Does the temperature sensitivity of aerobic decomposition in peatlands vary across vegetation types and with degradation state?

A large portion of the global soil carbon store is held in Northern peatlands, where low temperatures and anoxic conditions constrain rates of microbial decomposition. Projected increases in air temperature and evapotranspiration may relax these environmental constraints by exposing a greater thickness of peat to oxic conditions, as well as to higher (and possibly more variable) soil temperatures adding to atmospheric CO2 concentrations. However, the temperature sensitivity of aerobic decomposition is likely to differ across litter/peat derived from different vegetation types and in different states of degradation. Rates of decomposition of these types of soil organic matter (SOM) are commonly assumed to respond similarly to temperature increases. To date, the response of these substrate types remains unexplored in peatland systems. We tested the effects of vegetation type and degradation state on the temperature sensitivity of aerobic decomposition by tracking changes in litter/peat incubated in the laboratory for over a year. Litter/peat was collected from hollows in the fen lagg and bog plateau of a raised bog in central Sweden. At each location, two degradation states were sampled: fresh litter from near the moss surface and degraded peat from the upper part of the perennially anoxic layer. The samples were kept moist and incubated at 15 °C in a controlled environment chamber. At six-month intervals, temperature sensitivity of three replicates of each vegetation type-degradation state combination was assessed by measuring CO2 production at temperatures ramped up from 0 °C to 30 °C in daily 5 °C steps. The data were analysed using an Arrhenius-type equation: k = A exp(-Ea/(RT)), where k is respiration rate, A is frequency or pre-exponential factor, Ea is activation energy, R is the universal gas constant and T is temperature in Kelvin. Results so far suggest a complicated temperature response. Patterns were similar for temperature-sensitivity parameters A and Ea, with both increasing markedly over the period of incubation. Degraded peat from both the fen and bog sites responded similarly to temperature increases and was initially distinct from fresh litter types. The magnitude of the differences between litter/peat types, however, seemed to decrease over the incubation period. These results corroborate studies on mineral soils which have shown that parameters describing the temperature sensitivity of aerobic decomposition (A and Ea) vary across soil types and horizons. Our results illustrate the need to model temperature response on a per-substrate basis and point to an initial sensitivty of all peat types that may diminish over time.