Electron balance during steady‐state production of CH4 and CO2 in anoxic rice soil

Abstract

SummaryIn methanogenic rice‐field soil, organic matter (SOM) is anaerobically fermented to acetate, CO2 and H2 which then serve as substrates for methanogenesis. The whole process is a disproportionation reaction in which part of the SOM is oxidized to CO2 and part is reduced to CH4. We were interested in the electron balance during this process. The rates VCO2 and VCH4 at which CO2 and CH4 are produced by anaerobic degradation of SOM, and the fraction RH2 of the CH4 produced by reduction of CO2 with H2 (as opposed to acetotrophic methanogenesis), depend on (i) the presence or absence of inorganic oxidants and (ii) the electron balance ΔZ. Under pseudo steady‐state conditions, where inorganic oxidants are exhausted and the rate of SOM degradation is small compared with the size of the pool, VCO2, VCH4 and RH2 are constrained by ΔZ. Conversely, ΔZ may be determined from VCO2, VCH4 and RH2, all of which may be independently measured. We measured VCO2 (0.149–0.308 μmol g−1 day−1), VCH4 (0.169–0.466 μmol g−1 day−1) and RH2 (0.19–0.35) in eight soils and obtained values of ΔZ ranging from −0.918 to 0.035 μmol g−1 day−1. The majority (six) were negative, indicating a decrease in the oxidation state of the SOM carbon on degradation. This could be caused by humic acids acting as an electron acceptor and allowing more of the SOM to be oxidized to CO2 rather than reduced to CH4. Direct measurement of SOM carbon oxidation state Z produced values around zero (−0.1 ± 0.1), but is too insensitive to reveal changes of the magnitude of ΔZ.