Introduction

There is intense interest in utilizing plants to facilitate remediation of contaminated soils because ‘rhizoremediation’ offers a low-cost and ecologically acceptable approach to dissipating pollutants in soils (Anderson, Guthrie & Walton, 1993). The ability of a limited number of plant species, which are normally endemic to naturally metalliferous soils, to hyperac-cumulate metals is being explored with a view to remediating metal-contaminated soils; the process is termed phytoremediation (Cunningham et al., 1996). Phytoremediation as a technology has advantages and disadvantages, but as most hyperaccumulating species that are being explored with a view to commercial exploitation are in the Cruciferae and are generally non-mycorrhizal, these will not be considered in this review. The degradation of organic pollutants in the rhizosphere has also received considerable interest with a view to developing in situ remediation technologies (Anderson et al., 1993). It is here that mycorrhizal associations have to be considered (Donnelly & Fletcher, 1994; Meharg & Cairney, 2000a).

Rhizosphere degradation of organic pollutants

A wide range of organic pollutants are degraded more rapidly in the rhizospheres of most plant species tested than in bulk soils (Anderson et al., 1993). This ‘rhizosphere effect’ varies according to the chemical being degraded, the plant species used and the soil under study. The following explanations are normally put forward to explain enhanced rhizosphere degradation. First, rhizosphere carbon flow greatly stimulates microbial activity in soil surrounding plant roots, and this enhanced microbial activity results in an enhanced pollutant degradation rates.