Summary• Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain.• Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data.• We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased *These authors contributed equally to this work.

Boreal peatland ecosystems occupy about 3.5 million km 2 of the earth's land surface and store between 250 and 455 Pg of carbon (C) as peat. While northern hemisphere boreal peatlands have functioned as net sinks for atmospheric C since the most recent deglaciation, natural and anthropogenic disturbances, and most importantly wildfire, may compromise peatland C sinks. To examine the effects of fire on local and regional C sink strength, we focused on a 12 000 km 2 region near Wabasca, AB, Canada, where ombrotrophic Sphagnum-dominated bogs cover 2280 km 2 that burn with a fire return interval of 123 AE 26 years. We characterized annual C accumulation along a chronosequence of 10 bog sites, spanning 1-102 years-since-fire (in 2002). Immediately after fire, bogs represent a net C source of 8.9 AE 8.4 mol m À2 yr À1 . At about 13 years after fire, bogs switch from net C sources to net C sinks, mainly because of recovery of the moss and shrub layers. Subsequently, black spruce biomass accumulation contributes to the net C sink, with fine root biomass accumulation peaking at 34 years after fire and aboveground biomass and coarse root accumulation peaking at 74 years after fire. The overall C sink strength peaks at 18.4 mol C m À2 yr À1 at 75 years after fire. As the tree biomass accumulation rate declines, the net C sink decreases to about 10 mol C m À2 yr À1 at 100 years-sincefire. We estimate that across the Wabasca study region, bogs currently represent a C sink of 14.7 AE 5.1 Gmol yr À1 . A decrease in the fire return interval to 61 years with no change in air temperature would convert the region's bogs to a net C source. An increase in nonwinter air temperature of 2 1C would decrease the regional C sink to 6.8 AE 2.3 Gmol yr À1 . Under scenarios of predicted climate change, the current C sink status of Alberta bogs is likely to diminish to the point where these peatlands become net sources of atmospheric CO 2 -C.

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Abstract. Northern peatland ecosystems represent large carbon (C) stocks that are susceptible to changes such as accelerated mineralization due to water table lowering expected under a climate change scenario. During the growing seasons (1 May to 31 October) of 2011 and 2012 we monitored CO2 fluxes and plant biomass along a microtopographic gradient (hummocks-hollows) in an undisturbed dry continental boreal treed bog (control) and a nearby site that was drained (drained) in 2001. Ten years of drainage in the bog significantly increased coverage of shrubs at hummocks and lichens at hollows. Considering measured hummock coverage and including tree incremental growth, we estimate that the control site was a sink of −92 in 2011 and −70 g C m−2 in 2012, while the drained site was a source of 27 and 23 g C m−2 over the same years. We infer that, drainage-induced changes in vegetation growth led to increased biomass to counteract a portion of soil carbon losses. These results suggest that spatial variability (microtopography) and changes in vegetation community in boreal peatlands will affect how these ecosystems respond to lowered water table potentially induced by climate change.

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Across Canada's boreal forest, linear disturbances, including cutlines such as seismic lines and roads, crisscross the landscape to facilitate resource exploration and extraction; many of these linear disturbances cross peatland ecosystems. Changes in tree canopy cover and the compression of the peat by heavy equipment alter local thermal, hydrological, and ecological conditions, likely changing carbon exchange on the disturbance, and possibly in the adjacent peatland. We measured bulk density, water table, soil temperature, plant cover, and CO and CH flux along triplicate transects crossing a winter road through a wooded fen near Peace River, Alberta, Canada. Sample plots were located 1, 5, and 10 m from the road on both sides with an additional three plots on the road. Productivity of the overstory trees, when present, was also determined. The winter road had higher bulk density, shallower water table, higher graminoid cover, and thawed earlier than the adjacent peatland. Tree productivity and CO flux varied between the plots, and there was no clear pattern in relation to distance from the road. The plots on the winter road acted as a greater CO sink and greater CH source compared to the adjacent peatland with plots on the winter road emitting on average (standard error) 479 (138) compared to 41 (10) mg CH m day in the adjacent peatland. Considering both gases, global warming potential increased from 70 to 250 g CO e m year in the undisturbed area to 2100 g CO e m year on the winter road. Although carbon fluxes on any given cutline through peatland will vary depending on level of compaction, line width and vegetation community shifts, the large number of linear disturbances in Canada's boreal forest and slow recovery on peatland ecosites suggest they could represent an important anthropogenic greenhouse gas source.

Summary• Peat bogs have accumulated more atmospheric carbon (C) than any other terrestrial ecosystem today. Most of this C is associated with peat moss (Sphagnum) litter. Atmospheric nitrogen (N) deposition can decrease Sphagnum production, compromising the C sequestration capacity of peat bogs. The mechanisms underlying the reduced production are uncertain, necessitating multifactorial experiments.• We investigated whether glasshouse experiments are reliable proxies for field experiments for assessing interactions between N deposition and environment as controls on Sphagnum N concentration and production. We performed a meta-analysis over 115 glasshouse experiments and 107 field experiments.• We found that glasshouse and field experiments gave similar qualitative and quantitative estimates of changes in Sphagnum N concentration in response to N application. However, glasshouse-based estimates of changes in production -even qualitative assessmentsdiverged from field experiments owing to a stronger N effect on production response in absence of vascular plants in the glasshouse, and a weaker N effect on production response in presence of vascular plants compared to field experiments.• Thus, although we need glasshouse experiments to study how interacting environmental factors affect the response of Sphagnum to increased N deposition, we need field experiments to properly quantify these effects.

An electrically pumped three-level system was designed and fabricated using an AlGaAs/GaAs multiple quantum well structure. Under appropriate biases, the structure emits terahertz (THz) radiation, as a result of diagonal (or interwell) intersubband transition. The emission spectra were resolved using an external Fourier transform infrared spectrometer. The center frequency of the emission was voltage tunable. The emission spectra and the power-voltage relation showed clear evidence that the middle level was depopulated by fast longitudinal optical-phonon scattering, thus, a population inversion between the two upper levels is feasible.