Wood decomposition in Amazonian hydropower reservoirs: An additional source of greenhouse gases Gwenaël Abril a, b, c, * , Marcelo Parize d , Marcela A.P. Pérez e , Naziano Filizola a a Universidade Federal do Amazonas, Laboratório de Potamologia Amazônica, Departamento de Geografia, Manaus, Brazil b Université de Bordeaux, Laboratoire EPOC, Environnements et Paléoenvironnements Océaniques et Continentaux, CNRS UMR 5805, France c Institut de Recherche pour le Développement, Laboratoire GET, Geosciences et Environnement de Toulouse, CNRS, IRD, UPS, France d Instituto Nacional de Pesquisas da Amazonia, Manaus, Brazil e Universidade Federal do Amazonas, Laboratório de Geoquímica, Departamento de Geociencias, Manaus, Brazil article info Article history: Received 15 December 2011 Accepted 20 November 2012 Keywords: Hydroelectric dams Greenhouse gases Amazon basin abstract Amazonian hydroelectric reservoirs produce abundant carbon dioxide and methane from large quanti- ties of flooded biomass that decompose anaerobically underwater. Emissions are extreme the first years after impounding and progressively decrease with time. To date, only water-to-air fluxes have been considered in these estimates. Here, we investigate in two Amazonian reservoirs (Balbina and Petit Saut) the fate of above water standing dead trees, by combining a qualitative analysis of wood state and density through time and a quantitative analysis of the biomass initially flooded. Dead wood was much more decomposed in the Balbina reservoir 23 years after flooding than in the Petit Saut reservoir 10 years after flooding. Termites apparently played a major role in wood decomposition, occurring mainly above water, and resulting in a complete conversion of this carbon biomass into CO 2 and CH 4 at a timescale much shorter than reservoir operation. The analysis of pre-impounding wood biomass reveals that above-water decomposition in Amazonian reservoirs is a large, previously unrecognized source of carbon emissions to the atmosphere, representing 26e45% of the total reservoir flux integrated over 100 years. Accounting for both below- and above-water fluxes, we could estimate that each km 2 of Amazonian forest converted to reservoir would emit over 140 Gg CO 2 -eq in 100 years. Hydropower plants in the Amazon should thus generate 0.25e0.4 MW h per km 2 flooded area to produce lower greenhouse gas emissions than gas power plants. They also have the disadvantage to emit most of their greenhouse gases the earliest years of operation. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Hydroelectric reservoirs, particularly those that flood tropical forests, in which the stores of organic carbon are among the highest in the world, emit greenhouse gases (GHGs) to the atmosphere (Galy-Lacaux et al., 1997; St Louis et al., 2002; Delmas et al., 2001; Abril et al., 2005; Guérin et al., 2006; Kemenes et al., 2007 , 2011; Barros et al., 2011). Furthermore, anoxic conditions and high concentrations of CO 2 and CH 4 resulting from intense microbial activity have been consistently observed in Amazonian reservoirs (Galy-Lacaux et al., 1997; Abril et al., 2005; Kemenes et al., 2007 , 2011). The flooded soil and litter rapidly decompose underwater to GHGs, which reach the atmosphere through four distinct pathways (Abril et al., 2005): (1) ebullition (mainly of CH 4 ), from shallow areas of the reservoir; (2) diffusion of CO 2 and CH 4 from the reservoir surface; (3) degassing at the turbines and immediately below the dam; and (4) degassing from downstream rivers. As the carbon pool from the flooded soil and biomass is progressively consumed, the emissions decrease with time (Abril et al., 2005; Guérin et al., 2008). Most information available from the literature are gross GHG emissions and very few studies attempt to quantify net emissions, that is the difference between pre-impounding and post-impounding fluxes (Delmas et al., 2001; Guérin et al., 2008). Even though projections at a 100-year horizon are approximates, they suggest that lowland Amazonian reservoirs with a low power density (the ratio between the energy produced and the flooded area) could emit amounts of GHGs similar to or even higher than those from a gas power plant (Delmas et al., 2001). However, all these estimates have only considered the gross fluxes from waters upstream and downstream from dams, and the fate of the standing * Corresponding author. Université de Bordeaux, Laboratoire EPOC, Environne- ments et Paléoenvironnements Océaniques et Continentaux, CNRS UMR 5805, France. E-mail address: g.abril@epoc.u-bordeaux1.fr (G. Abril). Contents lists available at SciVerse ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames 0895-9811/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsames.2012.11.007 Journal of South American Earth Sciences 44 (2013) 104e107