Root Iron Plaque on Wetland Plants as a Dynamic Pool of Nutrients and Contaminants

Loading of nutrients and contaminants is increasing in wetlands due to anthropogenic activities. The scope of this paper is to (1) provide an overview of natural, cultivated, and constructed wetlands and hydrophytes, (2) characterize root iron plaque of hydrophytes, (3) show roles played by root iron plaque as a source and sink for nutrients and contaminants for hydrophytes, (4) present toxicity tolerance mechanisms employed by hydrophytes, and (5) offer implications of the findings about iron plaque, and (6) to suggest future research. Iron plaque deposits on hydrophyte root surfaces are a result of oxidation of ferrous iron in the oxic rhizosphere under waterlogged conditions in wetlands. The iron plaques mainly consists of amorphous and crystalline iron oxyhydroxides. They, therefore, can sequester nutrients and contaminants that can bind to iron oxides. Recently advanced spectroscopic techniques, such as synchrotron radiation techniques, have been used to identify and characterize iron plaque components. Sequestration and plant uptake of these materials mainly depend on the available nutrients and contaminants, oxygen diffusion capability of hydrophyte roots, and bio-physico-chemical properties of the rhizosphere. Root iron plaque plays a vital role in controlling the sequestration of excess loads of nutrients and contaminants in wetlands.