Toxicity Responses And The Mechanism Of The Earthworm Exposed To Heavy Metals In Greenhouse Soil

More global attentions have been paid on heavy metal contamination in greenhouse protected fields recently. Accumulation of heavy metals in greenhouse cultivated soils not only decreased the productivity and quality of greenhouse crops, but also posed a potential threat to ecosystem and human beings. Heavy metal contamination in greenhouse field soil and vegetables in Hangzhou has been investigated. The effect of earthworm activity on the speciation of Pb, Cu and Ni in greenhouse soils, and the toxicity responses of the earthworm exposed to Pb, Cu and Ni in greenhouse soils were studied in the present study via earthworm toxicity tests. Besides, this research also focused on obtaining some sensitive indicators in earthworm to demonstrate the heavy metal pollution levels in the greenhouse fields. The mainly conclusions were drawn as follows:(1) A survey and comprehensive assessment were conducted to investigate the heavy metal pollution in the topsoil and the corresponding greenhouse vegetables from the15typical greenhouse vegetable cultivation areas in eight districts of Hangzhou. The results showed that the soil in the studied area was acidic, fertile and well capacity of water supply, and Cd, Cu, Zn and Ni were exceeded the heavy metal safety limits of soil (GB15618-1995). The results of the single factor pollution index, Nemerow integrated pollution index and Hakanson potential ecological risk index all exhibited that the greenhouse soils were contaminated by the heavy metals and Cd was the contributing factor. Leafy vegetables were strongly contaminated by Cd, Pb and Zn, while fruit vegetables were heavily polluted by Ni. The main factors affecting the heavy metals in greenhouse vegetables were the soil heavy metal contents and soil pH value.(2) The earthworm toxicity test was conducted by adding different concentrations of Pb, Cu and Ni into soils, respectively. The accumulation ability of heavy metals on earthworms was discussed. The results indicated that the heavy metal enrichment capacity of earthworm was in an order of Pb> Cu> Ni. The earthworm toxicity test also showed that the earthworm activities had an effect on the speciation of heavy metal in greenhouse soils. The results demonstrated that earthworm actived the fraction of heavy metals in soil, Pb and Ni were easily transformed to the acid extractable fraction and the reducible fraction, while the oxidizable fraction of Cu was significantly affected by earthworm activities. The correlation analysis of earthworm heavy metal accumulation and the heavy metal speciation indicated that the easily reducible fraction of Pb and Ni, the oxidizable fraction of Cu significantly influenced the heavy metal accumulation in earthworms. Besides, the earthworm activities could strengthen the bio-availability of Pb and Cu, while decreased the bio-availability of Ni, those affected the safety of environment. Moreover, earthworm activities decreased the soil pH value (especially the pH value of Cu contaminated soil) and increased the DOC content, which had an effect on the speciation of heavy metals in soil and the accumulation of heavy metals in earthworms.(3) The mechanism of the toxicity responses of the earthworm exposed to the heavy metal in greenhouse soil was studied via the earthworm toxicity test. The results showed that the acute toxicity was different among Pb, Cu and Ni, and the median lethal concentrations (LC50) of Pb, Cu and Ni were2331.67,299.60and1757.05mg kg-1, respectively. The results of biochemical responses of earthworm exposed to Pb, Cu and Ni showed that the activity of SOD, GST, CYP450, Hsp20and Hsp90in earthworm were stimulated by slight heavy metal levels after long time exposure or high heavy metal levels in short time, while inhibited after long time exposure. The activities of the above biochemical indicator in earthworm were decreased with the increasing heavy metal concentration in soil.(4) The results of correlation analysis between earthworm sensitive indicators and heavy metal levels showed that there was a significant correlation between GST, CYP450and soil Pb content, and CYP450was more sensitive to Pb contamination in soil when compared with GST. A significant correlation existed between SOD activity and Cu or Ni, while SOD activity was more sensitive to Ni. The relationship between sensitive earthworm indicators and significant heavy metals speciations showed that the sensitive earthworm indicators of Pb (GST and CYP450avtivity) and Ni (SOD activity) had significant positive correlation with the acid extractable fraction and the reducible fraction of Pb and Ni, the same correlation were found between the oxidizable fraction of Cu and SOD activity in earthworm. While the correlation between SOD activity in earthworm and the acid extractable fraction of Cu was not significant.