Availability Of Deep Groundwater-Derived CO2 For Plant Uptake In A Costa Rican Rainforest

Abstract:

The role of export of carbon via surface waters has been increasingly appreciated as an important component of ecosystem carbon budgets. However the role of deep regional groundwater as a source of carbon to ecosystems is relatively poorly known. In a lowland rainforest in Costa Rica, inputs of elevated dissolved inorganic C (DIC) in regional groundwater greatly increase stream water C concentrations. Whether that groundwater-derived carbon represents a significant source of elevated CO₂ for photosynthesis of riparian plants is unknown. We compared the concentration and δ¹³C of CO₂ in the air above two weir-equipped streams with different inputs of high-DIC regional groundwater. The Taconazo has no inputs, whereas about 40% of stream discharge of the Arboleda is a result of regional groundwater. DIC from regional groundwater experiences little to no within-watershed sequestration and thus augments the C flux out of the watershed with stream flow and possibly the degassing flux from the stream.

CO₂ concentrations were recorded by Vaisala GMP343 gas analyzer over 24 hr periods above the weirs and in the splash zone below the weirs as well as at a small waterfall on the Sura, the higher order stream that the Arboleda joins, approximately 250 m downstream of the junction. Samples of air δ¹³C-CO₂ taken from unmixed (early morning) and mixed (afternoon) canopy air were measured by mass spectrometer. Concentrations of CO₂ at both sites on the Taconozo remained in the normal range of canopy storage of respiratory CO₂ (< 600 ppm). In contrast, [CO₂] above the Arboleda weir occasionally exceeded 1000 ppm and were generally above normal values of respiratory CO₂. Values below the weir by the splash zone were often higher than 1500 ppm and occasionally exceeded 2000 ppm. At the Sura waterfall pulses of high CO₂ > 1000 ppm occurred regularly throughout the day. We found higher δ¹³C-CO₂ above the Arboleda compared to the Taconazo, consistent with an enhanced flux of isotopically-heavy CO₂ from the Arboleda stream. Keeling plots of samples taken at the Arboleda and Sura deviated from those over the Taconazo and indicated a source of ¹³C other than atmospheric air and respired CO₂. Our data suggest that CO₂ from regional groundwater has the potential to be available to riparian plants, but primarily at areas of turbulent water flow.