Study On The Toxic Effects Of Soil Arsenic And Cadmium To The Crops And Their Threshold Values

Sensitive disparity of 23 vegetable species that commonly cultivated in Fujian Province to As and Cd was studied by water culture. The most As- and Cd- sensitive species were selected and used in soil culture to study the threshold values of As and Cd phyto-toxicity, respectively. The influence of soil properties on As and Cd toxicity to plants were investigated using 9 soils. The threshold values of As and Cd phyto-toxicity were modified by the normalization relationships between the threshold values and soil properties, which will provide scientific basis for studies on the regional threshold values of soil heavy metals phyto-toxicity. The results were as follows:1. Low concentrations of As and Cd in the culture solutions had hormesis effects to same vegetable species under water culture. However, when the concentrations of As and Cd increased, the toxicitys to the crops appeared. Rice was more sensitive than vegetables to As toxicity. The typical symptoms of As toxicity were chlorosis in new leaves, rust along leaf margins. Similarly, the typical symptoms of Cd toxicity were growth inhibition, chlorosis in new leaves, hyperplasia in fibrous roots, and black root.2. Base on the apparent symptoms and the EC2o value （effective concentration causing a 20% reduction of shoot biomass） of each species, rice （Oryza sativa）, cucumber （Cucumis sativus L.）, cowpea（V. unquiculata ssp. unguioulata W.）, three-coloured amaranth（Amaranthus mangostanus L）, hot pepper（Capsicum annuum L）, eggplant（Solanum melongena L.melongena L.） were screened as the most As sensitive vegetables, and Chinese cabbage （B. campestris L. ssp. Pekinensis （Lour.） Olsson）, pakchoi （B. campestris L. ssp. Chinensis （L.）Makino）, lettuce （Lactuca sativa L.） and leaf mustard（B. juncea Coss.） were screened as the most Cd sensitive vegetables. These vegetable species could be used as indicative plants in the following soil culture, which aimed at determining the threshold values of As and Cd toxicity to vegetables. Cucumber had a high resistance for Cd.3. Based on the EC10 and EC20 of rice under soil culture, the threshold values of As toxicity were calculated to be 1.42 mg·kg^-1 and 3.00 mg·kg^-1 （as NaH2PO4-As）, respectively. According to the regression between soil NaH2PO4-As and total As in the field, the corresponding thresholds of As toxicity （EC10 and EC20） were estimated as 14.07 mg·kg^-1 and 30.51 mg·kg^-1 （total As）, respectively. Based on the EC10 and EC20 of leaf mustard under soil culture, the thresholds of Cd toxicity were calculated to be 0.15 mg·kg^-1 and 0.32 mg·kg^-1 （as CaCl2-Cd）, respectively. According to the regression between soil CaCl2-Cd and total Cd in the field, the corresponding thresholds of Cd toxicity （EC 10 and EC20） were estimated as 0.40 mg·kg^-1 and 0.70 mg·kg^-1 （total Cd）, respectively.4. The disparity of soil As and Cd phyto-toxicity was studied by soil culture with 9 soils, using rice and water spinach as indicative plants, respectively. The EC10 （NaH2PO4-As） of rice shoot （fresh weight） and shoot height ranged from 3.22 to 27.96 mg·kg^-1 and 3.72 to 29.11 mg·kg^-1, representing 8.68 and 7.83-fold variations among soils respectively. The EC20 （NaH2PO4-As） of rice shoot （fresh weight） and shoot height varied between 5.83 and 38.87 mg·kg^-1,7.12 and 45.60 mg·kg^-1, representing 6.67 and 6.70-fold variations among soils respectively. The EC10 （CaCl2-Cd） of the shoot of water spinach （fresh weight） and shoot height ranged from 0.14 to 0.52 mg·kg^-1 and 0.21 to 0.63 mg·kg^-1, representing 3.71 and 3-fold variations among soils respectively. The EC20 （CaCl2-Cd） of the shoot of water spinach （fresh weight） and shoot height varied between 0.18 and 2.46 mg·kg^-1,0.44 and 3.25 mg·kg^-1, representing 13.67 and 7.39-fold variations among soils respectively.5. Stepwise regression analysis identified that free iron was the main factors affected soil Asbioavailability. The modified prediction model of the ECX（plant height） of soil As phyto-toxicity as NaH2PO4-As was [EC10]=-1.451+1.160 [free iron] （R2= 0.718, P= 0.004, n=9） [EC20]= 1.659+1.791 [free iron] （R2= 0.764, P= 0.002, n=9） [EC50]= 16.367+2.715[free iron] （R2= 0.827, P< 0.001, n=9）However, Soil pH was the main factors affected the availability of Cd in soil. The modified prediction model of the ECX （shoot fresh weight） of soil Cd phytoxicity based on CaCl2-Cd was [EC10]=-0.650+0.180 [pH] （R2= 0.791, P= 0.001, n=9） [EC20]=-4.072+0.954 [pH] （R2= 0.877, P< 0.001, n=9） [EC50]=-463.488+100.680 [pH] （R2= 0.747, P= 0.003, n=9）6. In order to protect 90 percent of soils from As and Cd pollution, the thresholds of As and Cd phyto-toxicity of agriculture soils were calculated from soil pH and free iron of Fujian field survey. The As thresholds of EC10 and EC20 by rice shoot height were 5.1 mg·kg^-1 and 10.1 mg·kg^-1 （NaH2PO4-As）, respectively, while the Cd thresholds of EC10 and EC20 by the water spinach shoot were 0.19 mg·kg^-1 and 0.36 mg·kg^-1 （CaCl2-Cd）, respectively.