Effect Study Of Earthworm On Transport And Transformation Of Zn And Pb In Soil-Plant System

With the development of industry and agriculture, soil contamination hasbecome a serious problem. Restoration of contaminated soil has been becoming an essentialproject to study. Phytoremediation of heavy metals in contaminated soil has been widelyemployed because of its exclusive advantages. But the small amount of aboveground biomassand the poor effectiveness of bioavailability of metals are the main limitations for application ofthis technology. To overcome the limitation, the technique of earthworm-inducedphytoremediation of soil contaminated by heavy metals was put forward by some researchers,but so far the research only focused on the plants tolerable to heavy metals, and the heavymetals are accumulated in the roots of plants. So it is necessary to study the hyperaccumulator ofheavy metals. Three kinds of soils, acid brown soil, aquatic brown soil from Taian, cinnamon soil fromJinan, were used in the experiments. Soils were amended respectively to contain 0,100,200,400mg Zn kg-1 and 0,200,400,800 mg Pb kg-1 by adding appropriate concentrations of ZnSO4·7H2Oand Pb(CH3COO)2·3H2O. Treatments with and without earthworm (Eisenia foetida) inearthworm culture experiment and planting experiment (Lolium multiflorum and Brassciajuncea L.), were compared. The objective of this study was to investigate the effects ofearthworm on the plant biomass and bioavailability of heavy metal in soils, to show themechanisms of earthworm effect on chemical behavior of heavy metal in soil-plant system, andto provide theoretic base for the application of earthworm in phytoremediation.The earthworms used in the experiments were able to tolerate heavy metals, but theirgrowth was obvious negatively affected by heavy metal pollution in soil. One month after weplanted Lolium multiflorum, all the earthworms' weight decreased in soils with the incrementrate in acid brown soil of -6%~-56%, in aquatic brown soil of -7%~-46% and in cinnamon soilof -9%~-35%. In addition, earthworms' growth was also found decreasing with increase of theconcentrations of Zn and Pb in soil. While in the soils with planting Brasscia juncea L, theincrement rate of earthworm was greater than in no plant soil with about +3%~-25% in acidbrown soil, -8%~-40% in aquatic brown soil and +2%~-29% in cinnamon soil. Earthwormactivity increased Lolium multiflorum aboveground biomass in all disposal soil added by Zn andPb, but had little effect when Brasscia juncea L was planted except the case of low density Znand Pb in the soils. Whether does earthworm increase plants biomass depends on the soil quality,varieties of heavy metals and the mechanisms of plants.Earthworm activity remarkably decreased the pH of cinnamon soil and aquatic brown soiltreated by Zn and Pb, regardless of planting Brasscia juncea L or Lolium multiflorum. In acidbrown soil, earthworm obviously increased the pH of the soil added by Pb when plantingBrasscia juncea L or added by Zn when planting Lolium multiflorum. When planting Loliummultiflorum, earthworm activity apparently increased the concentrations of DTPA-extractableZn in acid brown, but had little effect on cinnamon soil and aquatic brown soil. When plantingBrasscia juncea L, earthworm activity remarkably decreased the concentrations of DTPA-extractable Zn in aquatic soil, but increased the concentrations in acid brown soil when theconcentration of Zn was 400 mg·kg-1. While planting Lolium multiflorum or Brasscia juncea L,earthworm activity had little effect on concentrations of DTPA-extractable Pb in three kinds ofsoils. Soil pH was remarkably negatively correlated with concentration of Zn and Pb both inLolium multiflorum and in Brasscia juncea L tissues in three soils. So by affecting soil pH,earthworm activity affects bioavailability of heavy metal in soil or in earthworm cast.The omni-Pb, Zn and DTPA-extractable Pb and Zn in earthworm cast increased with theincrease of the concentrations of Pb and Zn added in soils whether planting Lolium multiflorumor planting Brasscia juncea L. The Pb and Zn concentrations in earthworm cast in plantingLolium multiflorum soils were more than those in soils planted by Brasscia juncea L. Theomni-Zn in earthworm cast was about 1.02 ~ 1.91 times as much as in soils;theDTPA-extractable Zn in earthworm cast was 0.52 ~ 52.9 times as much as in soils;the omni-Pbin earthworm cast was 0.87 ~ 8.25 times as much as in soils;the DTPA-extractable Pb inearthworm cast was 0.77 ~ 31.8 times as much as in soils. So the DTPA-extractable Zn and Pbin earthworm cast was one of the most important origins of assimilation in plant.Earthworm activity had little effect on heavy metal concentration absorbed by Loliummultiflorum except in cinnamon soil and aquatic brown soil with the concentration of Zn 200mg·kg-1 and 400 mg·kg-1. Earthworm activity could also increase Zn concentration absorbed byBrasscia juncea L in cinnamon soil and aquatic brown soil. While in aquatic brown soil with theconcentration of Pb 400 mg·kg-1and 800 mg·kg-1 or in acid brown soil with the concentration ofPb 800 mg·kg-1, earthworm activity remarkably increased the Pb concentration absorbed byBrasscia juncea L. In the soil contaminated by heavy metals, Brasscia juncea L and Loliummultiflorum had different absorbent mechanism, because the former is hyperaccumulater of Znand makes Zn transport to above biomass, but the latter is the tolerate plant for heavy metals andmakes Zn accumulate in root. Whether can the earthworm increase bioactivity of heavy metalsdepends on the soil quality, the varieties of heavy metals and the mechanisms of plants.The relations between the concentrations of Pb and Zn in the plants and those in earthwormcast faeces, pH and DTPA-extractable Pb and Zn in the soils were statistically analyzed in thisstudy. From the results it is inferred that through affecting soil pH, earthworm activity affectsbioavailability of heavy metal in soil and in earthworm cast, or through ingesting soils withmetal, rubbing, digest and excretion in tissue, and chelating heavy metal by secreting mucus,earthworm increased the contents of DTPA-extractable Pb and Zn.The research mentioned above show that the earthworm used in experiments could tolerateheavy metals and can also improve plant growth and enhance aboveground biomass in soilpolluted by heavy metals. The earthworm can activate heavy metal in soil and thus improvemetal bioavailability through several mechanisms, but the way of activation was different withthe different character of soils. The extents of earthworm activity improving metalbioavailability were different with the different plants, but earthworm can enhance the activationof heavy metals in soil, or increase total uptake of heavy metals by plants through improvingplant growth, which is important for enhancing efficiency of phytoremediation.