Isotopic Insights into Methane Production and Emission in Diverse Amazonian Peatlands

Tropical peatlands have the potential to be significant sources of methane (CH₄) to the atmosphere but their contribution to the global methane budget remains uncertain. Although much prior work has focused in Southeast Asia, other tropical regions, such as the Congo and the Amazon, have a much wider diversity of peatlands with more variable CH₄ emissions. Our work aims to better understand CH₄ production and emissions in these diverse peatlands, and how they are controlled by hydrology, geochemistry and vegetation. Using stable isotope and radiocarbon measurements, we assess the production pathway for methanogenesis and its carbon source at sites across the Pastaza-Marañon basin in Peru. As the largest peatland complex in the Amazon, this region is home to many peatland types, from palm swamps to open peatlands to pole forests. We find clear links between site geochemistry, hydrology, and CH₄ production. In rain-fed ombrotrophic sites (pH 3-4), we observe low emissions and highly depleted δ¹³CH₄ values (as low as -100‰). The lack of external nutrients and acidic conditions likely limit methanogenesis, and hydrogenotrophic methanogenesis dominates. In more minerotrophic sites (pH 5-6), more enriched methane (-75 to -60‰) suggests a contribution from acetoclastic methanogenesis. Emissions rates are also higher, likely fueled by external nutrient inputs from seasonal flooding. Across sites, modern, vegetation-derived inputs are the dominant carbon source for methanogenesis, with a limited contribution from old peat carbon in some ombrotrophic sites. The strong relationships we observe between peatland hydrology, vegetation, geochemistry and methane emissions will enable future work to upscale methane emissions across the region.