Experimental drought alters rates of soil respiration and methanogenesis but not carbon exchange in soil of a temperate fen
Introduction
Peatlands cover less than 3% of the earth's surface yet store about 30% of the world's soil carbon stocks (Gorham, 1991) and also contribute 2–10% to the global atmospheric methane (CH4) burden (Mikaloff Fletcher et al., 2004). Net ecosystem exchange (NEE) and CH4 emissions of peatlands are sensitive to changes in the soil temperature and hydrologic regime (Lafleur et al., 2005, Smemo and Yavitt, 2006). The impact of rising temperature and changes in the hydrologic regime on carbon dynamics is thus of interest. In particular, increases in precipitation in winters and drier summers with strong rainfalls driven by local and regional heat convection have been predicted for some northern regions (IPCC, 2001). These changes may decrease CH4 emissions and increase carbon release from peatlands (Moore et al., 1998).
Effects of mean changes in temperature and moisture on peatland carbon cycling have been identified but the impact of extreme weather is still uncertain. Soil moisture and CH4 emissions are, for example, not always related owing to complex interactions between CH4 transport, production, and oxidation (Walter et al., 1996). This raises the question what the net effect of short-term drought on CH4 production and emissions will be, as existing studies mostly focused on long-term changes in average water table position. The impact of hydrologic conditions on the carbon balance is even less understood due to the variable importance of individual processes and interactions between them. Soil and ecosystem respiration provide an example in this respect. In laboratory experiments with peat, presence of oxygen increased soil respiration by a factor of 2–6 (Moore and Dalva, 1997, Yavitt et al., 1997), and rewetting of aerated and dried soil samples resulted in short pulses of respiration (Clein and Schimel, 1994, Fierer and Schimel, 2003). Qualitatively similar results were obtained in mesocosm experiments that included part of the vegetation (Blodau and Moore, 2003, Blodau et al., 2004) and some field studies (Silvola et al., 1996a, Alm et al., 1999). However, the water table level, and thus aeration, was not related to ecosystem respiration (ER) in dry ombrotrophic bogs (Updegraff et al., 2001, Lafleur et al., 2005, Blodau et al., 2007) and a subalpine fen when the water table level dropped below 6 cm (Chimner and Cooper, 2003). The authors speculated that changes in soil respiration at greater depths had little impact on ER due to low temperatures, recalcitrant litter, incomplete aeration, and the predominance of autotrophic processes for ecosystem respiration. Uncertainty further arises from the response of autotrophic respiration of vascular plants and mosses to water availability and anaerobism, which can greatly differ for different types of vegetation. The productivity of mosses, for example, can be sensitive to drying (Robroek et al., 2007), whereas this may not be the case for vascular plants that access deeper soil layers.
To identify the impact of drought and rewetting on carbon cycling we characterized carbon exchange and belowground respiration in mesocosms from a weakly acidic temperate fen during a drying/rewetting cycle. Other controls, such as soil temperature and irradiation, were held constant. By incubating one mesocosm devoid of vegetation, the effect of plant cover on the dynamics of carbon fluxes and soil respiration was studied. We hypothesized that a simulated drought would decrease CH4 production and emission from the peat becoming aerated and results in prolonged periods of low or absent CH4 production after rewetting. We also expected that drought would shift the carbon balance towards losses to the atmosphere by increasing soil respiration, with peak respiration rates after changes in hydrologic conditions. Photosynthesis may on the other hand remain unaffected, as the dominating vascular vegetation can be expected to have access to deeper soil layers and is thus less sensitive to drying.
Section snippets
Sampling and treatment
Three peat cores (60 cm diameter, 60 cm depth, “mesocosms”) were collected in September 2005 at the Schlöppnerbrunnen fen site in northeastern Bavaria (50°08′38″N, 11°51′41″E, Fichtelgebirge, Germany). The site is moderately acidic (pH 3.5–5.5) and minerotrophic with highly decomposed soils rich in sulfur (up to 4.6 mg kg−1) and iron (up to >16 mg kg−1) and is dominated by graminoids and only few mosses. The site is located at 750 m, has a mean annual precipitation of 1150 mm, and a mean annual
Volumetric gas content during drying/rewetting
Initially, in phase I and, during the dry period, phase III, volumetric gas contents (VGCs) increased from about 2% near the water table to 9–12% at a depth of 10 cm in both dried and rewetted treatments (Fig. 1). Deeper in the unsaturated soil, VGCs remained low, particularly in the DW-V treatment. In this treatment VGCs decreased rapidly to 2–3% following rewetting, whereas in DW-D the complete filling of VGC to <4% was delayed by 30 days. The treatments DW-V and DW-D thus primarily differed
Respiration, photosynthesis, and net CO2 exchange
With respect to the impact of short-term drought, the key findings of the study were: (I) the relative resilience of surface carbon fluxes and (II) the clear impact of drying and rewetting on belowground respiration in the saturated and unsaturated peat. The latter impact was visible from the calculated diffusive fluxes (Fig. 6), minding the inherent uncertainty in these fluxes, and from the rapid change of DIC production following rewetting (Fig. 3, Fig. 4). Both results are in apparent
Acknowledgements
This study was funded by the Deutsche Forschungsgemeinschaft (DFG) grant Bl563/7-2 to C. Blodau and is part of the Research group FOR 562 “Soil processes under extreme meteorological boundary conditions”. Isotope measurements were performed with kind support from Dr. B. Glaser, Soil Physics Department, University of Bayreuth. The help of Karin Söllner, Martina Heider, and several research assistants is much appreciated. Two detailed and constructive anonymous reviews also significantly improved
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