Enhancement by soil micro-arthropods of phytoextraction of metal-contaminated soils using a hyperaccumulator plant species

There is little information available on the potential influence of soil micro-arthropods on the phytoremediation of metal-contaminated soils using hyperaccumulator plants. We hypothesized that soil micro-arthropods may promote organic matter decomposition, thereby releasing nitrogen and enhancing the growth of and metal uptake by hyperaccumulators and thereby increase phytoextraction efficiency. Four highly contaminated soils with a range of metal contents were used in a pot experiment to study the effects of the collembolan Folsomia candida and the mite Hypoaspis aculeifer on phytoextraction by Sedum plumbizincicola, a cadmium (Cd) and zinc (Zn) hyperaccumulator. Both micro-arthropods survived when the hyperaccumulator grew in the four soils. F. candida slightly increased the growth of S. plumbizincicola and shoot metal concentrations increased significantly in the presence of F. candida with or without H. aculeifer. Soil fauna increased shoot Cd accumulation by 27.9–66.3% compared to corresponding control soils. Similar shoot 15N abundances across treatments and decreasing shoot N content in two of the soils with addition of F. candida indicate that the promotion of phytoextraction was not associated with increasing plant nitrogen uptake from organic matter by F. candida. The mechanism of plant growth promotion might be an interaction between the soil fauna and the roots of the hyperaccumulator and this merits further investigation. This work highlights for the first time a potential role of soil micro-arthropods in the phytoextraction of metal-polluted soils.