Sustainable Land Management for Mitigating Climate Change

XIV. Management of Salt-Affected Soils

81. Secondary salinization is a major problem on irrigated lands. The irrigated land area in the world has increased 50 fold during the last three centuries from 5 Mha in 1700, 8 Mha in 1800, 48 Mha in 1900 to 255 Mha in 2000. Risks of secondary salinization are exacerbated by use of poor quality water, poor drainage and excessive irrigation, leakage of water due to defective delivery systems, impeded or slow soil drainage, and other causes. Salinization is a severe problem in China, India, Pakistan, and in the countries of Central Asia (Babaev, 1999). For example, the extent of salinized land area is 89% in Turkmenistan, 51% in Uzbekistan, 15% in Tajikistan, 12% in Kyrgyzstan, and 49% of the entire region (Funakawa et al., 2000; Khakimov, 1989; Pankova and Solovjev, 1995). Salinization is also a problem in southwestern USA, northern Mexico and in some dry regions of Canada (Balba, 1995). High salinity and water logging in South Asia (FAO, 1994), is caused mainly by excessive irrigation and lack of proper drainage (Lal, 2009d).

82. But irrigation is not the only reason for salinization of land; many coastal areas are threatened by climate change in ways that lead to salinization. More importantly, most if not all small island developing nations of the Pacific and Indian Oceans as well as the countries of the Caribbean are among the most vulnerable to global climate change (IPCC, 2007). While the severity of the impacts will vary from country to country, there are various key concerns directly linked to climate change that will affect countries across these regions. Projected sea level rise will combine a number of factors resulting in accelerated coastal erosion, increased flood risk and in some areas permanent loss of land. Any increase in the intensity and destructiveness of tropical storms will further accelerate land degradation along the coasts. The impacts of sea-level rise will be further exacerbated by the loss of protective coastal systems such as coral reefs in areas such as the Caribbean (Oxenford et al., 2007). More immediately, such sea-level rise is also directly associated with saline intrusion into coastal lands and aquifers, affecting the availability of farmland and freshwater. Hence using known SLM practices and technologies for reclamation and management of such salt affected lands is a crucial strategy to adapt to climate change in many areas around the world.

83. Despite the areal extent of salt–affected soils worldwide, the research information on SOC pool and flux under different management systems is rather scant (Wong et al., 2008). There are 3 major SLM strategies to reclaim salt-affected soils (Figure 13): (i) enhance tolerance to high salt concentrations either by choosing salt-tolerant species or by enhancing tolerance to excess salts through selective breeding, (ii) improve SOC concentration because even the slightest increase can have a major positive impact on soil structure aeration, permeability, water retention and microbial/enzymatic reactions, and accelerate soil desalinization by leaching excess salts out of the soil profile, and (iii) leach salts out of the root zone through improved drainage and irrigation with good quality water. The relative significance of each strategy depends on the soil-specific conditions.

A. Salt Tolerance

84. Vegetation is essential to enhancing the SOC concentration, which is low in salt-affected soils because of little or no vegetation cover. Establishment of plants increases SOC concentration by: (i) addition of leaf litter, (ii) growth and turnover of root biomass, (iii) addition of root and mucilage exudates, (iv) increase in activity of soil fauna especially microbial biomass. Some plant species are naturally adapted to saline conditions (Table 20) and should be considered for reclamation of salt affected areas. Most terrestrial plants evolved in saline ocean water with high salt concentration of about 500 mmol L^-1. However, most species eventually adapted to terrestrial environments with low salt concentrations around 450 million years ago (Rozema and Flowers, 2008). Yet, about 1% of the species growing under terrestrial environments have retained tolerance to high salt concentrations. These species, called halophytes, have a wide range of phonological characteristics, and include cereals, legumes, annuals, perennials, shrubs, trees, etc. Qadir et al. (1998) reported that there are 16 major halophytic plant families in Iran. These families, in terms of the number of species, are in the order Chenopodiaceae> Poaceae> Asteraceae> Brassucaceae> Plumbaginaceae> Cyperaceae> Tamar-ciacae> Zygophyllaceae> Polygonaceae> other families. The growth rates of halophytes are comparable to those of conventional plants. There are also some useful halophytes which can be used for industrial purposes and biofuel production. Halophytes can also be grown under arid conditions by irrigation with saline water, or mixing saline water with fresh water. In addition to exploration of natural genetic variations, development of transgenic plants is another option to enhance the degree of salt stress tolerance (Yamaguchi and Blumwald, 2005). And in coastal zones affected by saline intrusion partly due to sea level rise, introduction of such halophytes is a practical strategy to adapting to CC. In addition, plants such as coconuts, oil palm, guava, mango, mangroves, casuarina, can be an important addition to local livelihoods while helping to protect coastal lands (from erosion, storms) and adapt to CC.

Table 20. Some salt tolerant plants (Adapted from Lal et al., 1999).
	Plant Latin Name
I	Fruit trees
		Tamarind	(Tamarindus indica)
		Mango	(Mangifera indica)
		Loquat	(Eriobotyra japonica)
		Jamun	(Syzygirum cuminii)
		Coconut	(Cocos nucifera)
		Oil Palm	(Elaeis guineensis)
		Guava	(Psidium guajava)
II	Halophytes
		Pickle weed	(Salicornia spp) Turtle weed
		Salt Grass	(Distichlis palmeri) Seep weed
		NyPa Forage	(Distichlis spp)
		Salt bushes	(Atriplex numularia)
		Algae	(Spirulina geitleri)
III	Trees
		Gum trees	(Eucalyptus spp)
		Acacia	(Accacia spp)
		Shisham	(Dalbergia sissoo)
		Ye-eb	(Cordeeauxia edulis)
		Pine	(Pinus oocarpa)
		Mesquite	(Prosopis juliflora)
		Jojoba	(Simmondsia chinensis)
		Casuarina	(Casuarina equisetifolia)
		Albizia	(Albizia lebbeck)
		Ber Arjuna herb	(Zizyiphus mauritiana) (Terminalia Arjuna)
IV	Grasses and Forages
		Karnal Grass	(Leptochloa fusca)
		Vetiver	(Vetiveria spp)
		Narrow Leaf Lupin	(Lupinus angustifolius)
		Wheat grass	(Thynopyron ponticum)
V	Crops
		Triticale	(Secale spp)
		Bambara groundnut	(Voandzeia subteranea)
		Marama bean	(Tylosema esculentum)
		Tepary bean	(Phaseolus acutifolius)