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The effects of prilled urea(PU),sulfur coated urea(SCU),polymer coated urea(PCU),and sulphur-polymer-terresin coated urea(SPCU) were studied on the availability of Cd,Pb,Cu,Zn in contaminated red paddy soils under different nitrogen levels(100,200 and 400 mg/kg,respectively noted as 1,2,3) using the waterlogged incubation method.The results showed that application of different coated release-controlled urea could significantly affect the pH value and the contents of water-soluble SO2-4 in the soil.The pH values of red soil of each application nitrogen treatment increased with increasing nitrogen level(except PU treatment on 5th day and SCU treatment on 60th day).The application of different coated release-controlled urea could significantly affect the contents of water-soluble SO2-4 in the soil.The pH value and contents of water-soluble SO2-4 were significantly different among different coated release-controlled urea under the same nitrogen level.The pH values of soil of the treatments PU,SCU,PCU and SPCU increased by 0.17—0.38,0.08—0.27,0.07—0.36,and 0.10—0.21,respectively in the whole 60 days incubation duration compared to the control.The contents of water-soluble SO2-4 in soil of the treatments PU,SCU and PCU increased by 39.5%—157.3%,40.9%—94.5%,and 7.55%—55.8%,respectively whereas,decreased by 5.67%—90.7% in soil of the treatment SPCU.There were significant differences between different coated release-controlled urea types on the availability of Cd,Pb,Cu and Zn.The availability of Cd,Pb,Cu and Zn also varied sharply with the application rates of nitrogen fertilizer.Available Cd content in soil of treatment PCU at 100 mg/kg nitrogen level significantly decreased by 20.7%—69.8% and was the lowest during the whole 60 days incubation.Available Cd content in soil of treatment PU at 200 mg/kg nitrogen level was the highest and significantly increased by 163.5%—181.7% during the whole incubation.Available Pb,Cu and Zn content in soil of treatment PU at 400 mg/kg nitrogen level was the lowest and significantly decreased by 17.0%—54.2%,18.5%—34.6%,and 15.6%—59.5%,respectively.With increasing the application rates of nitrogen fertilizer,available Cd content in soil of treatment PU increased first and then decreased,while available Pb,Cu and Zn content decreased gradually.With increasing the application rates of nitrogen fertilizer,available Pb content in soil of treatment SCU decreased gradually,while there were not the same regularity of available Cd,Cu and Zn content among the three application nitrogen levels.With increasing the application rates of nitrogen fertilizer,available Cd content in soil of treatment PCU increased gradually,while there were not the same regularity of available Cd,Cu and Zn content among the three application nitrogen levels.With increasing the application rates of nitrogen fertilizer,available Cd,Pb,Cu and Zn content in soil of treatment SPCU decreased gradually.Soil available Pb and Zn were significantly negatively correlated with soil pH and water-soluble SO2-4 content.Soil available Cd was significantly positively correlated with water-soluble SO2-4 content.And the soil pH also plays a major role on the behavioral changes of Pb and Zn in the soil.In red paddy soil contaminated by multi-heavy metal,coupling use of different coated urea can effectively control the availability of heavy metals. |
