The overall question we are addressing is “What role does the evasion (outgassing) of CO2 from the river system to the atmosphere play in the carbon cycle of moist tropical forests?” Recent evidence suggests that the Amazon may be a net sink of atmospheric CO2 of 1-9 Mg C ha-1 y-1. We have made recent calculations that suggest that the evasion of CO2 from the river system of the central Amazon basin is on the order of 1.2 ± 0.3 1-9 Mg C ha-1 y-1. A flux of this magnitude suggests that the ecosystem processes involved in river corridors represent a significant pathway for the export of carbon fixed on land in the humid tropics at a globally significant level. The quantity of dissolved CO2 (pCO2) in surface waters of tropical river systems is the product of a long sequence of complex biological, hydrological, and geochemical processes.

Adsorption is an important geochemical process that removes dissolved organic carbon (DOC) from water percolating through soils, which eventually flows into groundwater and the river channel (where it is respired to CO2). Soil properties, such as texture, Fe- and Al-oxide content, and soil organic carbon content, are known to affect the concentration of DOC in soils (and, therefore, the quantity of DOC transported to river channels) and vary widely with soil type. We collected soil cores from various depths to use in continuous, unsaturated flow soil column adsorption experiments and for analysis of soil properties. We examined relationships between adsorption and a suite of soil properties in an attempt to estimated the quantity of DOC adsorbed based on soil type. Quantifying the DOC lost to adsorption as a function of soil properties is an important step in understanding the export of carbon fixed on land and transported to rivers.