Geochemical drivers of organic matter decomposition in arctic tundra soils

Climate change is warming tundra ecosystems in the Arctic, resulting in the decomposition of previously-frozen soil organic matter (SOM) and release of carbon (C) to the atmosphere; however, the processes that control SOM decomposition and C emissions remain highly uncertain. In this study, we evaluate geochemical factors that influence microbial production of carbon dioxide (CO₂) and methane (CH₄) in the seasonally-thawed active layer of interstitial polygonal tundra near Barrow, Alaska. We report spatial and seasonal patterns of dissolved gases in relation to the geochemical properties of Fe and organic C in soil and soil solution, as determined using spectroscopic and chromatographic techniques. The chemical composition of soil water collected during the annual thaw season varied significantly with depth. Soil water in the middle of the active layer contained abundant Fe(III), and aromatic-C and low-molecular-weight organic acids derived from SOM decomposition. At these depths, CH₄ was positively correlated with the ratio of Fe(III) to total Fe in waterlogged transitional and low-centered polygons but negatively correlated in the drier flat- and high-centered polygons. These observations contradict the expectation that CH₄ would be uniformly low where Fe(III) was high due to inhibition of methanogenesis by Fe(III)-reduction reactions. Our results suggest that vertically-stratified Fe redox reactions influence respiration/fermentation of SOM and production of substrates (e.g., low-molecular-weight organic acids) for methanogenesis, but that these effects vary with soil moisture. We infer that geochemical differences induced by water saturation dictate microbial products of SOM decomposition, and Fe geochemistry is an important factor regulating methanogenesis in anoxic tundra soils.