Evasion of CO₂ and dissolved carbon in river waters of three small catchments in an area occupied by small family farms in the eastern Amazon

Evasão de CO₂ e carbono dissolvido em águas fluviais de três pequenas bacias em área ocupada por pequenas propriedades de agricultores familiares na Amazônia oriental

Abstract

CO₂ effluxes from streams and rivers have been hypothesized to be a critical pathway of carbon flow from the biosphere back to the atmosphere. This study was conducted in three small Amazonian catchments to evaluate carbon evasion and dynamics, where land-use change has occurred on small family-farms. Monthly field campaigns were conducted from June 2006 to May 2007 in the Cumaru (CM), Pachibá (PB) and São João (SJ) streams. Electrical conductivity, pH, temperature, and dissolved oxygen measurements were done in situ, while water samples were collected to determine dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) concentrations, as well as carbon dioxide partial pressures (pCO₂) and CO₂ evasion fluxes. Instantaneous discharge measured by a current meter was used to calculate DOC fluxes. Considering all the sites, DOC, DIC, pCO₂, and CO₂ flux measurements ranged as follows, respectively: 0.27 - 12.13 mg L^-1; 3.5 - 38.9 mg L^-1; 2,265 - 26,974 ppm; and 3.39 - 75.35 μmol m^-2 s^-1. DOC annual flux estimates for CM, SJ and PB were, respectively, 281, 245, and 169 kg C ha^-1. CO₂ evasion fluxes had an average of 22.70 ± 1.67 μmol m^-2 s^-1. These CO₂ evasion fluxes per unit area were similar to those measured for major Amazonian rivers, thus confirming our hypothesis that small streams can evade substantial quantities of CO₂. As secondary vegetation is abundant as a result of family farming management in the region, we conclude that this vegetation can be a major driver of an abundant carbon cycle.

Keywords:
Amazon basin; biogeochemistry; carbon dioxide evasion.