Microbial Controls of Nitrous Oxide Fluxes in Three Peatlands of the Peruvian Amazon

Tropical peatlands are potentially major emitters of nitrous oxide (N2O), a potent greenhouse gas and the most important ozone-depleting emission. Despite their importance in climate change, the microbial nitrogen cycle and resulting N2O fluxes in tropical peatlands remain largely unstudied. The aim of this study was to assess the role of microbiological processes in N2O fluxes in the Iquitos region of Peru depending on land use and physicochemical parameters.

Soil samples were collected from four sampling sites near Iquitos, Peru. The first set of samples was collected in September 2019, the second one in March 2020. All sites were current or former palm swamps dominated by Mauritia flexuosa palms. Quantitative real-time PCR was used to measure the abundance of bacteria and archaea specific 16S rRNA, nitrification (AOA, AOB and COMAMMOX amoA), denitrification (nirK, nirS, nosZI and nosII), nitrogen fixation (nifH) and DNRA (nrfA) marker genes in collected soil samples. Spearman correlation analysis was used to determine significant relationships between gene abundances, physicochemical parameters and N2O emissions.

The results show that the abundance of microorganisms involved in the soil nitrogen cycle was most affected by soil moisture, and the wet sampling sites displayed abundances far greater than sampling sites with current or previous anthropogenic disturbances. The highest N2O was produced in the wet swamp peat after a heavy shower at the end of dry season. While most studies show 50-60% soil moisture content as the optimal range for N2O emissions, the results of this study might indicate a higher optimal range for N2O emissions. Relationships between N2O and marker gene abundance displayed clear differences between the sampling times. At the end of dry season, the swamp peat near the Symphonia globulifera trees emitted the highest amounts of N2O. These were positively correlated to archaeal amoA and nrfA gene abundances. During the wet season, the swamp forest emitted modestly while the dry manioc field produced fair amounts of N2O in no meaningful correlation with the marker gene abundances. Overall, the results suggest that nitrifiers produce much of N2O emissions from tropical peatlands, triggered by changes in soil moisture.