CH4 PRODUCTION VIA CO2 REDUCTION IN A TEMPERATE BOG - A SOURCE OF C-13-DEPLETED CH4

The flux and deltaC-13 of CH4 released from an acidic peat bog (pH = 3.5), located in the foothills of the Cascade Range in Washington state, USA, were measured over two annual cycles. The CH4 flux ranged from 1-380 mg CH4-C m-2 d-1, with an annual average of 73-mg CH4-C m-2 d-1, and correlated with soil temperature. Ebullition accounted for 89% of the methane flux. The average deltaC-13 of the CH4 flux was -74 +/- 5 parts per thousand, with no relationship between the deltaC-13 and flux of CH4. The deltaC-13 of CH4 and CO2 in bubbles disturbed from the peat soil was -73 +/- 4 and -2 +/- 3 parts per thousand, respectively. The deltaD of CH4 in bubbles was -308 +/- 35 parts per thousand, and the deltaD of soil water was -65 +/- 3 parts per thousand. Measurements of the rate of aceticlastic methanogenesis and CO2 reduction in peat soil, using C-14-labeled acetate and sodium bicarbonate, show that acetate was not an important CH4 precursor and that CO2 reduction could account for all of the CH4 production. The in situ kinetic isotope effect for CO2 reduction (alpha(r)), calculated using the deltaC-13 of soil water CO2 and the CH4 flux, was 0.932 +/- 0.007. The deltaC-13 of CH4 and CO2 coexisting in soil waters provides an isotopic tracer of CH4 Production via CO2 reduction.