Nickel Phytotoxicity As Affected By Soil Properties And Development Of A Predictive Model

Nickel toxicity in soils are affected by soil properties. Although the relationships between European soil properties and soil nickel toxicity thresholds have been studied, the results from European soils may be difficult to apply to Chinese soils because of European soils with permanent charges while quite a number of Chinese soils with variable charges. So that the relationships and regression equations between soil nickel toxicity thresholds and the properties od Chinese soils with and without leaching treatments were investigated.According to the distribution of soil pH and organic matter in China, the samples of topsoil were collected from 17 sites with a wide range of soil properties. The soil samples were treated by the unleaching and leaching after NiCl salt was spiked at eight doses. Barley root elongation of five days, tomato and bok choy plant growth of twenty-one days were used as endpoints.The results of barley root elongation test showed that the effective concentration of added Ni causing 50% inhibition (EC₅₀) ranged from 46 to 1729 mg·kg^-1 in leached soils and from 47 to 2519 mg·kg^-1 in unleached soils. The experiment of tomato shoot biomass indicated that the EC₅₀ ranged from 11 to 932 mg·kg^-1 in leached soil and from 7 to 2055 mg·kg^-1 in unleached soil for the tomato test. The results of bok choy shoot biomass test exhibited that the EC₅₀ ranged from 11.8 to 1768 mg·kg^-1 in leached soils and from 0.26 to 813 mg·kg^-1 in unleached soils for the bok choy test. The results of linear regressions showed that soil pH is a main factor controlling the toxicity of Ni in soils; the second factor is soil organic carbon content.Based on the relationship of between the soil properties and the threshold toxicity values of nickel, predictable models of barley toxicity were developed as follows: Log₁₀ (EC₅₀) = 0.363pH + 0.194Org-C - 0.195 (R² =0.829 unleached), Log₁₀ (EC₅₀) = 0.392pH + 0.173Org-C - 0.222(R² =0.794 leached). The models indicated a factor 2.5 difference between predicted and measured values. A predicted model of tomato toxicity was developed as follows: Log(EC₅₀) = 0.459pH+0.178Org-C - 1.281(R² =0.824 unleached), Log(EC₅₀) = 0.454pH + 0.165Org-C - 1.141(R² =0.925 leached), and the models indicated a factor 3 difference between predicted and measured values. A predicted model of bok choy toxicity was developed as follows: Log₁₀ (EC₅₀) = 0.580pH + 0.348Org-C - 2.668(R² =0.855unleached), Log₁₀ (EC₅₀) = 0.458pH + 0.174Org-C - 1.242(R² =0.940leached), and the models indicated a factor 3 difference between predicted and measured values. The model of Log₁₀ (EC₅₀) = 0.357pH + 0.024CEC - 0.184 could perfectly predict barley toxicity thresholds in European soils (R² = 0.974). The model of Log(EC₅₀) = 0.459pH+0.178Org-C - 1.281 could also perfectly predict barley toxicity thresholds in European soils (R², 0.7925).For the three plants in the experiment, the sensitivity of nickel toxicity to plants was as follows: bok choy > tomato > barley, which suggests that bok choy can be as a test plant for ecological risk assessment of Ni in soils.Compared to unleached soil, the leached soil can significantly reduce the nickel of plant toxicity, especially when soil's pH > 8(except Langfang). The results suggest that the leaching is important for risk assessment of Ni in soilsIn conclusion, the controlling factors of nickel toxicity to plants in a wide range of Chinese soils and its predictive model were studied in this paper. The results provide lots of base research data for the study of Biotic Ligand Model in soil ecological systerm and research experiences for ecological risk assessment of nickel pollution in soils. In addition, the achievement obtained in this study could contribute to the future revision of Chinese Ni standard in soil by giving a scientific base.