Poster, International Symposium Soil processes under extreme meteorological conditions, Bayreuth: 2007-02-25 - 2007-02-28
Abstract:
Covering about 450 million ha of the earth’s surface and storing substantial amounts of carbon, peatlands play an important role in the global carbon cycle. In the context of climate change and greenhouse gases, peatlands have gained increasing attention. According to recent climate scenarios, higher temperatures and an increasing frequency of extreme meteorological events – such as pronounced drying/rewetting cycles – may be expected. To date, however, the effects of the climate change and internal feedbacks of carbon turnover in peatlands are not well known. To evaluate the effect of pronounced drying/rewetting, three intact cores (60 cm diameter, 60 cm depth) from a northern temperate fen were incubated in a climate chamber (15°C; 12h/12h day/night cycle) for 9 months. The plants of one core had been removed, while they were kept on the other two cores (grasses and sedges, few mosses). First, the water table of all cores was adjusted and kept at 10 cm below surface (artificial precipitation) for 70 days. Subsequently, two cores were dried out (with and without vegetation) by stopping precipitation, while the third core (with vegetation) was kept at high water table as a control. Within 50 days, the water table dropped ~45 cm. Thereafter, we rewetted (>30 mm d-1) till the water table was back up at 10 cm within 2 days. Pronounced drying and rewetting had a substantial effect on internal C-turnover and electron acceptor pools. Profiles in CO2/CO32- followed closely the water table rise and drop (2-5 mmol L-1 below, 1-2 mmol L-1 above the water table), whereas methanogenesis lagged behind. While the electron acceptor pool (NO3-, Fe(III), SO42-) was renewed in the upper profile during drying out, there was still some methane detectable above the water table in the main root zone of the vegetated core, indicating anoxic micro-environments. After the rapid rewetting, thermodynamically preferred electron acceptors were consumed first. In the upper layers sulfate was present (>100 µmol L-1) for about 50-70 days before methane concentrations increased till >20% by volume in the gas samplers (eq. to 390 µmol L-1). The long lasting sulfate pool and high sulfate reduction rates in the upper layers (50 - >>250 nmol cm-3 d-1, radiotracer studies at 20°C) suggest a renewal of the electron acceptor pool by a yet not well identified mechanism. The CO2 release through mineralization) remained fairly constant during the drying/rewetting cycle (200-300 mmol m-2 d-1), while the type of vegetation had a substantial effect on photosynthesis (250–600 mmol CO2 m-2 d-1) and CH4 release (0-40 mmol m-2 d-1). This study demonstrates the impact of a changing climate on carbon turnover in peatland ecosystems. A permanently high water table allowed little renewal of electron acceptors and promoted methanogenesis, while a recycling of electron acceptors during drying and rewetting impeded methanogenesis. However, losses of CO2 through mineralization remained fairly constant, but photosynthesis showed a considerable effect.