Occurence and Characterization of Mn Nodules in a Toposequence at Coloso Valley Agricultural Reserve, Puerto Rico.

Manganese (Mn) nodules are redoximorphic features in soils with fluctuating high water tables. The objective of this study was to determine the occurrence, and physical and chemical properties of Mn nodules in the Coloso Valley Agricultural Reserve in Puerto Rico. A toposequence comprising three soil series, Rio Piedras (Typic Hapludults), Bajura (Vertic Endoaquolls), and Coloso (Vertic Dystrudepts) was selected for the study. A taxonomic evaluation of each soil profile was performed and the occurrence of Mn nodules in the soil horizons was evaluated.  The largest Mn nodules were observed in the Ap horizon of Rio Piedras soil series, but least in occurrence. Manganese nodules were quantified in an 800 g soil sample.  Two grams of manganese nodules per 800 g of soil were observed in the Ap horizon of Rio Piedras soil series. In Bajura and Coloso soil series, the largest occurrence of manganese nodules was observed in the Bwg horizon.  The nodule mass in Bajura soil was 11 g/800g and in Coloso soil it was 15 g/800g.  Manganese nodules from Rio Piedras soil were well formed with clearly defined concentric layers. In Coloso and Bajura soils the Mn nodules were not well formed and irregular masses of Mn oxides were observed on soil peds. XRD analyses of manganese nodules from the three soils indicate the presence of lithiophorite. Coloso and Bajura soils occur at the lower levels in the toposequence. In Bajura and Coloso soils a more active redox process of Mn is taking place. This process results in the formation of small manganese nodules and masses.  Rio Piedras soil occurs at higher elevation and the water table never saturates the soil profile.  The presence of Mn nodules in the Ap horizon of Rio Piedras soil indicates significant changes in climatic and hydrologic conditions in this toposequence.