Effects Of Earthworm On Available Pool Of Pb, Zn In Soils And Its Mechanism

Phytoremediation was a new arising biological technology; when adopted to dealwith the heavy-metal-polluted soils, the low efficiency of heavy metals in soils wasthe main handicap of the successful use of the technology. Though addition ofchelating agents has been shown to increase metal bioavailabilility andphytoremediation efficiency, it costs too much and shows environmental risk.Earthworm could improve soil quality and bioavailabililty of heavy metal in soils bymeans of feeding, metabolizabling, burrowing and excreting. Therefore, theapplication of earthworm would supply a new way of Phytoremediation ofheavy-metal-polluted soils more economically and safely.Three soils, Acid brown soil and Aquic brown soil from Taian, Cinnamon soilfrom Jinan, were used in the experiments in this thesis. Soils were amendedrespectively to contain 0,200,400,800 mg·kg^-1 Pb and 0,100,200,400 mg·kg^-1Zn by adding appropriate concentrations of ZnSO₄·7H₂O and Pb(CH₃COO)₂·3H₂O,By comparing with treatments with wild earthworm(Allolobaphora caliginosa)andcultured earthworms(Eisenia foetida),short culture time and long culture time, thisthesis studied the effect of earthworm on bioavailabililty of Pb,Zn, analyses andshowed the mechanisms of earthworm effect in soil from three perspectives of soilpH,DOC and earthworm cast and made a primary research on choice of earthwormspecies and earthworm culture time for Phytoremediation.Allolobaphora caliginosa and Eisenia foetida are the species that can tolerateheavy metals with the latter's ability to tolerate heavy metal better than the former.However, the pollution of Zn and Pb in soils will restrain the earthworm's growth,especially during the first 14 days when earthworm is influenced by heavy metals.The weight of all the Eisenia foetida in treatment decreased with the increase of heavymetal concentration. The earthworm in Zn-added Cinnamon soil Acid brown soil andAquic brown soil growed at an average rate of -10%～-38%,-13%～-48% and -8%～-52% respectively; While that in those Pb-added growed at an average rate of -10%～-38%,-13%～-39% and -8%～-46% respectively, The Eisenia foetida enrichmentcoefficients of Pb and Zn in the three kinds of soils were less than 1, which indicatedthat earthworm could only absorb part of heavy metals rather than enrich them. Eisenia foetida was able to increase notably DTPA-Pb in Acid brown soil andAquic brown soil, which also can be realized by the addition of Zn in Cinnamon soilthat is 400 mg·kg-1 concentration and Zn in Acid brown soil that is 200 mg·kg-1concentration. There is an obvious difference of Eisenia foetida from Allolobaphoracaliginosa in the aspect of activating Pb and Zn in soils treated by different heavymetals. Soil character is another factor that earthworm improves bioavailabililty ofheavy metal in soils. The activity of earthworm improved the pH in both Pb-treated and Zn-treated acidbrown soil and Zn-treated Cinnamon soil. Among the three Pb-added soils in theexperiment, the pH in the soil that contains Allolobaphora caliginosa was higher thanthat of the one that contains Eisenia foetida, while between Zn-treated acid brown soiland Cinnamon soil, the pH in the soil that contains Allolobaphora caliginosa waslower than that of the one containing Eisenia foetida and it is a different case underthe condition of aquic brown soil. The activity of the earthworm can decrease pH insoil, which can be another mechanism that earthworm could improve bioavailabililtyof heavy metal in soils. The earthworm can improve the quantity of DOC in the three Pb and Zn addedsoils. In Zn added aquic brown soil and acid brown soil, the quantity of DOC in thesoil that contains Eisenia foetida was higher than the one that contains Allolobaphoracaliginosa while it is a different case under the condition of Cinnamon soil.Amongthe three Pb added soils, the add of either Allolobaphora caliginosa and Eiseniafoetida obviously increased the quantity of DOC in soils. Such an increase led byearthworm activity may by another mechanism that earthworm could improve...