Double-funneling by trees, a two-stage hydrologic process is introduced in Chapter 1. In this coupled process, 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 groundwater recharge. 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.


Interactions between hydrology and the carbon cycle in forested Amazonian headwater catchments in Mato Grosso, Brazil are presented in Chapters 3 and 4. 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 flow paths.


Organic and inorganic C fluxes transported by water were evaluated for dominant hydrologic flowpaths on two adjacent headwater catchments in the Brazilian Amazon with distinct soils and hydrologic responses. Rainfall-runoff hydrologic responses of an Ultisol-dominated catchment were found to be more rapid and with larger quickflow volumes than for an Oxisol-dominated catchment due to lower subsurface hydraulic conductivities of the Ultisol. Overland flow was found to be an important feature on both watersheds.


Small volumes of quickflow correspond to large fluxes of DOC; DOC concentrations of the hydrologic flowpaths that comprise quickflow are an order of magnitude higher than groundwater flowpaths fueling base flow (19.6 ± 1.7 mg/L DOC for overland flow and 8.8 ± 0.7 mg/L DOC for shallow subsurface flow vs. 0.50 ± 0.04 mg/L DOC in emergent groundwater). Concentrations of dissolved inorganic C (DIC, as dissolved CO2-C plus HCO3--C) in groundwater were found to be an order of magnitude greater than quickflow DIC concentrations (21.5 mg/L DIC in emergent groundwater vs. 1.1 mg/L DIC in overland flow). The importance of deeper flowpaths in the transport of inorganic C to streams is indicated by the 40:1 ratio of DIC:DOC for emergent groundwater. Dissolved CO2-C represented 92% of DIC in emergent groundwater.


Results from this study illustrate a highly dynamic and tightly coupled linkage between the C cycle and the hydrologic cycle for both Ultisol and Oxisol landscapes: organic C fluxes strongly tied to flowpaths associated with quickflow, and inorganic C (particularly dissolved CO2) transported via deeper flowpaths. Groundwater flow paths, which comprise 96% of stream flow for the headwater catchments, are important DIC conduits at the terrestrial-aquatic interface, while quickflow contributes pulses of relatively unprocessed DOC to streams.