In situ determination of sulfate turnover in peatlands: A down-scaled push-pull tracer technique

Bacterial sulfate reduction (BSR) is a key process in anaerobic respiration in wetlands and may have considerable impacts on methane emissions. A method to determine sulfate production and consumption in situ is lacking to date. We applied a single-well, injection-withdrawal tracer test for the in situ determination of potential sulfate turnover in a northern temperate peatland. Piezometers were installed in three peat depth levels (20, 30, and 50 cm) during summer 2004, and three series of injection-withdrawal cycles were carried out over a period of several days. Turnover rates of sulfate, calculated from first-order-reaction constant k (-0.097 to 0.053 h^-1) and pore-water sulfate concentrations (approx. 10 μmol L^-1), ranged from -1.3 to -9.0 nmol cm^-3 d^-1 for reduction and from +0.7 to +25.4 nmol cm^-2 d^-1 for production, which occurred after infiltration of water following a heavy rainstorm. Analysis of stable isotopes in peat-water sulfate revealed slightly increasing δ³⁴S values and decreasing sulfate concentrations indicating the presence of BSR. The calculated low sulfur-fractionation factors of <2‰ are in line with high sulfate-reduction rates during BSR. Routine application will require technical optimization, but the method seems a promising addition to common ex situ techniques, as the investigated soil is not structurally altered. The method can furthermore be applied at low expense even in remote locations.