Do earthworms mobilize fixed zinc from ingested soil?

A wide range of organisms inhabit the soil and has to deal with soil-bound metals. The bioavailable fraction of metals may be estimated explicitly using the isotopic dilution technique. In the present paper, we evaluated the isotopic exchange technique for assessing the bioavailability of soil Zn (using 65Zn) to earthworms. To validate the technique, the worms were first exposed to various 65Zn levels, and errors due to soil entrained in the gut were evaluated. This exposure indicated no effect of gamma-radiation on growth (wet weight gain) of the organisms and that depuration of the earthworms minimized errors in labile pools determined by isotopic dilution. Our study further showed that the earthworms accessed 55-65% of the total Zn in the soil. The labile pool for the earthworms Eisenia andrei was similar to that for the plant Lactuca sativa, indicating that earthworms and plants to a large extent access the same fraction of soil Zn. Hence, the isotopic dilution technique has the potential to assess biologically available pool of Zn in soils. As lettuce is not known to significantly mobilize nonlabile metals in soil, this study indicates that Zn uptake by E. andrei is predominantly via the exchangeable pools (possibly the soil pore water) rather than dissolution of Zn held within soil particles or within soil organic matter or other food sources.