When peat burns: Wildfire and the fate of terminal electron acceptors.

The existence of peatlands relies on the balance of primary productivity and oxidation of organic matter. Oxidation requires a terminal electron acceptor (TEA). The most energetically favourable TEA is O₂ followed, in order of reducing energy return, by NO₃, Mn, Fe, and SO₄. Organic matter itself can become a TEA with the production of methane (CH₄). Organic matter will degrade faster the better access to the more energetically favourable TEAs. Therefore, the fate of the organic matter turnover in peatlands is related to the supply of TEAs. Typically, water tables are raised to limit the access of TEAs into the peat porewater, however, it is not only high water tables that are required but also stagnant water tables otherwise fresh TEAs are brought into the porewater.

This study looked at the hydrological and biogeochemical controls on organic matter turnover along a peat-covered hillslope using bunds. Bunds are used in peatlands to manipulate the water table to create environments for peat-forming species such as sphagnum mosses. To our knowledge, this is the only study with continuous pre-fire baseline data prior to a wildfire in a peatland system. Nine bunded plots and nine control plots were monitored monthly over a two year period, with sampling conducted upslope, within, and downslope of each bund. Measurements included water table depth, soil water chemistry (pH, conductivity, DOC, absorbance, cations and anions), and ecosystem CO₂ fluxes.

Pre-fire results showed significant differences absorbance down the hillslope, but no significant differences attributable to bund presence. Concentrations of DOC, iron and sulphate, conductivity, and water table depth did not differ significantly between bunded and control plots. Ecosystem respiration showed no significant variation related to bunds or hillslope position.

Following a wildfire, water table depths did not differ significantly from pre-fire conditions across the hillslope or between bunded and control plots. Similarly, concentrations of TEAs, including iron and sulphate, showed no statistically significant post-fire change. DOC concentrations, absorbance, conductivity, and CO₂ fluxes also remained within the range of pre-fire data.

Neither bund installation nor wildfire caused detectable changes in water table behaviour or TEA availability at this site over the two year study.