广西象州与横县碳酸盐岩分布区土壤中Cd形态分布特征及影响因素

广西壮族自治区碳酸盐岩分布面积为96 372 km2，约占全区陆地国土面积的407%。由碳酸盐岩风化形成的土壤中Cd、Pb、Hg等重金属元素普遍富集。初步评价发现，农作物籽实对Cd的吸收量与土壤Cd含量无对应关系，农作物Cd超标多出现在土壤Cd含量较低的地区。进一步研究发现，Cd高含量的土壤中普遍含有铝土矿、Fe Mn结核等颗粒。为了探索土壤中Cd存在形态及其影响因素，选择了含铝土矿和铁锰结核的横县土壤和无铝土矿与铁锰结核的象州土壤，系统研究了土壤Cd形态差异及其影响因素。结果表明：(1)象州土壤中Cd主要以活动态形式存在，生物可利用性高，横县土壤中Cd主要以稳定态形式存在，生物可利用性低。(2)pH值为60是象州土壤Cd活动态含量的突变点，pH值在60处含量达到最大值；而在横县，pH降低使活动态Cd比例增加。(3)象州土壤中活动态Cd随有机质增加而增加，表明Cd被有机质弱吸附；而横县土壤有机质含量与Cd形态无明显相关性。(4)由铝土矿导致的土壤高Cd含量，多以残渣态形式存在，不会对动植物造成危害。

Distribution and Influencing Factors of Cadmium Geochemical Fractions of Soils at Carbonate Covering Area in Hengxian and Xiangzhou of Guangxi

The carbonate covering area is about 96,372 km2, accounting for about 407% of district land area in Guangxi Zhuang Autonomous Region. The heavy metals in soil formed from the carbonate weathering were enriched commonly, such as cadmium (Cd), lead (Pb) and mercury (Hg). Preliminary evaluation results showed that there was no corresponding relation between Cd uptake of crops and Cd content in soil. Crops which exceed food safety standards always grew in the soil with low Cd content.Soil with high Cd content generally contained bauxite, Fe Mn nodules and other particles.In order to discuss factors affecting geochemical fractions of Cadmium, we determined geochemical fractions of cadmium and other soil properties in two study areas in relation to bauxite and Fe Mn nodules. Soils in the Hengxian area were with bauxite and Fe Mn nodules. Soils in the Xiangzhou area were without bauxite and Fe Mn nodules.The results show that: (1) In Xiangzhou area, the Cd content of soil was mainly bounded to mobile fraction and the bioavailability of Cd was high. In Hengxian area, Cd was mainly bounded to residue fraction, the bioavailability was low.(2) Soil pH and organic matter were important factors controlling the Cd mobility.Soil pH value 60 is the abrupt change point of the Cd mobility,contents of ion exchange Cd reached maximum in soil pH value 60 in Xiangzhou soils. The percentage of mobile fraction Cd was increasing with soil pH decreasing in Hengxian soils. (3) Cd in mobile fraction was correlated positively with organic matter in Xiangzhou soils, which indicated that Cd is weakly adsorbed by organic matter in soil. Cd fraction has no obvious correlation with organic matter in Hengxian soil. (4) The Cd content of soil with bauxite was high, which occurred in the residual fraction in Hengxian. So, it was not harmful to animals or plants.