B44A-02:
The Methane to Carbon Dioxide Ratio Produced during Peatland Decomposition and a Simple Approach for Distinguishing This Ratio
Thursday, 18 December 2014: 4:15 PM
Jeffrey Chanton1, Suzanne B Hodgkins2, William T Cooper3, Paul H Glaser4, J Elizabeth Corbett5, Patrick M Crill6, Scott R Saleska7, Virginia Isabel Rich7, Beth Holmes1, Mark E Hines8, Malak Tfaily9 and Joel E Kostka10, (1)Florida State Univ, Tallahassee, FL, United States, (2)Florida State University, Tallahassee, FL, United States, (3)Florida State University, Department of Chemistry & Biochemistry, Tallahassee, FL, United States, (4)University of Minnesota Twin Cities, geology, Minneapolis, MN, United States, (5)NASA Goddard Institute for Space Studies, New York, NY, United States, (6)Stockholm University, Stockholm, Sweden, (7)University of Arizona, Tucson, AZ, United States, (8)Univ Massachusetts Lowell, Lowell, MA, United States, (9)Pacific Northwest National Lab, Richland, WA, United States, (10)Georgia Institute of Technology Main Campus, Atlanta, GA, United States
Abstract:
Peatland organic matter is cellulose-like with an oxidation state of approximately zero. When this material decomposes by fermentation, stoichiometry dictates that CH4 and CO2 should be produced in a ratio approaching one. While this is generally the case in temperate zones, this production ratio is often departed from in boreal peatlands, where the ratio of belowground CH4/CO2 production varies between 0.1 and 1, indicating CO2 production by a mechanism in addition to fermentation. The in situ CO2/CH4 production ratio may be ascertained by analysis of the 13C isotopic composition of these products, because CO2 production unaccompanied by methane production produces CO2 with an isotopic composition similar to the parent organic matter while methanogenesis produces 13C depleted methane and 13C enriched CO2. The 13C enrichment in the subsurface CO2 pool is directly related to the amount of if formed from methane production and the isotopic composition of the methane itself. Excess CO2 production is associated with more acidic conditions, Sphagnum vegetation, high and low latitudes, methane production dominated by hydrogenotrophic methane production, 13C depleted methane, and generally, more nutrient depleted conditions. Three theories have been offered to explain these observations— 1) inhibition of acetate utilization, acetate build-up and diffusion to the surface and eventual aerobic oxidation, 2) the use of humic acids as electron acceptors, and the 3) utilization of organic oxygen to produce CO2. In support of #3, we find that 13C-NMR, Fourier transform infrared (FT IR) spectroscopy, and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) clearly show the evolution of polysaccharides and cellulose towards more decomposed humified alkyl compounds stripped of organic oxygen utilized to form CO2. Such decomposition results in more negative carbon oxidation states varying from -1 to -2. Coincident with this reduction in oxidation state, is the greater production of methane. Changing climatic conditions may alter the balance of the factors which affect the CO2/CH4 ratio by changing the water balance of the peatland, nutrient status, or temperature.
