Uptake Of Pb And Cd By 2 Rice Cultivars In 2 Soils Differing In Physico-chemical Properties Under Spiked Cd Treatment And Its Dynamics In Growing Period

The increasing concentration and subsequent accumulation of heavy metals in the soil due to human activities such as mining, fertilizers, and sewage waste may have serious consequences for mankind, environment and agriculture. Heavy metals can enter the plants by way of soil and directly or indirectly enter the human body leading to health risks. One of possible ways to reduce the amount of heavy metals entering the plants is to control the uptake by plant roots, in turn, to control the mobility in soils by pH control and addition of organic matter. Another alternative strategy may be the use of species or cultivars having low ability of uptake or translocation specific toxic metals.This study dealt with heavy metal dynamic bioavailibility in soil-rice system by field and pot experiments. The works included the dynamics of heavy metal uptake and accumulation during rice growth period, heavy metal distribution in organs, and heavy metal level of seed at maturity, as well as the dynamics of heavy metal availability in rhizosphere soil. The main results obtained are as follow:1. Field experiment of heavy metal uptake by different cultivars showed that the grain concentrations of Pb, Cd of 57 rice cultivars varied in the range of 0.337 + 0.078, 0.099 + 0.039mg.kg-1 respectively, demonstrating remarkable inter-species difference in heavy metal accumulation in rice grains. Hybrid cultivars had a high ability of uptake Pb and Cd. Co-uptake and accumulation of Pb and Cd in grains was observed for the rice cultivars. The Cd concentration of most of the samples was found over the permissible limitrecommended by USEPA. However, only a few samples showed Grain lead content over national grain standard.2. A pot experiment was conducted on uptake of Pb and Cd under Cd spiking by 2 cultivars in two soils differing in soil properties under continuously submerging condition. The result showed that significant difference of Pb ,Cd uptake and accumulation existed between two cultivars, Cultivar Suxiangjing No. 1 grown in both soils had a higher content of Pb and Cd than the cultivar of 99-22, Although there was no obvious influence on the biomass and yield of rice and no toxicity on rice by spiking Cd to soils, but grain Cd of two cultivars increased sharply with Cd spiking.. Spiking Cd increased grain Pb content. Both cultivars growing on red soil accumulated more Pb, Cd than these of paddy soil with and without Cd spiking. Thus, the bioavailability of Pb, Cd in red soil was higher than that of paddy soil.3.Pot experiment revealed that translocation of Cd was higher than Pb in plant organs. The concentration order of Pb and Cd in rice organs was root > stem and leaf > grain. The distribution and proportion of heavy metals in different organs of rice was different. The content of Pb, Cd of rice plant in early stage was higher than that in later stage.4. Pot experiment showed that the distributions of chemical speciation of Pb, Cd in two soils were obviously different. The amount of Cd accumulated by rice was positively related to fractions extracted by MgCl2, NaAc, HCl and DTPA respectively. The amount of Pb and Cd uptaked by rice only occupied small proportion of the soil total. The change of available concentration was a dynamic process during rice growth. In the whole growth of rice the rice, soil mobility of Pb and Cd was hardly affected by rice growth.5. The total accumulation of Pb and Cd in rice increased with growing time. While the content of Pb and Cd in rice decreased with growing time. The interspecies difference in Pb and Cd uptake was consistently observed in the whole growing period of rice. The content of Pb and Cd in plant was determined by the bioavailability of heavy metal in the soils, the interaction of soil and rice cultivar.