Role of Earthworms on Phytoremediation of Heavy Metal-Polluted Soils

Abstract

Heavy metal contamination of soil is a major concern in all parts of the world, in particular in emerging countries where there is an increasing need for soil for food. The accumulation of heavy metals in the environment can affect the health of humans and animals. This has led to the recent development of techniques for cleaning up polluted soils and sites. One such technique is phytoremediation, which exploits the ability of certain plants to accumulate large amounts of heavy metals. Phytoremediation has many advantages: (1) it is a method available for in situ extraction of heavy metals from soils, (2) it is economically viable, and (3) it has a low environmental impact. However, phytoremediation has limitations: (1) the slow growth and low biomass require a considerable investment in time and/or money, and (2) the heavy metals accumulate slowly in the plants as the pools of heavy metals available to the plants at a given time are small. To improve the performance of phytoextraction, hyperaccumulating plants with high biomass are used. Recent research has concentrated on the role of the rhizosphere, but few studies have considered the drilosphere compartment, the part of the soil influenced by earthworm secretions and castings. However, earthworms as ecological engineers play an important role in their environment. The positive effects of earthworms on plant production have been extensively documented as well as their effects on heavy metal solubility and availability. The interactions between heavy metals and earthworms depend on the earthworm species, the metal, and the physical and chemical properties of the soil. Earthworms have an effect on metal speciation in soils, changing the bioaccessibility and bioavailability of the metals for other organisms, such as plants.