5.15 NET ECOSYSTEM CO 2 EXCHANGE OF TWO PEATLANDS WITH CONTRASTING VEGETATION IN NORTHERN ALBERTA, CANADA Aaron J. Glenn*, Lawrence B. Flanagan, Kamran H. Syed, and Peter J. Carlson University of Lethbridge, Lethbridge, Alberta, Canada 1. INTRODUCTION Peatlands are terrestrial ecosystems that play a major role in the global carbon (C) cycle and associated climatic feedbacks (Gorham, 1991; Moore et al., 1998). In Canada, peatlands have been defined as wetland ecosystems with a minimum organic soil depth of 40 cm (NWWG, 1988). They form when net primary production consistently exceeds decomposition due to cool and anaerobic subsurface conditions (Vitt et al., 1995; Szumigalski and Bayley, 1996a; Thormann and Bayley 1997). Peatlands can be divided into bogs and fens, with bogs influenced by water input derived only from precipitation, whereas fens are also influenced by groundwater that has come in contact with mineral soils (Vitt et al., 1995). Peatlands are further classified along a bog-rich fen gradient, based on variation in plant species composition and water chemistry characteristics (Sjörs, 1952; Vitt et al., 1995; Szumigalski and Bayley, 1996a; Thormann and Bayley 1997). The water of bogs and poor fens has low pH, low electrical conductivity, and low base cation concentrations, and these three chemical components increase along the gradient to extreme-rich fens, which have water that is more alkaline with high electrical conductivities. Peat moss (Sphagnum spp.) dominates bogs and poor fens, while sedge (Carex spp.) and "brown moss" species are the dominant vegetation in rich fens (Vitt et al., 1995). Bog and fen ecosystems are very important because they contain approximately one-third of the world's soil C pool and represent the largest pool of C in the Canadian terrestrial biosphere. Canada contains between 30-40% of the world's peatlands, covering 10-14% of the country's entire land surface (NWWG, 1988; Gorham, 1991). The carbon dioxide (CO 2 ) exchange between an ecosystem and the atmosphere is the net result of the competing flux processes of gross primary *Corresponding author current address: Aaron J. Glenn, Dept. of Soil Science, Univ. of Manitoba, Winnipeg, MB, Canada R3T 2N2 e-mail: umglenn@cc.umanitoba.ca production (GPP, photosynthetic CO 2 uptake) and total ecosystem respiration (TER, autotrophic and heterotrophic CO 2 production).Like all terrestrial ecosystems, a number of environmental factors play important roles in governing the rate of net CO 2 exchange in peatlands and projected climate change can be expected to affect these regulating factors (Bubier et al., 2003). Accurately predicting the consequences of global climate change, as well as potential interactions and feedbacks with the atmosphere, requires a broader comprehension of the mechanisms influencing net ecosystem CO 2 exchange (NEE) in various types of peatlands. Northern Alberta, Canada is a region that has greater than 20% peatland cover (Vitt et al., 1998), with wetland types spanning the bog to rich fen gradient readily accessible by road, providing a convenient opportunity for conducting comparative ecological studies among contrasting peatland types experiencing a similar climate. Comparison studies of peatland water chemistry (Vitt et al., 1995), net primary production (Szumigalski and Bayley, 1996a; Thormann and Bayley 1997) and decomposition (Szumigalski and Bayley, 1996b) have been performed by others in the region and it was the aim of the present study to extend the comparative approach to analyses of NEE measurements. Two peatlands in northern Alberta were selected for this study, a peat moss dominated poor fen and a sedge dominated extreme-rich fen. It was hypothesized that rates of GPP and TER would differ between the peatland sites because of variation in nutrient availability and the dominant plant functional types present. Based on net CO 2 flux measurement campaigns and supporting meteorological measurements made at the two peatlands in northern Alberta during the 2004 growing season (May 1 – October 31), the following research objectives were addressed: (1) to determine if peak season rates of NEE were different between the two ecosystems; (2) to determine if growing season (6 month) CO 2 budgets were different between the sites; and (3)