Toxicity Thresholds Of Cadmium To Rice Cultivars In Soils And Species-sensitivity Distribution

Ecological risk to humans’ health of soil cadmium(Cd) pollution has received extensive attention, and it’s the lack of feasible system of paddy soil Cd risk assessment and it’s relatively weakness of study of soil Cd risk threshold insoils of different properties. Then, eight different soils, according to the difference of soil properties(e.g. soil pH values, organic carbon, etc.) in China, were selected and sampled for soil Cd toxicity bioassays. The toxicity thresholds(EC10, EC50)of Cd to rice roots were determined by using the dose-response curves fitted by Log-logistic model based the rice root elongation. Through stepwise regression analysis, the toxicity threshold predictive model were demonstrated between the rice Cd toxicity thresholds(EC10, EC50) and the soil properties, and found the main controlling factors of rice Cd toxicity. Meanwhile, twenty different varieties rice were selected to advance the study of rice Cd uptake sensitivity based on the difference of rice grains Cd concentration. The species-sensitivity distribution(SSD) were fitted by the Burr Type III distribution function to deduce the soil Cd limit concentration based on the rice Cd limit concentration of National Food Hygiene Standard of China. The projective was in order to provide the recommendation for the prevention and control of soil Cd pollution and updating current soil Cd limit values of Chinese soil environmental quality standard. The main results are as follows:1. The Cd toxicity to rice roots elongation varied significantly by effecting of different properties soils. The toxicity threshold EC10 of high-sensitive rice cultivar TY167 ranged from 1.40 mg·kg-1 to 5.25 mg·kg-1, EC50 ranged from 17.83 mg·kg-1 to 46.93 mg·kg-1; and EC10 of low-sensitive rice cultivar XZ45 ranged from 1.72 mg·kg-1 to 8.22 mg·kg-1,EC50 ranged from 26.96mg·kg-1 to 68.16 mg·kg-1. It demonstrated that soil p H, OC and CEC were the main controlling factors of rice Cd toxicity, and developed the predictive model of rice Cd toxicity threshold: Log(EC50) = 0.078 pH + 0.208 log(CEC) + 0.202 log(OC) + 0.705(R2=0.94, n=24), and pH, OC, CEC could control the 94.1% variability of the predictive model.2. It wasn’t significant reduced the yield of rice grains under the below soil Cd concentration 4.8 mg·kg-1, and the rice grain yield ranged from 20.69 g·pot-1to 37.22g·pot-1.The Cd concentration of roots, straws and grains of same rice cultivarsincreased gradually with the addition levels of Cd, and The Cd concentration of roots, straws and grains of different rice cultivars were significantly difference in the same Cd treatments. The high Cd concentration of roots, straws and grains of different rice cultivars were 1.52-4.11, 1.68-3.52 and 3.54-6.00 times of the low Cd concentration. The Cd transfer factors from root to straw(TFstraw) were significantly different among twenty rice cultivars, and the coefficient of variations of TFstraw ranged from 41.02% to 71.18%. While, the Cd transfer factors from straw to grain(TFgrain) weren’t significantly different among twenty rice cultivars, and the coefficient of variations of TFgrain ranged from 10.82% to 27.05%. Therefore, TFstraw could exactly describe the significant differences of Cd uptake and translocation among different rice cultivars than TFgrain, and the different ability of Cd transfer from root to straw led to the significant difference of rice grains Cd concentration.3. Cd bioconcentration factors from soil to grain(BCF) were significantly different among twenty rice cultivars, and ranged from 0.28 to 1.54. The coefficient of variations of BCF ranged from 10.79% to 27.72% under different Cd levels soil.There were high transfer factors(TFs) and high BCF for the rice cultivars of high Cd concentration in rice grainamong the different rice cultivars, conversely, the rice cultivarsof low Cd concentration in rice grain had the high transfer factors(TFs) and the high BCF.Then, the SSD was fitted by the BurrIII distribution function, and the R2 was 0.9977. Therefore, the soil Cd limit concentration was deduced by SSD based on the rice Cd limit concentration(0.2mg·kg-1) of National Food Hygiene Standard of China, it was 0.57mg·kg-1, to ensure 95% rice grains Cd concentration meeting the National Food Hygiene Standard of China.