Driving forces for sodium removal during phytoremediation of calcareous sodic and saline-sodic soils: A review

Sodic and saline-sodic soils are characterized by the occurrence of sodium (Na(+)) at levels that result in poor physical properties and fertility problems, adversely affecting the growth and yield of most crops. These soils can be brought back to a highly productive state by providing a soluble source of calcium (Ca(2+)) to replace excess Na(+) on the cation exchange complex. Many sodic and saline-sodic soils contain inherent or precipitated sources of Ca(2+), typically calcite (CaCO3), at varying depths within the profile. Unlike other Ca(2+) sources used in the amelioration of sodic and saline-sodic soils, calcite is not sufficiently soluble to effect the displacement of Na(+) from the cation exchange complex. In recent years, phytoremediation has shown promise for the amelioration of calcareous sodic and saline-sodic soils. It also provides financial or other benefits to the farmer from the crops grown during the amelioration process. In contrast to phytoremediation of soils contaminated by heavy metals, phytoremediation of sodic and saline-sodic soils is achieved by the ability of plant roots to increase the dissolution rate of calcite, resulting in enhanced levels of Ca(2+) in soil solution to replace Na(+) from the cation exchange complex. Research has shown that this process is driven by the partial pressure of CO2 (P(CO)2) within the root zone, the generation of protons (H(+)) released by roots of certain plant species, and to a much smaller extent the enhanced Na(+) uptake by plants and its subsequent removal from the field at harvest. Enhanced levels of P(CO)2 and H(+) assist in increasing the dissolution rate of calcite. This results in the added benefit of improved physical properties within the root zone, enhancing the hydraulic conductivity and allowing the leaching of Na(+) below the effective rooting depth. This review explores these driving forces and evaluates their relative contribution to the phytoremediation process. This will assist researchers and farm advisors in choosing appropriate crops and management practices to achieve maximum benefit during the amelioration process.