Phytoremediation of salt affected soils

Generally, chemical amendments are used to ameliorate sodic and saline-sodic soil by supplying readily available source of Ca2+ to replace excess Na+ on the cation exchange complex. In this respect, amendments such as gypsum (CaS04.2H20) supply soluble sources Ca2+ to the soil solution, which then replace excess Na+ on the exchange complex. Application of chemical amendments, particularly gypsum and dolomite, for management of sodic soil is a century-old practice. However, there are some constraints with chemical ameliration of sodic soils in several developing countries because of (1) low quality of amendments containing a large fraction of impurities; (2) restricted availability of amendments; and (3) increased costs due to competing demand. On the other hand, scientific research and farmer’s practices have demonstrated that sodic and saline-sodic soils can ameliorate through organic and plant based materials. The organic materials and the action of plant roots improve biological activity in the soil. The plant-assisted approach of amelioration of sodic and saline-sodic soils is also known as phytoremediation (Mishra et al. 2002, Qadir et al. 2002). The symonemous tenninology of phytoremediation includes vegetative bioremediation, phytoamelioration, and biological reclamation.

In case of typical phytoremediation strategies of metal contaminated soils, the contaminated soil is exhausted by cultivation of specific plant species capable of hyper accumulating targeted ionic species in their biomass, thereby removing them from the soil (McGrath et al. 2002, Salt et al. 1998). In contrast, phytoremediation of sodic and saline-sodic soils is achieved by the ability of plants’ roots to increase the dissolution rate of calcite, thereby resulting in enhanced level of Ca2' hi soil solution to effectively replace Na+. The salinity levels in soil solution dining phytoremediation improve soil structure through aggregate stability that facilitates the water movement through soil profile and enhances the amelioration process (Oster et al. 1999).

Phytoremediation mechanisms

Partial pressure of CО2 in the root zone

The mechanisms of phytoremediation of salt affected soil involve the enhancement of dissolution rate of calcite (i.e., Ca2+ ions in soil solution) in the calcareous soils. This phytoremediation mechanism by regulating the partial pressure of C02 in the root zone (Rpccu) involves dissolution and precipitation kinetics of calcite which is represented as

This kinetics involves 3 processes:

a) Conversion of C07 in aqueous matrix (soil solution) into H,C03 which then reacts with CaC03 is given by

b) Dissolution of H,C03 into H' and HC03 ions and CaCQ3 reacts with this H+ ion produced.

c) Dissolution of CaC03 to Ca2+ and COj~ ions through mineral hydrolysis. However, as calcite is less soluble, Ca2+ ion production through mineral dissolution is less compared to the processes discussed before.

In the above dissolution reactions, Ca2+, HC03~ and CO^~ are released into the soil solution. Pco2 may increase to a maximum level of 1 к Pa in aerobic soils, which is equivalent to 1% of the soil air by volume (Nelson and Oades 1998), while under anaerobic conditions of flooded soils it is much higher (Naxteli and Sahrawat 1999, Ponnamperuma 1972), where saturated conditions inhibit the escape of CO, to the atmosphere and increases Pc02 in the soil. In non-calcareous soils, an increase in CO, results in more of H" and thus pH reduces. However, no such decrease of pH to a great extent in calcareous soils was observed (Nelson and Oades 1998), since pH changes are buffered by the enhanced dissolution of calcite (Van den Berg and Loch 2000).

Also, other processes are involved in regulation of Pc02 other than root respiration, viz.

i) CO, production from oxidation of plant root exudates (polysaccharides, proteins) by microbes.

ii) Microbes produce organic acids which dissolve calcite.

All these individually or collectively increase C02 production and ultimately Ca2+ availability increases, which replaces exchangeable Na' at a rate higher than Pco, of atmosphere.

Proton release by plant roots

The release of H' from plant roots decreases the pH of the rhizosphere. As nitrogen source, when plants are supplied with ammonia (NHj), it acidifies, while with nitrate (NOj) it alkalizes the rhizosphere (Marschner and Romheld 1983, Schubert and Yan 1997). Though legumes rely on symbiotic N,-fixation and acidify the soil (Schubert et al. 1990), the acidification mechanism has been studied only under acid soils rather than its role in remediation of salt affected soils. The chemical reaction involved in this is similar to that of dissolution mechanism of calcite due to increased Pco,.

An electrochemical gradient develops between soil and root due to proton release and cation uptake increases net H'release which facilitates active H' pumping (Schubert and Yan 1997). Cytosolic pH increases due to H" release and organic anion synthesis is induced. The organic anion is thus a measure of net H" release at the root-soil and its complement in crop or tree litter is called ash alkalinity (Jtmgk 1968). Plants growing under high base status produce H" at the root-soil interface since they have high ash alkalinity. Therefore, such species that adapts to sodic soil conditions releases H' and can enhance the rate of calcite dissolution. Thus, proton release by plant can be effectively employed for salty soil amelioration by proper crop management, which facilitates high amounts of CO, and H' in root zone.

Physical effects of roots

Importance of plant roots lies in the fact that it maintains soil structure, macrospore fonnation and improves soil porosity by biopores or structural crusts (Czames et al. 2000, Oades 1993, Pillai and McGarry 1999, Yunusa and Newton 2003). Production of polysacchaiides and ftingal hyphae at root soil interface improves aggregate stability (Boyle et al. 1989, Tisdall 1991). Deep rooted plants withstand salinity and sodicity up to cextain levels by facilitating the leaching of Na+ to the deeper soils layers. This is evident from the improvenxent in soil structure by deep rooted legumes and perenixial grasses (Tisdall 1991). Phytoremediation effects of different crop rotation with aixd without gypsum was studied by Ilyas et al. (1993) on low permeable hard salixxe soil (pHs = 8.8, ECe = 5.6 dS m"1, SAR = 49). Results showed that alfalfa roots penetrated as deep as 1.2 m in the gypsum-treated plots as compared to 0.8 m in untreated plots and caused a twofold increase in saturated hydraulic conductivity (Ks). Similar increase in Ks was seen with sesbania-wheat- sesbania rotation up to 0.4 m depth.

Although deep tillage has been effective in ameliorating sub-soils with low porosity, the benefits are shoxt lived (Cresswell and Kirkegaard 1995). Biological drilling by plant roots was found as an alternative to deep tillage for the amelioration of dense sub-soils (Elkins 1985). Two stages in biological drilling are: (1) Creation of macro pores in the subsoil by the roots resulting in improved water and gas flow and (2) Benefits for the next crop (Cresswell and Kirkegaard 1995, Elkins 1985).

Salt removal by plant biomass

Phytoremediation of salt affected soil can also be done by removal of above ground biomass as they accumulate salts and Na' in their shoots. Halophytes are highly salt-resistant crops that also accumulate high amounts of salts and Na+ in their shoots. Salt concentration in leaf ash of atriplex was as high as 390 g salt kg-1 under salt affected soils (Malcolm et al. 1988), while under rangeland condition it was 130-270 g salts kg-1 (Hyder 1981).

Phytoremediation by salt accumulated shoot removal is only up to certain extent that it caimot ameliorate salt affected soils completely. This is evident from the research findings of Barrett- Lemiard (2002) that it would require about 20 consecutive years under non-irrigated conditions to remove half of the initial content of salts by halophytic crops having annual productivity of 101 ha-1. But halophytes such as artiplex hardly produce more than 2 mg ha-1 annually (Barrett-Lennard et al. 1990). Therefore, under non irrigated conditions, effect of halophytes is minimal to ameliorate salt affected soils, while under irrigated conditions there is excess Na+ removal by enhanced calcite dissolution making salt removal by shoots less effective comparatively. This is because salt removal by shoots is less than salts acmally accumulated thr ough irrigation water. For example, kallar grass (Leptochloa fusca L.) has forage salt levels of 40-80 g kg-1 under soil salinity level of 20 dSnr1. When they are irrigated with 107 litre ha-1 water having salinity of 1.5 dSnr1, salt added would be

9.6 t ha-1 but only 1-2 t ha-1 salt is removed by forage. So, a better way to remove salinity is by leaching the salts to greater depths than removal of shoot biomass.

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