The Biogeochemistry and Trace Metals Distribution of Mangrove Rhizospheres

Abstract

Soils and porewater of the rhizosphere of mangrove trees, Rhizophora mangle L. and Avicennia schaueriana Stapf & Leech., of the salt marsh grass Spartina alterniflora Loisel, and of unvegetated mud flats, were analyzed for pH, salinity, platinum electrode redox potential, organic matter content, sulfide concentrations, and the total and exchangeable concentrations of trace metals (Fe, Zn, Cu, Pb and Cd). The study was conducted in a fringe mangrove forest in Sepetiba Bay, Rio de Janeiro, Brazil. The rhizospheres differed in their biogeochemistry. Mud flats and Rhizophora soils were very reducing, with highest concentrations of sulfide. Avicennia soils showed the highest variability of the variables measured, with the rhizosphere changing from oxic to anoxic conditions. Spartina soils, on the other hand, were generally oxic, with very low sulfide concentration. The distribution of trace metals in these soils varied with the major physical and chemical characteristics. Mud flat soils presented the highest total trace metal concentrations followed by mangrove soils and Spartina soils. However, exchangeable trace metals were similar among the different soils with the exception of Avicennia soils, which due to their characteristic instability of redox conditions, presented much higher exchangeable trace metals concentrations.