Microbial communities and the putative interactions of methanogens with nitrogen oxides in diverse peatlands of the Amazon basin

High uncertainties and spatial variation in methane (CH4) fluxes from the Amazon basin pose a problem for predicting greenhouse gas sources and sinks in the wake of environmental change of the region. Tropical peatlands have been identified as hotspots of CH4 as they provide feasible conditions (anoxia, high abundance of organic matter) for microbial methanogenesis. We investigated the effects of soil NOx (nitrate, nitrite, nitric oxide) and nitrous oxide (N2O) on methanogenic abundance and activity in three geochemically diverse peatlands of the Pastaza-Marañón foreland basin. To characterize the soil geochemistry, we probed 1-m-soil profiles and measured concentrations of (in)organic nitrogen and carbon species, redox, and 13C enrichment in dissolved inorganic carbon and CH4 pools. The overall soil microbial community was characterized based on 16S rRNA and the functional gene mcrA was used to assess methanogen diversity as well as abundance deploying quantitative PCR (qPCR). We also conducted nitrite incubations with soil slurries and obtained N2O-spiked enrichments from two of the peat soils to evaluate cross-effects of nitrogen oxides on CH4 formation. Geochemical conditions were distinct, based on pH, DOC, NO3– pore water concentrations and C/N ratios of shallow soils. We observed a strong stratification in abundances of microbial groups putatively involved in NOx cycling. Thaumarchaeota and Nitrospira claimed similar ecological niches as producers of NOx species, with the Archaea being more abundant in more acidic peat. Our data revealed high potential of novel and diverse methanogens affiliated to the Methanomicrobiales order. Methanogenesis in contrasting soils showed differential sensitivity to nitrite and N2O which may have vast implications on net CH4 outputs. Overall, we present evidence that potential interactions of nitrogen species with methanogens contribute to variable CH4 fluxes.