喀斯特高镉地质背景区水稻镉的富集、转运特征与机理

西南喀斯特地区土壤镉(Cd)具有"高地质背景、低污染风险"特征,但超过《土壤环境质量农用地土壤污染风险管控标准》风险管制值的区域是否仍具有低风险性,是迫切需要回答的问题。本文在罗甸县喀斯特峰丛谷地选择一个高Cd土壤水稻种植区,开展土壤-水稻系统Cd含量特征和土壤Cd赋存形态等方面的研究,评价水稻各部位Cd的富集、转运特征。结果显示:土壤Cd含量平均值高达8.59 mg/kg,以可还原态为主(59%),弱酸提取态次之(22%);水稻糙米Cd平均含量为0.32mg/kg、超标率高达93%,富集系数仅为0.04;水稻根系平均含量为4.54 mg/kg,富集系数仅为0.52。研究区糙米、根系Cd的富集系数,远低于我国水稻主产区工业污染农田土壤及水稻,这种低富集性体现了喀斯特地区土壤Cd污染相对较低的风险性;土壤富钙、偏碱性及还原环境条件所产生多个交织的Cd解吸-吸附、溶解-沉淀作用过程,导致Cd的生物可利用性降低,是水稻根系、糙米低富集性的主导因素。本文建议加强喀斯特土壤Cd生物地球化学行为及机理的研究,探索适宜于喀斯特土壤Cd污染的消减技术与模式。

Translocation and Accumulation of Cadmium in Rice in a Karst Area with High Geochemical Background and Its Mechanism

The soil cadmium (Cd) in karst area of Southwest China has a characteristic of "high geochemical background and low pollution risk". Whether the area, that exceeds the risk control value of "Standard of soil environmental quality for soil pollution risk control of agricultural land", still has a low risk is an urgent question to answer. In this research, a high soil Cd rice-planting area located in the karst peak cluster valley of Luodian county was selected to reveal the modes of occurrences of Cd in soils, enrichment of Cd in different parts of rice plant, and the translocations of Cd between soil and rice. The results showed that, the average content of Cd in soil was 8.59 mg/kg, which is mainly in the reducible state (59%) and followed by the weak acid extraction state (22%); the average content of Cd in brown rice was 0.32 mg/kg, with an exceeding rate over the standard of 93%, whereas the enrichment coefficient was only 0.04; the average content of Cd in rice root was 4.54 mg/kg and the enrichment coefficient was only 0.52. Both the enrichment coefficients of Cd in brown rice and root are much lower than that in the main rice production areas in China where was polluted by industrial activities, indicating the relatively low risk of soil Cd pollution in karst area. Under the conditions of rich calcium, slightly alkaline, and reducting environment, multiple intertwined desorption-adsorption and dissolution-precipitation processes restrict the bioavailability of Cd in karst calcareous paddy soil, which is the main reason for the low enrichment of Cd in brown rice and root. Further studies to enhance the understanding of the biogeochemistry of Cd in karst area and to develop the soil Cd pollution abatement techniques and means are also proposed.