of soil erosion
- Aesthetic values
- Food and nutrition
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
Typical spatial scale
Typical time scale
- Irreversible (on 100 year time scale)
- Contemporary observations
Confidence: Existence of RS
- Well established – Wide agreement in the literature that the RS exists
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Links to other regime shifts
- Forest to Cropland
- Bush Encroachment
- Dryland degradation
Cultivated drylands are especially vulnerable to soil salinization, especially regions with variable rainfall. Evaporation of water following floods accumulates salts at the soil surface, and dry-spells prevent salts from being flushed back down into the water table. Older soils that contain great concentrations of salts are more prone to salinization. Dryland salinization occurs worldwide, but the greatest region is in Australia, in dryland areas with very old, salt rich soils.
The nonsaline topsoil regime is characterized by landscapes with normal topsoil salt levels, deep water tables, and healthy plant communities. Rainfall leaches high salt concentrations from topsoil to below the root zone. In cultivated areas, crop water-uptake is unhindered and crop growth is normal. Land produces high yields, supporting livelihoods and providing food and nutrition. Freshwater is not saline, supporting biodiversity and providing clean water (Anderies et al. 2006).
The saline topsoil regime is characterized by elevated water tables, significantly higher than normal soil salt levels and reduced plant growth across the landscape. High concentrations of salt in the topsoil reduce the uptake of water by plants and impede nutrient absorption. Some salts may also be toxic to plants when present in high concentrations, inhibiting plant growth. Under extreme salinization, a white crust of salt accumulates at the soil surface, and only salt tolerant plants are able to grow. Persistent high water tables change the hydrology of local aquifers considerably. Surface water becomes brackish, harmful to wildlife and unsuitable for irrigation. In cultivated regions, crop yields are restricted, threatening livelihoods and reducing food and nutrition. High levels of salt in freshwater can make water non-drinkable and harm wildlife and corrode water infrastructure, roads, and bridges (Neilsen et al. 2003; Pannell 2002).
Drivers and causes of the regime shift
The main direct driver that leads to the shift from normal to saline topsoil is elevation of dissolved salts within the water table to the soil surface. Typically, this rise in groundwater is due to a decrease in evapotranspiration due to the removal of deep-rooted perennial plants, such as trees. The water table can also rise due to intensive use of irrigation. Agriculture is the major indirect driver of both deforestation and irrigation (Anderies 2005).
How the regime shift works
Shift from Nonsaline to Saline Soil
Arid landscapes with normal topsoil salt levels have diverse communities of perennial plants that have evolved deep roots to survive periods of drought. These plants continually use and transpire water as it becomes available, so that little water infiltrates to the ground water. Where cultivation occurs, drought tolerant crops are grown and irrigation is not part of agricultural practice. The presence of deep-rooted native vegetation maintains the normal regime, as they have evolved to efficiently use the limited precipitation in arid and semi-arid areas. Deep rooted plants evaporate water from the soil, prevents water tables from rising and salinization of the root soil profile. The lack of salt in the soil profile in turn enables the persistence of deep-rooted vegetation. Crop plants that are grown are adapted to arid climates and are able to efficiently use available water. A social feedback that maintains this regime is the reproduction of agricultural and land management practices that maintain deep rooted plants and do not use irrigation (Pannell 2002; Anderies et al. 2006).
If deep-rooted plants are removed and replaced with shallow rooted annual crops, the regulation of the water table by deep-rooted plants is removed. However, the consequences of this removal may not become visible for decades as the water table rises slowly. The changes in water flow through a dryland ecosystem due to land use change are less visible, but can be many times larger than those due to irrigation (Gordon et al. 2003). The hydrology of the system can be further disrupted by the application of irrigation water, especially if this water is from outside the watershed. Crop irrigation exacerbates the risk of salinization not only because it contributes to the rise of groundwater, but also because the irrigation water can contain salts that leach from the soil and then are concentrated at the soil surface when the water evaporates. Once saline groundwater enters the root zone, plants quickly move the water and the salts it contains to the soil surface.
Salts continue to be deposited to the soil surface as long as the water table remains in the root zone. Salinity impedes the ability of plants to use water, which reduces growth rate. At higher levels of salinity plants are damaged, older leaves die and photosynthetic leaf area is reduced, until plants die (Munns 2002). If salts are not flushed below the root zone, land can becomes unproductive for the long term. A social feedback that can maintain the saline regime is an agricultural dependence on irrigation. Producing crops for large-scale markets often requires for continued intensive production to be viable. Investments in irrigation equipment can become a sunk cost, as their expense necessitates their continued use. As salt accumulate in the soil profile, producers become further dependent on irrigation to flush salts below the root zone. This agricultural system maintains the knowledge, infrastructure, and resources to conduct irrigated agriculture, but does not support other approaches to agriculture (Allison and Hobbs 2004).
Impacts on ecosystem services and human well-being
No ecosystem services are gained in this regime shift. Where crops are grown, the salinization regime shift negatively impacts ecosystem services such as food production, livestock feed, protection from soil erosion, human nutrition, livelihoods and economic activity. Freshwater becomes contaminated with salt, which can reduce the availability of drinking water, reduce fish and other aquatic populations, and damage infrastructure. In saline soil biodiversity is reduced as only salt-tolerant species can live in saline soil. Availability of wild plant and animal products are also reduced. Farmers lose the security of their livelihoods due to lost crop yield and degraded soil and water.
Hunters and outdoor recreation seekers lose due to a loss of wildlife. The general public loses availability of high quality, low cost good, as well as clean water. Salinized soil can benefit people who mitigate and adapt to salinization, such as desalinization technology providers and salt tolerant crop breeders.
Management to prevent soil salinization involves maintaining a mix of deep-rooted perennial vegetation and crops in order to prevent the rise of the water table, and limiting the amount of irrigation water that is applied to the system. However, the management of dryland salinity is complicated by the difficulty of understand groundwater dynamics and the long delays (decades to centuries) between action and response in some dryland groundwater systems. Long-term methods to keep the groundwater level below the root-zone include planting of deep-rooted vegetation and salt tolerant plants. However, while planting the halt the rise of a water table it will likely take decades to lower it. Apart from the direct effects of lowering the water table and reducing the salt concentration in the topsoil, if the planting of deep-rooted vegetation can be linked to economically beneficial activities this strategy can contribute to increasing the diversity of the agricultural system. This may improve soil health and make the ecosystem less vulnerable to disturbances (Anderies 2005).
Once the root zone has become saline, there are several short-term management options for removing the accumulated salts or preventing further salt accumulation. These include mechanically scraping surface salt (which leads to the problem of salt disposal), or flushing the topsoil using water (which has poor efficacy and might exacerbate the problem in situations with high water tables). A more efficient option is to create a surface water drainage system using field ditches to avoid the deposition of salt, combined with subsurface water pumping to decrease the water table level (Abrol et al. 1988). Another approach, used in Australia, is to pump groundwater to lower the water table. The expenses arising from the implementation and maintenance of such drainage or pumping systems are, however, substantial (Anderies 2005).Social systems also offer great potential for managing soil salinity. Water pricing systems, long-term tenancy of the land, use of appropriate technology and farmer's education can contribute significantly towards the goal of maintaining productive land. However, the substantial social resistance of the established irrigation regime can block many attempts at reform (Allison & Hobbs 2004, Anderies et al. 2006)
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