湿地植物根表铁膜研究进展

为了适应渍水环境,许多湿地植物都具有根系泌氧、形成铁膜的能力。因铁膜具有特殊的物理或化学结构,可以通过吸附和共沉淀作用影响元素在土壤中的化学行为和生物有效性,在植物吸收营养元素和重金属中起重要作用。综述了湿地植物根表铁膜的形成、影响因素以及根表铁膜对营养元素和重金属的生态环境效应,从表征技术方面阐述了根表铁膜的作用机制。对今后的研究方向给出如下建议:(1)扩大研究领域;(2)铁膜形成的动态变化过程;(3)铁膜对植物生理形态的影响;(4)利用先进的表征技术以确定铁膜的作用机制。

Progress in research of iron plaque on root surface of wetland plants

Waterlogged soils are frequently anaerobic, with low partial pressure of oxygen. To acclimate to an anaerobic condition when submerged in water, wetland plants must allow aerenchyma to transfer oxygen from aerial parts to the rhizosphere, which is termed radial oxygen loss. Radial oxygen loss also causes the oxidation of ferrous iron to ferric iron and the precipitation of iron oxides or hydroxides on the root surface. This material is termed "iron plaque". Iron plaque is a mixture of crystalline and amorphous iron oxide or hydroxide, and is mainly composed of ferric hydroxides, goethite and lepidocrocite. Given the high adsorption capacity of functional groups on iron oxides or hydroxides, iron plaque can influence the chemical behavior and bioavailability of heavy metals and nutrient elements in the rhizosphere. In recent years a number of studies have been conducted on the formation of iron plaque and uptake of heavy metals and nutrient elements in wetland plants. In this paper, progress in research on iron plaque was analyzed and summarized from four aspects, i.e., the formation of iron plaque and factors influencing its development, the nutritional effect of iron plaque on plants, the ecological and environmental effects of iron plaque on heavy metals, and techniques for characterization of iron plaque. The formation of iron plaque is controlled by a number of biotic and abiotic factors, such as Fe²⁺ concentration, the oxidizing capability of the root system, and plant species. Iron plaque is an enriched reservoir of plant nutrient elements, but some results indicate that this function is related to the amount of iron plaque. Many studies have been conducted on the relationship between iron plaque formation and uptake of heavy metals in wetland plants, but the results reported among the studies are inconsistent. Some studies have indicated that the formation of iron plaque acts as a physical "barrier" to uptake of heavy metals through immobilization and co-precipitation, whereas other studies have reported no difference in the presence or absence of iron plaque. Furthermore, iron plaque may promote the uptake of heavy metals. In addition, some results indicate that the inhibitory and promotive actions of iron plaque on uptake of heavy metals were related to the amount of iron plaque on the root surface. Thus our understanding of the control of iron plaque formation and the mechanism of action of iron plaque on heavy metals uptake remains incomplete at present. The following suggestions for future research are put forward: (1) expand the fields of research, such as lake wetlands; (2) consider the influence of environmental factors on dynamic changes in processes involving iron plaque; (3) pay close attention to the effect of iron plaque on the physiology and morphology of plants; and (4) use advanced characterization techniques to analyze the mechanism of action of iron plaque on heavy metals. For example, ion selective electrodes could be used to measure root oxygen release, and gas chromatography-mass spectrometry used for characterization and quantification of root exudates. In situ techniques should be employed to elucidate the mechanism of action of iron plaque on heavy metals. In particular, X-ray absorption fine structure technology has high potential for clarification of the molecular mechanism of action of iron plaque.