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We examined the concentration of nitrogen (N) and δ15N in vegetation and peat in the Mer Bleue bog, Ontario, Canada. Compared with other ecosystems, N concentration in bog plant foliage is low (generally < 1.2%), with more N stored belowground than in foliar tissues. The natural abundance of 15N varies by plant functional type, indicating different strategies for nutrient acquisition. δ15N values of evergreen shrubs and trees are lower (~− 5 to ~− 9‰) than sedges and herbs (~+ 1 to ~− 1‰), likely the result of ericoid mycorrhizal fungi, which retain 15N in fungal biomass, with preferential transfer of 14N to the host plant. Results suggest that N in Sphagnum moss capitula is derived from uptake of N from decomposing peat and litter, with δ15N values of − 8 to − 2‰, rather than from N2 fixation. Soil δ15N varies with depth, related to degree of decomposition and dominant plant species litter. Foliar N plays a critical role in influencing the rate of photosynthesis and CO2 exchange. Despite paucity of available inorganic N in the system, most bog plants (that is, evergreen shrubs) have adapted with conservative growth strategies. Owing to slow decomposition, N is stored in this system tightly bound with organic matter, with C:N ratios in deep peat ranging from 33:1 to 23:1 and small fluxes into, out of and within the system. An increase in N availability, through atmospheric N deposition or water table lowering, could jeopardize the C sink function of bogs by stimulating microbial decomposition of organic matter. In the long term, an increase in N cycling would shift plant community composition from Sphagnum and evergreen dominated to deciduous species, fundamentally altering bog ecosystem function. [ABSTRACT FROM AUTHOR]

Abstract Peat accumulation makes nutrient-limited ombrotrophic bogs important carbon sinks, while enhanced atmospheric nitrogen (N) deposition threatens this capacity. Here we show that the stoichiometry of peat and porewater, a control of plant growth and decomposition, has been altered in response to long-term N, phosphorus (P), and potassium (K) fertilization at Mer Bleue ombrotrophic bog in eastern Ontario, Canada. In N-fertilized peat, receiving up to an additional 6.4 g N m−2 y−1 above ambient deposition, we found N significantly (p < 0.05) greater, P both significantly greater and smaller dependent on treatment concentration, calcium (Ca) and K concentrations significantly lower, relative to control plots. In NPK-fertilized peat, P was greater than control while Ca was significantly smaller. In the porewater of N- and NPK-fertilized peat, we observed significantly more ammonium, nitrate, dissolved organic nitrogen, phosphate, and dissolved organic phosphorus than in the controls. Fourier-transform infrared (FTIR) spectroscopic analysis demonstrated that fertilization in all treatments increased decomposition, as indicated by residual enrichment of refractory moieties such as aromatics, carboxylates, and lignin, albeit only in the uppermost depths. Changes in peat quality may, however, not only relate to advanced decomposition but also to changes in litter input, as predominant vascular vegetation in the fertilized plots shared several features in FTIR spectra with samples of uppermost peat layers. Our results demonstrate that the peat in this ombrotrophic bog has responded to long-term fertilization with loss of Sphagnum moss and its filter function, increased shrub cover and enhanced decomposition. Highlights • Nutrient addition resulted in changes from moss to shrub dominance and a lowered peat surface. • There were significant increases in peat N and P and decreases in Ca and K concentrations. • Peat porewater had significant increases in N and P, with greatest changes in top 20 cm. • FTIR spectra of surface peat indicated advanced decomposition and altered vegetation. • These changes result from both direct nutrient additions and secondary vegetation changes. [ABSTRACT FROM AUTHOR]

We measured net ecosystem exchange of carbon dioxide (CO2) (NEE) during wet and dry summers (2000 and 2001) across a range of plant communities at Mer Bleue, a large peatland near Ottawa, southern Ontario, Canada. Wetland types included ombrotrophic bog hummocks and hollows, mineral-poor fen, and beaver pond margins. NEE was significantly different among the sites in both years, but rates of gross photosynthesis did not vary spatially even though species composition at the sites was variable. Soil respiration rates were very different across sites and dominated interannual variability in summer NEE within sites. During the dry summer of 2001, net CO2 uptake was significantly smaller, and most locations switched from a net sink to a source of CO2 under a range of levels of photosynthetically active radiation (PAR). The wetter areas—poor fen and beaver pond margin—had the largest rates of CO2 uptake and smallest rates of respiratory loss during the dry summer. Communities dominated by ericaceous shrubs (bog sites) maintained similar rates of gross photosynthesis between years; by contrast, the sedge-dominated areas (fen sites) showed signs of early senescence under drought conditions. Water table position was the strongest control on respiration in the drier summer, whereas surface peat temperature explained most of the variability in the wetter summer. Q 10 temperature-respiration quotients averaged 1.6 to 2.2. The ratio between maximum photosynthesis and respiration ranged from 3.7:1 in the poor fen to 1.2:1 at some bog sites; it declined at all sites in the drier summer owing to greater respiration rates relative to photosynthesis in evergreen shrub sites and a change in both processes in sedge sites. Our ability to predict ecosystem responses to changing climate depends on a more complete understanding of the factors that control NEE across a range of peatland plant communities. [ABSTRACT FROM AUTHOR]

Summary 1Above-ground biomass was measured at bog hummock, bog hollow and poor-fen sites in Mer Bleue, a large, raised ombrotrophic bog near Ottawa, Ont., Canada. The average above-ground biomass was 587 g m-2 in the bog, composed mainly of shrubs and Sphagnum capitula. In the poor fen, the average biomass was 317 g m-2 , comprising mainly sedges and herbs and Sphagnum capitula. Vascular plant above-ground biomass was greater where the water table was lower, with a similar but weaker relationship for Sphagnum capitula and vascular leaf biomass. 2Below-ground biomass averaged 2400 g m-2 at the bog hummock site, of which 300 g m-2 was fine roots (< 2 mm diameter), compared with 1400 g m-2 in hollows (fine roots 450 g m-2 ) and 1200 g m-2 at the poor-fen site. 3Net Ecosystem Exchange (NEE) of CO2 was measured in chambers and used to derive ecosystem respiration and photosynthesis. Under high light flux (PAR of 1500 µmol m-2 s-1 ), NEE ranged across sites from 0.08 to 0.22 mg m-2 s-1 (a positive value indicates ecosystem uptake) in the spring and summer, but fell to –0.01 to –0.13 mg m-2 s-1 (i.e. a release of CO2 ) during a late-summer dry period. 4There was a general agreement between a combination of literature estimates of photosynthetic capacity for shrubs and mosses and measured biomass and summer-time CO2 uptake determined by the eddy covariance technique within a bog footprint (0.40 and 0.35–0.40 mg m-2 s-1 , respectively). 5Gross photosynthesis was estimated to be about 530 g m-2 year-1 , total respiration 460 g m-2 year-1 , and export of DOC, DIC and CH4 10 g m-2 year-1 , leaving an annual C sequestration rate of 60 g m-2 year-1 . Root... [ABSTRACT FROM AUTHOR]