Temperature sensitivity of microbial Fe(III) reduction kinetics in subalpine wetland soils

Microbially-mediated iron (Fe) cycling controls the fate of organic matter, contaminants, and nutrients in terrestrial ecosystems including wetland soils. However, the effects of temperature variations due to seasonal differences on Fe(III) reduction rates and kinetics in such ecosystems remains poorly understood. To evaluate the potential temperature impact on dissimilatory microbial Fe(III) reduction it is crucial to determine environmentally-relevant reaction rates and kinetic parameters. Here, we investigate the relationship between soil temperature and microbial Fe(III) reduction kinetics in mineral soils from two subalpine wetlands with distinct hydrologic and edaphic conditions. We conducted flow-through experiments (FTR) at three temperatures (6, 12, and 18 °C) using intact soil cores collected from 30 cm [(higher organic carbon (Corg) and total nitrogen (TN)] and 70 cm (lower Corg and TN) soil depths in order to determine the apparent Fe(III) affinity constant (Km), apparent maximum Fe(III) reduction rates (Vmax) and temperature sensitivity (Q10 and Ea) of Fe(III) reduction. We used Fe(III)-NTA, a model compound for aqueous labile and complexed Fe present in natural organic matter. Our results show that changes in apparent Vmax and Km are driven primarily by temperature. Significant differences in apparent Vmax at 18 °C relative to 6 and 12 °C (P < 0.05) suggest that dissimilatory microbial Fe(III) reduction in wetland soils accelerates during warmer summer days. However, temperature alone fails to explain the large variability of the apparent parameters Q10 (1.5–8.9) and Ea (26–148 kJ mol−1) for the two wetland types and depths (30 and 70 cm). Strong relationship between both parameters of temperature sensitivity (Q10 and Ea) and reactive soil Fe content at 30 cm and Corg/TN at 70 cm depth demonstrate the notable impact of soil properties on the temperature sensitivity for mirobial Fe(III) reduction in these wetland soils. Our results emphasize the importance of soil temperature on Fe(III) reduction kinetics and must be considered when predicting dissimilatory Fe(III) reduction under different seasonal temperatures or in wetlands located at different temperature regimes.