Methane suppression by iron and humic acids in soils of the Arctic Coastal Plain

Methane-climate interactions are reasonably well understood; the biogeochemical controls on net methane fluxes are less so. Within anoxic soils, alternative electron acceptors such as iron and humic substances influence microbial metabolic function, and thus affect the amount of carbon lost as methane (CH4). We present three years of data from wet sedge tundra landscapes near Barrow, Alaska that show an inverse relationship between dissolved iron and CH4 concentrations. We found increasing organic layer thickness related to increases in active layer organic matter content, and decreases in both bulk density and extractable iron. Organic layer depth was also a good proxy for carbon dioxide (CO2) and CH4 dynamics, with increasing organic layer depths relating to lower dissolved iron, higher amounts of dissolved CH4, and lower CO2:CH4 ratios in the upper active layer. Net CH4 fluxes were also significantly suppressed following the experimental addition of iron and humic acids. Iron and humic acid treatment effects were indistinguishable for CH4 net flux; in contrast, post-treatment CH4 fluxes were an average of 0.74-fold the control treatment flux rates. These results suggest that in-situ CH4 production is tied to alternative electron acceptor availability, and that organic layer thickness is a good predictor of biogeochemical controls on CH4 fluxes in wet-sedge Arctic Alaskan tundra.