Effects Of Calcium On Chemical Speciation Of Cadmium In The Soil And Cadmium Absorbed By Rice

Cadmium is a heavy metal element with long decomposition period,strong transferability,high toxicity and difficulty in degradation.It seriously harms the growth and development of crops and also causes extremely damage to human health through the food chain.In China,rice is one of the main food products,and it is easy to uptake cadmium from contaminated soil.So,how to reduce the absorption of cadmium by rice and reduce the upward transport of cadmium from rice roots has become a research hotspot in the academic community.As an essential nutrient element for crop growth,calcium has an ion radius similar to that of cadmium,which will compete for the same ion transport channel and binding site.Therefore,adding calcium to soil will inevitably affect the uptake and transport of cadmium in rice and the chemical speciation of cadmium in soil.This paper aims to investigate the effect of calcium on the chemical speciation of cadmium in soil and on the uptake of cadmium in rice.The effects of calcium on the absorption of cadmium in rice and the distribution of cadmium in rice subcellular under cadmium stress were studied by hydroponic experiments.Meanwhile,contaminated soil was collected in the field to analyze the relationship between cadmium and calcium in contaminated soil.The effect of calcium on the chemical speciation of cadmium in soil and the absorption and accumulation of cadmium in rice were investigated by adding different proportions of calcium compounds in rice pot experiments.The main results are as follows:（1）There is a significant correlation between the calcium and the available content of cadmium in the soil.Correlation analysis showed that there is a significant positive correlation between soil available calcium content,correlation coefficients is 0.593（P<0.01）.Available cadmium was also positively correlated with available calcium,correlation coefficients is 0.601（P<0.01）.（2）With the application of exogenous calcium,the content of available cadmium was decreased in the soil.Among them,when calcium silicate was applied,the acid souble and reducible contents of cadmium in the soil decreased,and the oxidizable and residual contents increased.After applying calcium carbonate,calcium polysulfide and calcium acetate,the acid souble and oxidizable contents of cadmium in soil decreased,while the reducible and residual contents increased.（3）The indica rice“Taixiangyou 5318”is a cadmium-tolerant rice variety.Among the experiment types of rice,the indica rice“Taixiangyou5318”had the least absorption of cadmium in the aerial part,the cadmium content was 30.6 mg·kg^-1;the minimum transport coefficient from the underground part to the aerial part and the transport was 0.034,and under cadmium stress,the stem and leaf length,root length and dry weight relative biomass of rice were better than other varieties.（4）Calcium ions promote the growth of hydroponic rice and prevent the uptake of cadmium by rice root,altered distribution ratio of cadmium content in rice subcellular.The relative biomass of rice was 77.1%under cadmium stress.with the addition of 1 mmol·L^-1and 2 mmol·L^-1 calcium,the relative biomass of rice rised to 85.7%and 94.3%.Cdmium uptake by rice roots decreased by 12.4%and 22.1%,respectively.The distribution ratio of cadmium in the cell wall components of the aerial part increased by 5.2%and 12.5%,the distribution of cadmium in the soluble fraction decreased by 2.7%and 9.9%.And the distribution ratio of cadmium in the cell wall of the underground part increased by 4.4%and 8.9%The distribution of cadmium in the soluble fraction decreased by 4.1%and6.9%.（5）Exogenous calcium promotes growth of soil-cultured rice and alters cadmium transport in rice plants.Among them,within the application range of calcium silicate,the variation range of rice dry weight relative biomass is 109.3%～165.3%.Under the action of 0.1%and0.5%calcium carbonate,the relative biomass was 113.7%and 107.2%,respectively,with the addition of 0.1%calcium acetate,the relative biomass was 107.5%.The stem-to-leaf transport coefficient was significantly increased when the application of exogenous calcium at0.5%,at this dose,Especially after the application of calcium carbonate,the stem-leaf transport coefficient of rice was 0.53,and compared with the control,the maximum increase of the stem-leaf transport coefficient in rice was 82.8%.In conclusion,calcium can regulate the distribution of cadmium in rice subcellular,increase the proportion of cadmium in the cell wall,reduce the proportion of cadmium in the soluble fraction,and alleviate the inhibitory effect of cadmium on rice growth.The application of calcium silicate,calcium carbonate,calcium polysulfide and calcium acetate can make other speciations of cadmium in the soil change to the residual cadmium,reduce the activity of soil cadmium,enhance the stability of cadmium.Under the action of calcium,the absorption of cadmium by rice roots is significantly weakened,and the cadmium entering into rice plants has a tendency to be transported to stem-leaf.Using calcium to alleviate the absorption of cadmium by rice has certain application prospects.