| Chapter 1 presents the concept of hydrologic double-funneling by trees, a two-stage process in which nutrients and carbon (C) are first concentrated in stemflow fluxes as a function of tree canopy architecture. Stemflow fluxes are then routed through the soil along root-induced preferential flow paths, resulting in enhanced soil heterogeneity. In Chapter 2, the use of fire as a pasture-management strategy is shown to induce soil water repellency on pasture soils. Soil water repellency (hydrophobicity) was the strongest on recently burned pastures. Increasing soil water repellency was found be associated with lower nutrient status for the forage grass Brachiaria brizantha (Hochst.), indicating that soil water repellency and pasture productivity are inversely related. Chapter 3 focuses on C cycling in forested Amazonian headwater catchments in Mato Grosso. Brazil. At the soil surface, litterfall represents 95% of the C flux arriving at the soil surface, while C in streamflow is exported predominantly (59%) as dissolved organic C (DOC). Particulate organic C (FPOC, < 2 mm) and coarse particulate organic C (CPOC, > 2mm) are exported primarily in storm flow, but account for only 37% and 4% of annual C exports in stream water, respectively. Large litterfall pulses during the dry season and early part of the rainy season correspond to high DOC concentrations in throughfall and overland flow, which decrease over the course of the rainy season. The DOC concentrations of streamflow track the seasonal patterns of DOC concentrations in surface and near-surface these flow paths. In Chapter 4, rainfall-runoff responses of an Ultisol-dominated catchment are shown to be more rapid and with larger quick flow volumes than for an Oxisol-dominated catchment due to lower subsurface hydraulic conductivities of the Ultisol. DOC concentrations were found to be an order of magnitude higher in quick flow related flow paths compared to DOC in groundwater. Groundwater concentrations of dissolved inorganic C (DIC) were found to be much greater than groundwater DOC. Groundwater flow paths, which comprise 96% of stream flow for the headwater catchments, are important DIC conduits at the terrestrial-aquatic interface, while quick flow contributes pulses of relatively unprocessed DOC to streams. |
