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

In methanogenic rice-field soil, organic matter (SOM) is anaerobically ferm ented to acetate, CO2 and H-2 which then serve as substrates for methanogen esis. The whole process is a disproportionation reaction in which part of t he SOM is oxidized to CO2 and part is reduced to CH4. We were interested in the electron balance during this process. The rates V-CO2 and V-CH4 at whi ch CO2 and CH4 are produced by anaerobic degradation of SOM, and the fracti on R-H2 of the CH4 produced by reduction of CO2 with H-2 (as opposed to ace totrophic methanogenesis), depend on (i) the presence or absence of inorgan ic oxidants and (ii) the electron balance Delta Z. Under pseudo steady-stat e conditions, where inorganic oxidants are exhausted and the rate of SOM de gradation is small compared with the size of the pool, V-CO2, V-CH4 and RH2 are constrained by Delta Z. Conversely, Delta Z may be determined from V-C O2, V-CH4 and R-H2, all of which may be independently measured. We measured V-CO2 (0.149-0.308 mu mol g(-1) day(-1)), V-CH4 (0.169-0.466 mu mol g(-1) day(-1)) and RH2 (0.19-0.35) in eight soils and obtained values of Delta Z ranging from -0.918 to 0.035 mu mol 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 acc eptor and allowing more of the SOM to be oxidized to CO2 rather than reduce d to CH4. Direct measurement of SOM carbon oxidation state Z produced value s around zero (-0.1 +/- 0.1), but is too insensitive to reveal changes of t he magnitude of Delta Z.