Isotopic Insights into Methane Production and Emission in Diverse Amazonian Peatlands

Abstract

<p>Tropical peatlands have the potential to be significant sources of methane (CH<sub>4</sub>) 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 CH<sub>4</sub> emissions. Our work aims to better understand CH<sub>4</sub> 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 CH<sub>4 </sub>production. In rain-fed ombrotrophic sites (pH 3-4), we observe low emissions and highly depleted δ<sup>13</sup>CH<sub>4</sub> 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.</p>

From the Top: Surface-derived Carbon Fuels Greenhouse Gas Production at Depth in a Neotropical Peatland

“…Across all sites and depths, DOC was enriched in radiocarbon relative to the bulk peat indicating that it is largely derived from recent photosynthate as opposed to the solid peat, which became progressively older solid peat at depth. Similar behaviour has been reported in other tropical peatlands that preferentially use modern DOC as the source for microbial respiration ( Hoyt et al, 2020), while one site in Borneo reported respiration products from mixed sources (Hoyt, 2014). These contrasting results suggest either the Borneo peatland site is an exception to this behaviour or there is need to explore more tropical peatland sites to characterize source selection behaviour in the tropics.…”

δ 13 C methane source signatures from tropical wetland and rice field emissions

“…More 13 C-enriched CH 4 is expected where acetoclastic methanogenesis dominates, with more depleted CH 4 found in nutrient poor environments where CO 2 reduction dominates. For example, methane as depleted as −94‰ was observed in the porewater of an ombrotrophic peatland in Panama [ 49 ], and similar values have been observed in porewater in Borneo and Peru [ 50 ]. Thus, seasonal changes in δ 13 C could result from a changing relative contribution of different wetland types within a wetland complex throughout the year.…”