We would like to clarify some misunderstandings expressed in the 28 July SciDev.Net report (see Amazon basin stores carbon dioxide 'for just five years'). The article described our work, published in Nature on the same day (Mayorga et al 2005), addressing our understanding of carbon sinks and the local impacts of deforestation on river ecosystems in the humid tropics.

Our research shows that carbon dioxide released from streams and rivers of the Amazon basin - not directly from trees or forests - is about five-years old.

Although this carbon enters rivers as leaves and twigs and is thus ‘forest carbon’, most of the carbon in the forest — in wood and in soils — stays there for more than 100 years (Chambers et al 2001). In fact, each year a quantity of carbon equivalent to that being released by rivers is estimated to be sequestered in wood and soils for long term storage (Grace and Malhi 2002; Richey et al 2002).

Our work does not address the carbon released from wood per se. What we do show is that the large quantity of carbon released by rivers into the atmosphere does not come from old carbon that has been in storage. We also strengthen previous evidence that mature and old-growth rainforests accumulate on an annual basis less carbon into long-term storage than has been estimated recently (Grace and Malhi 2002).

Regarding changes to food supplies for aquatic life upon deforestation, our work suggests that pastures supplant forests as the main source of carbon consumed by organisms in rivers. While we are unable to say for sure how much the total carbon supply is changing, the shift from forest to pasture sources might dramatically alter the type of aquatic communities that can thrive in the rivers of the Amazon basin (Melack and Forsberg 2001).

Emilio Mayorga 
emiliom@u.washington.edu
University of Washington

Anthony K. Aufdenkampe
http://www.stroudcenter.org/about/aufdenkampe.htm
Stroud Water Research Center

Jeffrey E. Richey
http://www.riversystems.washington.edu
University of Washington

References:

Chambers, J. Q., N. Higuchi, E. S. Tribuzy and S. E. Trumbore, Carbon sink for a century. Nature 410, 429 (2001)

Grace, J. and Y. Malhi. Carbon dioxide goes with the flow. Nature 416, 594 (2002)

Mayorga, E., A. K. Aufdenkampe, C. A. Masiello, A. V. Krusche, J. I.
Hedges, P. D. Quay and J. E. Richey. Young organic matter as a source of carbon dioxide outgassing from Amazonian rivers. Nature 436, 538 (2005)

Melack, J. M. and Forsberg, B. R. Biogeochemistry of Amazon floodplain lakes and associated wetlands, p. 235-274. In: McClain, M. E., Victoria, R. L. & Richey, J. E. (ed.), The biogeochemistry of the Amazon basin. Oxford University Press, New York (2001)

Richey, J. E., J. M. Melack, A. K. Aufdenkampe, M. V. Ballester and L. L. Hess. Outgassing from Amazonian rivers and wetlands as a large tropical source of atmospheric CO2. Nature 416, 617 (2001)