Carbon biogeochemistry in northern peatlands : regulation by environmental and biogeochemical factors

Nitrogen and sulfur deposition and water table level fluctuations have the potential to influence the C biogeochemistry in peatlands. Processes in peatland mesocosms were examined under steady state and dynamic conditions at different rates of N and S deposition, and water table levels. Net turnover rates were calculated from diffusive-advective mass-balances of pore water constituents. The limitations of the approach were tested with tracer experiments, which showed that diffusive-advective transport adequately described the flow of dissolved substances in peat columns. Incubation experiments quantified potential CO2, CH4, DOC, H2S and Fe 2+ production rates.The vegetation assimilated most of the deposited nitrogen and sulfate when water table levels were high. Lowered water table levels resulted in seepage of sulfate to the water table, reduced the rates of photosynthesis, and increased the soil respiration rates. The potential for sulfate reduction was fairly large, despite small in situ sulfate concentrations, and the CO2 production could not be fully accounted for by known processes. Potential rates of sulfate reduction were large both in samples taken from the field site and from the controlled experiments. SO42- addition resulted partly in stimulation, partly in reduction of potential CH4 production rates suggesting that the relationship between sulfate reduction and methanogenesis is not exclusively competitive.Changes of the water table level had in situ effects on CO2 and CH4 production rates not explainable by a distinction in aerobic/anaerobic conditions. Anaerobic in situ rates at greater depths were much lower when the water table was at the surface of the mesocosms than when it was at greater depths. This might have been due to in situ accumulation of CO2 and CH 4 in the deeper peat, which lowers the energy gain of anaerobic C mineralization. Flooding and draining of peat soil resulted in a delayed onset of CH 4 production, in increased anaerobic CO2 production and decreased CH4 production rates, and in the decoupling of gas exchange from production rates. These results document that fluctuations of environmental variables on short time scales have an impact on rates of C turnover in peat soils, and also limit the predictability of fluxes by statistical models