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Information about the impact of nitrogen (N) deposition on the fate of deposited N in peatland ecosystems is lacking. Thus we investigated the fate of experimentally added N in long-term N-fertilized treatments in a Sphagnum-dominated ombrotrophic bog. Fertilization significantly stimulated vascular plant and suppressed Sphagnum and Polytrichum moss growth. N content in peat, mosses, and vascular plants was raised by the fertilizer addition and reached a maximum at 3.2 g m N input level with phosphorus (P) and potassium (K) addition. Most of N was retained in the vegetation and upper 10 cm of the peat. When N deposition equalled 1.6 g m and less, or 3.2 g m N with P and K addition, no inorganic N leaching was observed on the plots. This result indicates that co-fertilization with P and K raised the N retention capacity and that critical N loads with respect to N saturation depend on P and K availability. Most of the deposited N was recovered in the bulk peat, which may be related to a rapid immobilization of inorganic N by microorganisms and mycorrhizal assimilation. Increase of N, P, and K fertilization increased the contribution of vascular plants to N retention significantly and reduced those of mosses. The increase was mainly related to enhanced productivity, vascular biomass and N content in tissues; the reduced retention by mosses resulted from both reduced moss biomass and assimilation. The study shows that the N filter function of ombrotrophic bogs will be influenced by interactions with other nutrients and shifts in plant community structure. [ABSTRACT FROM AUTHOR]

We studied the effect of long-term water table drawdown on the vascular plant community in an ombrotrophic bog in central Finland by measuring aboveground biomass and belowground production (by in-growth cores) across plant functional groups including herbs, shrubs, and trees. We compared drained and undrained portions 45 years after the installation of a drainage ditch network, which has lowered water levels of 15–20 cm on average in the drained part of the site. Although shrub fine root production did not differ significantly between sites, water table drawdown increased belowground tree fine root production by 740% (3.8 ± 5.4 SD and 28.1 ± 24.1 g m−2 y−1 in undrained and drained sites, respectively) at the expense of herb root production, which declined 38% (27.62 ± 16.40 and 10.58 ± 15.7 g m−2 y−1 in undrained and drained sites, respectively) yielding no significant overall change in total fine root production. Drainage effects on aboveground biomass showed a similar pattern among plant types, as aboveground tree biomass increased dramatically with drainage (79 ± 135 and 2546 ± 1551 g m−2 in drained and undrained sites, respectively). Although total shrub biomass was not significantly different between sites, shrubs allocated more biomass to stems than leaves in the drained site. Drainage also caused a significant shift in shrub species composition. Although trees dominated the aboveground biomass following water table drawdown, understorey vegetation, mainly shrubs, continued to dominate belowground fine root production, comprising 64% of total root production at the drained site. Aboveground biomass proved to be a good predictor of belowground production, suggesting that allometric relationships can be developed to estimate belowground production in these systems. Increase in tree root production can counteract decrease in herb fine root production following water table drawdown, emphasizing the importance of plant functional type responses to water table drawdown. Whether these changes will offset ecosystem C loss via increased plant C storage or stimulate soil organic matter decomposition via increased above- and belowground litter inputs requires further study. [ABSTRACT FROM AUTHOR]

Changes of water table position influence carbon cycling in peatlands, but effects on the sources and sinks of carbon are difficult to isolate and quantify in field investigations due to seasonal dynamics and covariance of variables. We thus investigated carbon fluxes and dissolved carbon production in peatland mesocosms from two acidic and oligotrophic peatlands under steady state conditions at two different water table positions. Exchange rates and CO2, CH4 and DOC production rates were simultaneously determined in the peat from diffusive-advective mass-balances of dissolved CO2, CH4 and DOC in the pore water. Incubation experiments were used to quantify potential CO2, CH4, and DOC production rates. The carbon turnover in the saturated peat was dominated by the production of DOC (10–15 mmol m-2 d-1) with lower rates of DIC (6.1–8.5 mmol m-2 d-1) and CH4 (2.2–4.2 mmol m-2 d-1) production. All production rates strongly decreased with depth indicating the importance of fresh plant tissue for dissolved C release. A lower water table decreased area based rates of photosynthesis (24–42%), CH4 production (factor 2.5–3.5) and emission, increased rates of soil respiration and microbial biomass C, and did not change DOC release. Due to the changes in process rates the C net balance of the mesocosms shifted by 36 mmol m-2 d-1. According to our estimates the change in C mineralization contributed most to this change. Anaerobic rates of CO2 production rates deeper in the peat increased significantly by a factor of 2–3.5 (DOC), 2.9–3.9 (CO2), and 3–14 (CH4) when the water table was lowered by 30 cm. This phenomenon might have been caused by easing an inhibiting effect by the accumulation of CO2 and CH4 when the water table was at the moss surface. [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]

We compared the amount and composition of different aboveground biomass (BM) fractions of four mires with their net ecosystem CO2 exchange (NEE) measured by eddy covariance. We found clear differences in response of green biomass (GBM) of plant functional types (PFTs) to water table (WT), which resulted in larger spatial variation in GBM within a mire than variation between mires. GBM varied between mires from 126 ± 7 to 336 ± 16 g·m-2 (mean ± SE), while within mire variation at largest was from 157 ± 17 to 488 ± 20 g·m-2 (mean ± SE). GBM of dominant PFTs appeared to be better in explaining the peak growing season NEE than the total BM or GBM of a mire. The differences in photosynthetic capacity between PTFs had a major role, and thus a smaller GBM with different species composition could result in higher NEE than larger GBM. Vascular plant GBM, especially that of sedges, appeared to have a high impact on NEE. Eleven PFTs, defined here, appeared to capture well the internal variation within mires, and the differences in GBM between communities were explained by the water table response of PFTs. Our results suggest the use of photosynthesizing BM, separated into PFTs, in modelling ecosystem carbon exchange instead of using just total BM. [ABSTRACT FROM AUTHOR]