Terrestrial controls on the biogeochemistry of dissolved organic matter and inorganic nitrogen in streams of the central Amazon Basin, Brazil

ivers form a critical link in global element cycles by accumulating material of terrestrial origin and transferring it to the coastal ocean. Of the world's rivers, none carries more dissolved carbon and nitrogen than the Amazon River, which drains greater than 6 million square kilometers of northern South America. The overall objective of this dissertation is to examine, in a systematic manner, the sequence of processes regulating transfers of terrigenous dissolved organic matter and inorganic nitrogen to headwater streams of the central Amazon basin. I pursue this objective through a combination of in-depth field studies, laboratory analyses, and conceptual modeling.;In two contrasting first-order catchments characteristic of the central Amazon basin, I analyzed plant, litter, soil, groundwater, and stream chemistry over a period of two years. I show that clear and persistent differences exist in the concentration and composition of dissolved organic matter (DOM) and inorganic nitrogen in stream and groundwaters from the two catchments, due mainly to corresponding differences in soil texture and chemistry. In the terra firme/Oxisol terrain, which dominates the central Amazon, the majority of DOM contributions to the stream derived from the riparian zone, while in the more spatially restricted campinarana/Spodosol terrain, upland groundwater contributions could account for the concentration and composition of DOM in the stream. Combined sorption and decomposition reactions in upland soils of the terra firme/Oxisol terrain reduce DOM concentrations to levels below those of streams and rivers draining the region. Similarly, in the terra firme/Oxisol terrain, inorganic nitrogen (IN) chemistry differed substantially between upland, riparian, and stream hydrologic compartments. Nitrate dominated the IN chemistry of upland groundwater and stream water, while...