Availability Of Soil Cadmium And Its Uptake By Rice As Restrained By Using Magnesium, Zinc, Lime And Their Combinations

With social advances and rapid development in agriculture and industry, environmental pollution is getting deteriorated. Among the pollutants, the adverse impact of heavy metals on the environment is particularly alarming. Cadmium (Cd), because of its features of high mobility, high-toxicity, easy accumulation and difficult elimination from the environment, is usually regarded as the metal with the strongest toxicity. Once Cd enters the food chain, it will seriously affect human and animal health, causing various diseases or even cancer. Improper farming practices including land use, land management and improper fertilization can significantly increase availability of soil Cd and crop absorption, and lead to reduced yield and quality of agricultural products. Therefore, it is of great importance to conduct research to determine the factors affecting availability of soil Cd, search effective control measures, screen suitable fertilizers and application methods and other amendments to reduce availability of soil Cd and crop uptake to safeguard food safety and human health.Technology in controlling Cd pollution in soil-plant system has become a hotspot in the field of environmental pollution research around the world, and progress has been made through efforts of scientists both at home and abroad. Some studies have involved in magnesium (Mg), zinc (Zn), lime and other materials depressing availability of soil Cd and plant absorption. However, systematic studies dealing with effects of different lime materials, consecutive application of lime, interactions of lime with Mg, Zn, and activated carbon on soil available Cd and crop uptake are lacking. Therefore, the objectives of the studies were to investigate the effects of different lime materials and rates, consecutive application of lime in combination with Mg, silicate (Si), and activated carbon on soil pH and available Cd; to determine effects of continued liming to rice soil after wheat season with Mg, Zn and activated carbon on growth, Cd uptake by rice as well as distribution in the rice plant; and to finally develop fertilizer technology in controlling the over-limited Cd content in agricultural products in Cd polluted soil. The major conclusions are drawn as follows:1. The results from the incubation experiment showed that soil pH was approaching to neutrality for all the treatments under waterlogged condition, and the higher the initial pH the greater decline of the final pH was, indicating that the soil had a strong base buffering capacity. There was a negative correlation between soil available Cd and soil pH. Soil available Cd concentration on the whole was gradually reduced over time, indicating that flooding can effectively reduce the soil available Cd. All the treatments had significant lower available Cd than the control. The treatment effects on soil available Cd followed an order of lime+Si> lime+Mg> lime, and the higher the rates the better the effect. On the basis of lime application, addition of Mg or Si could further reduce amounts of soil available Cd. The better Cd-reducing effect was obtained by using the lower lime rates plus Mg or Si. Since soil pH and the available Cd concentration kept changing with incubation time, the optimal lime rates required to effectively control soil available Cd may need more experiments with longer testing time to determine rather than one crop season.2. Results from the continuous liming experiment showed that alkalinity of the three lime materials followed an order of CaCO3<Mg(OH)2<Ca(OH)2. Magnesium hydroxide showed a best impact on reducing availability of soil Cd, followed by calcium hydroxide but no pronounced effect for limestone powder. However, when Mg(OH)2was used as a basic material to nullify soil available Cd, the rate was already toxic to rice growth and thus it is cautious for its application rate when use in practice. During the60d waterlogged incubation study in second year, soil pH of the continued lime treatments increased significantly with increased lime rates but soil available Cd changed oppositely. Among the different lime rates, the best effect of Cd availability control was observed by consecutively using the low-rate lime (0.33g/kg) which is not only economic but also highly effective and can be recommended to use in practice. Magnesium, whether it was used in the second years following lime application in the previous year or it was used in combination with lime in the second year following lime application in the previous year, could further reduce availability of soil Cd on the basis of lime effect. Low rates of lime plus Mg could yield the better Cd-control result than any other treatment and thus, can be used in practice.3. The results from the rice culture experiment showed that magnesium sulfate, zinc sulfate and activated carbon, whether they were applied alone or in combination, could effectively inhibit Cd absorption by rice. Under the case of separate application of each material, the best effect was achieved from use of magnesium sulfate, which not only reduced Cd concentrations in grain and straw, but also restricted Cd transfer from straw to grain. Though magnesium sulfate can be applied to soil or as foliar spray, soil application was slightly superior to folia spray, implying the effect of Mg on inhibiting Cd absorption by rice occurred simultaneously both in soil and in plant, but more in soil. When magnesium hydroxide was used to nullify soil available Cd, its rates must be strictly IV controlled not in excess to avoid poisoning rice growth. Application of zinc sulfate could significantly reduce rice Cd absorption with a better effect from foliar spray than soil incorporation, indicating the mechanism of zinc sulfate inhibiting Cd absorption by rice mainly happened in rice plant. The activated carbon could also effectively inhibit the rice absorb Cd, too. Among zinc sulfate, magnesium sulfate and lime, when two or three of them applied together, the Cd-inhibiting effect was better than any single material application, showing positive Cd-depressing interactions of the three materials. Using low-Cd-absorption rice varieties, optimizing fertilization techniques, selecting the suitable soil amendments and the right agronomic practices are the key to reduce Cd content in’rice.