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Abstract:
The floodplain of the Amazon River is a large source for the greenhouse
gas methane, but the soil microbial communities and processes involved
are little known. We studied the structure and function of the
methanogenic microbial communities in soils across different inundation
regimes in the Cunia Reserve, encompassing nonflooded forest soil (dry
forest), occasionally flooded Igapo soils (dry Igapo), long time flooded
Igapo soils (wet Igapo) and sediments from Igarape streams (Igarape). We
also investigated a Transect (four sites) from the water shoreline into
the dry forest. The potential and resilience of the CH4 production
process were studied in the original soil samples upon anaerobic
incubation and again after artificial desiccation and rewetting.
Bacterial and archaeal 16S rRNA genes and methanogenic mcrA were always
present in the soils, except in dry forest soils where mcrA increased
only upon anaerobic incubation. NMDS analysis showed a clear effect of
desiccation and rewetting treatments on both bacterial and archaeal
communities. However, the effects of the different sites were less
pronounced, with the exception of Igarape. After anaerobic incubation,
methanogenic taxa became more abundant among the Archaea, while there
was only little change among the Bacteria. Contribution of
hydrogenotrophic methanogenesis was usually around 40%. After
desiccation and rewetting, we found that Firmicutes, Methanocellales and
Methanosarcinaceae became the dominant taxa, but rates and pathways of
CH4 production stayed similar. Such change was also observed in soils
from the Transects. The results indicate that microbial community
structures of Amazonian soils will in general be strongly affected by
flooding and drainage events, while differences between specific field
sites will be comparatively minor.
