Sensitivity of peatland respiration to vegetation community and temperature metric during a hot drought

2020 ◽  
Author(s):  
Julia Kelly ◽  
Natascha Kljun ◽  
Lars Eklundh ◽  
Leif Klemedtsson ◽  
Bengt Liljebladh ◽  
...  
Keyword(s):  
Temperature Sensitivity ◽  
Ecosystem Respiration ◽  
Gross Primary Production ◽  
Vascular Plant ◽  
Carbon Sources ◽  
Temperature Sensitive ◽  
Vegetation Community ◽  
Vegetation Communities ◽  
Drought Conditions ◽  
The Impact

<p>The majority of the world’s peatlands are located in northern regions where climate change is occurring most rapidly. Therefore, there is an urgent need to understand whether, and under what conditions, peatlands will remain carbon sinks or become carbon sources. The uncertainties in our predictions stem from a variety of sources, including uncertainty about the competing effects of rising air temperature on ecosystem respiration (R<sub>e</sub>) and gross primary production. Furthermore, peatlands contain a mixture of plant communities that respond differently to changes in temperature and precipitation. Such heterogeneity complicates attempts to upscale peatland carbon fluxes and predict the future peatland carbon balance.</p><p> </p><p>We focus on understanding the sensitivity of peatland R<sub>e</sub> to temperature and how it relates to vegetation community and the choice of temperature metric. We assess how these relationships changed during and after the severe heatwave and drought (‘hot drought’) in 2018. We conducted manual dark chamber CO<sub>2</sub> efflux measurements in Mycklemossen, an oligotrophic mire in southern Sweden in 2018 and in 2019, when weather conditions were closer to the long-term mean. The measurements covered the two main vegetation communities at the site: hummocks (vascular-plant dominated) and hollows (<em>Sphagnum</em>-dominated). We statistically compared the fluxes for both years and vegetation communities, then modelled them using three temperature metrics (air, surface, soil).</p><p> </p><p>We found that R<sub>e</sub> decreased during the hot drought for both vegetation communities, with maximum fluxes of 0.18 and 0.34 mgCO<sub>2</sub> m<sup>-2</sup> s<sup>-1</sup> in 2018 and 2019, respectively. However, the change in R<sub>e</sub> during the hot drought was dependent on vegetation community: hummock R<sub>e</sub> decreased substantially more than hollow R<sub>e</sub> (mean decrease: 48% and 15%, respectively). As a result, hollow R<sub>e</sub> was highest during drought whereas hummock R<sub>e</sub> was highest during non-drought conditions. Despite significant differences in R<sub>e</sub> between the vegetation communities, we found no significant differences in temperature between hummock and hollow vegetation, apart from in July and August 2018, at the peak of the hot drought. Nevertheless, hollow R<sub>e</sub> was more temperature-sensitive than hummock R<sub>e</sub> both during and after the hot drought. Furthermore, the temperature sensitivity of modelled R<sub>e</sub> depended on the choice of driving temperature, such that the surface temperature driven model produced the lowest whilst the soil temperature driven model produced the highest temperature sensitivity. Differences in temperature sensitivity of R<sub>e</sub> between the drought and non-drought conditions were similarly dependent on the temperature metric used to drive the R<sub>e</sub> model.</p><p> </p><p>We found that peatland R<sub>e</sub> almost halved during a hot drought. Our results show that predictions of peatland response to warming must account for the proportion of different vegetation communities present, and how this may change, due to their differing responses to warming. The choice of driving temperature in peatland R<sub>e</sub> models does not impact model accuracy but it does influence the temperature-sensitivity, and thus the impact of temperature variations on the modelled flux. Modellers should therefore base parameter choices on vegetation community and driving temperature. Furthermore, comparisons of R<sub>e</sub> sensitivity to warming between studies using different driving temperatures may be misleading.  </p>

scholarly journals Delayed responses of an Arctic ecosystem to an extremely dry summer: impacts on net ecosystem exchange and vegetation functioning

2013 ◽  
Vol 10 (12) ◽  
pp. 19189-19217 ◽  
Author(s):  
D. Zona ◽  
D. A. Lipson ◽  
J. H. Richards ◽  
G. K. Phoenix ◽  
A. K. Liljedahl ◽  
...  
Keyword(s):  
Extreme Events ◽  
Extreme Event ◽  
Ecosystem Respiration ◽  
Gross Primary Production ◽  
Vascular Plant ◽  
Net Ecosystem Exchange ◽  
Co2 Uptake ◽  
Tundra Ecosystem ◽  
Dry Conditions ◽  
The Impact

Abstract. The importance and mode of action of extreme events on the global carbon budget are inadequately understood. This includes the differential impact of extreme events on various ecosystem components, lag effects, recovery times, and compensatory processes. Summer 2007 in Barrow, Arctic Alaska, experienced unusually high air temperatures (fifth warmest over a 65 yr period) and record low precipitation (lowest over a 65 yr period). These abnormal conditions resulted in strongly reduced net Sphagnum CO2 uptake, but no effect neither on vascular plant development nor on net ecosystem exchange (NEE) from this arctic tundra ecosystem. Gross primary production (GPP) and ecosystem respiration (Reco) were both generally greater during most of this extreme summer. Cumulative ecosystem C uptake in 2007 was similar to the previous summers, showing the capacity of the ecosystem to compensate in its net ecosystem exchange (NEE) despite the impact on other functions and structure such as substantial necrosis of the Sphagnum layer. Surprisingly, the lowest ecosystem C uptake (2005–2009) was observed during the 2008 summer, i.e the year directly following the extremely summer. In 2008, cumulative C uptake was ∼70% lower than prior years. This reduction cannot solely be attributed to mosses, which typically contribute with ∼40% – of the entire ecosystem C uptake. The minimum summer cumulative C uptake in 2008 suggests that the entire ecosystem experienced difficulty readjusting to more typical weather after experiencing exceptionally warm and dry conditions. Importantly, the return to a substantial cumulative C uptake occurred two summers after the extreme event, which suggest a high resilience of this tundra ecosystem. Overall, these results show a highly complex response of the C uptake and its sub-components to atypically dry conditions. The impact of multiple extreme events still awaits further investigation.

scholarly journals Delayed responses of an Arctic ecosystem to an extreme summer: impacts on net ecosystem exchange and vegetation functioning

2014 ◽  
Vol 11 (20) ◽  
pp. 5877-5888 ◽  
Author(s):  
D. Zona ◽  
D. A. Lipson ◽  
J. H. Richards ◽  
G. K. Phoenix ◽  
A. K. Liljedahl ◽  
...  
Keyword(s):  
Extreme Events ◽  
Extreme Event ◽  
Ecosystem Respiration ◽  
Gross Primary Production ◽  
Net Ecosystem Exchange ◽  
Co2 Uptake ◽  
Lag Effects ◽  
Tundra Ecosystem ◽  
Co2 Sink ◽  
The Impact

Abstract. The importance and consequences of extreme events on the global carbon budget are inadequately understood. This includes the differential impact of extreme events on various ecosystem components, lag effects, recovery times, and compensatory processes. In the summer of 2007 in Barrow, Arctic Alaska, there were unusually high air temperatures (the fifth warmest summer over a 65-year period) and record low precipitation (the lowest over a 65-year period). These abnormal conditions were associated with substantial desiccation of the Sphagnum layer and a reduced net Sphagnum CO2 sink but did not affect net ecosystem exchange (NEE) from this wet-sedge arctic tundra ecosystem. Microbial biomass, NH4+ availability, gross primary production (GPP), and ecosystem respiration (Reco) were generally greater during this extreme summer. The cumulative ecosystem CO2 sink in 2007 was similar to the previous summers, suggesting that vascular plants were able to compensate for Sphagnum CO2 uptake, despite the impact on other functions and structure such as desiccation of the Sphagnum layer. Surprisingly, the lowest ecosystem CO2 sink over a five summer record (2005–2009) was observed during the 2008 summer (~70% lower), directly following the unusually warm and dry summer, rather than during the extreme summer. This sink reduction cannot solely be attributed to the potential damage to mosses, which typically contribute ~40% of the entire ecosystem CO2 sink. Importantly, the return to a substantial cumulative CO2 sink occurred two summers after the extreme event, which suggests a substantial resilience of this tundra ecosystem to at least an isolated extreme event. Overall, these results show a complex response of the CO2 sink and its sub-components to atypically warm and dry conditions. The impact of multiple extreme events requires further investigation.

scholarly journals Environmental and biological controls on Na∕Ca ratios in scleractinian cold-water corals

2019 ◽  
Vol 16 (18) ◽  
pp. 3565-3582 ◽  
Author(s):  
Nicolai Schleinkofer ◽  
Jacek Raddatz ◽  
André Freiwald ◽  
David Evans ◽  
Lydia Beuck ◽  
...  
Keyword(s):  
Temperature Sensitivity ◽  
Inductively Coupled Plasma ◽  
Cold Water ◽  
Inverse Correlation ◽  
Optical Emission ◽  
Emission Spectrometry ◽  
Temperature Sensitive ◽  
Lophelia Pertusa ◽  
Inductively Coupled ◽  
The Impact

Abstract. Here we present a comprehensive attempt to correlate aragonitic Na∕Ca ratios from Desmophyllum pertusum (formerly known as Lophelia pertusa), Madrepora oculata and a caryophylliid cold-water coral (CWC) species with different seawater parameters such as temperature, salinity and pH. Living CWC specimens were collected from 16 different locations and analyzed for their Na∕Ca ratios using solution-based inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements. The results reveal no apparent correlation with salinity (30.1–40.57 g kg−1) but a significant inverse correlation with temperature (-0.31±0.04 mmolmol-1∘C-1). Other marine aragonitic organisms such as Mytilus edulis (inner aragonitic shell portion) and Porites sp. exhibit similar results highlighting the consistency of the calculated CWC regressions. Corresponding Na∕Mg ratios show a similar temperature sensitivity to Na∕Ca ratios, but the combination of two ratios appears to reduce the impact of vital effects and domain-dependent geochemical variation. The high degree of scatter and elemental heterogeneities between the different skeletal features in both Na∕Ca and Na∕Mg, however, limit the use of these ratios as a proxy and/or make a high number of samples necessary. Additionally, we explore two models to explain the observed temperature sensitivity of Na∕Ca ratios for an open and semi-enclosed calcifying space based on temperature-sensitive Na- and Ca-pumping enzymes and transport proteins that change the composition of the calcifying fluid and consequently the skeletal Na∕Ca ratio.

scholarly journals Impact of water table level on annual carbon and greenhouse gas balances of a restored peat extraction area

2016 ◽  
Vol 13 (9) ◽  
pp. 2637-2651 ◽  
Author(s):  
Järvi Järveoja ◽  
Matthias Peichl ◽  
Martin Maddison ◽  
Kaido Soosaar ◽  
Kai Vellak ◽  
...  
Keyword(s):  
Greenhouse Gas ◽  
Water Table ◽  
Gross Primary Production ◽  
Vascular Plant ◽  
Plant Cover ◽  
Climate Impacts ◽  
N2o Emissions ◽  
Extraction Area ◽  
Peat Extraction ◽  
The Impact

Abstract. Peatland restoration may provide a potential after-use option to mitigate the negative climate impact of abandoned peat extraction areas; currently, however, knowledge about restoration effects on the annual balances of carbon (C) and greenhouse gas (GHG) exchanges is still limited. The aim of this study was to investigate the impact of contrasting mean water table levels (WTLs) on the annual C and GHG balances of restoration treatments with high (ResH) and low (ResL) WTL relative to an unrestored bare peat (BP) site. Measurements of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) fluxes were conducted over a full year using the closed chamber method and complemented by measurements of abiotic controls and vegetation cover. Three years following restoration, the difference in the mean WTL resulted in higher bryophyte and lower vascular plant cover in ResH relative to ResL. Consequently, greater gross primary production and autotrophic respiration associated with greater vascular plant cover were observed in ResL compared to ResH. However, the means of the measured net ecosystem CO2 exchanges (NEE) were not significantly different between ResH and ResL. Similarly, no significant differences were observed in the respective means of CH4 and N2O exchanges. In comparison to the two restored sites, greater net CO2, similar CH4 and greater N2O emissions occurred in BP. On the annual scale, ResH, ResL and BP were C sources of 111, 103 and 268 g C m−2 yr−1 and had positive GHG balances of 4.1, 3.8 and 10.2 t CO2 eq ha−1 yr−1, respectively. Thus, the different WTLs had a limited impact on the C and GHG balances in the two restored treatments 3 years following restoration. However, the C and GHG balances in ResH and ResL were considerably lower than in BP due to the large reduction in CO2 emissions. This study therefore suggests that restoration may serve as an effective method to mitigate the negative climate impacts of abandoned peat extraction areas.

scholarly journals Long-term drainage reduces CO<sub>2</sub> uptake and increases CO<sub>2</sub> emission on a Siberian floodplain due to shifts in vegetation community and soil thermal characteristics

2016 ◽  
Vol 13 (14) ◽  
pp. 4219-4235 ◽  
Author(s):  
Min Jung Kwon ◽  
Martin Heimann ◽  
Olaf Kolle ◽  
Kristina A. Luus ◽  
Edward A. G. Schuur ◽  
...  
Keyword(s):  
Primary Production ◽  
Ecosystem Respiration ◽  
Gross Primary Production ◽  
Thermal Characteristics ◽  
Growing Season ◽  
Co2 Uptake ◽  
Vegetation Community ◽  
Uptake Rates ◽  
Soil Temperatures

Abstract. With increasing air temperatures and changing precipitation patterns forecast for the Arctic over the coming decades, the thawing of ice-rich permafrost is expected to increasingly alter hydrological conditions by creating mosaics of wetter and drier areas. The objective of this study is to investigate how 10 years of lowered water table depths of wet floodplain ecosystems would affect CO2 fluxes measured using a closed chamber system, focusing on the role of long-term changes in soil thermal characteristics and vegetation community structure. Drainage diminishes the heat capacity and thermal conductivity of organic soil, leading to warmer soil temperatures in shallow layers during the daytime and colder soil temperatures in deeper layers, resulting in a reduction in thaw depths. These soil temperature changes can intensify growing-season heterotrophic respiration by up to 95 %. With decreased autotrophic respiration due to reduced gross primary production under these dry conditions, the differences in ecosystem respiration rates in the present study were 25 %. We also found that a decade-long drainage installation significantly increased shrub abundance, while decreasing Eriophorum angustifolium abundance resulted in Carex sp. dominance. These two changes had opposing influences on gross primary production during the growing season: while the increased abundance of shrubs slightly increased gross primary production, the replacement of E. angustifolium by Carex sp.  significantly decreased it. With the effects of ecosystem respiration and gross primary production combined, net CO2 uptake rates varied between the two years, which can be attributed to Carex-dominated plots' sensitivity to climate. However, underlying processes showed consistent patterns: 10 years of drainage increased soil temperatures in shallow layers and replaced E. angustifolium by Carex sp., which increased CO2 emission and reduced CO2 uptake rates. During the non-growing season, drainage resulted in 4 times more CO2 emissions, with high sporadic fluxes; these fluxes were induced by soil temperatures, E. angustifolium abundance, and air pressure.

scholarly journals Moderate increases in channel discharge are positively related to ecosystem respiration in forested Ozark streams

2021 ◽  
Author(s):  
Allyn K. Dodd ◽  
Daniel D. Magoulick ◽  
Michelle A. Evans-White
Keyword(s):  
Primary Production ◽  
Ecosystem Respiration ◽  
Gross Primary Production ◽  
Carbon Sources ◽  
Flow Regimes ◽  
Water Source ◽  
Stream Metabolism ◽  
Global Trends ◽  
Ecosystem Level ◽  
And Function

ABSTRACTThe natural flow regime is considered the “master variable” in lotic systems, controlling structure and function at organismal, population, community, and ecosystem levels. We sought to estimate forested headwater stream metabolism across two dominant flow regimes (Runoff and Groundwater) in northern Arkansas and evaluate potential differences in, and drivers of, gross primary production, ecosystem respiration, and net ecosystem metabolism. Flow regimes differed in intermittency, substrate heterogeneity, hyporheic connectivity, and dominant water source (subsurface runoff vs. groundwater), which we expected to result in differences in primary production and respiration. Average daily gross primary production (GPP) and ecosystem respiration (ER) estimated from field data collected from May 2015-June 2016 tended to be greater in Groundwater streams. Respiration was positively related to discharge (R2= 0.98 p< 0.0001) and net metabolism became more heterotrophic with increasing average annual discharge across sites (R2= 0.94, p= 0.0008). Characterizing ecosystem-level responses to differences in flow can reveal mechanisms governing stream metabolism and, in turn, provide information regarding trophic state and energy inputs as efforts continue to determine global trends in aquatic carbon sources and fates.

scholarly journals Drainage reduces CO<sub>2</sub> uptake and increases CO<sub>2</sub> efflux by a Siberian floodplain due to shifts in vegetation community and soil thermal characteristics

2016 ◽  
Author(s):  
M. J. Kwon ◽  
M. Heimann ◽  
K. A. Luus ◽  
E. A. G. Schuur ◽  
N. Zimov ◽  
...  
Keyword(s):  
Ecosystem Respiration ◽  
Gross Primary Production ◽  
Thermal Characteristics ◽  
Growing Season ◽  
The Arctic ◽  
Co2 Uptake ◽  
Vegetation Community ◽  
Closed Chamber ◽  
Soil Temperatures ◽  
Net Co2 Uptake

Abstract. With increasing air temperatures and shifts in precipitation patterns forecasted in the Arctic over the coming decades, thawing of ice-rich permafrost is expected to change the hydrological conditions in large parts of the region by creating mosaics of wetter and drier areas. The objective of this study is to investigate how lowered water table depths of formerly wet floodplain ecosystems affect CO2 fluxes measured with a closed chamber syst em, focusing on the roles of changes in vegetation community structure and soil thermal characteristics. We found that a decade-long drainage significantly increased the abundance of shrubs but decreased that of Eriophorum angustifolium, which subsequently made Carex species dominant. These two changes had opposing influences on photosynthetic uptake during the growing season: increased abundance of shrubs slightly increased gross primary production while replacement of Eriophorum by Carex significantly decreased it. Drainage also diminishes the heat capacity and thermal conductivity of soil, leading to increased soil temperatures in shallow layers during the daytime and decreased soil temperatures in deeper layers, and therefore reduced thaw depths. This soil temperature regime can intensify growing-season ecosystem respiration by up to 93 % theoretically. Overall, drainage increased net CO2 uptake (net ecosystem exchange) by 16 % over 20 days in 2013 but decreased it by 37 % over 66 days in 2014. During the frozen season, the drained transect emitted four times more CO2 than the undrained transect. In summary, the net effect of these complex changes recently weakened net CO2 uptake in the drained areas.

scholarly journals Watershed geomorphology modifies the sensitivity of aquatic ecosystem metabolism to temperature

2019 ◽  
Vol 9 (1) ◽  
Author(s):  
K. J. Jankowski ◽  
D. E. Schindler
Keyword(s):  
Temperature Sensitivity ◽  
Aquatic Ecosystem ◽  
Aquatic Ecosystems ◽  
Ecosystem Respiration ◽  
Ecosystem Metabolism ◽  
Temperature Sensitive ◽  
Carbon Quality ◽  
Carbon Cycles ◽  
Boreal River

AbstractThe regulation of aquatic carbon cycles by temperature is a significant uncertainty in our understanding of how watersheds will respond to climate change. Aquatic ecosystems transport substantial quantities of carbon to the atmosphere and ocean, yet we have limited understanding of how temperature modifies aquatic ecosystem metabolic processes and contributions to carbon cycles at watershed to global scales. We propose that geomorphology controls the distribution and quality of organic material that forms the metabolic base of aquatic ecosystems, thereby controlling the response of aquatic ecosystem metabolism to temperature across landscapes. Across 23 streams and four years during summer baseflow, we estimated variation in the temperature sensitivity of ecosystem respiration (R) among streams draining watersheds with different geomorphic characteristics across a boreal river basin. We found that geomorphic features imposed strong controls on temperature sensitivity; R in streams draining flat watersheds was up to six times more temperature sensitive than streams draining steeper watersheds. Further, our results show that this association between watershed geomorphology and temperature sensitivity of R was linked to the carbon quality of substrates that changed systematically across the geomorphic gradient. This suggests that geomorphology will control how carbon is transported, stored, and incorporated into river food webs as the climate warms.

Quantifying the responses of ecosystem production and respiration to drought time scale, intensity, timing and lagged response: A FLUXNET synthesis

2021 ◽  
Author(s):  
Wenzhe Jiao ◽  
Lixin Wang
Keyword(s):  
Ecosystem Respiration ◽  
Gross Primary Production ◽  
Multiple Time Scales ◽  
Drought Indices ◽  
Drought Severity ◽  
Multiple Time ◽  
Drought Impact ◽  
Drought Impacts ◽  
Drought Conditions ◽  
Ecosystem Production

&lt;p&gt;Drought is not only a multiscale (e.g., temporal, spatial) but also a multidimensional (e.g., onset, offset, duration, frequency, magnitude, intensity) phenomenon, and ecosystem production and respiration may respond to each drought dimension differently. &amp;#160;Although multiple reports exist in literature on the drought impact on ecosystem productivity, it remains unclear how each component of drought impacts ecosystem gross primary production (GPP), ecosystem respiration (R&lt;sub&gt;ECO&lt;/sub&gt;), and net ecosystem exchange (NEE) and how the different drought dimensions interacted with each other on their impacts. In this study, we conducted a comprehensive drought impact assessment on forest GPP, NEE, and R&lt;sub&gt;ECO&lt;/sub&gt; including all the drought dimensions using FLUXNET observations and multiple time-scales of Standardized Precipitation-Evapotranspiration Index (SPEI). Our results indicated that while most earlier drought studies focused on simultaneous and post-drought conditions, the cumulative drought impacts and drought timing are more significantly impacting forest carbon uptake than simultaneous drought severity. Temporal standardization based meteorological drought indices could be used to accurately reflect plant water stress if antecedent and cumulative drought conditions are considered.&lt;/p&gt;

Does nitrogen deposition lead to a weaker or stronger carbon sink in nutrient-poor peatlands?

2020 ◽  
Author(s):  
Tuula Larmola ◽  
Jani Antila ◽  
Liisa Maanavilja ◽  
Sari Juutinen ◽  
Jill L. Bubier ◽  
...  
Keyword(s):  
Ecosystem Respiration ◽  
Vascular Plant ◽  
N Deposition ◽  
Plant Litter ◽  
Nutrient Addition ◽  
N Addition ◽  
Plant Abundance ◽  
Gross Photosynthesis ◽  
The Impact

&lt;p&gt;Atmospheric nitrogen (N) deposition is increasing owing to fossil fuel burning and agriculture. In nutrient limited peatland ecosystems, the excess of reactive N has been found to increase vascular plant growth, but decrease Sphagnum growth. Higher vascular plant abundance and higher nutrient content alter decomposability of plant litter. These changes are likely to affect net imbalance of production and decomposition and thus carbon (C) accumulation in peatlands, which store about a third of global soil C. We studied whether the vegetation feedbacks of N deposition lead to stronger or weaker C sink in nutrient-poor peatlands. We investigated vegetation and ecosystem CO&lt;sub&gt;2 &lt;/sub&gt;exchange at two of the longest-running nutrient addition experiments on peatlands, Mer Bleue Bog, Canada and Deger&amp;#246; Stormyr poor fen, Sweden that have been fertilized with NH&lt;sub&gt;4&lt;/sub&gt;NO&lt;sub&gt;3&lt;/sub&gt; (2-15 times ambient annual wet deposition) for 12-23 years. Gross photosynthesis, ecosystem respiration and net CO&lt;sub&gt;2&lt;/sub&gt; exchange were measured weekly during June-August using chambers. To examine vegetation changes with increasing N influx, we determined the peak growing season aboveground biomass and coverage of vascular plants using the point intercept method. After 12-23 years of nutrient addition, the two sites revealed contrasting patterns: At Mer Bleue the highest nutrient additions were associated with up to 3-fold net CO&lt;sub&gt;2&lt;/sub&gt; uptake potential than in the control, whereas N addition treatments at Deger&amp;#246; Stormyr showed close to zero net CO&lt;sub&gt;2&lt;/sub&gt; uptake potential, only 0.3 fold compared to the control. The stronger C sink potential at Mer Bleue was mainly due to up to 50% increase in the gross photosynthesis and a diminished C sink potential at Deger&amp;#246; Stormyr due to down to 40 % lower gross photosynthesis. Ecosystem respiration showed similar trends at both peatlands: the rates were unaltered or increased to a lesser extent under N load. At both sites, the vegetation structure had changed remarkably. Most of the N addition treatments showed an increase of up to 90% in total vascular aboveground plant abundance and a concomitant loss of Sphagnum. At Mer Bleue along with the decrease in Sphagnum cover, the plots under highest N additions had become wetter, counterbalancing the impact of dry summer conditions in the study year whereas at Deger&amp;#246; Stormyr long term treatments did not alter wetness of the site. Thus, the contrasting C sink responses to long term N load may be explained by the type of vegetation and the water table depth. Shrubs were strong competitors at the dry Mer Bleue Bog while sedges had gained in abundance under N load at the wetter Deger&amp;#246; Stormyr. Our bog-fen comparison emphasizes the value of the long-term experiments in examining the ecosystem response of peatlands to N deposition, possible nonlinear responses and whether the key feedback mechanisms to ecosystem C sink potential differ in two main types of peatlands.&lt;/p&gt;

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