Vega-Pizarro, Geraldine N.
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Publication Nature and evolution of a soil toposequence at coloso valley agricultural reserve(2018) Vega-Pizarro, Geraldine N.; Muñoz, Miguel A.; College of Arts and Sciences – Sciences; O’Hallorans, Julia M.; Santos, Hernán; Department of Crops and Agro-Environmental Sciences; López RodrÃguez, Ricardo R.Soil physical, chemical and mineralogical properties of four profiles in a Rio Piedras-Bajura toposequence at Coloso Valley Agricultural Reserve were evaluated to theorize about the nature and evolution of these soil series over the last 49 million years. Rio Piedras soil series (Ultisol) formed from manganese enriched volcaniclastic and stratified sedimentary rocks, deformed by the compressive movement of tectonic uplift in western Puerto Rico. Bajura soil series (Mollisol) formed from alluvial deposits eroded from adjacent rocks. The Ultisol predominates in the highest parts of the valley, and the Mollisol in the lower parts. Soil profiles along the toposequence, at shoulder, backslope, footslope and valley landscape positions were evaluated. In the area occupied by the Rio Piedras soil series at shoulder position, a profile with an atypical vertical stratification was identified. Soil texture, aggregate stability, structure, color, pH, cation exchange capacity, organic matter content, presence of manganese masses or concretions, iron and aluminum oxides and the mineralogy of the clay fraction were evaluated. Yellowish and redish hues (2.5YR, 5YR, 7.5YR, 10YR and 10R), dominant in Rio Piedras profiles, were associated with an increase in iron and aluminum oxides with soil ageing. Brownish to yellowish and gley colors that dominated Bajura (2.5YR, 10YR and Gley) were associated to redox reactions during water table fluctuations. A clayey texture predominates along the toposequence, but a higher content of sand and silt was observed in Bajura, soil located at the lowest position in the toposequence. Aggregate stability (62% - 76%) and organic matter (0 - 3.3%) content in soil surface increased downslope, as we move from the Ultisol to the Mollisol. Surface pH also increased downslope (3.8 - 6.6). Soil exchangeable Al3+ content was very high in Rio Piedras profiles, reaching values over 30 cmolc kg−1 in some instances. Bajura had a cation exchange capacity over 20 cmolc kg−1as expected for a Mollisol. The oxides content in the crystalline fraction was considerably higher than amorphous fraction. X-ray diffraction analysis of the clay fraction indicated the presence of micas, expansive (2:1) clays, chlorite, kaolinite, gibbsite, goethite, hematite and quartz minerals in all soil profiles and possible presence of manganese minerals pyrolusite or birnesite in Bajura, Rio Piedras footslope and some strata of the vertically stratified profile. Oxides ratios, mineralogy and color reflect the advanced weathering stage of Rio Piedras Ultisol. Main differences in soil properties are attributed to differences in parent materials and topography. Nature and evolution of Coloso Valley is associated to geological processes of Cerrillos Belt, Eocene in age (~49 Ma). Manganese nodules occurrence in Rio Piedras soil may be a weathering product from these rocks while masses of Bajura are of more recent origin due to water table fluctuations. Geological and pedological processes of the area resulted in the properties and evolution of this soil toposequence over time.