CO₂ and CH₄ isotope compositions and production pathways in a tropical peatland

While it is widely recognized that peatlands are important in the global carbon cycle, there is limited information on belowground gas production in tropical peatlands. We measured pore water methane (CH₄) and carbon dioxide (CO₂) concentrations and δ¹³C isotopic composition and CH₄ and CO₂ production rates in peat incubations from the Changuinola wetland in Panama. Our most striking finding was that CH₄ was depleted in ¹³C (-94‰ in pore water and produced at -107‰ in incubated peat) relative to CH₄ found in most temperate and northern wetlands, potentially impacting the accuracy of approaches that use carbon isotopes to constrain global mass balance estimates. Fractionation factors between CH₄ and CO₂ showed that hydrogenotrophic methanogenesis was the dominant CH₄ production pathway, with up to 100% of the CH₄ produced via this route. Far more CO₂ than CH₄ (7 to 100X) was measured in pore water, due in part to loss of CH₄ through ebullition or oxidation and to the production of CO₂ from pathways other than methanogenesis. We analyzed data on 58 wetlands from the literature to determine the dominant factors influencing the relative proportions of CH₄ produced by hydrogenotrophic and acetoclastic methanogenesis and found that a combination of environmental parameters including pH, vegetation type, nutrient status, and latitude are correlated to the dominant methanogenic pathway. Methane production pathways in tropical peatlands do not correlate with these variables in the same way as their more northerly counterparts and thus may be differently affected by climate change.