Inorganic and Organic Carbon Fluxes from Tropical Andisols and Andesitic Saprolite in a Pre-Montane Rainforest

Tropical rainforests are highly productive ecosystems that play a pivotal role in global carbon cycling. The fate of carbon in these systems is not clear due to lack of direct measurements of transformations and fluxes, especially in response to climate change and land use. The objective of this study is to measure the fate and transport of dissolved inorganic (DIC) and organic carbon (DOC) within the major soil horizons of a first-order mountain stream in Alajuela Province, Costa Rica. Surface and subsurface DIC fluxes to stream water were previously measured in this watershed, but clear end-member chemistries were not well-defined, which limited the delineation of major sources of DIC. The main soil horizons with their conceptualized hydrologic flow pathways in parentheses are: 1) leaf litter (quick flow); 2) mineral soil (interflow); and 3) saprolite (baseflow). We hypothesize that DIC is primarily produced from decomposition of organic matter during leaching of leaf litter. The leaf layer additionally releases DOC acids, which weathers silicates within the underlying mineral soil releasing dissolved calcium (Ca). Lastly, the saprolite layer acts as a DIC sink by precipitating carbonates, which is facilitated by a rise in pH. This hypothesis was tested utilizing duplicate sequential flow column experiments packed with the three soil horizons in columns (ID=6.3, length=20 cm). Steady-state (2.3 ml/min), storm (9.2 ml/min), and drought (0 ml/min) flow rates were simulated for this study. The influent rain water pH and DIC concentration was 7.6 and 3.0 mg/L, respectively. Under steady-state (low) flow conditions downstream of the leaf litter, mineral soil, saprolite, DIC was generated at a rate of +5.7, +17.9, and -10.2 (mg/L DIC)/day, respectively, indicating saprolite acts as a DIC sink. The simulated storm enhanced kinetic rates of DIC production. The specific conductance measurements closely correlate to alkalinity indicating that carbon mineralization as water seeps through the rainforest floor was responsible for generating not only DIC but also played a role in releasing other ions. Our results critically inform reforestation programs, such as Payment for Ecosystem Services, to evaluate effectiveness of rainforests as long-term atmospheric carbon sinks.