The existing century is marked with global scarcity of water resources, environmental pollution and increased salinization of soil and water (1). An increasing human population and the reduction of land availability for cultivation are threats for agricultural sustainability (2). Various environmental stresses, such as high winds, extreme temperatures, soil erosion, soil salinity, drought and flood, have affected the production and cultivation of agricultural crops (3). Among these, soil salinity is one of the most devastating environmental stresses, which causes major reductions in cultivated land area, crops productivity, and harvests quality (4).
Salinity taking over farmland
Soil salinity is a term used to describe the salt content within the soil. Salt is a naturally occurring mineral within soil and water, which affects the growth and vitality of plants. Soil salinity can be influenced through several different factors, ranging from human influence to environmental causes (5). Soil salinization is the accumulation of water-soluble salts within soil layers above a certain level, which adversely affects crop production, environmental health, and economic welfare (6). Soil salinity is described and characterized in terms of the concentration and composition of the soluble salts (7).
Even though soluble salts are inherent in all soils, there are many processes that can contribute to the build-up of salts in a given soil layer (8). Weathering of soil minerals; salts added by rainfalls; agronomic practices, such as fertilizer and pesticide application; saline groundwater intrusion; irrigation with saline water; recycled or waste waters; dumping of industrial and municipal wastes into soils; and other soil conditions leading to reduced leaching of salts from the soil layers; can all lead to soil salinization (9). Seawater intrusion onto land, especially, when sea levels elevate, can deposit a large amount of salts in soils of coastal lands (10). The particular processes contributing salt, combined with the influence of other climatic, hydrological, and landscape features, as well as the effects of human activities, farming practices, and plant interactions, determine where salinization occurs (11).
Palm trees farm and Tamarix bushes on saline soils in Iran’s central desert
Water salinity is the amount of salt contained in the water. It is also called the “salt concentration” and may be expressed in grams of salt per liter of water (grams/liter or g/l), or in milligrams per liter (which is the same as parts per million, p.p.m). However, the salinity of both water and soil is easily measured by means of an electrical device. It is then expressed in terms of electrical conductivity: millimhos/cm or micromhos/cm. A salt concentration of 1 gram per liter is about 1.5 millimhos/cm. Thus, a concentration of 3 grams per liter will be about the same as 4.5 millimhos/cm. The salt concentration in the water extracted from a saturated soil (called saturation extract) defines the salinity of that soil. If this water contains less than 3 grams of salt per liter, the soil is said to be non-saline. If the salt concentration of the saturation extract contains more than 12 g/l, the soil is said to be highly saline (12).
Ruined farmland due to salinity problems in Iran’s central desert
Worldwide, more than 831 million hectares of land are estimated to be salt-affected, covering a range of soils defined as saline, saline sodic and sodic in every climatic zone and on every continent except Antarctica (13). All soil types with diverse morphological, physical, chemical, and biological properties may be affected by salinization. Generally, salt-affected soils are predominant in arid and semi-arid regions (14). It has been estimated that worldwide 20% of total cultivated and 33% of irrigated agricultural lands are afflicted by high salinity (15). Furthermore, the salinize areas are increasing at a rate of 10% annually for various reasons, including low precipitation, high surface evaporation, weathering of local rocks, irrigation with saline water, and poor cultural practices. Globally, irrigated lands cover some 310 million ha, an estimated 20% of it is salt-affected (62 million ha). The inflation-adjusted cost of salt-induced land degradation in 2013 was estimated at $441 per hectare, yielding an estimate of global economic losses at $27.3 billion per year (16).
Degraded farms because of salinity problem in Yazd province, Iran
It is estimated that salinization affects around 3.8 million ha in Europe. There are different causes of salinization, but irrigated areas in particular can be affected by salinization. It is estimated that 25% of irrigated cropland in the Mediterranean area is affected by moderate to high salinization leading to moderate soil degradation. Projected temperature increases and changes in precipitation characteristics in the Mediterranean are only likely to enhance the problem of salinization (17). Of the about 4.5 million acres of irrigated cropland in California, more than half are affected to some degree by salinization. Most of the seriously affected acreage is in the Imperial Valley in Southern California and the Western San Joaquin Valley in Central California (18). A certain level of salinity exists in all untreated irrigation water, although this natural salinity tends to be low in California. Most crops take up little salt, so the evapotranspiration process concentrates salt in the soil. A semi-arid climate exacerbates the degree and rate of salinization because the state’s main agricultural areas receive relatively little precipitation and have high evapotranspiration rates. This is especially true in the Imperial Valley, which coincidentally meets its water demand from the Colorado River, the most saline irrigation water in the state (19, 20).
Saline irrigation water contains dissolved substances known as salts. In much of the arid and semi-arid regions of the United States, most of the salts present in irrigation water are chlorides, sulfates, carbonates, and bicarbonates of calcium, magnesium, sodium, and potassium. While salinity can improve soil structure, it can also negatively affect plant growth and crop yields (21).
Invasion of salinity and desert over the rangelands, Great desert, Iran
Problem of soil salinization:
Accumulation of high levels of salts in the soil is characteristic of arid and semi-arid regions. Although different curative and management measures are being used to render salt-affected soils fit for agriculture, they are extremely expensive and do not provide permanent solutions to overcome the salinity problem. In contrast, a biotic approach for overcoming salinity stress has gained considerable recognition within the past few decades in view of the vast experimental evidence from what has happened in nature concerning the evolution of highly salt-tolerant ecotypes of different plant species, and also from the remarkable achievements that have been made in improving different agronomic traits through artificial selection (22).
Soil salinization limits the agricultural potential of soils in many environments and has been considered to be a significant component of desertification processes in the world’s drylands. Taking the case of Africa in particular, the nature of salinization processes and previous estimates of extent are considered. UNEP’s GLASOD program and the GRID database are used to reassess the problem. Secondary (human induced) salinization accounts for only 10 percent of Africa’s saline soils though it does affect half the area of irrigated land. When all African drylands agricultural land is considered, nutrient depletion is a far greater constraint on food production (23).
Salt affected soils limit crop yields around the world. Knowledge of how nutrient availability is affected in plants growing on salt affected soils is important in adopting appropriate management practices to satisfy plants’ nutritional requirements and improve yields to meet food demands of increasing world populations. In the salt affected environment, plants required to absorb essential nutrients from a dilute source in the presence of highly concentrated nonessential nutrients. Nutrient uptake in salt affected soils is low due to salt stress and negative interactions of present cations and anions. Hence, a higher amount of nutrients is necessary in salt affected soils compared to normal soils. Salts also reduce the activity of many enzymes, which supply energy for nutrient uptake. Application of manures to create favorable plant growth environments, leaching salts from the soil profile, planting salt tolerant crop species or genotypes within species, and the addition of fertilizers, especially potassium, may also help in reducing salinity effects and improving nutrient use efficiency (24).
Typical wet saline soils plant community, near Karaj city, Iran
Different Classes of Salt Affected Soils:
Salt-affected soils can be broken into three categories based on general EC, SAR, ESP, and pH guidelines: saline, sodic and saline-sodic. Properties of each of these soils are discussed below (25).
Salinity refers to the total concentration of all salts in the water or the soil. Saline soils contain excessive concentrations of soluble carbonate, chloride and sulfate salts that cause EC levels to exceed 4 mmhos/cm. Although relatively insoluble salts, such as Ca and Mg carbonates, do not cause high EC levels, but they are present in saline soils and may result formation of a white crust on the soil surface. The primary challenge of saline soils on agricultural land is their effect on plant/water relations. Excess salts in the root zone reduce the amount of water available to plants and cause the plant to spend more energy to exclude salts and take up water. Additionally, if salinity in the soil solution is high enough, water may be pulled out of the plant cell into soil solution, causing root cells to shrink and collapse. The effect of these processes is ‘osmotic’ stress for the plant. Osmotic stress symptoms are very similar to those of drought stress, and include stunted growth, poor germination, leaf burn, wilting, and possibly plant death. Salinity can also affect plants, causing specific ion effects, such as nutrient deficiencies or toxicities. Salt itself is toxic to plants at elevated concentrations. Thus, any increase in salinity can be at the expense of plant health, and decrease in crop productivity (26,27, 28).
Although excessive salts can be hazardous to plant growth, low to moderate salinity may actually improve some soil physical conditions. Ca2+ and Mg2+ ions have a tendency to ‘flocculate’ (clump together) soil colloids (fine clay and organic matter particles), thus increasing aggregation and macro-porosity. In turn, soil porosity, structural stability, and water movement may actually be improved in saline soils. However, benefits of structure improvement are likely to come at the cost of reduced plant health (29,30, 31).
Soil sodicity represents the relative preponderance of exchangeable sodium compared to other exchangeable cations, chiefly calcium, magnesium, potassium, hydrogen and aluminium (32). Terms for more complex relations are used to describe the potential effects of sodium (33). Sodicity refers specifically to the amount of sodium present in irrigation water. Irrigating with water that has excess amounts of sodium can adversely impact soil structure, making plant growth difficult (34). Highly saline and sodic water qualities, depending on the type and amount of salts present, can cause serious problems in agriculture. Various factors, such soil type being irrigated, the specific plant species, growth stage, and the amount of water being able to pass through the root zone, have significant roles (35).
In contrast to saline soils, sodic soils have relatively low EC, but a high amount of Na+ occupying exchange sites, often resulting in the soil having a pH at or above 8.5. Instead of flocculating, Na+ causes soil colloids to disperse, or spread out, if sufficient amounts of flocculating cations (i.e., Ca2+ and Mg2+) are not present to counteract the Na+. Dispersed colloids clog soil pores, which effectively reducing the soil’s ability to transport water and air. The result is soil with low water permeability and slow infiltration that causes ponding and then crusting when it dries. These conditions tend to inhibit seedling emergence and hinder plant growth. Sodic soils are also prone to extreme swelling and shrinking during periods of drying and wetting, thus further breaking down soil structure. The subsoil of a sodic soil is usually very compact, moist and sticky, and may be composed of soil columns with rounded caps. Fine-textured soils with high clay content are more prone to dispersion than coarser textured soils, because of their low leaching potential, slow permeability, and high exchange capacity. Other symptoms of sodic soils include less plant available water, poor tilth, and sometimes a black crust on the surface formed from dispersed organic matter (29, 30, 31).
Saline-sodic soils are soils that have chemical characteristics of both saline soils (EC greater than 4 mmhos/cm and pH less than 8.5) and sodic soils (ESP greater than 15). Therefore, plant growth in saline-sodic soils is affected by both excess salts and excess Na+. Physical characteristics of saline-sodic soils are intermediate between saline and sodic soils; flocculating salts help moderate the dispersing action of Na+ and structure is not as poor as in sodic soils. The pH of saline-sodic soils is generally less than 8.5; however, this can increase with the leaching of soluble salts unless concentrations of Ca2+ and Mg2+ are high in the soil or irrigation water (29, 30, 31).
Effects of Salinity on Plant Growth:
The property of salinity tolerance is not a simple attribute. It is an outcome of various features that depend on different physiological interactions, which are difficult to determine. The morphological plant appearance in response to salinity may not be enough to determine its effect. So, it is important to recognize other physiological and biochemical factors, including toxic ions. Osmotic potential, along with other physiological and chemical imbalances, as well as interactions between various stresses are important to know. The effect of salinity on plants includes not only the reduced ability of the plant to take up water, but also reduction in the growth rate. This is referred to as the osmotic or water-deficit effect of salinity. Salinity becomes a problem when enough salts accumulate in the root zone to negatively affect plant growth (29, 32).
Plants exposed to salt stress undergo changes in their environment. The ability of plants to tolerate salt is determined by multiple biochemical pathways that facilitate retention and/or acquisition of water, protect chloroplast functions, and maintain ion homeostasis. Essential pathways include those that lead to synthesis of osmotically active metabolites, specific proteins, and certain free radical scavenging enzymes that control ion and water flux and support scavenging of oxygen radicals or chaperones. The ability of plants to detoxify radicals under conditions of salt stress is probably the most critical requirement. Many salt-tolerant species accumulate methylated metabolites, which play crucial dual roles as osmo-protectants and as radical scavengers. Their synthesis is correlated with stress-induced enhancement of photorespiration (30).
A very salt tolerant plant, Limonium iranicum, in Iran’s central desert
Despite a wealth of published research on salinity tolerance of plants, neither the metabolic sites at which salt stress damages plants nor the adaptive mechanisms utilized by plants to survive under saline conditions are well understood. As a result, there are no well-defined indicators for salinity tolerance available to assist plant breeders in the improvement of salinity tolerance of important agricultural crops. Although plant breeders have successfully improved salinity tolerance of some crops in recent decades, however, there is a need to determine the underlying biochemical mechanisms of salinity tolerance, so as to provide plant breeders with appropriate indicators (31, 33).
Soil water salinity is dependent on soil type, climate, water use and irrigation routines. For example, immediately after the soil is irrigated, plant available water is at its highest and soil water salinity is at its lowest. However, as plants use soil water, the remaining water is held tighter to the soil and becomes progressively more difficult for plants to obtain. As the water is taken up by plants through transpiration or lost to the atmosphere by evaporation, soil water salinity increases because salts become more concentrated in the remaining soil water. Thus, evapotranspiration (ET) between irrigation periods can further increase salinity. Increased salinity due to ET is rarely taken into account in salinity charts (34, 35).
A Halocnemum strobilaceum, another salt tolerant plant, Iran’s central desert
Salinity has a dual effect on plant growth via an osmotic effect on plant water uptake, and specific ion toxicities. Plant access to soil water is decreased by the decrease of osmotic potential of the soil solution due to decrease in total soil water potential. As the soil dries, the concentration of salt in the soil solution increases which further decreases osmotic potential. Plants must osmotically adjust themselves in order to maintain water uptake from a saline soil. This is done either by taking up salts and compartmentalizing them within plant tissue or synthesizing organic solutes. Plants, which take up salts, have generally a higher salt tolerance and a greater ability to store high salt concentrations in plant tissue without affecting cell processes and are known as halophytes (36, 37, 38).
Plants, which synthesize organic solutes, are known as glycophytes. They can tolerate much lower concentrations of salt in plant tissues before being adversely affected. Glycophytes tend to be salt sensitive, although this is not always the case (39).
While above mentioned categories are the two extremes, most plants utilize a combination of different strategies. A reduction in growth may occur, even with complete osmotic adjustment, due to the metabolic demands of maintaining osmotic adjustment. While increased uptake of salts may contribute to osmotic adjustment, but may result in Na+ and Cl- toxicity. Regarding this issue, a range of symptoms common across many species have been described with chlorosis on the tips of older leaves and development to necrosis followed by leaves’ death. Excess Na+ may cause metabolic disturbances in processes where low Na+ and high K+ or Ca2+ are required for optimal metabolic functions. A decrease in nitrate reductase activity causes inhibition of photosystem II and chlorophyll breakdown, which are all associated with increased Na+ concentration. Cell membrane function may become compromised as a result of Na+ replacing Ca2+, resulting in increased cell leakiness (40).
Symptoms of Cl- toxicity are frequently documented, but much less information is available regarding the specific effects of high Cl-. High Cl- concentration in leaf tissue may disrupt photosynthetic function through the inhibition of nitrate reductase activity. Once the capacity of the cell to store salts is exhausted, salts build up in the intercellular space leads to cell dehydration and death. A better understanding of the involved dominant effects in plant response to salinity will facilitate development of improved salt resistant varieties and crop management practices (41).
Atriplex leucoclada a very salt tolerant camels and game forage plant, Iran’s central desert
Effects of Salinity on Soil Physical Properties:
Salt affected soils are characterized by rising water tables and water-logging of lower lying areas in the landscape. Soil water salinity can affect soil physical properties by causing fine particles to bind together into aggregates. This process is known as flocculation and is beneficial in terms of soil aeration, root penetration, and root growth. Although increasing soil solution salinity has a positive effect on soil aggregation and stabilization, however at high levels, salinity can have negative and potentially lethal effects on plants. As a result, an increase in salinity would not maintain soil structure without potential impacts on plant health (42).
Sodicity is a term given to the amount of sodium held in a soil. Sodium is a cation (positive ion) that is held loosely on clay particles in the soil. It is one of many types of cations that are bound to clay particles. Other cations are calcium, magnesium, potassium and hydrogen. The chief characteristic of sodic soils in agricultural land is the content of exchangeable sodium, which adversely affects the growth of most crops. Sodic soils are defined soils having an exchangeable sodium percentage (ESP) of more than 15. Excess exchangeable sodium has an adverse effect on the physical and nutritional properties of the soil with significantly or entirely consequent of reduction in crop growth. These soils lack appreciable quantities of neutral soluble salts, but contain measurable quantities of salts capable of alkaline hydrolysis, such as sodium carbonate. The saturated soil extracts electrical conductivity are likely to be variable but often less than 4 dS/m at 25 °C. The pH of saturated soil pastes is 8.2 or more and in extreme cases may be above 10.5. Dispersed and dissolved organic matter of highly sodic soils solution may deposit on the soil surface by evaporation causing a dark surface. Thus, these soils have also been termed as black sodic soils (43).
Changes in salinity and sodicity affect soil physical and chemical properties, which subsequently alter nutrient cycles and decomposition processes. As soil physical and chemical properties are altered, the risk of soil erosion will also increase, impacting soil aggregation, and nutrient cycle, as well as biotic activity. Therefore, there is a clear linkage between land mismanagement practices and increase in soil salinity and sodicity (44).
There are not many studies available to demonstrate unambiguously the effect of increasing salinity and sodicity on soil carbon dynamics. Responses to salinization are two general approaches: 1) Engineering the environment to manage increased salt in the soil by irrigation and drainage management, 2) “Engineering” the plants to increase their salt tolerance. Salt tolerant plants may also ameliorate the environment by lowering the water table in salt affected soils (45, 46).
Excess sodium on the cation exchange sites of fine-textured soils forms a condition in which irrigation water entering the soil is attracted to small pores with a great amount of force resulting in soil swelling. Slaked particles from aggregates and dispersion preclude drainage. Dispersed soil particles upon drying undergo a reorientation resulting in loss of soil structure, lower hydraulic conductivity, surface crusting that can break plant stems, inhibits seeds germination, emergence, and slows infiltration (47, 48).
Effects of Sodium and Sodicity on Soil Physical Properties
Sodium has the opposite effect of salinity on soils. The primary physical processes of high sodium concentrations are soil dispersion, as well as clay platelet and aggregate swelling (49). The forces that bind clay particles together are disrupted when too many sodium ions locate between them. When this happens, clay particles expand, causing swelling, and soil dispersion. Soil dispersion causes clay particles to plug soil pores resulting in reduced soil permeability. When soil is repeatedly wetted and dried and clay dispersion occurs, it then reforms itself and solidifies into almost cement-like layer with little or no structure (50). Three main problems are induced by soil dispersion: reduced infiltration, reduced hydraulic conductivity, and surface crusting (51). Salts that contribute to salinity, such as calcium and magnesium, do not have this effect because they are smaller and tend to cluster closer to clay particles. Calcium and magnesium will generally keep soil flocculated, because they compete for the same spaces as sodium to bind to clay particles. Increased amounts of calcium and magnesium can reduce the amount of sodium-induced dispersion (52).
Infiltration Soil dispersion hardens soil and blocks water infiltration, making it difficult for plants to establish and grow. The major implications associated with decreased infiltration due to sodium-induced dispersion include reduced plant available water, increased runoff and soil erosion (53, 54).
Soil dispersion not only reduces the amount of water entering the soil, but also affects hydraulic conductivity of soil. Hydraulic conductivity refers to the rate at which water flows through soil. For instance, soils with well-defined structure will contain a large number of macro-pores, cracks, and fissures, which allow for relatively rapid flow of water through the soil. The hydraulic conductivity is reduced when soil structure is destroyed due to sodium-induced dispersion. If water cannot pass through the soil, then the upper layer becomes swollen and water logged. This results in anaerobic soils, which reduces or prevents plant growth and decrease organic matter decomposition rates. The decrease in decomposition causes soils to become infertile, such as black alkali soils (55, 56, 57).
Surface Crusting Surface crusting is a characteristic of sodium affected soils. The primary causes of surface crusting are 1) physical dispersion caused by impact of raindrops or irrigation water, and 2) chemical dispersion, which depends on the ratio of salinity and sodicity of the applied water.
Surface crusting, due to rainfall, is greatly enhanced by sodium induced clay dispersion. When clay particles disperse within soil water, they plug macro-pores in the soil surface by two means. First, they block avenues for water and roots to move through the soil. Second, they form a cement like surface layer when soil dries. The hardened upper layer, or surface crust, restricts water infiltration and plant emergence (58, 59, 60).
Relationship Between Salinity and Sodicity and Soil Physical Properties (EC/SAR):
The relationship between soil salinity and its flocculating effects, soil sodicity and its dispersive effects under various salinity, and sodicity combinations influence soil aggregation or dispersion. As irrigation water with low salinity is applied to the soil, this water flows into the spaces between clay particles (micropores). If salinity of the applied water is lower than the soil salinity, it results in swelling and the dispersion of clay particles. In contrast, irrigation water with higher salinity than the soil tends to cause particles to stay together, maintaining soil structure (61) .
More than fifty years of research have been conducted to determine the relationship between salinity (EC) and sodicity (SAR) of irrigation water and its effects on soil physical properties. This relationship is now understood well enough to make accurate predictions of how specific soils will behave when irrigated water containing different levels of salts and sodium. The main concerns related to the relationship between salinity and sodicity of irrigation water are the effects on soil infiltration rates and hydraulic conductivities (62).
Sodium absorption ratio (SAR) is a measure of the relative preponderance of dissolved sodium in water compared to the amounts of dissolved calcium and magnesium. The mathematical form of this measure follows from a theoretically derived and frequently observed relationship with the preponderance of sodium held in exchangeable form in soil, which is in equilibrium with the subject water. Exchangeable sodium percentage (ESP) is the amount of sodium held in exchangeable form on the soil’s cation exchange complex expressed as a percentage of the total cation exchange capacity (CEC). These terms are described in the following discussion (63 ).
Exchangeable sodium percentage (ESP) = Conc. Na+ x 100/Sum of conc. all cations,
or ESP (%) = Conc. Na+/CEC; where the units of concentration are in cmol(+) kg-1 ( or meq/100 g).
Electrical conductivity is measured using a conductivity meter and electrical conductivity (EC) is reported in the international units of measure as deciSiemens per meter (equivalent to milliSiemens per centimeter).
Smaller units of microSiemens per centimeter are commonly used for low salinity recordings. 1 dS m-1 = 1 mS cm-1 = 1000 microS cm-1
The Swelling Factor: The ratio of salinity (EC) to sodicity (SAR) determines the effects of salts and sodium on soils. Salinity promotes soil flocculation and sodicity promotes soil dispersion. The combination of salinity and sodicity of soils is measured by the swelling factor, which is the amount a soil is likely to swell with different combinations of salinity and sodicity. Essentially, the swelling factor predicts whether sodium-induced dispersion or salinity-induced flocculation will more greatly affect soil physical properties (64, 65, 66).
The Role of Soil Texture:
Soil texture plays an important role in all aspects of irrigated agriculture and the soil texture role with respect to effects of salinity and sodicity is no exception. Soil texture helps determine how much water will be able to pass through the soil, how much water the soil can store, and the ability of sodium to bind to the soil (67). Clay soils can hold more water, because they are composed of small particles and drain slower than course textured soils. Smaller particles can pack closely together block the spaces between particles and prevent water from passing through. Sand particles are larger and, therefore, have larger pore spaces for water to pass through. Under normal irrigation practices, sandy soils will naturally be able to flush more water through the root zone than clay soils. The end result is that sandy soils can withstand higher salinity irrigation water, because more dissolved salts will be removed from the root zone by leaching (67).
An important aspect of clay soil textures have to do with surface area. Because of clay’s particle tiny size, a given volume of clay particles has far more surface area than the same volume of a larger sized particle. This simply means that clay soils are at a greater risk than course textured soils for excess sodium to bind to them and cause dispersion. Sands have larger particle sizes resulting in less surface area. They cannot correspondingly accept as much sodium as clay particles (68).
Role of Clay Type:
The three main clay types are montmorillonite, illite, and kaolinite clays. On the microscopic scale, each of these clays has a different lattice structure, i.e., different building blocks. This directly affects the ability of sodium to bind to each type. Basically, the more sodium a certain type of clay is able to hold, the more infiltration and hydraulic conductivity will be reduced. Montmorillonite clays are affected by sodium the most, while kaolinite is least affected. This same pattern is also true for the swelling factor. Montmorillonites are the most prone to swelling and dispersion, whereas kaolinites are the least likely to swell and disperse (69).
Clay soils have relatively high water holding capacities and are slow to drain because of their smaller pore diameters. Conversely, sandy soils retain less water and are faster to drain. Under normal irrigation practices, sandy soils will have naturally occurring greater leaching fractions (loss of water from the root zone) than clay soils when both soils are irrigated with equal volumes of water. Sandy soils can withstand higher salinity irrigation water as more of the water, and hence salts, will be leached beneath the root zone. A second important aspect of soil texture is the fact that clays generally compromise the majority of cation exchange sites in soils. This is because clays, by virtue of their small particle size, have the most surface area and, therefore, the most exchange sites. Consequently, clay soils have the greatest risk for excess sodium binding and dispersion. Sands, with their substantially larger particle size, have less total surface area and, therefore, fewer exchange sites (68).
Dispersion of soil particles, and building of crusts
Impact of salinity on plants:
Salts in the soil water may inhibit plant growth for two reasons. First, the presence of salt in the soil solution reduces the ability of the plant to take up water and this leads to reductions in the growth rate. This is referred to as the osmotic or water-deficit effect of salinity. The difficulties caused by salt stress arise from the disruption of cellular aqueous and ionic equilibria. Second, the process of ion toxicity is followed. So, tolerance determinants include effectors that function to restore cellular homeostasis (70). High salinity exerts its negative impact mainly by disrupting the ionic and osmotic equilibrium of the cell (71).
Plant responses to salt and water stress have much in common. Salinity reduces the ability of plants to take up water and then quickly causes reductions in growth rate along with a suite of metabolic changes identical to those caused by water stress (72). The initial reduction in shoot growth is probably due to hormonal signals generated by the roots. There may be salt-specific effects that later have an impact on growth (73). Once excessive amounts of salt enter the plant, salt will eventually rise to toxic levels in the older transpiring leaves, causing premature senescence, and reduce the photosynthetic leaf area of the plant to a level that cannot sustain growth (74).
Plants have evolved biochemical and molecular mechanisms that may act in a concerted manner and constitute the integrated physiological response to soil salinity (75). These include the synthesis and accumulation of compatible solutes to avoid cell dehydration, maintain root water uptake, the regulation of ion homeostasis to control ion uptake by roots, compartmentation and transport into shoots, the fine regulation of water uptake and distribution to plant tissues by the action of aquaporins, the reduction of oxidative damage through improved antioxidant capacity and the maintenance of photosynthesis at values adequate for plant growth (76, 77). Photosynthetic response to drought and salinity stress is highly complex. It involves the interplay of limitations taking place at different sites of the cell/leaf and at different time scales in relation to plant development (78). The intensity, duration and the rate of progression of the stress will influence plant responses to water scarcity and salinity, because these factors will dictate whether mitigation processes associated with acclimation will occur or not. Acclimation responses under drought, which indirectly affect photosynthesis, include those related to growth inhibition or leaf shedding, that by restricting water expenditure by source tissues will help to maintain plant water status and, therefore, plant carbon assimilation (79).
Plant response to salinity stress
Arbuscular mycorrhizal (AM) symbiosis can help the host plants to cope with the detrimental effects of high soil salinity. There is evidence that AM symbiosis affects and regulates several of the above-mentioned mechanisms, but the molecular bases of such effects are almost completely unknown (75).
Physiological or Osmotic Stress of Plants in Saline Soils:
Osmotic shock or osmotic stress is a sudden change in the solute concentration around a cell causing a rapid change in the movement of water across its cell membrane. Under conditions of high concentrations of either salts, substrates, or any solute in the supernatant water is drawn out of the cells through osmosis. Osmotic adjustment is generally regarded as an important adaptation to drought and salinity, because it helps to maintain turgor and cell volume. It is often thought to promote growth, yield, or survival of plants in dry and saline soils. Osmotic adjustment itself cannot promote growth. The solutes, which account for it, must be diverted from essential processes, such as protein and cell wall synthesis. Further, it now appears that turgor does not control cell expansion or stomatal conductance (80).
Osmotic adjustment or accumulation of solutes by cells is a process by which water potential of a cell can be decreased without an accompanying decrease in cell turgor. It is a net increase in solute content per cell that is independent of the volume changes that result from loss of water. Osmotic adjustment in plants subjected to salt stress can occur by the accumulation of high concentrations of either inorganic ions or organic solutes or both. In some cases, accumulation of solutes is so high that it goes beyond the limits of regulation of cytoplasmic content with growth impairment (81).
Osmotic stress in the initial stage of salinity stress causes various physiological changes such as interruption of membranes, nutrient imbalance, impairs the ability to detoxify reactive oxygen species (ROS), differences in the antioxidant enzymes, decreased photosynthetic activity, and decrease in stomatal aperture. Salinity stress is also considered as a hyper ionic stress (82).
Environmental factors, such as drought, salinity, extreme temperatures, chemical toxicity and oxidative stresses, are difficult to control and engineer changes, but globally and regionally very important. Drought and salinization are usually manifested as osmotic stress. They are most serious threats to agriculture and to maintaining a safe environment in many parts of the world. Abiotic stress is expressed in plants by a series of morphological, physiological, biochemical, and molecular changes that adversely affect plant growth and productivity (83, 84). The major interacting domains of stress tolerance include whole plant and cell physiology, molecular biology, genetics, and breeding (85). The use of molecular tools for elucidating the molecular control mechanisms of osmotic stress tolerance and for engineering more tolerant plants is based on the expression of specific stress-related genes. These genes regulate osmoprotection, water, ion movements, a variety of functional and structural stress-induced proteins, signal perception and transduction, free radical scavenging, and many others (80).
Mechanisms of Plants Tolerance in Saline Soils:
Soil salinity as a major abiotic stress of crops in agriculture worldwide has led the research into improving of salt tolerance crops. However, tolerating salt might have much wider implications. Transgenic salt-tolerant plants often also tolerate other stresses, including chilling, freezing, heat, and drought (86, 87). The ability of plants to detoxify radicals under conditions of salt stress is probably the most critical requirement. Many salt-tolerant species accumulate methylated metabolites, which play crucial dual roles as osmoprotectants and as radical scavengers. Their synthesis is correlated with stress-induced enhancement of photorespiration (88, 89). Plant growth responds to salinity in two phases: a rapid, osmotic phase that inhibits growth of young leaves, and a slower, ionic phase that accelerates senescence of mature leaves. Plant adaptations to salinity are of three distinct types: osmotic stress tolerance, Na+ or Cl− exclusion, and the tolerance of tissue to accumulated Na+ or Cl− (90).
Increased soil salt concentrations decrease the ability of a plant to take up water and once Na+ and Cl− are taken up in large amounts by roots, both Na+ and Cl− negatively affect growth by impairing metabolic processes and decreasing photosynthetic efficiency. Thus, plant salt stress can be subdivided into early-occurring osmotic stress and slowly increasing ionic Na+ stress with additional Cl− stress. Plants enact mechanisms to mitigate osmotic stress by reducing water loss while maximizing water uptake. Furthermore, plants minimize the harmful effects of ionic Na+ stress by exclusion of Na+ from leaf tissues and by compartmentalization of Na+, mainly into vacuoles (91, 92).
Salinity has many different effects on a plant. So, there are also many mechanisms for plants to tolerate these stresses. These mechanisms can be classified into three main categories. First, osmotic tolerance, which is regulated by long distance signals, that reduces shoot growth and is triggered before shoot Na+ accumulation. Secondly, ion exclusion where Na+ and Cl- transport processes in roots reduce the accumulation of toxic concentrations of Na+ and Cl- within the leaves. Finally, tissue tolerance where high salt concentrations are found in leaves, but are compartmentalized at the cellular and intracellular level (93).
Halophytes of the Chenopodiaceae family respond to salinity by taking up sodium and chloride at high rates and then accumulating these ions in their leaves. These plants use the accumulated salt for osmotic adjustment to lower water potential in the soil (94). An important feature of this kind of osmotic adjustment is the isolation of the accumulated salt in the vacuoles of the leaf cells, keeping the salt concentration in the cytoplasm and organelles at a low level that does not interfere with the functions of their enzymes and metabolic machinery. This compartmentation has great significance for the performance of halophytes in a saline environment. As for the cytoplasm, osmotic adjustment in it is accomplished mainly by means of dissolved substances compatible with enzymes and metabolism (95). These “compatible solutes” are mostly organic compounds, such as the nitrogenous compounds glycinebetaine, proline, and in some plants, sugar alcohols, such as sorbitol. In addition, potassium is thought to be maintained in the cytoplasm at a concentration on the order of 4,000 mg/L (100 mM) (96, 97).
Limonium iranicum, an extremely halophytic plant on saline soils
Haxoylon aphyllum a halophytic tree, well establishes itself on saline soils
Amelioration of Saline and Sodic Soils:
Soil salinization is one of the major causes of declining agricultural productivity in many arid and semiarid regions of the world. Excessive salt concentrations in soils, in most cases, cannot be reduced with time by routine irrigation and crop management practices. Such situations demand soil amelioration. Various means used to ameliorate saline soils include: (a) movement of excess soluble salts from upper to lower soil depths via leaching, which may be accomplished by continuous or intermittent ponding or sprinkling; (b) surface salts flushing from soils that contain salt crusts at the surface, a shallow water-table or a highly impermeable profile; (c) biological reduction of salts in areas with negligible irrigation water or rainfall available for leaching by harvesting of high-salt accumulating aerial plant parts; and (d) amelioration of saline soils under cropping and leaching. Among these methods, cropping in conjunction with leaching has been found as the most successful and sustainable way to ameliorate saline soils (98).
Productivity enhancement of salt-affected land and saline water resources through crop-based management has the potential to transform them from environmental burdens into economic opportunities. Research efforts have led to the identification of a number of field crops, forage grasses and shrubs, aromatic and medicinal species, bio-fuel crops, and fruit tree and agroforestry systems, which are profitable and suit a variety of salt-affected environments. Several of these species have agricultural significance in terms of their local utilization on the farm. Therefore, crop diversification systems based on salt-tolerant plant species are likely to be the key to future agricultural and economic growth in regions where salt-affected soils exist (99).
Ameliorating sodic soils requires the application of a calcium (Ca2+) source, which replaces excess sodium (Na+) at the cation exchange sites. The displaced Na+ is then leached from the root zone through excess irrigation (100).
Many sodic and saline–sodic soils contain inherent or precipitated sources of Ca2+, typically calcite (CaCO3), at varying depths within the profile. Unlike other Ca2+ sources used in the amelioration of sodic and saline-sodic soils, calcite is not sufficiently soluble to affect the displacement of Na+ from the cation exchange complex. Phytoremediation has shown promise for the amelioration of calcareous sodic and saline–sodic soils in recent years. Phytoremediation of sodic and saline–sodic soils is achieved by the ability of plant roots to increase the dissolution rate of calcite. This process results in enhanced levels of Ca2+ in soil solution to replace Na+ from the cation exchange complex. This process is driven by the partial pressure of CO2 (PCO2) within the root zone. The generation of protons (H+) released by roots of certain plant species and to a much smaller extent the enhanced Na+ uptake by plants and its subsequent removal from the field at harvest (101).
Various research has indicated that sodic lands could be rehabilitated effectively to restore degraded environments through appropriate mixed tree cropping systems. The biological rejuvenation potential of sodic land is related to the distribution of tree roots in the soil profile. To obtain better results, a combination of Prosopis juliflora, Tamarix articulata, Dalbergia sissoo and Acacia nilotica tree species have provided maximum and constant litter mulch throughout the year. It was found that there was a greater circulation of Ca, Mg and Fe than the other nutrients by all four-tree species, which is a desirable factor. It is reported enhanced microbial activities due to the accumulation of humus through decomposition of leaf litter and root decay occur as well (102, 103, 104, 105).
Remediation of salt-affected soil using chemical agents, including gypsum (CaSO4.2H2O), calcite (CaCO3), calcium chloride (CaCl2.2H2O) and organic matters, such as farmyard manure, green manure, organic amendment, and municipal solid waste, were successful approaches that has been implemented worldwide (106).
The physical, chemical, and biological properties of soil in salt-affected areas are improved by the application of organic material leading to enhanced plant growth and development. Therefore, the application of organic material for soil remediation is important for sustainable land use and crop productivity (107). Various organic amendments, such as Municipal Solid Waste (MSW) compost or Sewage sludge, have been investigated for their effectiveness in saline soil remediation. The application of organic matter to saline soils can have different effects, such as speeding up of NaCl leaching, decrease of the exchangeable sodium percentage, electrical conductivity, and increase of water infiltration. It is also reported that application of biosolids increases soil microbial biomass and some soil enzymatic activities, such as urease, alkaline phosphatase, and β- glucosidase linked to C, N, P and S soil cycles (108).
In addition to facilitate adequate leaching of the salts added through saline water irrigation, soil and water management approaches should attempt to reduce unproductive water losses associated with evaporation from soil surfaces as well (109). One must also take measures to increase soil moisture storage, maintain soil physical properties in root zone area, enhance soil organic matter inputs and nutrient availability status, and maintain soil salinity and sodicity levels within acceptable crop production limits. Several approaches and techniques have been used to address the aforementioned issues and include irrigating at night to reduce evaporation from the soil surface and mulching of soil surface with different materials to reduce evaporation losses and reduce salt build-up in the soil. Mulching, out of all the above-mentioned soil and water management approaches, has potential to enhance soil quality over the long-term, as well as increase production (110).
The above referenced vast literature indicates the importance of salinity and sodicity matters. The problem is that even though it is more enhanced in arid and semi-arid regions, it is a global issue without discriminating any continent. The problem at hand gets its significance due to its complex factors of creation and its cure, also due to man losing valuable farming land to salinity year after year, the land man uses to produce food for increasing world population.
Estimates suggest that about 34 million ha, including 4·1 million ha of the irrigated land, are salt-affected as the consequence of naturally occurring phenomena and anthropogenic activities in Iran only. The annual economic losses are more than US$ 1 billion due to salinization in that country (111).
Tamarix ramosissma, a tree well establishes on saline soils
Salinity, sodicity, and toxicity of soils not only reduces crop productivity and quality of farming land, but also limits the choice of crops cultivation. There are two major approaches for improving and sustaining productivity in saline environments: modifying the environment to suit the growing plants or modifying the plants to suit the environment. Man had more extensively tried the first approach. The available options are mediated through the management and sequencing crops, irrigation water, adding chemicals and other practices. However, all efforts must be concerted and integrated to achieve specific goal, higher agricultural yields and sustainable ecosystem.
Some important interventions regarding reclamation of saline and sodic soils have been implemented, including appropriate crop variety selection, as well as blending saline/alkali and fresh water to keep the salinity’s sum below threshold. Cyclic application of irrigation with salty water at less salt sensitive stages, proper policy measures, encouragement for co-operation, seeking domestic expertise and education of local residents are extremely valuable in the success of all reclamation projects. Also, other viable options that could be considered are salinity and sodicity tolerant agro-forestry and bio-saline agriculture.
Salinity tolerance has been well defined and classified during past decades and research advances in the physiology, cellular biology, molecular biology, and biotechnology of salt tolerant crops are in full swing. Additionally, transgenic breeding advances has been profiled and studies on quantitative traits are identified, but an outlook for future salinity resistance research is still not on a clear path. Salinity issue is complex and multivariate. Its research requires concentrated efforts and a team of scientists of all related fields. In order to overcome technical applications and scientific barriers, frequent meetings and exchange of positive or negative field experiences and research findings are required.
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