Response of anaerobic carbon cycling to water table manipulation in an Alaskan rich fen

To test the effects of altered hydrology on organic soil decomposition, we investigated CO₂ and CH₄ production potential of rich-fen peat (mean surface pH = 6.3) collected from a field water table manipulation experiment including control, raised and lowered water table treatments. Mean anaerobic CO₂ production potential at 10 cm depth (14.1 ± 0.9 μmol C g⁻¹ d⁻¹) was as high as aerobic CO₂ production potential (10.6 ± 1.5 μmol C g⁻¹ d⁻¹), while CH4 production was low (mean of 7.8 ± 1.5 nmol C g⁻¹ d⁻¹). Denitrification enzyme activity indicated a very high denitrification potential (197 ± 23 μg N g⁻¹ d⁻¹), but net NO^-₃ reduction suggested this was a relatively minor pathway for anaerobic CO₂ production. Abundances of denitrifier genes (nirK and nosZ) did not change across water table treatments. SO^2-₄ reduction also did not appear to be an important pathway for anaerobic CO₂ production. The net accumulation of acetate and formate as decomposition end products in the raised water table treatment suggested that fermentation was a significant pathway for carbon mineralization, even in the presence of NO^-₃. Dissolved organic carbon (DOC) concentrations were the strongest predictors of potential anaerobic and aerobic CO₂ production. Across all water table treatments, the CO₂:CH₄ ratio increased with initial DOC leachate concentrations. While the field water table treatment did not have a significant effect on mean CO₂ or CH₄ production potential, the CO₂:CH₄ ratio was highest in shallow peat incubations from the drained treatment. These data suggest that with continued drying or with a more variable water table, anaerobic CO₂ production may be favored over CH₄ production in this rich fen. Future research examining the potential for dissolved organic substances to facilitate anaerobic respiration, or alternative redox processes that limit the effectiveness of organic acids as substrates in anaerobic metabolism, would help explain additional uncertainty concerning carbon mineralization in this system.