Effects of Micronutrient and Trace Elements on Arid, Saline, and Saline Sodic Soils and Their Relationships with Different Crop Types.

Arid, saline and saline sodic soils are common in many parts of the world, especially in arid and semi-arid regions. These soils pose significant challenges for crop production, as they limit the availability of water and nutrients, and cause salt stress and toxicity to plants. Micronutrient and trace elements are essential for plant growth and development, but their availability and uptake in these soils are often affected by various factors, such as soil pH, salinity, sodicity, organic matter, cation exchange capacity, and soil microbial activity. Moreover, different crop types have different requirements and responses to micronutrient and trace elements, depending on their physiological and biochemical characteristics, as well as their adaptation and tolerance mechanisms. Therefore, understanding the effects of micronutrient and trace elements on arid, saline and saline sodic soils and their relationships with different crop types is important for developing effective management practices and improving crop productivity and quality in these soils.

Micronutrient and Trace Elements in Arid, Saline and Saline Sodic Soils

Micronutrient and trace elements are those elements that are required by plants in small amounts, usually less than 100 mg kg-1 of dry matter. They include zinc (Zn), iron (Fe), manganese (Mn), copper (Cu), boron (B), molybdenum (Mo), nickel (Ni), chlorine (Cl), and cobalt (Co). Some of these elements, such as Zn, Fe, Mn, and Cu, are also essential for human and animal nutrition, and their deficiency can cause health problems. Trace elements are those elements that are present in soils in very low concentrations, usually less than 1 mg kg-1 of dry matter. They include selenium (Se), iodine (I), arsenic (As), cadmium (Cd), mercury (Hg), lead (Pb), and chromium (Cr). Some of these elements, such as Se and I, are beneficial for human and animal health, while others, such as As, Cd, Hg, Pb, and Cr, are toxic and can cause environmental and health hazards.

The availability and distribution of micronutrient and trace elements in arid, saline and saline sodic soils are influenced by various factors, such as soil texture, organic matter, pH, redox potential, cation exchange capacity, clay minerals, carbonate, gypsum, salt content, and irrigation water quality. Generally, arid, saline and saline sodic soils tend to have low levels of micronutrient and trace elements due to the following reasons:

- Low organic matter content, which reduces the chelation and complexation of micronutrient and trace elements and their retention in soil.

- High pH, which increases the precipitation and adsorption of micronutrient and trace elements and reduces their solubility and mobility in soil.

- High salinity, which increases the competition and antagonism between micronutrient and trace elements and other ions, such as sodium (Na), calcium (Ca), magnesium (Mg), and sulfate (SO4), and reduces their uptake by plants.

- High sodicity, which deteriorates the soil structure and permeability, and affects the diffusion and mass flow of micronutrient and trace elements to plant roots.

- Low rainfall and irrigation, which limits the leaching and replenishment of micronutrient and trace elements in soil.

- High evaporation and transpiration, which concentrates the salts and micronutrient and trace elements in the soil surface and creates salinity and toxicity problems.

However, some arid, saline and saline sodic soils may also have high levels of micronutrient and trace elements due to the following reasons:

- High parent material content, which contributes to the natural occurrence and accumulation of micronutrient and trace elements in soil.

- Low crop removal, which reduces the depletion and export of micronutrient and trace elements from soil.

- High irrigation water content, which adds micronutrient and trace elements to soil from external sources, such as fertilizers, pesticides, sewage, industrial effluents, and atmospheric deposition.

- Low pH, which increases the solubility and mobility of micronutrient and trace elements in soil.

Arid, saline and saline sodic soils can affect the availability and uptake of micronutrient and trace elements in different ways, as discussed below.

• Soil salinity refers to the concentration of soluble salts in the soil, mainly sodium (Na), chloride (Cl), calcium (Ca), and magnesium (Mg). Soil salinity can reduce the availability and uptake of micronutrient and trace elements by crops, as it can cause osmotic stress, ion toxicity, ion imbalance, and nutrient antagonism. For example, high levels of Na and Cl can reduce the uptake of Fe, Zn, Cu, and Mn by crops, as they can compete with them for the transport sites in the roots and shoots. High levels of Ca and Mg can reduce the uptake of B and Mo by crops, as they can form insoluble complexes with them in the soil. High levels of soil salinity can also lower the soil pH, which can affect the solubility and mobility of micronutrient and trace elements in the soil.
• Soil sodicity refers to the proportion of sodium (Na) in the soil exchange complex, relative to other cations such as calcium (Ca) and magnesium (Mg). Soil sodicity can affect the availability and uptake of micronutrient and trace elements by crops, as it can cause physical and chemical degradation of the soil structure, such as dispersion, swelling, crusting, and sealing. These processes can reduce the soil porosity, permeability, aeration, and drainage, and thus limit the root growth and water and nutrient uptake by crops. Soil sodicity can also increase the soil pH, which can affect the solubility and mobility of micronutrient and trace elements in the soil. For example, high soil pH can reduce the availability of Fe, Zn, Cu, and Mn by crops, as they can form insoluble hydroxides and oxides in the soil. High soil pH can also increase the availability of B and Mo by crops, as they can become more soluble and mobile in the soil.

Relationships between Micronutrient and Trace Elements and Different Crop Types

Different crop types have different requirements and responses to micronutrient and trace elements, depending on their physiological and biochemical characteristics, as well as their adaptation and tolerance mechanisms. Some crop types are more sensitive and susceptible to micronutrient and trace element deficiency or toxicity than others, and some crop types can accumulate and translocate micronutrient and trace elements more efficiently and effectively than others. Therefore, understanding the relationships between micronutrient and trace elements and different crop types is important for selecting suitable crops and optimizing their growth and yield in arid, saline and saline sodic soils.

• Cereals: Cereals are the most widely grown and consumed crop types in the world, and they include wheat, rice, maize, barley, sorghum, millet, and oats. Cereals are generally tolerant to salinity and sodicity, but they vary in their sensitivity and susceptibility to micronutrient and trace element deficiency or toxicity. Wheat and barley are more prone to Zn and Fe deficiency than rice and maize, while rice and maize are more prone to B and Mo toxicity than wheat and barley. Sorghum and millet are more tolerant to Zn and Fe deficiency than wheat and barley, but they are more sensitive to Mn and Cu deficiency than rice and maize. Oats are more tolerant to B and Mo toxicity than wheat and barley, but they are more sensitive to Zn and Fe deficiency than rice and maize. Cereals can accumulate and translocate micronutrient and trace elements more efficiently and effectively than other crop types, especially Zn, Fe, Mn, and Cu, and they can enhance the bioavailability and biofortification of these elements in the food chain.
• Legumes: Legumes are the second most widely grown and consumed crop types in the world, and they include soybean, bean, pea, lentil, chickpea, peanut, and alfalfa. Legumes are generally sensitive to salinity and sodicity, but they vary in their sensitivity and susceptibility to micronutrient and trace element deficiency or toxicity. Soybean and bean are more prone to Zn and Fe deficiency than pea and lentil, while pea and lentil are more prone to B and Mo toxicity than soybean and bean. Chickpea and peanut are more tolerant to Zn and Fe deficiency than soybean and bean, but they are more sensitive to Mn and Cu deficiency than pea and lentil. Alfalfa is more tolerant to B and Mo toxicity than soybean and bean, but it is more sensitive to Zn and Fe deficiency than pea and lentil. Legumes can accumulate and translocate micronutrient and trace elements more efficiently and effectively than other crop types, especially B, Mo, and Ni, and they can enhance the bioavailability and biofortification of these elements in the food chain. Legumes can also fix atmospheric nitrogen and improve the soil fertility and quality in arid, saline and saline sodic soils.
• Oilseeds: Oilseeds are the third most widely grown and consumed crop types in the world, and they include sunflower, rapeseed, canola, safflower, sesame, and flax. Oilseeds are generally tolerant to salinity and sodicity, but they vary in their sensitivity and susceptibility to micronutrient and trace element deficiency or toxicity. Sunflower and rapeseed are more prone to Zn and Fe deficiency than canola and safflower, while canola and safflower are more prone to B and Mo toxicity than sunflower and rapeseed. Sesame and flax are more tolerant to Zn and Fe deficiency than sunflower and rapeseed, but they are more sensitive to Mn and Cu deficiency than canola and safflower. Oilseeds can accumulate and translocate micronutrient and trace elements more efficiently and effectively than other crop types, especially Se, Co, and Cl, and they can enhance the bioavailability and biofortification of these elements in the food chain.
• Fruits and vegetables: Fruits and vegetables are the fourth most widely grown and consumed crop types in the world, and they include tomato, potato, onion, garlic, carrot, beetroot, spinach, lettuce, cabbage, cauliflower, broccoli, apple, pear, peach, apricot, plum, grape, citrus, banana, and date. Fruits and vegetables are generally sensitive to salinity and sodicity, but they vary in their sensitivity and susceptibility to micronutrient and trace element deficiency or toxicity. Tomato and potato are more prone to Zn and Fe deficiency than onion and garlic, while onion and garlic are more prone to B and Mo toxicity than tomato and potato. Carrot and beetroot are more tolerant to Zn and Fe deficiency than tomato and potato, but they are more sensitive to Mn and Cu deficiency than onion and garlic. Spinach and lettuce are more tolerant to B and Mo toxicity than tomato and potato, but they are more sensitive to Zn and Fe deficiency than onion and garlic. Cabbage, cauliflower, and broccoli are more prone to Zn and Fe deficiency than apple and pear, while apple and pear are more prone to B and Mo toxicity than cabbage, cauliflower, and broccoli. Peach, apricot, and plum are more tolerant to Zn and Fe deficiency than apple and pear, but they are more sensitive to Mn and Cu deficiency than cabbage, cauliflower, and broccoli. Grape and citrus are more tolerant to B and Mo toxicity than apple and pear, but they are more sensitive to Zn and Fe deficiency than cabbage, cauliflower, and broccoli. Banana and date are more tolerant to salinity and sodicity than other fruits and vegetables, but they are more prone to Zn and Fe deficiency and B and Mo toxicity than other fruits and vegetables. Fruits and vegetables can accumulate and translocate micronutrient and trace elements more efficiently and effectively than other crop types, especially Zn, Fe, Mn, Cu, B, Mo, and Se, and they can enhance the bioavailability and biofortification of these elements in the food chain.

Pros and Cons of Micronutrient and Trace Elements in Arid, Saline and Saline Sodic Soils and Their Relationships with Different Crop Types

Micronutrient and trace elements in arid, saline and saline sodic soils and their relationships with different crop types have both pros and cons, depending on the type, level, and balance of these elements, as well as the crop type, growth stage, and environmental condition. Some of the pros and cons are summarized below.

• Pros: Micronutrient and trace elements can enhance the plant growth and development, crop productivity and quality, stress tolerance and adaptation, and human health and nutrition in arid, saline and saline sodic soils and their relationships with different crop types. For example, Zn and Fe can improve the photosynthesis, respiration, enzyme activation, hormone synthesis, and chlorophyll formation in plants, and the protein, carbohydrate, and vitamin content in crops, as well as the anemia, immunity, and cognitive function in humans. B and Mo can improve the cell wall structure, membrane stability, sugar transport, and nitrogen fixation in plants, and the sugar, oil, and amino acid content in crops, as well as the bone, teeth, and thyroid health in humans. Se and Co can improve the antioxidant defense, detoxification, and metabolism in plants, and the oil, fatty acid, and vitamin content in crops, as well as the cancer prevention, thyroid function, and blood pressure regulation in humans.
• Cons: Micronutrient and trace elements can also impair the plant growth and development, crop productivity and quality, stress tolerance and adaptation, and human health and nutrition in arid, saline and saline sodic soils and their relationships with different crop types. For example, Zn and Fe deficiency can cause chlorosis, stunting, wilting, and yield reduction in plants, and the low quality and quantity of crops, as well as the growth retardation, immune deficiency, and cognitive impairment in humans. B and Mo toxicity can cause necrosis, leaf curling, root damage, and yield reduction in plants, and the low quality and quantity of crops, as well as the skin rash, diarrhea, and liver damage in humans. Se and Co toxicity can cause oxidative stress, chlorosis, necrosis, and yield reduction in plants, and the low quality and quantity of crops, as well as the hair loss, nail deformation, and nerve damage in humans.

Management Practices to Improve the Effects of Micronutrient and Trace Elements on Arid, Saline and Saline Sodic Soils and Different Crop Types

Several management practices can be adopted to improve the effects of micronutrient and trace elements on arid, saline and saline sodic soils and different crop types, such as:

• Soil testing and plant analysis, which can help to diagnose the status and requirement of micronutrient and trace elements in soil and crops, and to guide the application of appropriate amendments and fertilizers.
• Soil amendment and reclamation, which can help to improve the physical, chemical, and biological properties of arid, saline and saline sodic soils, and to enhance the availability and uptake of micronutrient and trace elements by crops. For example, the addition of organic matter, gypsum, lime, sulfur, and biochar can help to improve the soil structure, reduce the soil pH, increase the soil cation exchange capacity, and chelate and solubilize the micronutrient and trace elements in soil.
• Fertilizer application and management, which can help to supply the adequate and balanced amount and form of micronutrient and trace elements to crops, and to avoid the deficiency or toxicity problems. For example, the use of chelated, coated, or slow-release fertilizers can help to increase the efficiency and effectiveness of micronutrient and trace elements application, and to reduce the losses and environmental impacts.
• Irrigation water management, which can help to control the salinity and sodicity of soil and water, and to leach the excess salts and micronutrient and trace elements from soil. For example, the use of good quality water, drip irrigation, deficit irrigation, and alternate wetting and drying can help to reduce the water and salt stress on crops, and to conserve the water and nutrient resources.
• Crop selection and breeding, which can help to choose and develop the suitable and tolerant crop types and varieties for arid, saline and saline sodic soils, and to enhance the uptake and utilization of micronutrient and trace elements by crops. For example, the use of salt-tolerant, drought-tolerant, and nutrient-efficient crops, such as halophytes, can help to improve the crop productivity and quality, and to reduce the input and management costs.
• Crop rotation and intercropping, which can help to diversify and optimize the crop production system, and to improve the soil and crop health. For example, the use of legumes, cereals, oilseeds, and forages can help to enhance the soil organic matter, nitrogen fixation, nutrient cycling, and pest and disease control, and to increase the yield and quality of crops.

Pros and Cons of Various Management Practices and Interventions to Improve the Soil Quality and Crop Productivity

Various management practices and interventions have been proposed and implemented to improve the soil quality and crop productivity in arid, saline and saline sodic soils, with respect to micronutrient and trace elements. Some of the most common and effective management practices and interventions are discussed below, along with their pros and cons.

• Soil leaching and drainage: This practice involves the application of good quality water to the soil surface, and the removal of excess water and salts through a drainage system. This practice can reduce the soil salinity and sodicity levels, and improve the soil physical and chemical properties, such as porosity, permeability, aeration, and drainage. This practice can also increase the availability and uptake of micronutrient and trace elements by crops, as it can lower the soil pH, and reduce the ion toxicity, ion imbalance, and nutrient antagonism. However, this practice can also have some drawbacks, such as the high cost and maintenance of the irrigation and drainage infrastructure, the scarcity and competition of good quality water, the environmental pollution and health risks of the drainage water, and the possible leaching and loss of micronutrient and trace elements from the soil.
• Soil amendment and amendment: This practice involves the addition of organic or inorganic materials to the soil, to improve the soil physical and chemical properties, and to supply or enhance the availability of micronutrient and trace elements. Some of the common soil amendments and amendments are gypsum, lime, sulfur, biochar, compost, manure, and crop residues. These materials can reduce the soil salinity and sodicity levels, and improve the soil pH, organic matter content, cation exchange capacity, soil texture, and soil moisture. These materials can also increase the availability and uptake of micronutrient and trace elements by crops, as they can provide or release these elements in the soil, or increase their solubility and mobility in the soil. However, this practice can also have some drawbacks, such as the high cost and availability of the soil amendments and amendments, the variability and uncertainty of their quality and efficacy, the possible negative interactions or side effects of some materials, and the possible accumulation and toxicity of some elements in the soil or plant tissues.
• Crop selection and improvement: This practice involves the selection or development of crop species and cultivars that are tolerant and efficient in growing and producing in arid, saline and saline sodic soils, and in acquiring and utilizing micronutrient and trace elements. Some of the traits and mechanisms that can confer tolerance and efficiency to crops are salt exclusion or sequestration, iron reduction or chelation, zinc or copper transporters or translocators, boron or molybdenum regulators or redistributors, and selenium or manganese accumulators or detoxifiers. These traits and mechanisms can be selected or improved by conventional breeding, genetic engineering, or gene editing techniques. However, this practice can also have some drawbacks, such as the long time and high cost of the crop selection or improvement process, the possible trade-offs or negative effects of some traits or mechanisms, the possible gene flow or biosafety issues of some techniques, and the possible consumer or market acceptance or preference of some crops.
• Crop management and nutrition: This practice involves the optimization of the crop management practices and inputs, to enhance the crop performance and productivity in arid, saline and saline sodic soils, and to improve the crop nutrition and quality with respect to micronutrient and trace elements. Some of the aspects and factors that can be optimized are the irrigation water quality and quantity, the fertilizer type and rate, the soil amendment and amendment rate, the crop rotation and intercropping, and the pest and disease control. These aspects and factors can be optimized by using the best available knowledge, technology, and tools, such as soil and water testing, crop modeling, precision agriculture, and integrated pest management. However, this practice can also have some drawbacks, such as the high cost and complexity of the optimization process, the variability and uncertainty of the environmental and economic conditions, the possible trade-offs or conflicts of some aspects or factors, and the possible overuse or misuse of some inputs or tools.

Micronutrient and trace elements are important for crop production and human and animal health, but their effects on arid, saline and saline sodic soils and their interaction and relationships with different crop types are complex and variable. There is a need for more research and development on this topic, to improve the understanding and management of micronutrient and trace elements in these soils, and to optimize their benefits and minimize their risks for crop production and food security. Some of the research priorities and challenges include:

- Developing and validating reliable and cost-effective methods for soil testing and mapping of micronutrient and trace elements in arid, saline and saline sodic soils.

- Identifying and selecting crop species and cultivars that are adapted and responsive to micronutrient and trace element application in these soils, and evaluating their agronomic and nutritional performance.

- Exploring and exploiting the potential of halophytes and other salt-tolerant plants for phytoremediation and biofortification of micronutrient and trace elements in these soils.

- Investigating and optimizing the effects of various agronomic practices, such as irrigation, fertilization, liming, biochar application, and intercropping, on the availability and uptake of micronutrient and trace elements, and their interaction and relationships with different crop types in these soils.

- Assessing and mitigating the environmental and health risks of micronutrient and trace element accumulation and leaching in these soils, and developing and implementing appropriate regulations and guidelines for their safe and sustainable use.

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