||Project Title: Water-carbon links in a tropical forest: how interbasin groundwater flow affects carbon fluxes and ecosystem carbon budgets
This new project is quantitatively exploring the importance, for carbon cycling, of the link between the tropical rainforest and the deeper hydrogeological system on which it sits. Tropical forests are an important component of the global carbon cycle, and yet even the basic issue of whether they operate as net sources or sinks with respect to carbon remains a primary research question and the subject of sometimes-conflicting analyses. Full accounting of carbon inputs and outputs is a critical issue and one closely linked to the hydrologic cycle. Recent water and carbon data from the lowland rainforest at La Selva Biological Station in Costa Rica suggest that interbasin groundwater flow (IGF, regional groundwater flow beneath watershed topographic boundaries) strongly influences concentrations and fluxes of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). The influence of IGF on carbon dioxide degassing from streams, wetlands, and soils is presently not known but may be significant. The project focuses on the potentially significant links between IGF (a common groundwater process detected at watersheds world-wide) and carbon cycling in tropical forests.
We have begun collection of new data (carbon concentrations, chemical and isotopic characteristics, soil respiration, net ecosystem exchange (NEE) of carbon dioxide between the atmosphere and forest, carbon dioxide degassing rates in streams, and stream discharge) to show how IGF influences carbon fluxes and the overall carbon budget of this tropical rainforest ecosystem. Two small watersheds (one with significant inputs by IGF, the other without) are being used as the field study units for flux and budget accounting in the ecosystem. Preliminary analyses show, for example, that IGF increases streamwater DIC concentration by a factor of about 12 and stream-based watershed export of DIC by a factor of about 70; for DOC, concentration was actually reduced by a factor of about 0.67 while export increased by a factor of 3.5. A significant quantity of carbon is moving into the ecosystem via IGF, and out of the ecosystem via other fluxes (including streamflow and atmospheric exchange of carbon dioxide). Data collection and analysis related to carbon chemistry and fluxes is ongoing. We are beginning to study the impact of IGF on DOC quantity and quality, including whether “old” DOC from IGF is relatively “protected” from biological degradation, thus facilitating its hydrologic export from watersheds via stream flow.