MASS FLUXES OF NITROGEN AND CARBON FROM SOIL WATER TO A FIRST-ORDER MOUNTAIN STREAM IN A PRISTINE COSTA RICAN RAIN FOREST IN RESPONSE TO INDIVIDUAL RAIN EVENTS

Watersheds within the tropical montane rainforests of Costa Rica are generally understudied and their hydrological and biogeochemical fluxes have been only sparsely characterized. This study focuses on the sources, fate and transport of dissolved carbon (C) and nitrogen (N) entering a 75 m control reach of a first-order mountain stream from hillslope soil water and a persistent groundwater spring. The volumetric flux of this stream typically doubles over this short reach length owing to soil water and surficial runoff within the forest litter. The volumetric and mass fluxes entering and exiting the reach were calculated over a 48-hour, high-frequency (every 2 hour) sampling period that spanned a range of intensities in individual rainfall events. Volumetric fluxes from the spring, soil water and rainfall were estimated through end-member mixing analysis (EMMA) using SO₄, Cl, δ¹⁸O, and δD as conservative tracers. Once the end-member mixing proportions were estimated, the expected mass fluxes of N species (inorganic nitrate and ammonium, and total organic nitrogen) and C (inorganic and organic) from those end-members could be compared to the mass fluxes observed across the 75 m control reach of the stream. These numbers were used to calculate excess or missing mass fluxes of N and C species. Geochemical inverse modeling was then performed in PHREEQC to identify the chemical reactions that can likely explain the chemical transformations in the end-member chemistry during rain events. Modeling and field data indicate that the study reach is highly responsive to event water and highlight the role of soil water as a major vector for N and C fluxes into surface waters during dry and wet periods. Soil water chemistry varied spatially and underlined the potential challenges of traditional mixing model methods in forested headwater catchments. These results have implications for future water management of tropical rainforests and provide a framework for studying the long-term patterns of streamflow generation in similar, understudied watersheds.