Impact of manipulated drought and heavy rainfall events on peat mineralization processes and source-sink functions of an acidic fen

Climate change models predict changes in precipitation patterns over the next several decades for northern temperate regions. Resulting fluctuations of the water level may drastically affect the source-sink functions of peatlands. Here, we manipulated the water table level in an acidic, minerotrophic fen using drying and rewetting experiments to simulate summer drought and heavy rainfalls to estimate changes in peat decomposition and source-sink functions. We found that carbon dioxide (CO2) formation rates and exoenzymatic activities increased in the most active surface layer during the initial water table drawdown; however, extreme drying did not further increase these activities. Activity stimulated in deeper oxygenated peat layers did not substantially contribute to CO2 emissions. Additionally, no phenol oxidase activity was determined. Rewetting of peat after drying did not lead to a CO2 flush like in mineral soils. Water table manipulations yielded a higher availability of nitrate, ferric iron, and sulfate and prolonged the onset of methane formation. Sulfate was exported to a nearby stream. We concluded that the increasing frequency of extreme weather conditions like summer droughts and heavy rainfalls might not affect carbon storage but instead strengthen the sink function for nitrate and ferrous iron and the source function for sulfate in peatlands.