CH4 production via CO2 reduction in a temperate bog: A source of 13C-depleted CH4

The flux and δ13C 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/m2d, with an annual average of 73-mg CH4-C/m2d, and correlated with soil temperature. Ebullition accounted for 89% of the methane flux. The average δ13C of the CH4 flux was -74 ± 5 per-thousand, with no relationship between the δ13C and flux of CH4. The δ13C of CH4 and CO2 in bubbles disturbed from the peat soil was -73 ± 4 and -2 ± 3 per-thousand, respectively. The δD of CH4 in bubbles was -308 ± 35 per-thousand, and the δD of soil water was -65 ± 3 per-thousand. Measurements of the rate of aceticlastic methanogenesis and CO2 reduction in peat soil, using 14C-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 (αr), calculated using the δ13C of soil water CO2 and the CH4 flux, was 0.932 ± 0.007. The δ13C of CH4 and CO2 coexisting in soil waters provides an isotopic tracer of CH4 production via CO2 reduction