Pérez Castro, Sherlynette

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    Occurrence and characterization of Mn nodules in a Toposequence at Coloso valley agricultural reserve
    (2010) Pérez Castro, Sherlynette; Muñoz, Miguel A.; College of Agricultural Sciences; de la Torre, Winston; O’Hallorans, Julia M.; Department of Crops and Agro-Environmental Sciences; López, Ricardo R.
    Manganese (Mn) nodules are sedimentary rocks, generally spherical or irregularly round in shape with concentric layers formed from manganese and iron mineral precipitation. These are redoximorphic features in soils with fluctuating high water table. This study aimed to: (i) evaluate taxonomically the soil profiles, (ii) determine the occurrence of manganese nodules in the soil horizons, (iii) evaluate physical and chemical properties of Mn nodules, and (iv) theorize about manganese nodules formation mechanism in the Coloso Valley Agricultural Reserve. A toposequence comprising three soil series, Rio Piedras (Typic Hapludults), Bajura (Vertic Endoaquolls), and Coloso (Vertic Dystrudepts) was selected for the study. The largest Mn nodules were observed in the Ap horizon of the Rio Piedras soil series; however this site had the fewest nodules. Two grams of manganese nodules per 800 g of soil were observed. 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/800 g and in Coloso soil it was 15 g/800 g. 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 and todorokite. Coloso and Bajura soils occur at the lower levels in the toposequence, where they are subjected to wet and dry cycles. 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 highest rate of oxidation creates stability and well formed nodules. The presence of Mn nodules in the Ap horizon of Rio Piedras soil indicates significant changes in hydrologic conditions in this toposequence, suggesting a soil uplifting event.