Podzolisation and exportation of organic matter in black waters of the Rio Negro (upper Amazon basin, Brazil)

In the Rio Negro basin, podzols develop at the expense of clay-depleted laterites through localised and spectacular weathering fronts. This natural process leads to the remobilization of previously accumulated organic matter (OM) and elements (Al, Fe, Si). OM is thus redistributed within soil profiles and exported towards rivers, giving them their typical black coloration. In soils, this redistribution leads to a strong vertical variation of OM characteristics in podzols. However, little is known about the structure of OM transferred to rivers, and about the relative contributions of the different soil horizons to this exportation. To investigate the fate of OM in the soil – water continuum in the Rio Negro basin, we have water-extracted OM from seven key soil samples collected in a soil sequence representative for the transition between latosol and podzol on the low-elevation plateaux of the Rio Negro basin. These samples enable following both vertical and lateral differentiations and they were compared with OM from the water-tables and rivers draining the soil sequence. Specific UV Absorbance, pH and conductivity of aqueous samples were measured. OM chemical structure was investigated through solid-state C nuclear magnetic resonance (NMR) and thermally assisted hydrolysis and methylation (THM). OM water-extracted from different soil horizons was thus shown to exhibit contrasting features and we could identify compounds released upon THM specific to OM waterextracted from the different soil horizons investigated. Comparison with water from the drainage network suggests that, in the early rainy season, OM is mainly remobilised in deep horizons of well-developed podzols. This study opens new perspectives to trace the origin and evolution of OM in black rivers of the Rio Negro basin. Structure, composition, size and reactivity of biogenic nano-uraninite JOHN R. BARGAR, RIZLAN BERNIER-LATMANI, DANIEL E. GIAMMAR, APURVA MEHTA, ELEANOR SCHOFIELD, JONATHAN O. SHARP, ELENA I. SUVOROVA, KAI-UWE ULRICH AND HARISH VEERAMANI Stanford Synchrotron Radiaion Lightsource, SLAC National Accelerator Laboratory, CA, USA Environmental Microbiology Laboratory, École Polytechnique Fédérale de Lausanne, Switzerland Department of Energy, Environmental and Chemical Engineering, Washington University, MO, USA