[Phytoavailable lead in rhizosphere of lettuce]

A greenhouse experiment was performed to investigate the influence of pH TOC, and fractionation of Pb on phytoavailability of Pb in the rhizosphere of lettuce. And the Low molecular-weight organic acids (LMWOAs) were used to evaluate the phytoavailability of soil Pb to lettuce. The results show that the pH is lower in the rhizosphere compared to the bulk soil, and the rhizosphere becomes more acidic with increasing Pb (NO3)2 applied. In contrast, the TOC content is higher in the rhizospherer compared to the bulk soil, and more TOC accumulates in the rhizosphere with increasing Pb (NO3)2 applied. The concentrations of different fractions in rhizosphere soils follow the order of exchangeable > carbonate bound > Fe-Mn oxide bound > organic bound > residual, and for bulk soil the order is carbonate bound > Fe-Mn oxide bound > exchangeable > organic bound > residual. Compared with bulk soils, exchangeable Pb increases significantly in rhizosphere. With increasing Pb (NO3)2 applied, the transformation of residual Pb to carbonate bound and exchangeable Pb may have happened and results in the accumulation of carbonate bound and exchangeable Pb. The air-drying increases the concentration of exchangeable, carbonate bound and Fe-Mn oxide bound Pb and decreases organic bound and residual Pb. Rhizosphere and bulk soils were extracted by low molecular-weight organic acids(LMWOAs) to fractionate Pb fraction of soil pools, which is considered as the phytoavailable Pb. The results show that phytoavailable Pb in rhizosphere is much higher compared to bulk soil, and phytoavailable Pb increases with increasing Pb (NO3)2 applied due to acidification processes and accumulated TOC in the rhizosphere. Air-drying process also increases the phytoavailable Pb. The root-induced changes of Pb availability in the rhizosphere might also be related to the Pb species, especially carbonate bound and exchangeable Pb. Correlation analysis indicates that application of wet fresh rhizosphere soils and LMWOAs should be recommended in the future study on the phytoavailability of soil Pb to lettuce.