Tropical Peatlands in the Amazon basin: An underestimated CH4 emission source and their unexplored microbial structure

Background/Question/Methods

Peatlands are a type of wetlands acting as important net CO₂-fixing (primary productivity) and CH₄-producing (decomposition) ecosystems. Tropical peatlands were thought as mainly important in South Asia’s humid forest and rarely considered in the Amazon basin. Recent expeditions to the Pastaza- Marañon basin in the western Amazon of Peru, reported over 295 peatlands. The newly found volumes (3-6 Gt of organic C) and rates (up to 195 g C m^-2) of C accumulation over a large area (21,929 Km²), have lay to rest the idea that prevailing conditions in the Amazon are unfavorable for peat formation. This raises questions about the significance of these environments for CH₄ formation and release from the Amazon basin. Satellite measurements suggest the tropics, especially the western Amazon basin, are a major source for CH₄ emission. In this study we address (a) whether Amazon peatlands have significant levels of greenhouse gases emissions (particularly CH₄), (b) whether ecological gradients have an influence in their rates and (c) also in the make up of microbial communities including methanogens. We evaluated six peatlands and one forest site in the Western Amazon of Peru. Gas fluxes, soil geochemistry, and microbial communities were measured with standard methods.

Results/Conclusions

All peatlands emitted CH₄, N₂O and CO₂ and forest was a CH₄-consumer. CH₄ fluxes were variable within sites but in average ranged from 6-15 mg C m^-2 day^-1. These rates are comparable to those in peatlands from Indonesia or interfluvial wetlands in the central Amazon basin. Also, all sites produced significant levels of N₂O and CO₂ gases with averages of 0.2-1.2 and 0.5-2 mg N₂O and g CO₂ m^-2 day^-1, respectively. pH was variable among sites ranging from 2.5 to 6, and a relationship of acidic pH/oligotrohic versus near neutral pH/minerotrophic was observed from sites’ geochemistry.  The richness and community distribution of Bacteria and Archaea was catalog with ~580 000 16S rRNA gene reads. Bacteria in Amazon peatlands are highly diverse with over 40 phyla, but dominated by Proteobacteria, Acidobacteria, Actinobacteria and Chloroflexi. Archaea were made up by a diverse community dominated by putative H₂-utilizing methanogens with most sequences represent uncultured novel methanogens. The overall microbial community clustered in groups related to ecosystem type (forest vs peatland) and pH (acidic to near neutral).

Amazon peatlands are sources of CH₄ emissions likely influenced by site gradients shaping the composition, and possible activity, of Bacteria and CH₄-producing Archaea.