Magnetic properties of iron plaque on roots of Phragmites Australis in tidal flat of the Yangtze Estuary

Iron plaque on the wetland plant roots plays an important role in the elemental cycle(e.g., heavy metals), and has therefore received wide attention regarding its iron oxide composition. However, characterization of iron plaque composition using the method of environmental magnetism, as far as we know, has not been reported. In the present study, analyses of magnetic properties, particle size and diffuse reflectance spectrophotometry were carried out on a core from the tidal flat of Chongming Island, Yangtze Estuary. The purpose was to investigate the magnetic mineralogy of iron plaque on roots of Phragmites australis as well as the surrounding sediments. It is indicated that both magnetite and hematite are present in the iron plaque. The magnetic mineralogy shows vertical variations due to change in redox conditions. Hematite is more abundant in iron plaque in the upper 24 cm of the core due to relatively oxidizing environment, whereas magnetite dominates magnetic properties in the layer below 24 cm due to relatively reducing environment. In comparison to the surrounding sediments, ferrimagnetic mineral content is lower in the layer above 24 cm depth while higher below 24 cm depth. It is indicated that magnetic minerals in iron plaque have different source from those in the sediments. Our results demonstrate that magnetic measurement can be used to characterize the mineral composition of the iron plaque on the roots of wetland plants, which will contribute to the study of biogeochemical process in the wetland system.