Trace metals in the soil-plant system and beyond

Metals in the soil-plant system and beyond Metallophytes can play an important role in remediating contaminated land either by decreasing soil metal levels via phytoextraction or through restoring a vegetation community to prevent soil erosion or re-establish ecosystem function/services. As even non-hyperaccumulating metallophytes can accumulate considerable concentrations of metals in their tissues, the ecological consequences of growing plants with enhanced metal concentration requires consideration. Metals accumulated by metallophytes can potentially be transferred to higher trophic levels through consumption of the root/shoot tissue or sap, through consumption of the seed or through consumption of litter. Invertebrates are most likely to consume root/shoot tissues or sap, but many are able to discern high metal levels in their food, which in turn has a strong antifeedant effect. Indeed, it is hypothesised that hyperaccumulation may have a function in plant defence, but non-hyperaccumulating metallophytes may also benefit from reduced herbivory due to the metal content of their tissues. Although this may generally be effective in restricting the transfer of accumulated metals to higher trophic levels, there are arthropod herbivores that feed despite high metal concentrations. These species can in turn accumulate very high levels of metals from their metallophyte hosts. Consequently, there is the potential for critical pathways to be formed that may transfer high concentrations of metals from the soil to higher trophic levels, resulting in secondary toxicity. Granivory is the most likely pathway through which vertebrate animals such as birds and small mammals can be exposed to metals and hence secondary toxicity. Most plants effectively exclude metallic elements from their seeds, but metallophytes can contain high levels of toxic metals such as cadmium. Carnivorous & omnivorous vertebrates may also be exposed by consuming contaminated herbivorous invertebrates and vertebrates. The senescence or death of plants will return metals to the soil in the resulting litter. Contaminated litter can induce toxicity in invertebrate detritivores, effectively excluding them from contaminated ecosystems with the result that litter builds up, severely curtailing key ecosystem functions/services. Thus, the use of metallophytes in remediation/restoration can negatively affect the fauna on or adjacent to the site. Unfortunately, the scale of the potential problem is not yet clear.