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

Maria Beatriz Silva Da Rosa,Fabíola Fernandes Costa,Ricardo De Oliveira Figueiredo,Pedro Gerhard,Alex Vladimir Krusche,Daniel Markewitz

doi:10.4136/ambi-agua.2040

Maria Beatriz Silva Da Rosa, Fabíola Fernandes Costa + Show 4 more

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https://doi.org/10.4136/ambi-agua.2040

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Abstract

CO2 effluxes from streams and rivers has been hypothesized to be a critical pathway of carbon flow from the biosphere back to the atmosphere. This study was conducted in three Amazonian small 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. 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 ranged from 3.39 to 75.35 μmol m-2 s-1, with 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.

Highlights

Riverine CO2 concentrations in Amazonian lowlands are 5-30 times supersaturated with respect to atmospheric equilibrium; such conditions may be prevalent throughout Amazonian streams and rivers (Richey et al, 2002)
It was estimated by Richey et al (2002) that CO2 outgassed from the Amazon River is more than ten times the amount of carbon exported to the ocean in the form of total organic carbon or dissolved inorganic carbon (DIC), and that this CO2 evasion from rivers and wetlands of the central Amazon basin constitutes an important carbon loss pathway
The predominance of small family farming resulted in a landscape where secondary vegetation of different ages comprised the largest area in all catchments

Summary

Introduction

Riverine CO2 concentrations in Amazonian lowlands are 5-30 times supersaturated with respect to atmospheric equilibrium; such conditions may be prevalent throughout Amazonian streams and rivers (Richey et al, 2002). Stream channels are linked to the land by groundwater flows, surface and subsurface runoff, infiltration from riparian zones, and by direct inputs of throughfall and terrestrial detritus affecting C dynamics at the terrestrial-aquatic interface as observed by Johnson et al (2006a) in two Amazonian headwater catchments. The rates of these processes determine the main C sources and the magnitudes of land-water C transfers in small watersheds (Neill et al, 2006). Zanchi et al (2015) pointed out that further studies are needed to understand the processes that lead to dissolved organic carbon (DOC) formation in soils, especially in the poorly drained valley soils of Amazonian rainforest catchments, and that it would be important to include the dissolved inorganic C measurement to calculate the total carbon export from such areas

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