Rhizosphere effect on Pb solubility and phytoavailability in Pb-Contaminated soils

The effect of plant roots in modifying Pb solubility and bioavailability in an historically contaminated orchard (Hudson) and a Pb phosphate-spiked (Arkport) soil was determined by measuring soluble Pb in the soil solutions as well as content of Pb in radish shoots grown in these soils. Soluble Pb and dissolved organic carbon (DOC) contents were greater in the rhizospheres of both Pb-contaminated soils than in the unplanted high-Pb soils. The rhizosphere effect increased soluble Pb 15-fold in the field-contaminated orchard soil, whereas the effect was much smaller in the Pb phosphate-spiked soil. The rhizosphere effect persisted in the Pb-phosphate spiked soil after adjustment of the soil pH from 7.8 to 6.7. The results indicate that Pb phosphate added to a non-acid soil has lower solubility than Pb in an orchard soil contaminated by historical Pb arsenate applications; nevertheless, some uptake of Pb into plant shoots resulted from both sources of soil Pb contamination. The rhizosphere effect was observed for trace metals in addition to Pb, with the solubility of Al, Fe, Cu and Ni all increasing in the rhizosphere soil. In contrast, the solubility of alkali and alkaline earth metals (K, Ca, Mg, Sr, Ba) all decreased in the rhizosphere soil. The results indicate that the rhizosphere effect associated with plant roots can raise the solubility of Pb in soils contaminated by legacy Pb and by insoluble Pb phosphate.