The Phytotoxicity Based On Extractable Ni In Soils As A Function Of Soil Properties

The current ecological risk assessment of heavy metal was usually based on total concentration of heavy metal in soil.The aim of the study is to improve the accuracy of ecological risk assessment of heavy metal with different sources in soil.The toxicity of added water-soluble nickel(Ni)to barley root elongation,tomato and bok choy plant growth was studied in 17 representative Chinese soil samples with or without artificial rainwater leaching using single or sequential three EDTA or Mehlich-3 extractions.One single EDTA extraction can extract about 46% of the total Ni added to the unleached and leached soils;and three EDTA extractions can extract about 70%-74% on average.One single Mehlich-3 extraction can extract about 36%-40% of the total Ni added to the unleached and leached soils;and three Mehlich-3 extractions can extract about 64%-75% on average.Linear regressions were used to study the relationships between EDTA or Mehlich-3 extractable Ni and total Ni added to soils as affected by soil properties.The results showed that pH was the most important factor affecting the relationship between EDTA or Mehlich-3 extractable Ni and total Ni added to soils,incorporating soil pH the relationship between EDTA or Mehlich-3 extractable Ni and added total Ni was established significantly.The phytotoxicity thresholds based on EDTA or Mehlich-3 extractable Ni was significantly affected by soil properties.For example,the phytotoxicity thresholds of Mehlich-3 extractable Ni to barley root elongation(the effective concentration causing 50% inhibition)varied with ? 38-fold difference among soils.The leaching effects on decrease in Ni toxicity depended on the soil properties.Leaching had a comparatively stronger influence on alkaline soils than on acidic and neutral soils.And the effect of leaching on decreasing metal toxicity depends on the ecoloxicological endpoints.Based on EDTA or Mehlich-3 extractable Ni,the phytotoxicity thresholds prediction models for different ecoloxicological endpoints was established.Regression models for toxicity thresholds based on EDTA extractable Ni to barley root elongation against soil properties showed that soil citrate dithionate extractable Fe was more important than soil pH in predicting Ni toxicity.Regression models for toxicity thresholds based on EDTA extractable Ni to tomato and bok choy plant growth against soil properties showed that soil pH and organic content(OC)were the most important factors to affect the phytotoxicity.The phytotoxicity thresholds prediction models based on Mehlich-3 extractable Ni indicated that the soil citrate dithionate extractable Fe,Al and Mn and clay were the most important factors to affect the phytotoxicity based on single Mehlich-3 extractable Ni,and that soil pH and organic content(OC)were the most important factors to affect the phytotoxicity based on sequential three Mehlich-3 extractable Ni.The influence of soil properties on the phytotoxicity based on Mehlich-3 extractable Ni was associated with extraction times.Regression models for toxicity thresholds based on Mehlich-3 extractable Ni to tomato and bok choy plant growth against soil properties showed that soil pH was the most important factors to affect the phytotoxicity.The results not only indicated the possibility of extractable metals replacing total contents as risk assessment criteria but also provided a basis for improving the accuracy and scientificity of Ni ecological risk assessment.