Net ecosystem CO2 exchange of two peatlands with contrasting vegetation in northern Alberta, Canada

Net ecosystem exchange (NEE) of CO₂ was measured in two peatlands in northern Alberta, Canada using the eddy covariance technique during the 2004 growing season (May to October) as part of the Fluxnet-Canada Research Network. The two ecosystems, a poor fen and an extreme-rich fen, differed significantly in plant species composition and water chemistry characteristics. The poor fen (surface water pH = 4.0) had a plant community dominated by Sphagnum spp., while the extreme-rich fen (surface water pH = 6.5) was dominated by the perennial sedge species, Carex lasiocarpa. Both sites had maximum daily rates of net CO₂ uptake of approximately 5 mmol m^-2 s^-1, and typical nighttime rates of CO₂ loss of 2 mmol m^-2 s^-1 during measurement campaigns conducted at the peak of the growing season (July to mid-August). During this time at the poor fen, the continuous ground cover of Sphagnum spp. made up approximately 66 % of the total leaf area index (LAI) and had a total nitrogen content of 7.8 mg g^-1. In contrast, at the extreme-rich fen, C. lasiocarpa accounted for approximately 60 % of the total LAI and had a total leaf N content of 19.3 mg g^-1 at peak biomass. Calculations of maximum photosynthetic capacity and basal respiration rate at midseason were both higher for the extreme-rich fen and appeared to offset the net CO₂ flux, resulting in a similar pattern of NEE as the poor fen during this period. Empirical models driven by simple meteorological inputs fit to eddy covariance measurements and parameterized by day of year were used to calculate 6 month CO₂ budgets for the two sites. The poor fen was shown to be a net sink for CO₂ during four of the six months (peaking at 44 g C m^-2 in July), while only slight net losses of CO₂ (3 g C m^-2) occurred in May and September. In contrast, the extreme-rich fen was calculated to be a significant net CO₂ sink for only two months of the growing season (peaking at 30 g C m^-2 in August), while significant net losses of CO₂ occurred in May (8 g C m^-2) and in October (13 g C m^-2). When integrated over the 6 month study, the poor fen was a net sink (90 g C m^-2) that was approximately three times larger than the extreme-rich fen (31 g C m^-2). The difference in the magnitude of the growing season CO₂ budgets was mainly attributed to the seasonal timing of biological activity in the contrasting peatland plant communities. The plant species at the poor fen were active earlier and later in the growing season, while at the extreme-rich fen it took longer for C. lasiocarpa to develop leaf tissue in the early season and leaf senescence and reduction in photosynthetic activity occurred earlier in the fall. The ratio of cumulative growing season total ecosystem respiration (TER) to gross primary production (GPP) was 0.7 at the poor fen and 0.9 at the extreme-rich fen. Reasons for the larger ratio of TER to GPP at the extreme-rich fen compared to the poor fen could include greater belowground respiration (autotrophic and heterotrophic), greater lability of C. lasiocarpa tissue and derived peat than the more recalcitrant Sphagnum spp. tissue, and the coupling of photosynthetic and respiratory processes at the ecosystem scale