Temperature Sensitivity of Soil Organic Matter Decomposition in Subtropical Wetlands: Assessing the Role of Microbial Carbon Use Efficiency.

Temperature sensitivity of anaerobic carbon processing in wetlands remains poorly represented in most climate models, especially for warmer systems which account for a significant proportion of global CH₄ emission. Several studies of experimental warming have documented acclimation of soil respiration involving adjustments in microbial physiology or the efficiency with which microbes use carbon substrates (i.e. carbon use efficiency, CUE). These studies have observed an initial decline in CUE with increasing temperature followed by a partial recovery in CUE at a later stage, implies that the rate of warming may impact the microbial CUE and the rate of CO₂ and CH₄ production. Here, we have assessed the effects of warming rate on decomposition of subtropical peats using either a large single-step (10°C) or a slow ramping (0.1°C day^-1) temperature increase over the range of 15°C to 25°C. Results demonstrated the slow warming rate resulted in lower production of CO₂ and CH₄, whereas the rapid single-step warming resulted in both higher loss of gaseous carbon as well as a higher proportion of CH₄ over CO₂ (On average, CO₂-C:CH₄-C range over 0.71-1.4 and 0.73-1.6 in +10°C step vs. +10°C ramp, respectively). Microbial CUE declined from an initial value of 0.59-0.96 in the control to 0.30-0.36 and 0.44-0.50 unit in response to the single-step and slow-ramping temperature treatment, respectively. These results indicate that microbes are better able to adjust their physiology under a slow temperature increase. This has implications for the estimation of seasonal patterns in wetland systems emission.