Anaerobic respiration pathways and response to increased substrate availability of Arctic wetland soils

Changes in the net radiative forcing of greenhouse gas emissions from Arctic soils in response to warming and permafrost thaw will depend on the balance of CO2 and CH4 emitted during anaerobic organic matter decomposition. While soil water content limits aerobic respiration, the factors controlling methanogenesis and anaerobic respiration are poorly defined in suboxic Arctic soils. We conducted incubation experiments on two Alaskan soils with contrasting pH and geochemistry to determine the pathways of anaerobic microbial respiration and predict changes with increasing substrate availability upon warming. In the circumneutral Teller soil incubations, the ratio of CO2 to CH4 dropped rapidly from 10 to <2 after 60 days, indicating alternative terminal electron acceptors (TEAs) were rapidly consumed. In contrast, the ratio in the acidic Council soil remained above 4, indicating pathways other than methanogenesis contributed to CO2 production. Relatively small changes in dissolved Fe(II), NO3-, and SO42- concentrations suggest alcohol fermentation or respiration using organic TEAs. In the Teller soil, acetate stimulated production of CO2 and CH4 in a nearly 1:1 ratio, indicating methanogenesis rather than respiration. However, acetate addition to the Council soil reduced both CO2 and CH4 production, but fermentative bacteria increased in relative abundance. Selective inhibition of methanogens did not increase respiration. The response of CO2 and CH4 production to increased substrate availability in wet Arctic soils will vary widely depending on soil geochemistry: acidic soils with alternative TEAs may experience smaller changes than circumneutral soils with low TEA availability, which could become much larger sources of CH4.