In Situ Determination of Sulfate Reduction in Peatlands - A Downscaled Push-Pull Tracer Technique

Microbial dissimilatory sulfate reduction is a key process for carbon cycling in nutrient-poor peatlands. For an estimation of a future behaviour of these carbon-rich ecosystems under altered environmental conditions, reliable methods for a quantification of biogeochemical processes still have to be developed. In general, sulfate reduction is being quantified via laboratory-scale incubation experiments with field samples. A main disadvantage and source of uncertainty of this approach is the use of disturbed samples and the determination of not actual but potential turnover rates, which can not easily be extrapolated to the field scale. In order to find an alternative for measuring sulfate reduction in peat soils we developed an experimental design derived from hydrogeological practice, where single-well push-pull tracer tests are widely used for in situ determination of microbial activities in aquifers. A mixture of a conservative tracer and a reactant is injected into a well and regained after a certain time of in situ incubation. Assuming that sulfate reduction follows mainly first-order reaction kinetics, the analysis of the obtained breakthrough curve data is simple. The slope of a log-linear regression of the ratio of relative reactant vs. tracer concentrations plotted against time since injection delivers the reaction rate coefficient k. Nine piezometers installed at three depth levels (20-30, 30-40 and 50-60cm below ground surface) in Mer Bleue Bog, Ontario, Canada were used to conduct three series of push-pull tracer tests. In both uppermost depth-levels, increased hydraulic conductivity causes a dominance of lateral flow and leads to lower recovery rates. Nevertheless, with restrictions due to a higher sensitivity to operational errors, data from these replicates could be used for method evaluation. Mean k values ranged from -7.2*10-3 to -11.7*10-3 h-1. Positive k values representing increasing sulfate concentrations were determined in several series of the upper levels. We concluded that sulfate was not net consumed in the shallow peat. These first results indicate that small scale push-pull tests are a suitable technique for the determination of biogeochemical in situ reaction rates.