Considered, until now, a source of greenhouse gas emissions, capturing the CO2 fixed by the tropical forest through the soils of the watershed to release it into the atmosphere, the Amazon River actually has a balanced carbon footprint. In fact, a new study shows that the CO2 outgassed by the river is only drawn from the river system itself, by the semi-aquatic vegetation on the flood plains. Therefore, the Amazon recycles the CO2 from its own river system, and not that fixed by the tropical forest, releasing as much carbon dioxide into the atmosphere as it absorbs. This study, coordinated by researchers from the GET (IRD, OMP, CNRS, CNES, UPS) and EPOC laboratories (OASU, CNRS, Université Bordeaux 1) and published in the journal Nature, changes the order for global carbon footprints.
The Amazon recycles its own CO2
The study contributes a new conception of the carbon cycle in the Amazon, and more generally on the continents. Until now, researchers thought that rivers were supplied with carbon by trees and other land plants through the soils of the watershed. This carbon was then transformed into CO2 and released by outgassing into the atmosphere. Watercourses, and in particular the giant Amazon, were thus considered as net sources of emissions, releasing more CO2 than they absorbed. Now, researchers have just shown that the CO2 outgassed by the waters of the Amazon is in reality only drawn from the river system itself. This CO2 comes from the decomposition of the organic matter produced by semi-aquatic vegetation in the Amazon wetlands. Conversely to what we thought, the river thus acts as a "CO2 pump."
The link between aquatic vegetation and CO2emission
Ten French and Brazilian teams within the framework of the ANR-CARBAMA project and the HYBAM environmental research observatory conducted many field-studies in the Amazon region and analysed satellite images. The measurements of CO2 concentrations dissolved in the water compared to the satellite map of vegetation showed a very strong correlation between the intensity of CO2 outgassing and the area of flooded vegetation and floating aquatic plants. This proportional relationship can be verified at two levels: over time, as the water level varies during the year; and in space, as the proportion of vegetation diminishes from upstream of the study area, where flooded forests dominate, to downstream, where the majority of the lakes are found.
All emissions covered by the wetlands
The Amazon emits some 200,000 tons of carbon per year through outgassing. According to the researcher's estimates, the majority of these emissions come from the respiration of the roots and the fall and decay of the semi-aquatic vegetation in the flood plains. In fact, the researchers showed a very high export ratio toward the aquatic environment of the gross primary production of the Amazon wetlands: half of this carbon, in the form of dissolved CO2 and biodegradable organic matter, is transferred directly to the river. This quantity of CO2 is equivalent to the 200,000 tons of carbon outgassed annually. Therefore, the carbon footprint of the river system in the central Amazon region is close to equilibrium: its waters release the same quantity of carbon into the atmosphere as is fixed by its vegetation.
Nevertheless, this study highlights the very heavy contribution of inland waters to CO2 emissions. It sheds light on the need to consider the specific properties of wetlands in global carbon footprints.
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