Covariation of redox potential profiles and water table level in peatland sites representing different drainage regimes: implications for ecological modelling

Abstract. Reduction-oxidation (redox) reactions are ubiquitous in nature, responsible for the energy acquisition of all organisms. Redox reactions are electron transfer reactions and necessarily involve two participants: one being oxidised (electron donor) and one being reduced (electron acceptor). Availability of terminal electron acceptors (TEAs) is a major determinant of the extent to which the carbon in OM can be oxidised in an ecosystem. This is the most important under waterlogged conditions, such as in peatlands, where diffusion of O2, the most effective common TEA, into the soil is blocked by water. Under these conditions, available alternate TEAs are used by microbiota to continue OM oxidation. The decomposition processes in soil can be characterised by its redox state, ie. which TEA is responsible for oxidation of OM at a given time. This can in principle be measured as a voltage between the soil solution and a known reference electrode, known as the redox potential. Current soil ecosystem models do not depict the use of alternate TEAs well. This limits their applicability for predicting soil carbon loss under different drainage regimes, and thus their usefulness for assessing best management practices for soil carbon preservation and water course protection. Water table level (WTL) is the most common determinant of the mode of decomposition in ecosystem models, implying the assumption that the redox state of a peatland ecosystem responds predictably to changes in WTL. We conducted a two-year redox monitoring experiment in a boreal mesotrophic peatland under three drainage regimes: undrained, short-term drainage and long-term drainage. In addition, an ombrotrophic long-term drained plot was monitored. Snapshot assessments of the activities of three major metabolic enzymes, arginine deaminase, protease and urease, were also done in the mesotrophic plots, indicating differences in microbial activities between the drainage regimes. We found that WTL was a poor temporal predictor of redox potential, but that the position of major transition zones between oxic and anoxic states as well as enzymatic activities within the peat profile were somewhat determined be the dominant WTL depth. In the undrained plots especially redox potential values reflecting oxic or suboxic conditions were often found below the WTL, whereas on the drained plots anoxia was present above the WTL. Preceding redox potential was found to affect enzymatic activities of protease and urease, but not arginine, in all measured plots.