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Improvement of soil properties through phytoremediation

The ability of different plant species used in phytorernediation of sodic and saline-sodic soils has been found to be highly variable. In general, plant species with greater biomass production potential together with the ability to withstand soil salinity and sodicity have been found suitable for soil amelioration (Qadir et al. 2002). The study revealed that Na' uptake by above ground biomass of several plant species constitutes 2-20% of the total salt uptake (Qadir et al. 2007). It was also found that Na' removal by plant biomass sitch as alfalfa would contribute to only 1-2% of the Na' removed during phytorernediation (Qadir et al. 2003). However, phytorernediation has two major advantages: (1) no financial liabilities to purchase chemical amendments, and (2) financial benefits from cultivation of crops during amelioration. In various parts of the world, several studies have been conducted aimed at amelioration of sodic and saline-sodic soils through phytorernediation approaches (Ahmad et al. 1990, Ghaly 2002). Apart from the amelioration of soil sodicity and salinity, this approach has been compared for its effect on soil drainage, nutrients dynamics and environmental benefit in terms of carbon sequestration (Garg 1998, Kaitr et al. 2002).

4.2.1 Amelioration of soil sodicity

Various field experiments revealed that phytorernediation and chemical amelioration approaches are equally effective in terms of ability to decrease the soil sodicity and salinity. Field experiment indicated that amelioration efficiency of two grass species, Para grass (Brachiaria mutica) and Kamal grass (Dichanthium annulatum), was comparable with soil application gypsum @ 12.5 Mg ha"1 (Kumar and Abroal 1984). It was also found that amelioration efficiency of Kallar grass was greater in the pots leached after 6 days of harvesting and it was comparable with the gypsum-treated soil (Hamid et al. 1990). However, some field trials indicated that phytoremediation approaches were not successful because a salt-resistant crop was not the first crop of the rotation. Several crop rotations have been evaluated to ameliorate sodic soils. The study revealed that the entire crop rotation could ameliorate only upper 0.15 m of soil after 1 year as did amelioration by the gypsum treatment (Qadir et al. 2003). It is peitinent to note that growing rice in submerged soil has been recognized as promising phytoremediation approach for amelioration of moderate sodic and saline- sodic soils. Under submerged condition, rice rhizosphere accumulated CO, in the soil atmosphere to react and neutralize alkalinity (Qadir et al. 2002). Field experiment (n = 17) conducted in different parts of the world concluded that chemical amelioration is able to decrease 60% of the initial sodicity level, whereas 48% was observed in case of phytoremediation approaches (Singh and Singh 1989, Ahmad et al. 1990, Muliaimned et al. 1990, Rao and Bums 1991, Helalia et al. 1992, Qadir et al. 1996, 2002, Ghaly 2002, Ahmad et al. 2006). In general, phytoremediation approach worked well on moderately sodic and saline-sodic soils provided: (1) excess irrigation was done to facilitate adequate leaching and (2) excess irrigation was applied when crop growth and Pc02 were at their peak. On such conditions, the potential of phytoremediation was comparable with gypsum application (Qadir et al. 2003).

4.2.2 Amelioration throughout the root zone

The anticipated zone of amelioration is an important parameter to determine relative efficiency of the chemical amelioration and phytoremediation approaches. In most of the comparative studies, it was found that amelioration occurred primarily in the root zone where chemical amendment (gypsum) was incorporated (Qadir et al. 1996, Ilyas et al. 1997). In case of phytoremediation, amelioration of sodic and saline-sodic soils occurs throughout the root zone. However, different plant species have variable capacity and depth of soil amelioration which is influenced by the root morphology (Ahmad et al. 1990, Akhter et al. 2003). So, deep-rooted crop with tap root system have advantages in terms of greater depth of soil amelioration. For example, alfalfa root can penetrate as deep as

  • 1.2 m in the soil.
  • 4.2.3 Soil nutrient dynamics

Apart from reducing soil salinity and sodicity levels of sodic and saline-sodic soils, phytoremediation provides additional benefits over other amelioration approaches. A 20-year study with tree plantation (Prosopisjuliflora, Acacia nilotica, Eucalpytus tereticornis and Albizia lebbeck) on alkali soil resulted in considerable decrease in soil pH and increase in soil organic carbon (SOC) content, and available phosphorus (P) and potassium (K) in surface soil (Singh and Gill 1990). There was an increase in phosphorus (P), zinc (Zn) and copper (Cu) availability in the phytoremediation soils probably due to the production of root exudates and likely dissolution of some nutrient-coated calcite. Soil microbial biomass (MBC), dehydrogenase (DHA) and respiration are related to microbial populations and provide an index of the overall soil biological status. The levels of DHA in post amelioration soil were found higher in phytoremediated soils than gypsum treated soil. Permanent vegetation such as grasses resulted in significant increase in urease and DHA in alkali soils (Rao and Ghai 1985). The greater microbial activity in upper 0.6 m soil under the tree species (A. nilotica, D. sissoo, P. juliflora and I arjuna) due to the increased soil organic C from leaf litter biomass decomposition (Garg 1998). The inorganic C of axid and semiarid soils is converted to organic form by plants through photosynthesis, and in soils through the reaction of C032- with decomposing organic matter (added via phytoremediation). Thus, the transfer of inorganic C from inorganic to organic form provides a better environment for C sequestration and soil quality (Sahrawat et al. 2005). The rate of C sequestration through this pathway ranged between 0.25-1.0 Mg C ha-1 year1 (Wilding 1999). The amelioration of saline and saline-sodic soil through phytoremediation approaches could lead to both organic and inorganic C sequestration simultaneously.

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