镉在土壤-香根草系统中的迁移及转化特征

以无植物组处理为对照,采用盆栽试验方式探讨不同Cd浓度胁迫条件下香根草根际土壤中重金属Cd的积累、迁移及转化特征。土壤Cd处理设4个浓度梯度,分别为0、2、20、80 mg/kg土壤干重。结果表明:(1)香根草可以显著降低土壤中生物有效态Cd和总Cd含量。(2)香根草各部分Cd积累量随处理浓度的增加和处理时间的延长而增加,90 d时80 mg/kg处理组地上部分和根的Cd积累量分别高达180.42 mg/kg和241.54 mg/kg。(3)各浓度Cd处理下,富集系数随着Cd处理浓度的增加而显著降低,随处理时间的延长而升高。(4)香根草地上部分Cd含量小于根部,各处理转移系数均小于1。随着处理时间的延长,中低浓度处理组的转移系数稍有降低,高浓度处理组的转移系数则显著上升。(5)种植香根草使其根际土中残渣态的Cd转化为生物有效态Cd,提高Cd清除效率。研究结果表明,香根草能够有效地吸收土壤中的Cd,降低土壤中总Cd含量,提高土壤安全性,可作为Cd污染地区植物修复的备选物种。

Transfer and transformation characteristics of cadmium from soil to Vetiveria zizanioides

Soil cadmium (Cd) contamination poses serious environmental concerns. The arable land area contaminated with Cd has been increasing in recent years, and it has now reached 2 × 10⁵ km², which accounting for 1/5 of the country''s arable land. Soil Cd contamination has already jeopardized food security in China. Cd is one of the most phytotoxic heavy metals, and it is easily taken up by plants owing to its high bioavailability. Thus it easily enters the food chain and poses considerable threat to human health. This environmental concern can be overcome by phytoremediation- a powerful environment-friendly technique that involves growing plants for the extraction of toxic metals from contaminated soil. The ideal plant species for phytoremediation would be a high biomass-producing plant that can both tolerate and accumulate the desired soil contaminants. Exploitation of heavy metal uptake into plant biomass as a method of soil decontamination is limited by plant productivity and the metal concentration achieved. Therefore, more information is needed to identify the optimum plant species for phytoremediation, particularly in Cd contaminated soils. This study aimed to bridge this gap and evaluated the usefulness of Vetiveria zizanioides as a candidate species for phytoremediation. V. zizanioides is a tall (1-2m), fast-growing and perennial tussock grass, and eurytopic in terms of habitat choice. It develops a long (3-4m), massive and complex root system, which can penetrate the deeper layers of the soil. This species is known to have potential applications for phytoremediation of Cd-contaminatedsoil. Here, we conducted a microcosm experiment to determine the transfer and transformation characteristics of Cd from soil to V. zizanioides. The experiment involved two groups, each comprising four soil Cd-contaminated treatments, with or without plants, and 5 replicates per treatment. These included control, low, medium, and high treatment groups treatments, containing Cd concentrations of 0, 2, 20, 80 mg/kg soil dry weight, respectively. Groups containing soils planted with V. zizanioides were designated CKP, LP, MP, and HP, respectively, and those without plants were designated CK, L, M and H respectively. Cd contents in soil and plant tissues were further analyzed by ICP (inductively coupled plasma) method. The different forms of Cd, including neutral exchange form of Cd concentration extracted by CaCl₂, chelate form of Cd concentration extracted by chelant DTPA, and residual fractions of Cd in soils were examined to determine the effect of V. zizanioides on bioavailable and total Cd in soil. Cd accumulation in the aboveground parts and root of V. zizanioides, bio-concentration factor (BCF), translocation factor (TF), and bioavailability factor (BF) were calculated as indicators of the translocation and accumulation capability of plants. The results showed that V. zizanioides significantly decreased the bio-available Cd and total Cd in soils. Furthermore, the Cd concentration in aboveground parts and root of V. zizanioides increased with increasing soil Cd addition.After 90 days of treatment, Cd concentrations reached 180.42 mg/kg and 241.54 mg/kg in aboveground parts and roots, respectively, under HP treatment. With increasing concentrations of Cd, the BCF of V. zizanioides decreased significantly, but it increased with prolonged time. Moreover, Cd contents of aboveground parts were lower than those in roots for all treatment, and all TFs were lower than 1. Planting V. zizanioides could facilitate the transformation of the residual Cd fractions to the bio-available form. These results demonstrate that V. zizanioides is suitable for phytoremediation of Cd-contaminated soils and aids in improvement of soil quality from the aspect of food safety.