Understanding Methane Cycling Dynamics across Tropical African Wetland and Upland Ecosystems

Abstract:

The majority of the world’s wetlands (~64%) are located in tropical and subtropical humid regions and it is estimated that 47-89% (median 73%) of global wetland methane (CH₄) emissions originate in the tropics. While extensive research has been conducted in northern zones to understand biogeochemical controls on wetland CH₄ emissions, little research has been conducted across tropical regions. We investigated anaerobic and aerobic CH₄ cycling dynamics across a variety of ecosystem types in Gabon, Africa using a combination of in-situ field measurements and controlled laboratory incubations. We found African landscapes to possess highly variable CH₄ flux rates both within and across ecosystems, with sources producing up to 155 mmol CH₄/m²/day and sinks consuming as much as 53 mmol CH₄/m²/day. Gabonese wetlands have CH₄ production rates 1-6 orders of magnitude greater than that of higher latitude wetlands and, additionally, a much larger proportion of anaerobic carbon (C) mineralization is converted to CH₄ over CO₂. Mineral soil wetlands were dominated by acetoclastic methanogenesis (53-87% of total CH₄), while the hydrogenotrophic pathway was determined to be the principal pathway in organic soil wetlands (78-96% of total CH₄). Finally, we found rates of CH₄ oxidation under high CH₄ concentrations to be comparatively higher in wetlands, while CH₄ oxidation rates under low CH₄ concentrations tended to be higher in upland sites. The observed relationships in CH₄ production and consumption are not solely explained by temperature or pH, but are likely a result of differences in the dynamics and composition of the microbial communities responsible for the regulation of these processes. In this study, we have provided biogeochemical data that demonstrate the importance of tropical wetlands to the global CH₄ cycle and which are vital in paving the way for research investigating the underlying mechanisms responsible for the high CH₄ efficiency of this region.