Citation: Sen, H.S. and Bal, A.R. (2009). Plants facing nutritional disorders in coastal saline soils and management options. In "Enhancing Nutrient Use Efficiency in Problem Soils",Gurbachan Singh, Ali Qadar, N.P.S. Yaduvanshi & P. Dey (eds.), pp. 52-83. Central Soil Salinity Research Institute, Karnal, India.
Plants Facing Nutritional Disorders in Coastal Saline Soils and Management Options1
H.S.Sen2 and A.R.Bal3
Nearly 40 % of cities larger than 500,000 population are located in the coast in India. Overall about 50-70 % of the global population live within 100 km of the coastline covering only about 4 % of earth’s land (Poyya and Balachandran, 2008), thereby drawing heavily on coastal and marine habitats for food and other needs. According to another estimate (Wikipedia, 2009a), coastal areas (within 200 km from the sea) share less than 15 % of the earth surface area, and also predicts that three-fourths of the world population are expected to reside in the coastal areas by 2025. By one estimate (Poyya and Balachandran, 2008), the combined global value of goods and services from coastal ecosystems is about US$ 12-14 trillion annually - a figure larger than the United States' Gross Domestic Product worked out in 2004.
According to World Resources Institute (2006) coastal areas may be commonly defined as the interface or transition areas between land and sea, including large inland lakes. Coastal areas are diverse in function and form, dynamic, and do not lend themselves well to definition by strict spatial boundaries. Unlike watersheds, there are no exact natural boundaries that unambiguously delineate coastal areas at the global scale. According to them, the world coastline extends from 350,000-1,000,000 km in length, depending upon how finely the ‘length’ is resolved. More comprehensively, the coastal ecosystem has been defined by Sen et al. (2000) as representing the transition from terrestrial to marine influences and vice versa. It comprises not only shoreline ecosystems, but also the upland watersheds draining into coastal waters, and the nearshore sub-littoral ecosystems influenced by land-based activities. Functionally, it is a broad interface between land and sea that is strongly influenced by both.
Soils in the coastal ecosystem along with their characteristics have been described comprehensively on a global scale by Finkl (2005) but no attempt has been made to delineate the zones from inlands based on scientific criteria. Estimates made world over have generally been arbitrarily done based on length of the coastline times a fixed distance landward, varying from 50 to 200 km, as followed by different countries, from the shore assuming the zone representing coastal ecosystem different from that for inland part of the country. Velayutham et al. (1998) for the first time described soil resources and their potentials for different Agro-ecological Sub Regions (AESR) in the coastal ecosystems of India showing total of 10.78 million hectare area under this ecosystem (including the islands) in India.
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1 Prepared for the SAARC Workshop on “Nutrient Management in Problem Soils”, held at CSSRI,
Karnal 132 001, India on 8-11 September, 2009
2 Former Director, Central Research Institute for Jute & Allied Fibres (ICAR), Barrackpore, West Bengal, PIN 700 120, India; Address for correspondence: 2/74, Naktala, Kolkata 700 047, West Bengal, India;
Email: hssen.india@gmail.com, hssen2000@hotmail.com)
3 Principal Scientist, Central Soil Salinity Research Institute, Research Station Canning Town 743 329, South 24-Parganas, West Bengal, India; Email: asitbal@yahoo.com
Salient features of coastal problem soils
Coastal soils are constrained by various technological factors limiting the agricultural productivity and therefore, merit attention. Salinity in the soils and ground waters has, however, become a major environmental issue, and excessive salinity in the soil or irrigation water has been considered as the main limiting factor for the distribution of plants in natural habitats. The salient factors in the coastal plains are: (i) Excess accumulation of soluble salts and alkalinity in soil, (ii) Pre-dominance of acid sulphate soils, (iii) Periodic inundation of soil surface by the tidal water, and (iv) Eutrophication and hypoxia. All the above factors affect nutrient balance in soil and, in turn, plant growth.
Saline soil or solanchak
Soils contain excess soluble salts (ECe> 4 dSm-1) with pH below 8.5 and ESP lower than 15. Salts are composed mainly of sodium, calcium and magnesium among the cations, and chloride, sulphate, carbonate and bicarbonate among the anions. In majority of the situations, salt concentration along with its composition at the crop root zone varies not only spatially but also temporarily depending upon the soil type, salt-rich ground water characteristics and its rate of recharge into the root zone, and nature and distribution of rainfall along with other relevant climatic parameters.
Alkaline or sodic soil or solonetz
These soils contain exchangeable sodium in a quantity sufficient (ESP > 15) to interfere with the growth of most plants. In such soils ECe is generally < 4 dSm-1 and the pH higher than 8.5. The soil colloids are usually in a state of deflocculation. The dispersive effect of exchangeable sodium will be observed, however, only if the electrolyte concentration in the soil solution is smaller than that required to flocculate the clay particles, i.e. the critical coagulation concentration (Sposito, 1989). The state of deflocculation has been observed even in soils having ESP much lower than 15, and therefore this limit for ESP is not without dispute. High concentration of Mg in relation to Ca, observed in some coastal salt affected soil solution, has been responsible for such soils behave differently in respect of some physico-chemical properties, the fact not yet very clearly understood. The saline-sodic soil, upon alternate wetting and drying, develop dense subsoil with a columnar structure, and such soils are designated as solonetz. The alkaline or sodic clay colloids in a dispersed state render poor physical properties primarily in respect of moisture and solute transport, aeration and thermal flux, thereby adversely affecting the plant growth. It is difficult to differentiate toxic effect of sodium on plant from its effect on soil properties. Nature and extent of the adverse effect depend not only on the soil type including nature and amount of clay, moisture and fertility level, but also upon nature and amount of salt in the soil solution and pH of the soil, as well as their interactions.
Acid sulphate soils
Acid sulphate soils either contain sulphuric acid or have the potential to form sulphuric acid when exposed to oxygen in the air. These soils occur naturally in both coastal (tidal) and inland or upland (freshwater) settings, as a consequence of the deposition of large amounts of organic matter, such as decaying vegetation in a waterlogged setting. These waterlogged wetlands and mangroves or highly reducing environments are ideal for the formation of sulphide-containing minerals, predominantly iron pyrite (FeS2) in sulphidic material, which can react with the oxygen in the air to form sulphuric acid (sulphuric materials). Left undisturbed, these soils are harmless, but when excavated or drained, the acid drains into waterways, or reacts with carbonates and clay minerals in soils and sediments to form sulphates – liberating dissolved iron, calcium, magnesium and other elements such as copper (CSIRO, 2009). It is generally believed that the H2S is formed by sulphate reducing bacteria acting on sulphate from seawater, rather than the introduction of sulfide with the dredge sediments. Groundwater and soil samples from soils and ground water in fishermen islands in Australia were collected and analyzed for sulphide, sulphate, and organic carbon contents (O’Sullivan et al., 2005). Elevated concentrations of sulphides coincide with elevated concentrations of sulphate in the groundwater and elevated concentrations of organic carbon in the sediments, supporting the hypothesis that sulphide formation is the result of heterotrophic, sulphate reducing organisms. Most acid sulphate soils occur in the tropics, in low lying coastal land formerly occupied by mangrove swamps. Their most important characteristics are a field pH of below 4, owing to the oxidation of pyrite to sulphuric acid, and a generally high clay content. Other properties such as organic matter content and cation exchange capacity may vary widely. As sulphuric acid moves through the soil, it strips iron, aluminium and sometimes manganese from the soil. In some cases it also dissolves heavy metals such as cadmium. In the soil this mixture can make the soil so acid and toxic that few plants can survive. In some cases, where peat overlying the iron sulphide layer has burnt away, the iron sulphide layer is completely exposed to air. It produces so much sulphuric acid that nothing will grow, giving the soil surface a bare, scalded appearance. The soils reduce farm productivity. The sulphuric acid, apart from lowering pH, makes several soil nutrients less available to plants. The acid dissolves iron and aluminium from the soil so that they become available to plants in toxic quantities in soil water.
The total area of actual and potential acid sulphate soils is rather small: about 10 million hectares are known to occur in the tropics, and the world total probably does not exceed 14 million hectares in South and Southeast Asia, West and Southern Africa, and along the South American and Australian coastlines. In addition, some 20 million hectares of coastal peats, mainly in Indonesia, are underlain by potential acid sulphate soil.
In the near-neutral potential acid sulphate soils (Sulfaquents, Sulfic Fluvaquents), high salinity, poor bearing capacity, uneven land surface, and the risk of strong acidification during droughts are the main disadvantages. Young acid sulphate soils (Sulfaquepts) in which the pyritic substratum occurs near the surface are often more acidic than those soils (Sulfic Tropaquepts, Sulfic Haplaquets) in which this horizon is found at greater depths.
Inundation and flooding of soils
A flood is an overflow or accumulation of an expanse of water that submerges land. In the sense of ‘flowing water’, the word may also be applied to the inflow of the tide. ‘Coastal flood’ is caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A storm surge, from either a tropical cyclone or an extratropical cyclone, falls within this category (Wikipedia, 2009b) Coastal flooding is a problem wherever development has occurred adjacent to, or on, beach systems. The problems of maintaining these areas are accentuated by naturally rising sea levels due to global climate change. Floods usually occur when storms coincide with high tides. Very often the problem becomes much more severe with increase in salinity in the flood water caused by breaching or overflowing of the sea dykes, etc. Flooding thus causes significant change in soil properties depending on the soil, hydrological properties of the flood water, and duration of flood. Among others the most significant changes in soil properties of relevance to plant growth are silt deposition, accumulation of salts, erosion of top soil, organic C status in soil, depletion of soil oxygen resulting in lack of plant metabolic activities, and overall reduced soil atmosphere causing significant change in soil nutrient dynamics.
Eutrophication, hypoxia, dead zones and nutrient cycle
Overuse of fertilizer can result in eutrophication, and in extreme cases, the creation of ‘dead zones’. Dead zones occur when excess nutrients—usually nitrogen and phosphorus—from agriculture or the burning of fossil fuels seep into the water system and fertilize blooms of algae along the coast. As the microscopic plants die and sink to the ocean floor, they feed on bacteria, which consume dissolved oxygen from surrounding waters. This limits oxygen availability for bottom-dwelling organisms and the fish that eat them. In dead zones, huge growths of algae reduce oxygen in the water to levels so low that nothing can live. There are now more than 400 known dead zones in coastal waters worldwide, compared to 305 in the 1990s, according to a study undertaken by the Virginia Institute of Marine Science. Those numbers were up from 162 in the 1980s, 87 in the 1970s, and 49 in the 1960s. In the 1910s, four dead zones had been identified (Minard, 2008).
The occurrence of hypoxia in shallow coastal and estuarine areas has been increasing worldwide, most likely accelerated by anthropogenic activities. Hypoxia in the Northern Gulf of Mexico, commonly named the 'Gulf Dead Zone', has doubled in size since researchers first mapped it in 1985, leading to very large depletions of marine life in the affected regions (Portier, 2003). He studied changes in microbial communities as a result of oxygen depletion, the potential contribution of increasing hypoxia to marine production and emission of N2O and CH4, and the effect of hypoxic development on methyl mercury formation in bottom sediments at the Gulf of Mexico's Texas-Louisiana Shelf during the summer months.
The World Resources Institute reported that driven by a massive increase in the use of fertilizer, the burning of fossil fuels, and a surge in land clearing and deforestation, the amount of nitrogen available for uptake at any given time has more than doubled since the 1940s. In other words, human activities now contribute more to the global supply of fixed nitrogen each year than natural processes do, with human-generated nitrogen totaling about 210 million metric tons per year, while natural processes contribute about 140 million metric tons (Table 1).
This influx of extra nitrogen has caused serious distortions of the natural nutrient cycle, especially where intensive agriculture and high fossil fuel use coincide. In some parts of northern Europe, for example, forests are receiving 10 times the natural levels of nitrogen from airborne deposition, while coastal rivers in the Northeastern United States and Northern Europe are receiving as much as 20 times the natural amount from both agricultural and airborne sources.
Nutrient acquisition by plants in saline environment
Plants acquire mineral nutrients from their native soil environments. Most crop plants are glycophytes and have evolved under conditions of low soil salinity. Consequently, they have developed mechanisms for absorbing mineral nutrients in non-saline soils. Salinity in soil has a dual effect on plant growth via an osmotic effect on plant water uptake, and specific ion toxicities (Sheldon et al., 2004). By decreasing the osmotic potential of the soil solution, plant access to soil water is decreased, because of the decrease in total soil water potential. In order to maintain water uptake from a saline soil, plants must osmotically adjust. This is done either by taking up salts and compartmentalizing them within plant tissue, or synthesizing organic solutes. Plants which take up salts generally have a higher salt tolerance and greater ability to store high salt concentrations in plant tissue without affecting cell processes, and are known as halophytes. Plants which synthesise organic solutes are known as glycophytes, and they try to prevent excess salt uptake because they can tolerate much lower concentrations of salt in plant tissues before cell processes are adversely affected. Even with complete osmotic adjustment, a reduction in growth may occur due to the metabolic demands of maintaining osmotic adjustment.
Under saline conditions, which are characterized by low nutrient-ion activities and extreme ratio of Na+/Ca2+, Na+/K+, Ca2+/Mg2+ and Cl−/NO3−, nutritional disorders can develop and crop growth may be reduced. A list of nutrient disorders in coastal soils is listed in Table 2.
Additions of N and P generally increase the growth of plants grown in N- and P-deficient environments, provided that the plant is not experiencing severe salt stress. When salinity and nutrient deficiency are both factors limiting growth, relief of the most limiting factor will promote growth more than the relief of the less limiting factor. Therefore, addition of a limiting nutrient can either increase, decrease or have no effect on relative plant tolerance to salinity, depending on the level of salt stress. Failure to account for the severity of salt stress when interpreting salinity x nutrient interactions has caused considerable confusion among researchers. According to Berstein et al. (1974) and Bernstein (1975), however, while N or P were severely growth limiting, salinity was found to affect to growth of some crops [broccoli (Brassica oleracea var. capitata), cabbage (B. oleracea var. botrytis)] less. Conversely, when salinity severely limited growth, nutritional responses of some crops decreased. Salinity did not aggravate N or P deficiency as judged by leaf N and leaf P contents. Effects of salinity and N or P deficiency on other mineral constituents were highly crop specific.
Salinity, however, disrupts mineral nutrient acquisition by plants in two ways. First, the ionic strength of the substrate, regardless of its composition, can influence nutrient uptake and translocation. Evidence for this is salinity-induced phosphate uptake in certain plants and cultivars. The second and more common mechanism by which salinity disrupts the mineral relations of plants is by reduction of nutrient availability by competition with major ions (i.e. Na+ and Cl−) in the substrate. These interactions often lead to Na+-induced Ca2+ and/or K+ deficiencies and Ca2+-induced Mg2+ deficiencies.
Table 2. Major abiotic stresses common to coastal problem rice soils in Asia
Ecosystem Major problem Deficiencies Toxicities Other stresses
Rainfed and irrigated Acid and acid sulphate P, N Acidity, sulfate, Al, Fe, salinity Inhibition of nutrient uptake, flooding
Peat (Histosols) N, P, K, Zn, Mo, Cu, and B Acidity, Fe, H2S, organ ic substances Waterlogging, low thermal conductivity
Salinity P, Zn, N Salts (Ca, Mg, Na) Submergence, stagnant flooding, drought
Adapted from Ponnamperuma (1994); FAO Problem Soils Database http://www.fao.org/ag/agl/agll/prosoil; Ismail et al. (2007)
In sodic soils when pH of the soil solution exceeds 8.5, availability of some nutrients may be restricted resulting in nutrient imbalances. Bicarbonate toxicities occur primarily from reduced iron and other micronutrient availabilities at high pH while high Na+ may lead to Ca2+ and Mg2+ deficiencies (Arshad, 2008). In sodic soil total soluble salt concentrations are low and consequently, Ca2+ and Mg2+ concentrations are nutritionally inadequate. The deficiencies rather than Na+ toxicity are usually the primary influence on plant growth among the non-woody species (Maas, 1986).
Seasonal crops
Nitrogen: Under salt-stress conditions, the uptake of N by crop plants is generally affected (Alam, 1999). Reports show inhibitory and stimulatory effects on the plant N uptake under salinity stress. A substantial number of laboratory and greenhouse studies have shown that salinity reduces N accumulation in plants. It has been reported that an increase in Cl- uptake and accumulation is accompanied by a decrease in shoot nitrate concentration. Examples of these effects are found in barley, cotton, watermelon (Citrullus lanatus), and wheat. In his experiment, Aslam et al. (1984) have reported that Cl- inhibited NO3- uptake more than SO42- when these anions were present on an equal osmolarity basis. In contrast to the effect of Cl- on NO3- uptake, reported data indicated that increased NO3- in the substrate decreased Cl- uptake and accumulation. The possible decrease in N uptake by increasing salinity has been partly attributed to a probable substitution of Cl- for NO3-. For example, the N-deficiency symptom increased the Cl level in corn, barley, and some other crops. Sodium chloride salinity significantly decreased the amount of total N in all parts of the wheat plants possibly as a result of the antagonism of nitrate by chloride in the growth medium (Abdul-Kadir and Paulsen, 1982). Both the chloride salts of Na and K inhibited the nitrate uptake similarly, suggesting that the process was more sensitive to anionic salinity than to cationic salinity (Aslam et al.,1984).
Although, Cl- salts were primarily responsible for reduced NO3- uptake by plants, NO3- reduction in plants was not affected by salinity in studies with barley (Aslam et al.,1984). Salinity also stimulated nitrate reductase activity in peanut plants as well as decreased the nitrate reductase activity in tomato and cucumber (Cucumis sativus L.) plants, and reduction in NRA may be due to inhibition of NO3-uptake by Cl- in plant species (Abdul-Kadir and Paulsen, 1982) .
The source in which N is applied to salt-treated plants also is important. In an experiment, the NH4-fed maize and wheat plants were more sensitive to salinity than NO3-fed plants grown in nutrient solution culture. Supplementation of Ca2+ to the growth media improved the growth rate of the plants in the NO3 treatment but not those treated with NH4. Based on the results of their nutrient solution experiments, Leidi et al. (1991) suggested that NO3- is a better N source than NH4+ for wheat grown in salt-affected soils.
Potassium, Sodium, Calcium & Magnesium: The higher K/Na ratio in shoots of barley cultivars compared with that in root medium solution indicated selective uptake of K, which seems to be among the processes involved in tolerance of cultivars to salinity stress (Alam, 1999, Niazi et al., 1992). Addition of K suppressed the uptake of other cations by rice and tomato plants in the order of Na>Mg>Ca. The depression of Na uptake by K could be due to the antagonism between the two cations. It is widely recognized that a high Na concentration inhibits K uptake by plants. On the other hand, Na appeared to stimulate the K uptake by plants.
Salinity stress has significant inhibitory effects on the concentrations of K, Ca, and Mg as well as stimulatory effects of these nutrient elements on different crop plants (Alam,1999). With the increasing concentration of NaCl salts, K concentration decreased in the leaves, stems, and roots and was accompanied by a substantial increase of Na in the organs. In Na2SO4–treated sorghum plants, an increase in Na and SO4 and a decrease in K- uptake was observed with increasing concentrations of the salt (Khan et al., 1995).
In lowland rice plants adapt to saline conditions and avoid dehydration by reducing the osmotic potential of plant cells. Antagonistic effects on nutrient uptake may occur, causing deficiencies, particularly of K and Ca under conditions of excessive Na content. For example, Na is antagonistic to K uptake in sodic soils with moderate to high available K, resulting in high Na:K ratio in the rice plant and reduced K transport rates. Sodium-induced inhibition of Ca uptake and transport limits shoot growth. Increasing salinity inhibits nitrate reductase activity, decreases chlorophyll content and photosynthetic rate, and increases the respiration rate and N content in the plant. Plant K and Ca contents decrease but the concentrations of NO3-N, Na, S, and Cl in shoot tissue increase.
The factors related to nutrient disorder that affect different rice varieties are:
• Differences in nutrient uptake under Na stress. Tolerant cultivars have a narrower Na:K ratio (higher K uptake) and greater leaf Ca2+ content than susceptible cultivars.
• Efficient exclusion of Na+ and Cl-. Salt-tolerant rice varieties have a reduced Na+ and Cl- uptake compared with less tolerant cultivars.
Calcium plays a vital nutritional and physiological role in plant metabolism. Calcium, which like K also is an essentail mineral nutrient, helps in maintaining membrane integrity, is important in senescence processes, and is known to counteract the harmful effects of Na on crops (Lahaye.and Epstein, 1971). Plant growth is dependent on Ca2+, and both cell division and cell elongation processes are affected by the Ca2+ ion concentration.
The presence of Ca2+ as the dominant cation in agricultural soils generally ensures that the absolute Ca2+ level is not a primary growth-limiting factor. As salinity increases, the requirement of plants for Ca2+ increases. The uptake of Ca2+ from the soil solution may decrease because of ion interactions, precipitation, and increases in ionic strength that reduce the activity of Ca2+. In citrus, Ca was found to be effective in reducing the transport of both Na and Cl from the roots to leaves. Maintaining an adequate supply of Ca2+ in the soil solution is an important factor in controlling the severity of specific ion toxicities.
The magnesium content of the leaves of saline-treated bean plants increased, whereas it decreased in the root. Hodson et al. (1982) found potentially toxic concentrations of Mg in salt-marsh soil solution samples and demonstrated that a salt-marsh clone, Agrostis stolonifera, was considerably more tolerant to Mg2+ than was an inland clone. Magnesium concentration of avocado leaves was decreased with an increase in the exchangeable Na in the soil. In rice, Mg transport to the tops was suppressed by Na compared with Mg uptake (Song and Fujiwara, 1996). The Mg content in the roots revealed the competition between Mg and Na uptake and transport to the tops (Alam, 1999, Alfocea et al., 1993).
Phosphorus: Phosphorus, which has a crucial role in the energy metabolism of cells, is involved in a number of anabolic and catabolic pathways (Alam, 1999). A recent study indicates that salinity may increase the P requirement of certain plants. Awad et al. (1990) found that when NaCl increased in the substrate from 10 to 50 or 100 mM, the P content in the tomato leaf increased from 58 to 70 and 97 mmol kg-1 dry weight. The influence of salinity on P accumulation in crop plants is variable and depends on the plant and experimental conditions. In many cases, salinity decreased the P concentration in plant tissue (Sharpley et al., 1992). It is unlikely that Cl and H2PO4 ions are competitive in terms of plant uptake. However, it has also been observed that Cl may have a suppressing effect on P uptake in tomato shoots (Papadopoulos and Rendig, 1983). The presence of Cl as well as SO4 reduced P uptake in barley and sunflower plants. In other cases, a reduction in plant P concentration by salinity may result from the reduced activity of P in the soil solution due to the high ionic strength of the growth media (Awad et al.,1990). Phosphate solubility and its availability are reduced in saline soils not only because of ionic strength effects that reduce the activity of phosphate but also because the P concentration in soil solution is tightly controlled by sorption processes and by the low solubility of Ca-P minerals. It is, therefore, understandable that P concentrations in field-grown agronomic crops decreased as salinity increased in the media. When plants are P-deficient, they may be more sensitive to salinity (Sharpley et al., 1992).
The effects of salinity on corn plants (Zea mays L.) are influenced by the concentration of nutrient orthophosphate. Salinity (–2 bars each of NaCl and CaCl2) was more injurious in combination with a high concentration of orthophosphate (2 mM) (that gave optimum yields in the absence of salinity) than it was with a lower concentration (0.1 mM) (Nieman and Clark, 1976). With 2 mM orthophosphate, salinity seemed to damage the plant mechanisms that normally regulate the internal concentration of orthophosphate resulting in excessive accumulation and P toxicity. On the other hand, with 0.1 mM orthophosphate, salinity decreased orthophosphate concentration in mature leaves. This effect was paralleled by decreases in the concentration of adenosine 5'-triphosphate and in the energy charge of the adenylate system, indicating an orthophosphate deficit. Even so, plants survived salinity better under these conditions than in the presence of 2 mM orthophosphate. The data indicated that salinity affected the phosphorylated state of the adenine nucleotides only indirectly through its effect on the concentration of orthophosphate in the cells. Salinity, especially in the presence of 2 mM orthophosphate, resulted in an increase in the concentrations of sugar phosphates in mature photosynthesizing leaves, suggesting that translocation rather than photosynthesis was a limiting process. Decreased translocation could be a secondary effect of decreased growth. However, a decreased translocation rate could cause decreased growth by limiting the supply of essential metabolites reaching growing tissues (Nieman and Clark, 1976). Another study was undertaken by Navarro et al. (2001) on melon seedlings (Cucumis melo L. cv.Galia) in hydroponics to understand the effect of salinity (80 mmol L-1NaCl) on phosphate (Pi) uptake and translocation at two levels of Pi (25 μmol L-1 and 1 mmol L-1). Salinity decreased Pi uptake at low Pi (high affinity uptake mechanism), 25 μmol L-1, although no specific competitive inhibition of Pi uptake by Cl− was observed. When plants were grown with high Pi (1 mmol L-1), the uptake of Pi through the low affinity system was increased by 80 mmol L-1 NaCl. Salinity also reduced the phosphorus flux, as Pi, through the xylem. It was hypothesised that high levels of NaCl decreased the mobility of Pi stored in vacuoles, and as a result, inhibited export from this storage compartment to other parts of the plant.
The effects of external salt and inorganic phosphate (Pi) on the concentrations of vacuolar Pi, and cytoplasmic Pi, ATP, glucose-6-phosphate and UDP-glucose in maize root tips were examined using 31P nuclear magnetic resonance spectroscopy. It was observed that more than two-fold stimulation of Pi uptake from 10 millimolar KH2PO4 solutions took place when root tips were exposed to 100 millimolar NaCl + CaCl2. This stimulation of Pi uptake was associated with an increase in the concentration of cytoplasmic Pi in root tip cells. Thus, the molar ratio of cytoplasmic Pi to Pi + ATP + glucose-6-phosphate + UDP-glucose increased greatly in root tips exposed to salt and Pi. It has been thus speculated that it is this disturbance in relative concentrations of cytoplasmic phosphates (which are normally tightly regulated) that is responsible for both the greater rate of uptake of Pi by vacuoles of excised maize root tips, and the previously documented stimulation of Pi translocation from root to shoot in whole maize plants exposed to salt and Pi (Roberts et al., 1984).
Gibson (1988) observed that adequate phosphorus nutrition was essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed wheat.
Micronutrients: The concentrations of micronutrients in the soil solutions, with the exception of Cl, seem to be low and depend on the physical and chemical characteristics of the soil bodies. The availability of most micronutrients depends on the pH of the soil solution as well as the nature of binding sites on organic and inorganic particle surfaces. In saline soils, the solubility of micronutrients such as Fe, Mn, Zn, and Cl is particularly low and plants grown in these soils often experience deficiencies in these elements (Alam, 1999, Page et al., 1990). Nevertheless, the micronutrient concentration in plant shoots may increase, decrease, or have no effect depending on the type of plant tissue, salt tolerance of plant species, salinity, micronutrient concentration, environmental conditions, and/ or abrupt changes in the permeability of the crop cell membranes.
Both Fe and Mn contents were reported to increase in all parts of the salt-treated peanut plants (Chavan and Karadge, 1980). The increase in Fe contents was more prominent than that of Mn. Salinity increased the Fe concentration in the shoots of pea and rice and decreased its concentration in the shoots of barley and corn (Hassan et al., 1970). In other investigations with barley, salinity had no effect on shoot Fe concentrations, but at low Ca, salinity increased root Fe in certain barley (Hordeum vulgare L.) species (Suhayda et al., 1992). Salinity increased the manganese concentration in the shoots of barley, rice sugar beet (Beta vulgaris L.), soybean, and tomato plants, but decreased in concentrations in the shoots of barley, squash, pea, and corn (Alam, 1999) plants. Saline solutions rich in divalent cations increase shoot Mn concentration, whereas a saline environment dominated by monovalent cations reduces shoot Mn concentration.
A range of soil conditions have been associated with binding of Zn in less plant available forms: high pH (7.0), prolonged submergence and low redox potential common in coastal soils, high organic matter and bicarbonate content, high Mg:Ca ratio, and high available P. High soil pH and bicarbonate appear to be the main factors associated with the widespread Zn deficiency in calcareous soils of the Indo-Gangetic Plains of India and Pakistan (Qadar, 2002), whereas perennial wetness and low redox potential are the major causes of Zn deficiency in peat and coastal saline soils (Neue and Lantin, 1994; Quijano-Guerta et al., 2002). One remarkable feature of Zn-deficiency tolerance in rice is that the trait seems to be associated with tolerance of other abiotic stresses. For example, on-site selection on calcareous soils of northern India produced several cultivars with tolerance of high pH, high bicarbonate, and low Zn availability (Singh et al., 2003); while rice germplasm identified with tolerance of Zn deficiency from large-scale screening at IRRI often showed cross-tolerance of salinity, P deficiency, and peat soils (Quijano-Guerta et al., 2002). The reasons behind this cross-tolerance are still unknown and could be attributed to better Zn acquisition when Zn is the most limiting factor.
Zinc concentration has been found to increase in salt-stressed bean, barley, soybean, squash, and tomato plants but to decrease in corn and mesquite (Prosopis juliflora L.) plants. Zinc (Zn) deficiency is the most widespread micronutrient disorder in rice (Oryza sativa), but efforts to develop cultivars with improved tolerance have been hampered by insufficient understanding of genetic factors contributing to tolerance. Wissuwa et al. (2006) observed that low-Zn nutrient solution and Zn-deficient field did not produce similar tolerance rankings in a set of segregating lines, which suggested that rhizosphere effects were of greater importance for lowland rice than internal Zn efficiency.
Boron is highly toxic to almost all plants. Sensitive plants may be affected at concentrations > 0.50 gm-3 in soil saturation extract (Maas, 1986).
Trees, grasses and halophytes, etc.
Yuncai et al. (2005) discussed current knowledge of the characteristics of the spatial distribution patterns of the mineral elements along the growing grass leaf and of the impact of salinity on these patterns. Their hypothesis was that a causal link exists between ion deficiency and/ or toxicity, and the inhibition of leaf growth of grasses in a saline environment was tested by them at CSIRO. Trees growing along windy coasts often have canopies that are greatly reduced in size by the sculpting effects of wind and salt spray. Trees with environmentally reduced stature are called elfinwood (windswept shrub-form or krummholz) and are ecologically important because they represent outposts growing at the limit of tree success. The purpose of this study (Barrick, 2009) was to assess if Banksia grandis elfinwood growing at Cape Leeuwin had a different nutrient status than normal low-form (LF) trees growing nearby, and if nutrient deficiencies, toxicities and/or imbalances were among the limiting factors imposed on elfinwood. The concentrations of N, P, K, Ca, Mg, Na, Cl–, Fe, Mn, Zn, Cu, Mo and B were analysed for mature green foliage, immature foliage, foliage litter, flowers and soil. When the elfinwood and LF trees were compared, the foliar nutrient status was generally similar, except that elfinwood foliage had significantly higher mean concentrations of N, Zn and Cu, while LF trees had higher Fe and Mn contents. Many nutrients were conserved before leaves were shed in both elfinwood and LF trees, including N, P, K, Na, Cl–, Mn and Cu (LF trees also conserved Ca and Mg). However, elfinwood and LF tree-litter contained significantly higher Fe concentrations than green foliage (elfinwood litter also had higher levels of Mg and B). It was suggested that the translocation of Fe into leaves before they were shed is a regulation mechanism to prevent Fe toxicity, or imbalance in the Fe : Mn ratio. Proteoid roots strongly acidify the soil to mobilize P, which also chemically reduces Fe+3 to plant-available Fe+2. The increased supply of Fe+2 in the rhizosphere, caused by the action of proteoid roots, might tend to defeat self-regulation of Fe uptake. It is possible that excess Fe accumulation in the plant might be regulated, in part, by exporting Fe into the leaves before they are shed. The nutrient status of B. grandis elfinwood could be compared with mountain elfinwood of North America. The extreme habitat of coastal elfinwood provides many theoretical pathways for nutrient limitation, but B. grandis elfinwood at Cape Leeuwin does not appear to be nutrient deficient.
Halophytes have not received the attention glycophytes have in the area of salinity-mineral nutrient relations. Nevertheless, some halophytes may show symptoms selectively from soil solutions dominated by Na+ and Cl− (Grattan and Grieves, 1992). There is increasing interest in superior halophytes for use on saline turfgrass, forage, and land reclamations sites. Lee et al. (2007) investigated halophytic seashore paspalum (Paspalum vaginatum Swartz) ecotype responses for inorganic ion uptake in shoot tissues and identify total and individual inorganic ion contributions to total solute potential ( s) adjustment under increasing salinity. It was observed that increasing salinity resulted in reduced uptake of K, Ca, and Mg with Mg most affected. Sodium tissue content increased with salinity, while Cl increased but then declined. The least salt-tolerant ecotypes, Adalayd, exhibited lower Cl uptake at high salinity compared to the most salt-tolerant types (SI 93-2, HI 101), but trends of other nutrients and Na under increasing salinity were not related to salinity tolerance of ecotypes. Shoot and root growth were highly correlated to K tissue content. The shoot-tissue content relationships of K, Mg, and Ca to increasing salinity provided insight into nutritional programs on salt-affected sites for this species. Inorganic ions contributed 57 to 97% to osmotic adjustment, indicating that seashore paspalum can readily use inorganic ions in a saline site for maintenance of more favorable plant water relations. In terms of physiological traits that may be useful in salt-screening protocols for this species, determination of K contribution to total s in either percent or MPa units was not useful and the same conclusion could be made for the other inorganic ions. While it appears that high percent or MPa contributions of K by the least tolerance type may be a negative screen for salt sensitivity, there appears to be a secondary effect related to the inability of salt-sensitive ecotypes to produce organic photosynthates.
Interactive and antagonistic effects of salinity and nutrients
In an exhaustive review Reddy and Iyengar (1999) observed that some cations influence the uptake of other cations by plants. Such antagonism occurs between K+ and Na+ and Ca2+ or Mg2+. These effects may be involved in the occurrence of nutritional disorders in plant tissues. There is abundant evidence that Na+ and the Na/Ca ratio can affect K+ uptake and accumulation within plant cells and organs. Salt tolerance appears to be correlated with the selectivity of K+ uptake over Na+. It was found that the growth of cultured citrus cells in various NaCl and CaCl2 concentrations was a function of the internal K+ concentration independent of the NaCl concentration. K+ application can reduce the deleterious effect of salinity on plant growth and development. However, contradictory results on the effects of K+ fertilization under saline conditions on field crops have been reported. Potassium uptake by plants is affected by high salinity and the Na+ concentration in the soil. Under saline conditions, a high Ca2+ supply alleviated the inhibition of NO3 uptake and increased Na/K selectivity (Reddy and Iyengar, 1999).
P translocation from roots to young shoots increased in the presence of an additional supply of Ca2+ (Reddy and Iyengar,1999). The effect of salinization on P nutrition depends on the available P in the substrate. A low supply of P to young tissues could become a limiting factor to their growth under saline conditions. An increased Ca2+ supply to the plant could be more efficient than P fertilization in restoring the P supply to young tissues under saline conditions. High levels of CaCl2 in the nutrient media resulted in a greater increase in the Ca concentration and reduced in the K+ and Mg2+ levels in the tissues of bean plants. On the other hand, the addition of K as KCl increased K+ and decreased Ca2+ and Mg2+ concentrations in maize plants. Corn (Z. mays L.) plants tested in the same study responded differently to different Cl salts. Plant growth was better in the presence of CaCl2 than with any combination of NaCl, MgCl2, or KCl at a comparable osmotic pressure (Reddy and Iyengar,1999). A high Ca2+ content depressed the K+ and Mg2+ levels; however, a mixed solution (Ca, K, Mg, Na) corrected the imbalance. Elevated Ca2+ levels may protect the plant from NaCl toxicity by reducing the displacement of membrane-associated Ca2+ (Cramer et al.,1985) by reducing Na+ uptake and transport to the shoots (Cramer et al., 1989) or by a combination of these effects. The Ca2+ also improves K+ uptake under NaCl salinity, effectively increasing the Na/K ratio in the tissues. An increase in the Cl- concentration in the nutrient media led to a reduction in the NO3 content of tomato plants (Kafkafi, 1984). However, an increase in the concentration of NO3 in the nutrient media, from 7.5 to 20 meq L-1, in the absence of Cl- had no effect on the NO3 concentration. It seems unlikely that the composition between H2PO4 and Cl- ions is important because of the greater differences in the physical and physiological properties of these ions (Champagnol, 1979).
The effect of salinity and fertilizer on grains and several vegetables may be either independent or additive when stress is imposed on them under moderate levels of nutrient deficiency and salinity. When either of these factors severely limits growth, the other has little influence on yield. Nutrient application increased the growth of the halophyte under saline conditions, presumably because salinity was moderately growth limiting. On the other hand, nutrient application did not improve the growth of the glycophyte under saline conditions, presumably because salinity was severely growth limiting. Under low salinity stress, nutrient deficiency limits plant growth more than salinity and a positive interaction or increased salt tolerance response occurs. Under moderate salinity, nutrient deficiency and salinity stress may equally limit plant growth and no interaction occurs. Under highly saline conditions, salinity limits growth more than fertilizer. According to Grattan and Grieve (1994), the plant performance would always exhibit a negative interaction or a decreased salt tolerance response if nutrient element was limiting growth under saline conditions and the upper salinity treatment was lethal or severely growth limiting. Salinity tolerance under suboptimal conditions is important only under dry land conditions, where high levels of fertilizers are not economical or the availability of fertilizer is limited (Okusanya and Unger, 1984). The disagreement between some of the publications dealing with the salinity-fertility relationship may stem from the use of different salinization techniques and the use of different chemicals to change the level of salinity. The difference between species varieties, the duration of the experiments, and growth conditions also could explain the variations in crop responses. Standardization of methodology, when feasible, could reduce variations between experiments. Chemical analysis of plant tissues and studies of the physiological and biochemical process involved in salinity-fertility relationship are essential to understand the nature of interactions.
Nutrient availability in acid sulphate soils
For potential and young acid sulphate soils improved nutrient availability is intimately connected with the reclamation strategies and the stage of reclamation. Following approaches are usually suggested for reclamation: (i) pyrite and soil acidity can be removed by leaching after drying and aeration, and (ii) pyrite oxidation can be limited or stopped and existing acidity inactivated by maintaining a high water table, with or without (iii) additional liming and fertilization with phosphorus, though liming may be often uneconomic in practical use. Upon waterlogging under item (ii) above, soil reduction caused by microbial decomposition of organic matter lowers acidity and may cause the pH to rise rapidly to near-neutral values. The less toxic and deeper developed older acid sulphate soils are moderately suitable for rice and can be improved by sound agronomic practices (Beek et al., 1980). In the Muda irrigation project in Malaysia, where patches of Sulfaquepts occur among better soils, improved water management and intensive irrigation have dramatically increased the productivity of these highly acid soils through improved nutrient availability.
In Sri Lanka addition of Gliricidia muculata in combination with phosphate and a small dose of inorganic fertilizer was effective to secure high rice yields (Deturckl et al., http://www2.alterra.wur.nl/Internet/webdocs/ilri-publicaties/publicaties/Pub53/pub53-h8.pdf). In India, for the coastal acid sulphate soils of Sundarbans, application of lime, superphosphate and rock phosphate have been found beneficial in improving the soil properties and rice growth (Bandyopadhyay, 1989). Application of Ca-rich oystershell, which is available in plenty, was found beneficial, if applied in powdered form, as an inexpensive alternative soil ameliorating agent. In this soil continuous submergence for one year could not improve the soil properties substantially.
Mongia et al. (1989), while reporting for two soils in Andaman Islands, observed that application of lime and phosphorus may be beneficial for lowland rice. In another study on mangrove (Avecenia marina) muds in this island, it has been reported that liming significantly depressed the concentrations of Al, Mn and Fe. Exchangeable Al content also decreased with lime application. The depression of exchangeable Al may be due to precipitation of trivalent Al and Al (OH)3 in the presence of high concentration of OH- ions. Lime application, in general, also reduced the exchangeable and extractable Fe contents of the soil.
In the rice-based systems of the Mekong Delta in Viet Nam, there is a trend towards replacing the traditional rice double cropping with a rotation of wet season rice and dry season upland crops (vegetables and tuber crops). However, under the prevailing acid sulphate soils, the build-up of excessive concentrations of exchangeable aluminium (Al3+) during the aerobic soil phase strongly limit upland cropping to few relatively Al-tolerant tuber crops that farmers grow on raised beds to enhance Al3+ leaching process. Study was conducted on the performance of major tuber crops (cassava, sweet potato and yam) in relation to soil exchangeable Al3+ concentration and as affected by the application of locally produced biogas sludge on farmers’ fields on a typical acid sulphate soil with observation plots laid out on raised beds and the same categorised based on the initial exchangeable Al3+ content of the topsoil in classes of <10, 10–15, and >15 meq Al3+ 100−1 g. Biogas sludge was applied at 3 Mg ha−1 (dry matter) to tuber crops and compared with an unamended control. It was found that biogas sludge tended to reduce soil exchangeable Al3+ concentrations but significantly increased the tolerance to given Al concentrations with higher tuber yield and P uptake in all tuber crops. However, Al-tolerant cassava showed stronger responses to amendment than Al-sensitive yam. It was concluded that in the absence of soil liming, the application of organic wastes can improve the performance of Al-tolerant while permitting the cultivation of more Al-sensitive crop on acid sulphate soils. Further research aims at identifying most appropriate substrate types and application rates for specific acid soil conditions and (Saleh et al., http://www.tropentag.de/2005/abstracts/links/Saleh_gFbcXBF1.pdf) crop tolerance levels.
Nutrient acquisition by plants under flooding
The ways in which flooding influences plant mineral nutrition are very complex, being determined by several concomitant flooding effects on the soil, initial soil conditions, and nutrient absorption mechanisms, as well as other physiological processes and responses of the particular plant species under study (Alam, 1999). Soil-and plant-produced C2H4 were suggested as factors in reducing root permeability and increasing resistance to water uptake by tobacco (Hunt et al., 1981). It has been reported that soil flooding stress may severely reduce water and ion uptake directly by increasing the permeability of roots to water and indirectly by reducing the size and volume of the plant roots, as well as by decreasing the ion uptake per unit weight of roots. Such uptake implies a disruption in root metabolism and reducing the effective root surface area available for ion uptake. Much evidence indicates that dysfunction in nutrient absorption by roots under flooding conditions is largely caused by a lack of O2 and attendant deleterious metabolic effects. The accumulation of dioxygen in the earth’s atmosphere allowed for evolution of aerobic organisms that use O2 as the terminal electron acceptor, thus providing a higher yield of energy compared with fermentation and anaerobic respiration. For example, in aerobic metabolism, the complete breakdown of one molecule of glucose yields a total of 38 molecules of adenosine triphosphate (ATP), whereas the anaerobic breakdown of this same glucose molecule to ethanol and CO2 yields only 8 molecules of ATP (Scandalios, 1993). Oxygen stress may inhibit guttation either by increasing the root resistance to water movement or by inhibition of ion transport to the xylem and arrest of vegetation and reproductive growth.
Nitrogen: Ammonia volatilization, denitrification, and leaching can lead to a major loss of nitrate from the soil solution, so that plants may suffer from nitrogen-deficiency symptoms. The normal aerobic N cycle is arrested in the mineralization stage in anaerobic soils, since oxygen is not available for oxidation of NH4+-N. On the other hand, under aerobic conditions, the NH4+ -form of soil mineral N is oxidized to NO3, which may accumulate in the soil or be utilized by crops.
In a series of experiments, Drew and Lynch (1986) studied growth and nitrogen uptake patterns of wheat subjected to several anaerobic regimens. Waterlogged wheat plants exhibited reduced nitrogen concentration, chlorosis, and generally accelerated senescence of older leaves, with the latter two occurring with the onset of remobilization of N from old to young leaves. Generally a lack of O2 in the root environment causes an immediate decrease in nutrient uptake and redistribution of N from older to younger leaves (Epstein, 1972).
The crop plants, tolerant of flooding stress, often grow well and take up more available nutrients in response to flooding compared with the well-watered control plants. Such beneficial responses have been reported for rice (Oryza sativa L.) and for several species of flood-tolerant woody angiosperms and conifers (Hook et al., 1983). Several morphological adaptations of flood-tolerant species allow continued nutrient absorption under waterlogging conditions. Many flood-tolerant species initiate vigorous adventitious roots that proliferate most abundantly in the upper, well-aerated portion of submerged soil (Hook et al., 1983).
It is an accepted fact (Alam, 1999) that, under flooding stress, the N concentrations in plant parts of wheat (Triticum aestivum L.), barley (Hordeum vulgare L.), field corn (Zea mays L.), pea (Pisum sativum L.), cotton (Gossypium hirsutum L.), sunflower (Helianthus annuus L.), soybean (Glycine max. L. Merrill), subterranean clover (Trifolium subterraneum L.), bent grass (Agrostis stolonifera L.), orchard grass (Dactylis glomerata), perennial ryegrass (Lolium perenne L.), timothy (Phleum pratense L.), fescue (Festuca arundinacea L.), avocado (Persea americana L.), sweet gum (Liquidambar styraciflua L.), hackberry (Celtis laevigata), and orange (Citrus sinensis L.) are significantly decreased. One contributor to the reduction of N in tissues of flooded plants is that, in waterlogged soils, NO3-N is rapidly depleted as oxygen is quickly consumed by soil biota and anaerobic conditions develop. As a result, volatilization and loss of N are promoted through denitrification in which nitrates serve as a terminal electron acceptor for anaerobic microbes (Ponnamperuma, 1972).
Phosphorus: Phosphorus is one of the most important nutrient element in the growth and development of plants. It plays a key role in cellular energy transfer, respiration, and photosynthesis. Phosphorus is present in nucleic acids, phospholipids, and sugar phosphate. A phosphorus deficiency causes immediate and severe disruptions of metabolism and development (Epstein,1972)
The P composition of flooded plants, like the N composition, is greatly influenced by both soil conditions and plant uptake responses to soil inundation. Where the amounts of soluble P available in the soil are adequate, flooding stress of non-tolerant plants generally lowers both the tissue concentration and total content of P (Kozlowski and Pallardy, 1984). Flood-sensitive plants (Alam, 1999) that have shown the lower uptake of P are barley, cowpea, Citrus sinensis, corn, wheat, Helianthus annuus, Liquidambar styraciflua, maize, Persea americana, ryegrass, and jojoba (Simmondsia chinensis). These declines in P concentration have been attributed to the inhibited uptake under anaerobiosis (Drew and Lynch,1986). However, the situation is more complex for soils that are moderately or severely deficient in P. In such well-aerated soils, much P may be held in unavailable forms. When soil is flooded, soil pH moves toward neutrality and soil reduction levels increase; as a result, P can be released from insoluble adsorbed and bound forms (Ponnamperuma, 1972), thereby becoming more available for uptake by roots. The increase in the concentration of water-soluble phosphate and desorption of sulphate caused by flooding the soils may be the result of a decrease in anion exchange capacity and an increase in the bicarbonate concentration. Hence, if P uptake is not severely limited by the imposed level of anaerobiosis and the levels of soil P before flooding are not inordinately high, flooding can result in temporarily increased plant P content (Devitt and Francis, 1972)
Potassium: In general, the inhibitory effects of flooding stress on potassium uptake are similar to those for N (Alam, 1999). Severe inhibition of K uptake characteristically follows soil submergence, and this response may limit plant growth in certain flooded crops. It has generally been reported that on giving flooding stress to soils, the K content in crop plants, such as barley, wheat, corn, subterranean, clover, avocado (Persea americana), orchard grass and bent grass generally decreased. Reduction in K absorption is most likely attributable to the effects of anaerobiosis on uptake mechanisms of roots (Alam, 1999). If organic matter is available and cation exchange capacity of the soil is low, submergence may increase soluble K somewhat in the soil solution through displacement of exchangeable K from the exchange complex by competing ions (Jones and Etherington, 1970). However, the flood associated increases in K available to the plants are generally too small to overcome the large inhibitory effects of anaerobic conditions on K uptake by crop plants. There are, however, other reports to find greater efficiency of K fertilizer with increasing soil moisture of the growth medium. The K content also decreased even in flooded tolerant rice plants. Lack of K was found drastically to reduce the oxidizing power of rice roots.
Calcium, Magnesium & Sulphur: Flooding stress appears to have much less pronounced inhibitory effects on the accumulation of Ca and Mg than on N, P, or K (Alam, 1999). Hence, Ca and Mg concentrations are not altered as much by flooding as are those of N, P, and K. However, concentrations may decrease slightly and their total contents may decline appreciably because of severely reduced growth. It has been suggested that the absorption of Ca and Mg may be metabolically mediated, and therefore may be dependent on an adequate supply of O2, but some data suggested that Ca and Mg ions are actively extruded from the plasmalemma (Lauchli, 1979). Moreover, based on comparisons of mineral element, analyses of xylem exudate, and culture solutions, Trought and Drew (1980) suggested that Ca and Mg were excluded relative to water movement in anaerobically cultured wheat root systems and that the Ca and Mg contents of the exudate could be accounted for by simple mass flow. Accordingly, the lack of close coupling between active uptake mechanisms and Ca and Mg concentrations by crop plants may explain the reduced effect of flooding on tissue concentrations of these two secondary elements. Flooding of rice increased the tissue concentrations of Ca, N, P, and Fe, whereas those of Mg were not significantly altered. On the other hand, the concentrations of Ca and Mg were generally decreased in crop plants (Alam, 1999), such as wheat, corn, subterranean clover (Devitt and Francis, 1972), Persea americana, Celtis laevigata, Citrus sinensis, Helianthus annuus, and Agrostis stolonifera.
Sulphur occurs in most soils largely in organic forms and is subject to numerous biological transformations (Alam, 1999). All forms of organic S in soil contribute to S mineralization and greater availability to crops. Elemental S, which is biologically oxidized to H2SO4 under aerobic conditions, is often applied to reduce soil pH and dissolve insoluble nutrients. In calcareous soils with low organic matter, addition of organic matter with S stimulates S oxidation. Sulphur deficiency has been recognized as an important growth limiting factor for both dryland crops and wetland rice (Blair et al., 1979). Sulphur deficiency of wetland rice has been also reported in many Asian countries. Sulphate concentrations in most of the soil solutions decreased to 1.8 mg L-1 within 8 weeks of submergence (Islam and Ponnamperuma, 1982).
Sulphate-containing fertilizers applied to flooded or submerged rice soils may undergo reduction to sulphide in the paddy fields with the subsequent problem of plant availability and/ or H2S production. Reduced or flooded conditions in the paddy soil may also inhibit the oxidation of elemental S and render this form of fertilizer useless to rice. Incubation studies of S transformations in flooded soils are of limited value in predicting fertilizer reactions in the presence of rice plants because of the modifying effect of oxidized zone adjacent to the rice root. The change in pH range that occurs when a soil is flooded means a drastic change in sulphate adsorption. It is pH-dependent, with adsorption being negligible above the pH value of 6.5. The flooding of an acid soil raises the pH to an equilibrium value ranging from 6.7 to 7.2 (Zhu and Alva, 1993), and at this pH, sulphate adsorption is negligible. Since crop plants can take S only in the sulphate form (SO42-), the oxidation state of the S present in the paddy soil is important. Because of the reduction of sulphate to S, the availability of S to rice has been found to be lower under flooded than under upland conditions.
Flooding X Salinity interactions on H2S gas evolution: Bandyopadhyay et al. (1995) measured H2S production in coastal saline and flooded rice field in Sundarbans, West Bengal, India. They observed that sulphates constitute about 40-50 percent of salts in these soils undergoing reduction reactions resulting in transformation of sulphate salts into sulphides including H2S gas which is toxic to plants. It was estimated that during peak periods about 390 to 1180 cm3 of H2S gas per m2 per 24 hours (day time) escaped to the atmosphere from waterlogged rice field, extent of which depended on the soil salinity (ECe 5-9 dSm-1) values and dose of green manure(Sesbania aculeata) applied (0-5 Mg ha-1) in the experiment. Higher soil salinity and application of green manure increased the production of H2S. The concentration of P and micronutrients in both shoot and root of rice decreased considerably at higher concentration of H2S in irrigation water. The insoluble sulphides precipitated on root surfaces (Table 3) and possibly interfered with the growth and nutrient uptake by plants. The acid soluble metallic sulphide (FeS equivalent) content of soil increased from 221 mg per kg soil to 381 mg per kg soil as the H2S content of irrigation water increased from 0 to 80 ppm. The primary cationic constituent of the precipitate on root was Fe, the solubility of which increased considerably after submergence of soil accompanied by a sharp fall in the redox potential. It was inferred that besides the direct toxic effect of H2S gas the indirect effect of sulphate reduction in interfering with the nutrient uptake by plants was also responsible for decrease in yield of rice.
Table 3. Nutrient content in the extract of root coating (10 g fresh root per 20 ml 6N HCl) and the insoluble metallic sulphides (FeS equivalent) in soil at flowering stage of rice in pot culture experiment
H2S in irr. water
(ppm) Soil salinity, ECe 7.4 dSm-1 Soil salinity, ECe 3.8 dSm-1
Insol.
sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm) Insol. sulp.
(mg kg-1 soil) Conc. in the extract of root coating (ppm)
Fe
Mn
Zn
Cu Fe
Mn
Zn
Cu
0 221 2913 63 68 13 204 2217 52 71 11
20 253 8492 121 142 22 244 7992 98 125 20
40 280 11948 144 181 24 265 10731 127 174 20
60 310 15316 175 218 26 309 14986 160 215 25
80 381 17723 225 268 32 355 17265 196 257 30
LSD
(p=0.05) 26 150 4 5 3 21 161 4 6 2
Adapted from Bandyopadhyay et al., 1995
Micronutrients: Under submerged soil conditions, the iron and manganese solubilities generally increase as first the ferric and manganic forms are converted to the more reduced and soluble ferrous and manganous forms (Ponnamperuma, 1972). The
change in availabilities of these two elements in waterlogged soils is reflected in increased tissue concentrations by several plants, including wheat, cotton, and some other flood-intolerant plants. Some workers have not found any change or a decrease in the plant uptake of Fe and Mn (Alam, 1999) depending, of course, on the their concentrations in soil. However, the total content of Fe and Mn most often declines because of severely inhibited growth. Flooding of tolerant plants such as rice also increased the content of Fe and Mn.
The concentrations of several nutrients (Fe, Mn, Zn, Cu, and P) were higher in mycorrhizal wetland rice than in non-mycorrhizal rice under flooded conditions (Alam, 1999). Reduced aeration also restricts carbon dioxide escape from the soil causing, in alkaline soils, an increased bicarbonate concentration in the soil solution, which is known to reduce the availability of Fe in the plants. The bicarbonate ion is reported to inhibit the activity of cytochrome oxidase in the roots of soybean (Glycine max L.) and spinach (Spinacea oleracea L.).
A high soil moisture content caused poor utilization of Fe by peanut (Arachis hypogaea L.) from alkaline soil. However, Alam and Azmi (1989) have reported an increase in available Fe, Mn, and P in alkaline calcareous soil due to flooding. Factors including poor soil aeration, high concentration of phosphate, presence of heavy metals (Ni, Zn, Co, and Cr), extreme light, plant root damage, and viral incidence have also been reported to cause plants to fail either to absorb Fe from the soil or to utilize it efficiently.
Under flooding soil organic matter contributes to Fe and Mn availability through the formation of metallo-organic complexes with organic substances. This phenomenon may be attributed to the production of chelating agents from compost that generally keep the micronutrient elements soluble and, consequently, more available to crop plants. Increased Fe and Mn solubility in flooded soils benefits rice, which has a higher requirement for these elements, than the other plants. There was an increase in pH, CO32-, and DTPA extractable Fe and Mn on the submergence of a lowland rice plants (Dutta et al., 1989).
It has been observed that generally high levels of Fe are found in the soil solution of submerged soil. In dry soil that has low pH and abundant sulphate, extremely high amounts of soluble ferrous-Fe are found soon after submergence, causing bronzing of rice leaves (Tanaka et al., 1969). As the pH rises toward neutrality with prolonged flooding, Fe availability decreases, as does Fe toxicity. Excessive Fe can also interfere with the uptake of other nutrients. High concentrations of Fe in flooded soil can induce P-deficiency symptoms in rice plants. Manganese concentrations of flooded rice plants grown on certain soils may reach 3000 mg L-1, but visual toxicity symptoms on plants are unusual. Under flooding conditions, the solubility of Zn, Cu, B, and Mo generally change with time and growth environmental conditions. It has been found that, under flooded conditions, the production of organic complexing compounds and reductions of Fe and Mn tend to enhance the solubility of Zn and Cu in the growth media. Increase in soil pH in acidic to near neutral soils on submergence plays the most dominant role in depressing Zn and Cu availability in flooded rice soils, whereas the role of increased concentrations of CO2 and S, although considerable, is less than that of pH. Alkaline soils subjected to prolonged flooding exhibit increased Zn availability as pH declines with increasing soil reduction, which was also observed by Ponnamperuma (1972). On calcareous soils, the rice plants frequently exhibit Zn deficiency possibly as a result of Zn fixation.
Deficiency occurs primarily during early growth of the crop and during this period may be exacerbated by immobilization of Zn in roots by bicarbonate ions that are produced in alkaline soils soon after submergence (Tanaka et al., 1969). Submergence increases the accumulation of CO2 in soil solution resulting in an increase in the formation of H2CO3, HCO3, and CO3, and found to depress the Zn availability in flooded soils. In an experiment, the percentage decrease in Zn and Cu to 57 and 59%, respectively, in soil on submergence are partially due to their insoluble precipitation as sulphides, hydroxide, carbonate, phosphate, oxide, and chelate and their adsorption-precipitation by iron compounds (Dutta et al., 1989).
Long-term flooding of noncalcareous soils generally tends to increase the availability of Cu and Mo and depress that of Zn (Tanaka et al., 1969). It has been observed that tissue concentrations and the total content of Zn generally decline in flood-intolerant plants, such as wheat, corn, bent grass, and subterranean clover. In an experiment, it was found that urea and ammonium sulphate had more effects than ammonium nitrate on the availability of Zn under flooded conditions. The total contents of Cu and B decreases in plants and the growth is markedly inhibited. It has been reported that, under flooding conditions, the tissue concentration of Mo increased in the ear leaf of corn. The behaviour of Cu and Zn under flooded conditions seems to be complex; both decreases and increases in readily available forms have been reported. When a soil has undergone reduction by flooding, the breakdown of Fe and Mn oxides can provide an increased surface area with a high adsorptive capacity onto which Cu and Zn may be firmly adsorbed (Lu et al., 1982).
It has been reported by a number of workers that (Alam, 1999), under flooding stress or anaerobiosis conditions, the uptake and transport of Na ions generally increased in a number of crop plants, such as Persea americana, Citrus sinensis, cotton, sunflower, and jojoba. Contrary to this, some workers have reported substantial decrease in the Na content by crop plants like subterranean clover and Persea americana under flooding stress. Waterlogging of the root zone of tomato resulted in significantly higher concentration of Na ions in plant parts when tested at temperatures of 20 and 28 C (West and Taylor, 1980). This response is consistent with the current understanding of nutrient element metabolism in that plant roots are thought to extrude Na ions at the plasmalemma. It is also possible that under flooding stress, with the O2 depletion, exclusion of Na from the growth media becomes less efficient and the tissue Na concentrations rise away to a considerable extent.
There is some evidence that flooding stress effects on nutrient contents vary among the plant organs (Alam, 1999). Although not apparent in all studies, high concentration of elements in the roots of flooded plants may be coupled with decreased shoot concentration of some minerals like N, P, and K. To account for these responses, it has been suggested that reduced O2 availability to roots inhibited translocation of ions from roots to shoots, which decreased ion uptake. In control, the Na concentration in flooded plants was sometimes increased in shoots and decreased in roots. As root system of non-flooded plants in these studies had a higher Na concentration compared with shoots, metabolically related reduction in the efficiency with which Na is excluded from the shoot may occur in flooded plants.
Management options for improved nutrient status
For correcting nutrient disorder in plants grown on coastal problem soils, the basic approach remains that the soil may be reclaimed, in the first instance, through reclamation of the soil concerned, through various proven technologies identified, details of which will not be discussed being outside the domain of this paper, for improving the physical and physico-chemical properties conducive for sustainable and higher nutrient uptake by plants. It should be borne in mind that the effect of salinity and fertilizer on grains and several vegetables is independent or additive when stress is imposed on them. When either of these factors severely limit growth, the other has little influence on yield. The salient research findings are summarized below to work out future strategies in research and their applications for improved nutrient uptake in coastal saline soils.
I. Options for further research
Research may be intensified in the following areas based on the clues obtained so far.
A. Attempts should be made with long term strategies to decrease eutrophication, hypoxia and dead end zones by moderating particularly the use of fertilizers and pesticides in agriculture all over the globe. This influx of extra nitrogen has caused serious distortions of the natural nutrient cycle, especially where intensive agriculture and high fossil fuel use coincide.
B. Adequate phosphorus nutrition has been found essential for effective ion compartmentation by contributing to efficient carbohydrate utilization in salt-stressed plants. P translocation from roots to young shoots should increase in the presence of an additional supply of Ca2+. An increased Ca2+ supply to the plant could be more efficient than P fertilization itself in restoring the P supply to young tissues under saline conditions.
C. Elevated Ca2+ levels may protect the plant from NaCl toxicity by reducing the displacement of membrane-associated Ca2+ by reducing Na+ uptake and transport to the shoots or by a combination of these effects. The Ca2+ also improves K+ uptake under NaCl salinity, effectively improving on the Na/K value in the tissues. An increase in the Cl- concentration, on the other hand, in the nutrient media may lead to a reduction in the NO3 content of plants, observed in case of tomato. Under saline conditions, a high Ca2+ supply should alleviate the inhibition of NO3 uptake and increase Na/K selectivity.
D. Supplementation of Ca2+ may also improve the growth rate of the plants in the NO3 treatment based on which it may be suggested that NO3- is possibly a better N source than NH4+.
E. In trees increased supply of Fe+2 in the rhizosphere, caused by the action of proteoid roots, might tend to defeat self-regulation of Fe uptake. It is possible that excess Fe accumulation in the plant might be regulated, in part, by exporting Fe into the leaves before they are shed. The nutrient status of B. grandis elfinwood could be compared with mountain elfinwood of North America. The extreme habitat of coastal elfinwood provides many theoretical pathways for nutrient limitation.
F. In halophytes little work has been done in this area. It has been suggested from limited works so far that shoot-tissue content relationships of K, Mg, and Ca to increasing salinity should provide insight into nutritional programs on salt-affected sites for different species.
G. In coastal flooded saline soils measures should be taken to reduce volatilization loss, in particular, either through placement of N-source (urea) at sub-surface depth, through application of slow release source, through use of urease inhibitor, or by adjusting the time of application coinciding with the plants’ active growth stage, for higher N-uptake.
H. Under flooded condition, soil organic matter contributes to Fe and Mn availability through the formation of metallo-organic complexes with organic substances. This phenomenon may be attributed to the production of chelating agents from compost that generally keep the micronutrient elements soluble and, consequently, more available to crop plants. Increased Fe and Mn solubility in flooded soils benefits rice, which has a higher requirement for these elements, than the other plants. There is concomitant increase in pH, CO32-, and DTPA extractable Fe and Mn on the submergence of a lowland rice plants.
I. Under flooded conditions, the production of organic complexing compounds and reductions of Fe and Mn tend to enhance the solubility of Zn and Cu in the growth media. Contradictory results are also, however, available. When a soil undergoes reduction by flooding, the breakdown of Fe and Mn oxides may provide an increased surface area with a high adsorptive capacity onto which Cu and Zn may be firmly adsorbed in some soils. Thus, long-term flooding of noncalcareous soils though generally tends to increase the availability of Cu and Mo may depress that of Zn. Deficiency may also occur primarily during early growth of the crop due to immobilization of Zn in roots by bicarbonate ions that are produced in alkaline soils soon after submergence with increase in CO2 concentration in soil.
J. Plant roots and their interaction with the different abiotic and biotic soil components represent a key point in the acquisition of water and essential nutrients. However, anthropogenic effects on the environment – including soil and water deterioration and contamination – could alter these relationships. In addition to these, vegetable production presents diverse problems, which could be mitigated by the use of plant-growth promoting microorganisms (PGPMs). On the soil, PGPMs could contribute to solubilize and/ or acquire essential minerals, making scarce nutrients more available to the plant. On the host, they stimulate several physiological changes that could lead to a better growth and render the plant more tolerant to abiotic stresses. Amongst PGPMs, Azospirillum is one of the most studied genera. Even though it colonizes different plant species in an ample variety of soils, its favourable effects on vegetable germination, emergence and growth, have not been thoroughly studied. The review (Barrasi et al., 2007) describes the beneficial effects PGPM inoculation could have on vegetables growing either under normal or stressful conditions including salinity, with an emphasis on the use of Azospirillum. Focus was made on the recent advances on Azospirillum-plant interactions and the bacterial mechanisms of plant growth promotion.
K. For sustainable soil health in order to ensure improved plant nutrient status and its use by the plants the importance of improved soil quality in the coastal plains through higher SOC level of the soils, for which C sequestration is one of the important pathways, has been highlighted by various workers since low lying coastal soils may be a useful sink for higher organic carbon pool for the terrestrial system. Very recently, IRRI characterized lowland rice soils (excluding deepwater rice) in Asia in respect of soil quality (Haefele and Hijmans, 2009). However, the issue remains to be solved as to whether a coastal problem soil should be assessed for quality regardless of factors rendering it ‘problematic’ in nature.
II. Field tested technologies: Integrated plant nutrient management
With regard to soil fertility, the coastal soils are usually rich in available K and micronutrients (except Zn), low to medium in available N, and are having variable available P status (Bandyopadhyay et al., 1985, Bandyopadhyay, 1990, Maji and Bandyopadhyay, 1991). Major portion of the applied N fertilizer is, however, lost through volatilization (Sen and Bandyopadhyay, 1987) owing particularly to increase in soil salinity.
Effect of salinity on the microbial and biochemical parameters of the salt affected soils in Sundarbans (India) was studied at nine different sites. The study revealed that the average microbial biomass C (MBC), average basal soil respiration (BSR), and average fluorescein diacetate hydrolyzing activity (FDHA) were lowest during the summer season, indicating the adverse effect of soil salinity (Tripathi et al., 2006). It was therefore suggested that integrated approach towards nutrient management should be very effective for increasing use efficiency for higher and sustainable yield of crops (Bandyopadhyay et al., 2006, Tripathi et al., 2007). Bandyopadhyay and Rao (2001) were of the opinion to introduce systems approach involving organic, inorganic and biofertlizers compatible with the farmers’ practice.
In coastal soil at Tamil Nadu (India), application of agro-industrial wastes significantly improved soil organic carbon, pH, EC, and soil bacteria, fungus and actinomycetes population, and enhanced the soil fertility status (macro and micronutrients) along with improvement in crop productivity of finger millet. Application of pressmud @12.5 t ha-1 recorded better growth and yield of finger millet followed by composted coirpith @ 12.5 t ha-1 (Rangaraj et al., 2007).
Use of green manure crops like Sesbania spp. (S. aculeata) has been proved very suitable for saline alkali soils (Keating and Fisher, 1985, Rao, 1986, Evans and Rotar 1986). In favourable climate with proper management, these crops accumulate well over 100 kg N ha-1, mostly through biological N fixation, in 50-55 days, thereby increasing the yield of the following rice crop significantly (Singh et al., 1991). Likewise, application of green leaves of Glyricidia maculata @ 10 tonne ha-1 to the puddled soil before rice transplanting has been found comparable in terms of yield with inorganic fertilizer application of N, P and K at 100, 50 and 50 kg ha-1, respectively (Chavan and Dongale 1994). Buresh and De Datta (1991), however, were of the view that leguminous green manure and its residue normally are able to meet only partially the N requirement for the following high yielding rice variety. They focused attention on the high loss of N under the anaerobic-aerobic soil cycles typical of legume-lowland rice sequence, as well as on higher production of methane and nitrous oxide from the low land rice field. However, in view of rising energy cost and limited input availability, recycling of organic wastes and use of renewable sources of biofertilizers, viz. rhizobium cultures for pulse or legume, and blue-green algae for waterlogged rice field may play a significant role in terms of integrated nutrient management for rice in coastal saline soils (Kundu and Pillai, 1992).
Long term field experiment in coastal saline soils in India showed that rice and wheat yield could be maintained even at 50% NPK used in conjunction with FYM or green manure (DARE, 2003-04). In a detailed long term experiment conducted (CSSRI, 1990) in Sundarbans (India) it was observed that grain yield of crops in a rice-barley rotation increased significantly only due to the application of N. Application of P did not show any significant increase in the yield of crops in the initial 8 years, after which the yield of barley alone increased due to P application. Available K content was high in the soil. The experiments conducted so far showed that a basal dose of 11 kg P ha-1 for rice and 5.5 kg P ha-1 for barley or for similar upland crops should maintain the fertility status of the soil, whereas the K application may be omitted without any detrimental effect on soil fertility or crop growth. The K removal by the crop was compensated by K added through accumulation and release of non-exchangeable sources.
For acid sulphate soils addition of Gliricidia muculata in combination with phosphate and a small dose of inorganic fertilizer was effective to secure high rice yields in Sri lanka. In India, for the coastal acid sulphate soils of Sundarbans, application of lime, superphosphate and rock phosphate have been found beneficial in improving the soil properties and rice growth (Bandyopadhyay, 1989). Use of biogas sludge or other organic substrates may be further explored for removing Al-toxicities in acid sulphate soils and improving soil nutrient availabilities in acid sulphate soils as observed in Mekong delta in Viet Nam.
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