Bioremediation of Salt Affected Soils: An Indian Perspective

Food and nutritional security of the ever-growing world population in an era of climate change, widespread environmental degradation, water scarcity and the loss of productive lands has heightened the need for harnessing the potential of degraded lands and marginal quality water resources in agricultural production. Consistent with the fact that even marginal gains in the productivity of deteriorated, salt-affected lands could significantly improve the farmers’ livelihoods and environmental quality, a suit of agronomic and biological measures have been suggested to augment their agricultural efficiency. However, an analysis of the literature reveals drawbacks in the potential applications of many such conventional techniques rendering them somewhat ineffective in addressing these challenges. As fresh water is an indispensable input in the reclamation of saline and sodic lands, severe freshwater shortages have necessitated a refurbishing of the existing approaches to ensure the sustained use of marginal quality water in soil restoration. Climate change has emerged as a grave threat to the land and water resources, and agriculture in salt-affected environments is likely to be worst affected by the projected alterations in crop growth conditions. In this backdrop, this paper presents an analysis of the loopholes in currently pursued salinity management practices and delineates the future research priorities to augment and sustain the agricultural productivity in salt-affected soils.

Salt-affected soils include saline or sodic soil that covers over 400 million hectares, i.e. over 6 % of the world land area. In India, about 6.73 million hectares (Mha) of land is affected by salinity and sodicity problems. Coastal soils in a number of situations are constrained by various technological factors limiting the agricultural productivity and, therefore, merit attention. The numerous constraints need proper diagnosis of the problem and its appropriate remediation. The chapter highlights the soil resources of India and the distribution of problematic soils with their specific constraints for agricultural production. This gives complete background information on the properties, issues and problems associated with inland and coastal saline and sodic and waterlogged soils.

It is conjectured that about 1.2 billion ha of land around the world are affected with different levels of salinity. In India, about 6.7 million ha of land are affected by salt, including salinity and sodicity. Of this, about 3.77 million ha of land is affected specifically by sodicity. After reclamation of these soils, rice is recommended as a first crop due to certain adaptive traits. However, complementing the reclamation intervention with salt-tolerant high-yielding and acceptable rice varieties holds great promise and has delivered many dividends and impact under such conditions. Developing varieties that are suitable for specific environments is one of the major challenges for improving food security in rural resource poor communities. Thus, breeding rice plants for adaptation to salt stress situations has been proved to be an economically feasible and environmentally viable plant-based approach. There are a large number of rice varieties that are grown from coastal to inland saline and sodic ecosystems. Varieties suitable for one ecosystem may or may not be suitable for other ecosystems. Since the ecology of salt-affected environments differs from one area to another, it is imperative to develop cultivars that are adapted to specific environments. Farmers’ continued cultivation of traditional low-yielding rice varieties is due to the non-availability of improved salt-tolerant varieties. Plant breeders often consider yield, flowering duration, height and the ability to withstand salt stress as important traits while developing new varieties. However, farmers prefer other traits such as higher straw yield, suitable plant height, threshability, earliness, grain quality and disease resistance. Farmer’s participatory varietal selection (FPVS) approach adopted for developing salt-tolerant varieties has been proven to be an effective approach in harnessing productivity potential of varieties and increasing genetic diversity in crops. Keeping with these advantages as well as fast adoption of selected varieties and farmer-to-farmer seed networks, on-station and on-farm varietal development and dissemination studies were conducted under varying sodic environments in Uttar Pradesh, India, to harness productivity potential of salt-affected soils in the Indo-Gangetic river basin with available technologies. This paper covers the success story of farmer’s participatory varietal development, dissemination and adoption programme initiated at the Central Soil Salinity Research Institute, Regional Research Station, Lucknow, to enhance production and productivity of rice in the target sodic areas of Indo-Gangetic plains.

Agriculture is the principal lever of economic and social development. A significant amount of arable land is becoming lost to urban sprawl, forcing agricultural production into marginal areas. Salinity-related land degradation is becoming a serious challenge to food and nutritional security in developing countries. Many crops cannot be grown on salt-affected lands, but nature has provided us with a unique group of plants, that is, halophytes. Halophytes, plants that survive to reproduce in environments where the salt concentration is around 200 mM NaCl or more, constitute about 1 % of the world’s flora. Some halophytes show optimal growth in saline conditions; others grow optimally in the absence of salt. However, the tolerance of all halophytes to salinity relies on controlled uptake and compartmentalization of Na⁺, K⁺, and Cl⁻ and the synthesis of organic “compatible” solutes, even where salt glands are operative. The cultivation of economically useful halophytes has the potential to remediate saline wastelands and to meet the demands for fodder, fuel, etc., from saline lands, thereby helping the farming community to improve their livelihood.

The salt-affected soils are dominated by many types of halophilic and halotolerant microorganisms, spread over a large number of phylogenetic groups. The biotic approach ‘plant-microbe interaction’ to overcome salinity problems has recently received a considerable attention throughout the world. The halophilic microbes have potential for bioremediation of salt-dominant soils. Halophilic bacteria having plant growth promotion potential were isolated that could tolerate up to 15 % NaCl in liquid media. Soil inoculation showed their sustenance and activity up to electrical conductivity (EC) of 8 dS/m. Also, plant growth-promoting endophytic halophiles from leaves of halophyte plants have potential to remediate salt-affected soils. The efficient plant growth-promoting isolates were inoculated in seeds of maize and wheat to mitigate salt stress. There was 10–12 % increase in yield attributes and yield of wheat at 6 % NaCl irrigations in soil as compared to 2 % NaCl irrigations in experiments.

Salinization, recognized as one of the most devastating soil degradation threats on earth, has endangered the potential use of soil on almost an estimated land area of about 1 billion ha globally, representing about 7 % of earths continental extent of which about 20 % is cultivated land area. It is not only suppressing plant growth but is also disturbing the sustainability of beneficial microorganisms associated with the plant rhizosphere. The agricultural crops under salinity are known to exhibit a spectrum of responses ranging from crop yield declines to disturbance in ecological balance of the region. It is a major cause of land abandonment and aquifers for agricultural purposes. The impacts include poor agricultural productivity, low economic returns and soil erosions. PGPRs, which live in association with plant roots that alleviate salt stress for better growth and yield, through their own mechanisms for osmotolerance, osmolyte accumulation, asymbiotic N₂ fixation, solubilization of mineral phosphate and other essential nutrients, enhanced NPK uptakes, production of plant hormones, ACC production, scavenging ROS, ISR and IST, are an important alternative to traditional agricultural techniques. The present chapter focuses on the advantages of PGPR-based mechanics through an engineered increase in tolerance to salinity and conceptual understanding of crop productivity as a complex product of plant genetics and microbial community function. The direct and indirect mechanics of PGPR through bio-fertilization, stimulation of root growth, rhizo-remediation and plant antibiosis and induction of systemic resistance, nutrient competition and niches that assists to sustain healthy growth of plants enhancing the crop productivity are also accentuated.

Continuous utilization of quality land in civilization and industrialization has gained interest in the utilization of salt-affected soils for crop production. However, crop growth and productivity is severely affected in saline soil. Many strategies were proposed to overcome the salt detrimental effects like development of salt-tolerant cultivars through breeding and/or genetic engineering, removal of excessive salt accumulation in soil, desalinization of irrigation water etc. Though these strategies are efficient but costly. Hence, a cost-effective new alternative attempt has taken up to mitigate soil salinity which involves inoculation of salt-tolerant arbuscular mycorrhizal fungi (AMF) in agricultural crop. Mechanisms of amelioration of salt stress in AMF-plant symbiosis involve enhancing the uptake of less mobile phosphorus, increasing nutrient acquisition, maintaining osmotic balance, enhancing antioxidants and polyamines, altering hormonal status, reducing ion toxicity and enhancing photosynthetic efficiency. AMF colonization induces an increase in root hydraulic conductivity of the host plants under osmotic stress conditions. Furthermore, AMF symbiosis also alters expression of cation channels and transporters, late embryogenesis abundant protein and aquaporins. AMF symbiosis not only changes plant physiology but also changes nutritional and physical properties of the rhizosphere. In the mycorrhizosphere, AMF interact with natural and introduced microorganisms and affect soil properties and quality. The quality of soil largely depends on its physical and chemical properties as well as diversity and activity of soil biota. Thus, AMF have been considered as bio-ameliorators of saline soils.

The use of plants remedy saline and sodic soils is a low-cost, emerging method, but with little acceptance because of its low profitability. However, some farmers have improved the condition of their soil salinity planting trees resistant to salt. Low-productivity saline soils are not only due to their toxicity resulting salt or were caused by excessive amounts of soluble salts and low soil fertility. Fertility problems are evidenced by the absence of an organic material and minerals, especially N and P. In the latest years, saline soils received a great attention because of the general shortage of arable land and of the increasing demand for ecological restoration of areas affected by secondary salinization processes. This is due to the fact that naturally salt-affected soils have a biotechnological potential in their microbial communities, which represent not only a gene reserve for future exploitation in biotechnological applications, assuming they could be used in some kind of restoration or conservation techniques of saline environments, but they can also serve as model systems for exploring the relationships between diversity and activity at the soil level in selective/limiting situations. As outlined in the introduction, very few studies succeeded in addressing the beta diversity of the microbial species in soils, according to the different salt concentrations and, at a different scale, to bacterial taxa distribution in relation to salinity gradients.

About 340 million ha to 1.2 billion ha land worldwide is salt-affected. A large part of these salt-affected soils are suited for agricultural production but are unexploited because of salinity/sodicity and other soil and water-related problems. In India salt-affected soils occupy about 6.73 million hectares. Indo-Gangetic plains that lie between 21°55′–32° 39′N and 73°45′–88^°25′E comprising of the states of Punjab, Haryana, Uttar Pradesh and part of Bihar (North), West Bengal (south) and Rajasthan (north) have about 2.7 million hectare salt-affected soils. Majority of these lands is treated as wastelands as their productivity is low due to soil-based constraints. As no additional resources are available for horizontal expansion of agriculture, we need to find out viable technologies for utilization of existing land resources including degraded wastelands in order to meet the future requirement of food, fodder, timber and fuel. There is a need to revegetate these wastelands and prevent their further degradation. Growing of multifunctional agroforestry tree species which is a widespread alternate land use adaptation may support the restoration of these lands and potentially support livelihood improvement of resource poor farmers through simultaneous production of food, fodder and firewood as well as mitigation and adaptation to climate change. Trees growing in combination to agriculture as well as numerous other vegetation management regimes in salt-affected soils can be integrated to take advantage of services provided by adjacent natural, seminatural or restored ecosystems. This paper presented the contribution of agroforestry systems as a potential option for (1) restoring salt-affected soils, (2) mitigating climate change, (3) enhancing the fertility status of soil, (4) producing biomass and bioenergy and (5) providing social and economic well-being of the people.

Sodic soil needs chemical reclamation and also organic amendments for making it useful to produce crops. Reclamation of sodic soil requires removal of part or most of the exchangeable sodium, improvement of the soil physical structure and lowering of pH value. Apart from gypsum, there are certain inorganic amendments that can be used to reclaim sodic soils. Among organic amendments, pressmud, a waste product from sugar factories, is one such material commonly used for soil improvement. Organic amendments on decomposition result in high partial pressure of CO₂ and produce organic acids. These processes help to increase electrolyte concentration, mobilize calcium through enhancing the solubility of soil calcite and lower pH and ESP of the soil. Most commonly used amendments are crop residues, farm yard manure (FYM), green manure, poultry manure, etc. Organic materials, when applied in conjunction with inorganic amendments or when applied alone in soils of mild sodicity, have proved beneficial, and therefore their use in the reclamation of sodic soils occupies an important place. Phytoremediation approach is also found to be effective where cultivation of salt-tolerant plant species helps in dissolving native CaCO₃ to provide adequate calcium for effective Na⁺-Ca²⁺ exchange at the exchange sites.

Modern industrialization, rapid urbanization, and excessive fertilization generate huge amounts of hazardous heavy metals and harmful salts leading to various degrees of soil contamination. The metal contamination, salinity, and sodicity are the prime examples of soil pollution that contribute as potential threat to soil health. The existing conventional technologies to remediate contaminated soil based on physicochemical approaches are highly cost intensive and could upset the biological component consequently productive function of soil in a long run. The magnitude of soil contamination can be minimized through the use of viable technology by means of using suitable plant species; the approach is called phytoremediation. In the recent past, phytoremediation received great attention because of its eco-friendly and economic approaches. Several hyperaccumulator and halophyte plants are known to decontaminate the soil polluted with various hazardous metals and salts. Several heavy metals such as lead, cadmium, copper, manganese, etc. have been commonly chosen as representative metals for which their concentrations in the environment may be used as reliable indices of environmental pollution. Salinity and sodicity are described as major causes of land degradation process that retards plant growth and productivity particularly in the arid and semiarid regions. By virtue of various interactions in the process of phytoremediation and salt removal, the plants can reduce soil contamination to a great extent and re-established the productive potential of the soil. Still, there is demand of research on co-contamination of inorganic and organic contaminants and various salts by means of phytoremediation strategies or plant-rhizosphere microbe interaction.

Heavy metals are naturally present in the soil, but higher concentration of these elements is harmful to plants, animals, and humans. Prolonged exposure of such heavy metals can have deleterious health effects on human life. Bioremediation of these heavy metals like As, Cd, Cr, Hg, Ni, Hg, and Zn can be done by either plants or microorganisms or by the combination of two. In this chapter emphasis has been given to its microbial methods. There are certain disadvantages associated with physicochemical methods of remediation; thus bioremediation is arising as alternative to these methods. It is an environment friendly approach because it is achieved via natural processes. In this chapter efforts have been made to give brief introduction of available physicochemical and biological methods of heavy metal remediation. Bioremediation by bacteria and fungi is discussed in detail.

Soil sodicity is a major problem in arid and semiarid regions of Indo-Gangetic plains in India. A large proportion of sodicity-affected soils in Indo-Gangetic areas occur on land inhibited by resource-poor small farmers. Several efforts have been made by central and state governments to check soil degradation and increase agricultural productivity through sodic land reclamation programmes. Currently, India is losing annually around 11 million Mg of farm production valued at ₹150 billion from sodic soils. The severity of sodicity problem received the attention of policy makers and development agencies. A significant advancement in the sodic land reclamation technology has been made in India to reclaim the degraded sodic soils and to enhance crop productivity for improving farm income of the farmers. The successful application of soil reclamation technologies at the farmer’s fields has encouraged many states to launch ambitious programmes of land reclamation through land reclamation and development corporations by providing necessary inputs to augment the food and livelihood security of resource-poor farmers. Over the past few decades, with the support of the World Bank, European Union and other developmental agencies, India has reclaimed 1.95 Mha sodic soils, which contributed substantially to improve the economic conditions of millions of small farmers.

A lot of work has been done on improving practices for remediation of coastal and inland salt-affected soils. This has resulted in improving crop yields in these degraded lands and thereby improving the socioeconomic status of the resource-poor farmers. Keeping in view the limited availability of good quality waters for flushing out salts and scarce mineral gypsum availability for reclaiming sodic soils, vegetative and microbial bioremediation of salt-affected soils has emerged as a promising technique. Cultivation of economically useful halophytes, salt-tolerant plants, and crop varieties capable of growing under salt-stress environments has enabled conversion of saline and sodic wastelands. The high potential for bioremediation of salt-affected soils using applications of halophilic bacteria has been reported by some researchers. The applications of halophilic bacteria include recovery of saline soil by directly supporting the growth and stress tolerance of vegetation, thus indirectly increasing crop yields in saline soil. The biotic approach “plant-microbe interaction” to overcome salinity problems has received considerable attention from many workers throughout the world recently. Plant-microbe interactions are beneficial associations between plants and microorganisms and also a more efficient method for reclamation of salt-affected soils. However, there are many challenges to overcome for widespread adoption of these techniques and opportunities for the future to reclaim salt-affected soils through bioremediation approach.