Preliminary Study On Toxic Effect Of Exogenous Chromium On Soil Microorganisms And Its Ecological Safety Threshold

In this paper,the toxic effects of chromium?Cr?on microbial parameters and root elongation of plants in Shandong fluvo-aquic soil and Jiangxi red soil were studied.The toxicology database of Cr???was constructed based on the data obtained from experiment results and collected literature.The intraspecific variation analysis,interspecific extrapolation and normalization were used.According to the species sensitivity distribution?SSD?curves of Cr in Shandong fluvo-aquic soil and Jiangxi red soil,the ecological safety thresholds of Cr under two typical soil conditions were deduced preliminarily.It was expected to provide some reference for ecological risk assessment and remediation management of soils polluted with Cr.?1?The microbial parameters including potential nitrification rate?PNR?,activity of dehydrogenase,urease,and phosphatase,abundance of the bacterial groups related with the function,Cr???reduction bacteria,and total bacteria,and bacterial community structure were selected in this study.Based on the effects of exogenous Cr???under eight concentration levels ranging from 0 to 428 mg·kg^-1 on these microbial indexes,the dose-effect curves reflecting their relationships were respectively fitted for the two soils.And the changes of Cr???,Cr???and available Cr concentrations were analyzed and compared in these two soils.The results showed that only the abundance of chromium reduction bacteria was promoted by Cr at low concentration in Shandong fluvo-aquic soil,and the rest microbial indicators were all inhibited by Cr in both soils.The bacterial community structure was also significantly influenced by Cr added into soils.On condition that the same concentration of chromium was added,the concentration of Cr???and available chromium was higher in Shandong fluvo-aquic soil than that in Jiangxi red soil.Compared with total Cr,microbial indicators are more closely related to the available Cr.Based on the absolute value of slope?b?of dose-response curve,it was found that PNR and abundance of ammonia-oxidizing bacteria were the most sensitive microbial indicators to the change of Cr concentration in Shandong fluvo-aquic soil and Jiangxi red soil respectively.Quantitative models of the median effective concentration(EC₅₀)and the 10%effective concentration(EC₁₀)of Cr were respectively established based on microbial data of two typical soils,which could be used to calculate the EC₁₀ value of Cr according to its EC₅₀ value in the same soil.By comparing the EC₅₀ values of Cr to different microbial parameters,it was found that the abundance of ammonia oxidizing bacteria and PNR could be used as diagnostic indicators of microbial toxicity caused by Cr contamination in Shandong fluvo-aquic soil and Jiangxi red soil,respectively.?2?Root elongations of 19 plant varieties under eight Cr???concentrations ranging from 0 to 428 mg·kg^-1 were determined by plant screening experiments in two typical soils.The dose-effect curves between root elongations of various plants and Cr concentration were fitted and used to obtain the toxicity threshold of Cr to plant root elongation index.The results showed that Cr at low concentration could promote the root elongations of rape,radish,carrot,tomato and pepper in Shandong fluvo-aquic soil and those of wheat,maize,lettuce,leek,rape and radish in Jiangxi red soil,and inhibit the root elongations of other plant varieties.Based on the absolute value of slope?b?of dose-response curve,it was found that root elongations of barley and tomato were the most sensitive plant indexes to the change of Cr concentration in Shandong fluvo-aquic soil and Jiangxi red soil,respectively.The quantitative models of EC₅₀ and EC₁₀ of Cr were respectively established based on plant root elongation data of two typical soils,which could be used to calculate the EC₁₀ value of Cr according to its EC₅₀ value in the same soil.By comparing the EC₅₀ values of Cr to root elongations of different plants,it was found that root elongations of leek and soybean could be used as diagnostic indicators of plant toxicity caused by Cr contamination in Shandong fluvo-aquic soil and Jiangxi red soil,respectively.?3?Based on the ecological toxicological data of Cr???in Chinese soil obtained from literatures and experiment,the prediction model reflecting the relationship between EC₅₀ of Cr and soil properties was established.After model prediction and intraspecific variability analysis,suitable EC₁₀ models were selected to extrapolation between species and normalization processing.The normalized literature data and the experimental results of this study were used to construct SSD curves of Cr based on Burr III model under the conditions of Shandong fluvo-aquic soil and Jiangxi red soil,then the ecological safety thresholds of Cr for these two soils are preliminarily deduced.The results showed that there was no obvious regularity in the ecological toxicity sensitivity of Cr to soil plants,animals and microorganisms;soil bacterial abundance and PNR were the most sensitive biological indicators to Cr pollution in Shandong Fluvo-aquic soil and Jiangxi red soil,respectively;and the concentration values of protecting 95%biological species?HC₅?in Shandong fluvo-aquic soil and Jiangxi red soil were 1.90 and 0.44 mg·kg^-1,respectively,which were far lower than the current soil environmental quality standards of China.