Interactive effects of earthworm Eisenia fetida and bean plant Phaseolus vulgaris L on the fate of soil selenium

Selenium (Se) is an essential micronutrient for animals with a narrow margin between essentiality and toxicity. Se toxicity is largely related to inorganic forms of Se in soil, i.e., selenite and selenate that enter food chains through plant uptake, threatening higher trophic level organisms. This experiment investigated effects of earthworm activity on Se bioavailability in soil and the subsequent plant uptake, using earthworm Eisenia fetida and bean plant Phaseolus vulgaris L, both exposed to either selenite or selenate at 1 or 4 mg Se kg⁻¹ for 16 weeks. Plants took up selenate (up to 221-fold) faster than selenite, with up to 84% of the Se rapidly transported to shoots. In the presence of earthworms, Se accumulation obviously increased for selenate-supplied plants, leading to an up to 4% increase in Se translocation factor for all treatments except for 1 mg kg⁻¹ selenite treatment. Earthworms also concentrated Se faster in tissues (up to 274 mg kg⁻¹ DW) at exposure to selenate. For Se toxicity, Se speciation analysis was conducted on the plants and earthworms using XAS. Compared to worm-free treatments, the percentage of organo-Se, i.e., SeMet and CysSeSeCys, increased in beans (up to 34%) in the presence of earthworms for selenate, while the elemental Se portion was significantly reduced or absent, opposite to the results for selenite. Surprisingly, elemental Se (up to 65%) dominated earthworms, regardless of the form of Se supplied. In conclusion, earthworms clearly enhanced Se uptake and translocation in plants, leading to elevated Se levels in shoots. To prevent resulting hazards to humans and other animals, caution should be taken while consuming the shoots, particularly beans, harvested from the Se contaminated soil where earthworm activity is high. Finally, the significant reduction in soil Se suggests phytoextraction of Se from the soil could be improved using earthworms as an aid to plants.