| Farmland soils were polluted by chromium emission via three wastes from chromium mining and smelting,the production of chromium salt,leather and electroplating,and other industries,thereby affecting the growth of crops and the safty of agricultural products.There are significant differences in the absorption and accumulation of heavy metals by different crops due to the different external morphology,internal structure,and physiological and biochemical mechanisms of heavy metal absorption.The study aimed to compare the differences in the accumulation ability of heavy metals by different crops and distinguish the sensitivity of different crops to Cr-contaminated soil.It is proposed to provide theoretical basis for the rational and safe cultivation of crops in Cr-contaminated soil as well as to provide data for the formulation of soil threshold of Cr.Firstly,based on the collection of reference involving Cr contents in the edible part of crops that planted in Chinese soils,the bioconcentration factors（BCF）were calculated,and the prediction model between BCF and soil properties was established.The species sensitivity distribution（SSD）curve was also constructed by normalization of different BCF using the prediction model,and the difference in Cr bioconcentration ability and the related influencing factors were also analyzed.Then,the stress response and enrichment ability of six crops（wheat,radish,cucumber,Chinese cabbage,oilseed rape and lettuce）to Cr（Ⅵ）were studied by hydroponic experiments.Finally,soils subjected to exogenously Cr（Ⅵ）were used to perform pot experiments.The BCF was caculated from the ratio of Cr content in the edible part of the crop and the bioavailable Cr in soil.The SSD method was used to compare the enrichment ability of Cr by different crops.The main findings are as follows:BCF is significantly affected by crop species and soil properties（pH,cation exchange capacity（CEC）or soil organic carbon（OC）content）.Under the conditions of pH=7、OC=1.5g·kg-1,and CEC=15 cmol·kg-1,the Cr bioconcentration ability of wheat grain is the weakest.The bioconcentration ability of the 9 vegetable species（the edible parts）follows the order of celery>water spinach>edible amaranth>leaf lettuce>bok choy>oilseed rape>spinach>lettuce>crown daisy.In soils with different pH,the bioconcentration of Cr by crops follows the order of alkaline soil>neutral soil>acidic soil,and the BCF values decrease with increasing soil cation exchange capacity.Under the conditions of hydroponic experiments,the root surface area and tissue biomass of the six crops decreased with the increase of Cr（Ⅵ）concentration.The biomass reduction of wheat was the smallest,and that of lettuce was the largest.The activities of superoxide dismutase（SOD）and peroxidase（POD）in the leaves of wheat,radish,Chinese cabbage,and lettuce were significantly inhibited at 5 mg·L-1 of Cr（Ⅵ）.Moreover,for Chinese cabbage and radish,significant decrease in SOD activity was observed at 1 mg·L-1 of Cr（Ⅵ）treatment,suggesting that these two crops are sensitive to the stress of Cr（Ⅵ）.For Chinese cabbage and oilseed rape,significant activation effect on the POD was observed at 1 mg·L-1 or 5 mg·L-1 of Cr（Ⅵ）treatment.However,the SOD and POD activities in cucumber leaves did not significantly change at the two levels of Cr（Ⅵ）concentrations,indicating that cucumber is not sensitive to the stress of Cr（Ⅵ）.The contents of Cr in the tissues of the six crops increased with the increase of Cr（Ⅵ）concentration,and Cr accumulated mainly in the roots.Under 5 mg·L-1 of Cr（Ⅵ）treatment,Cr content in the aerial parts of the six crops follows the order of Chinese cabbage>oilseed rape>lettuce>wheat>radish>cucumber.The transfer coefficient of Chinese cabbage was the largest,while that of lettuce was the smallest.In conclusion,the growth of wheat showed strong tolerance to the stress of Cr（Ⅵ）,with the lowest accumulation of Cr in the shoots.Lettuce presented to be the most sensitive vegetable in terms of the inhibition on the growth,and Cr is mainly accumulated in roots.Chinese cabbage had a certain tolerance to Cr（Ⅵ）and exhibited the highest accumulation of Cr in the edible part.Therefore,when planting crops in low and medium Cr contaminated soil,concerns should be addressed on the food safety issue from leafy vegetables especially Chinese cabbage.The Cr content in the edible parts of the eight crops did not exceed the limit value specified in the food safety standard under soil exogenous addition of 2.8 mg·kg-1 and 5.6mg·kg-1 Cr（Ⅵ）.The absorption capacity of leaf lettuce was weak in both Shandong Cambisols and Jiangxi Ferralsols,which indicated that it has strong tolerance to Cr pollution in soil.The sensitivity of carrots in the two types of soil was quite different.Lettuce was relatively sensitive to Cr in leafy vegetables.The two-way analysis of variance showed that the interaction of varieties and varieties and soil types had a vital influence on the Cr content of crops.Therefore,when crops are planted in low Cr-contaminated farmland,crops with low enrichment ability of Cr should be screened according to soil types.The safety thresholds based on the contents of total Cr were 79.5 mg·kg-1 and 135 mg·kg-1 in Shandong Cambisols and Jiangxi Ferralsols,respectively,which are very different.While the safety thresholds based on the contents of available Cr were 0.85 mg·kg-1 and 0.80 mg·kg-1,respectively,which are very close.Therefore,it is more scientific and reasonable to establish soil safety thresholds based on the contents of available Cr. |
