Influence of water table levels on CO2 emissions in a Colorado subalpine fen: an in situ microcosm study

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Abstract

We quantified the relationship between water table position and CO2 emissions by manipulating water table levels for two summers in microcosms installed in a Colorado subalpine fen. Water levels were manipulated in the microcosms by either adding water or removing water and ranged from +10 cm above the soil surface to 40 cm below the soil surface, with ambient water levels in the fen averaging +3 and +2 cm above the soil surface during 1998 and 1999, respectively. Microcosm installation had no significant effect on CO2 efflux; the 2 year means of natural and reference CO2 efflux were 205.4 and 213.9 mg CO2-C m−2 h−1, respectively (p=0.80). Mean CO2 emissions were lowest at the highest water tables (water +6 to +10 cm above the soil surface), averaging 133.8 mg CO2-C m−2 h−1, increased to 231.3 mg CO2-C m−2 h−1 when the water table was +1 to +5 cm above the soil surface and doubled to 453.7 mg CO2-C m−2 h−1, when the water table was 0–5 cm below the soil surface. However, further lowering of the water table had little additional effect on CO2 emissions, which averaged 470.3 and 401.1 mg CO2-C m−2 h−1 when the water table was 6–10 cm, and 11–40 cm beneath the soil surface, respectively. The large increase in CO2 emissions as we experimentally lowered the water table beneath the soil surface, coupled with no increase in CO2 emissions as we furthered lowered water tables beneath the soil surface, suggest the presence of an easily oxidized labile carbon pool near the soil surface.

Introduction

Peatlands accumulate carbon because mean annual primary production exceeds annual organic matter decomposition (Clymo, 1983). Peat accumulation in boreal and high mountain regions is a function of low decomposition rates rather than high net annual primary production (Malmer, 1986, Francez and Vasander, 1995), and only a small fraction of the carbon fixed by plants each year accumulates in the soil. More than 90% of the fixed carbon is re-released (Clymo, 1983), with up to 95% of the output being CO2 (Francez and Vasander, 1995, Waddington and Roulet, 2000). This small net carbon storage can be offset by increases in CO2 emissions, converting peatlands from sinks to sources of carbon to the atmosphere (Gorham, 1991, Francez and Vasander, 1995).

Air and soil temperature, water table level, and the quality of organic substrates are the main local controls of CO2 emissions from peatlands (Bridgham et al., 1995). Higher concentrations of labile organic matter can result in higher carbon mineralization rates while higher concentrations of recalcitrant organic matter decrease mineralization rates (Hogg et al., 1992, Updegraff et al., 1995). Warmer air and soil temperatures stimulate microbial activity resulting in higher CO2 emissions (Crill et al., 1988, Frolking and Crill, 1994, Silvola et al., 1996a). The temperature response of CO2 is modified by substrate quality, with higher substrate quality resulting in higher temperature responses (Valentine et al., 1994, Updegraff et al., 1995).

Water table levels can have important effects on CO2 emissions from peatlands, because saturated soils limit the diffusion of atmospheric oxygen into the peat, limiting microbial activity and decomposition rates (Clymo, 1983). Conversely, a water table decline increases oxygen diffusion into soils allowing aerobic decomposition, which increases CO2 emissions (Moore and Knowles, 1989, Bubier, 1995, Silvola et al., 1996a, Nykänen et al., 1998).

Rocky Mountain National Park has a number of water diversion ditches that pre-date the formation of the Park in 1915, and yet continue to alter water table levels in many wetlands (Cooper, 1990, Graf, 1997, Cooper et al., 1998, Woods, 2000, Chimner, 2000). We have found that the largest of these ditches, the Grand Ditch, has lowered water table levels in at least one fen, causing high organic matter decomposition rates as quantified by high CO2 emissions (Chimner, 2000). Although we have evidence to support the concept that lowered water tables can significantly alter carbon cycling in fens, it is unclear how much the water table must be lowered before changes occur. The sensitivity of fens to water level changes is critical to understand for evaluating the long-term effects of hydrologic changes due to water diversions, groundwater pumping or climate change. The objective of this study was to quantify in the field how changes in water table position influence CO2 emissions. Our approach involved manipulating water levels in microcosms installed for two-summers in a Colorado subalpine fen and measuring CO2 emissions.

Section snippets

Study site

This study was conducted in Moose fen, located at 2655 m elevation, on the western side of Rocky Mountain National Park, Colorado (Fig. 1). The fen is approximately 1 ha in size and the peat is 2.3 m thick at our study location. The fen is fed primarily by groundwater discharging from the toe of an adjacent mountain slope. Water table levels in this site have been above the soil surface for the 4 years that this site was analyzed (Cooper and Kennedy, unpublished data). Surface water has a pH of

Water table dynamics

Water table levels in the undisturbed fen (natural) and reference microcosms remained at or above the soil surface for the duration of the 2 year study period, averaging +3 and +2 cm, respectively, for 1998 and 1999 (Fig. 3). The natural water table levels measured for Moose Fen are typical for C. aquatilis dominated fens in the region (Chimner, 2000). The water addition treatment during 1998 increased water levels to between +5 and +8 cm above the soil surface (Fig. 3).

In 1998 the water level

Acknowledgements

Financial support for his project came from Rocky Mountain National Park, and the Society of Wetlands Scientists Student Grant. We thank Ken Czarnowski of Rocky Mountain National Park for his help and support. We thank Dan Reuss and Natural Resources Ecology Laboratory (NREL) Ft Collins, CO. for his generosity in letting us use the Infrared Gas Analyzer. We also thank Michel de Luz for field assistance and Jason Kaye, Sigrid Resh, Tom Stohlgren, Lee MacDonald, Eugene Kelly, and anonymous

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