Peatlands and the carbon cycle: from local processes to global implications - a synthesis

Peatlands cover only 3% of the Earth's land surface but boreal and subarctic peatlands store about 15-30% of the world's soil carbon ( C) as peat. Despite their potential for large positive feedbacks to the climate system through sequestration and emission of greenhouse gases, peatlands are not explicitly included in global climate models and therefore in predictions of future climate change. In April 2007 a symposium was held in Wageningen, the Netherlands, to advance our understanding of peatland C cycling. This paper synthesizes the main findings of the symposium, focusing on (i) small-scale processes, (ii) C fluxes at the landscape scale, and (iii) peatlands in the context of climate change. The main drivers controlling most are related to some aspects of hydrology. Despite high spatial and annual variability in Net Ecosystem Exchange ( NEE), the differences in cumulative annual NEE are more a function of broad scale geographic location and physical setting than internal factors, suggesting the existence of strong feedbacks. In contrast, trace gas emissions seem mainly controlled by local factors. Key uncertainties remain concerning the existence of perturbation thresholds, the relative strengths of the CO2 and CH4 feedback, the links among peatland surface climate, hydrology, ecosystem structure and function, and trace gas biogeochemistry as well as the similarity of process rates across peatland types and climatic zones. Progress on these research areas can only be realized by stronger co-operation between disciplines that address different spatial and temporal scales

EU Peatlands: Current Carbon Stocks and Trace Gas Fluxes

Peatlands in Europe has formed a significant sink for atmospheric CO2 since the last glacial maximum. Currently they are estimated to hold ca. 42 Gt carbon in the form of peat and are therefore a considerable component in the European carbon budget. Due to the generally wet soil conditions in peatlands they are also significant emitters of the strong greenhouse gas (GHG) methane (CH4) and in some cases also of nitrous oxide (N2O). The EU funded CarboEurope-GHG Concerted Action attempts to develop a reliable and complete greenhouse gas budget for Europe and this report aims to provide a review and synthesis of the available information about GHG exchanges in European peatlands and their underlying processes. A best estimate for all the European countries shows that some are currently sinks for atmospheric CO2 while others are sources. In contrast, for CH4 and N2O, only the sources are relevant. Whilst some countries are CO2 sinks, all countries are net GHG emitters from peatlands. The results presented, however, carry large uncertainties, which cannot be adequately quantified yet. One outstanding uncertainty is the distribution of land use types, particular in Russia, the largest European peat nation. The synthesis of GHG exchange, nevertheless, indicates some interesting features. Russia hosts an estimated 41% of European peatlands and contributes most to all GHG exchanges (CO2: 25%, CH4: 52%, N2O: 26%, Total: 37%). Germany is the second-largest emitter (12% of European total) although it contains only 3.2% of European peatlands. The reason is the use of most of the peatland area for intensive cropland and grassland. The largest CO2 emitters are countries with large agricultural peatland areas (Russia, Germany, Belarus, Poland), the largest N2O emitters are those with large agricultural fen areas (Russia, Germany, Finland). In contrast, the largest CH4 emitters are concentrated in regions with large areas of intact mires, namely Russia and Scandinavia. High average emission densities above 3.5 t C-equiv. ha-1 are found in the Southeast Mediterranean, Germany and the Netherlands where agricultural use of peatlands is intense. Low average emission densities below 0.3 t C-equiv. ha-1 occur where mires and peatland forests dominate, e.g. Finland and the UK. This report concludes by pointing at key gaps in our knowledge about peatland carbon stocks and GHG exchanges which include insufficient basic information on areal distribution of peatlands, measurements of peat depth and also a lack of flux datasets providing full annual budgets of GHG exchanges

Issues Related to Incorporating Northern Peatlands into Global Climate Models

Northern peatlands cover ~3–4 million km2 (~10% of the land north of 45°N) and contain ~200–400 Pg carbon (~10–20% of total global soil carbon), almost entirely as peat (organic soil). Recent developments in global climate models have included incorporation of the terrestrial carbon cycle and representation of several terrestrial ecosystem types and processes in their land surface modules. Peatlands share many general properties with upland, mineral-soil ecosystems, and general ecosystem carbon, water, and energy cycle functions (productivity, decomposition, water infiltration, evapotranspiration, runoff, latent, sensible, and ground heat fluxes). However, northern peatlands also have several unique characteristics that will require some rethinking or revising of land surface algorithms in global climate models. Here we review some of these characteristics, deep organic soils, a significant fraction of bryophyte vegetation, shallow water tables, spatial heterogeneity, anaerobic biogeochemistry, and disturbance regimes, in the context of incorporating them into global climate models. With the incorporation of peatlands, global climate models will be able to simulate the fate of northern peatland carbon under climate change, and estimate the magnitude and strength of any climate system feedbacks associated with the dynamics of this large carbon pool

Can a bog drained for forestry be a stronger carbon sink than a natural bog forest?

This study compares the CO2 exchange of a natural bog forest, and of a bog drained for forestry in the pre-Alpine region of southern Germany. The sites are separated by only 10 km, they share the same soil formation history and are exposed to the same climate and weather conditions. In contrast, they differ in land use history: at the Schechenfilz site a natural bog-pine forest (Pinus mugo ssp. rotundata) grows on an undisturbed, about 5 m thick peat layer; at Mooseurach a planted spruce forest (Picea abies) grows on drained and degraded peat (3.4 m). The net ecosystem exchange of CO2 (NEE) at both sites has been investigated for 2 years (July 2010-June 2012), using the eddy covariance technique. Our results indicate that the drained, forested bog at Mooseurach is a much stronger carbon dioxide sink (-130 ± 31 and -300 ± 66 g C m-2 a-1 in the first and second year, respectively) than the natural bog forest at Schechenfilz (-53 ± 28 and -73 ± 38 g C m-2 a-1). The strong net CO2 uptake can be explained by the high gross primary productivity of the 44-year old spruces that over-compensates the two-times stronger ecosystem respiration at the drained site. The larger productivity of the spruces can be clearly attributed to the larger plant area index (PAI) of the spruce site. However, even though current flux measurements indicate strong CO2 uptake of the drained spruce forest, the site is a strong net CO2 source when the whole life-cycle since forest planting is considered. It is important to access this result in terms of the long-term biome balance. To do so, we used historical data to estimate the difference between carbon fixation by the spruces and the carbon loss from the peat due to drainage since forest planting. This rough estimate indicates a strong carbon release of +134 t C ha-1 within the last 44 years. Thus, the spruces would need to grow for another 100 years at about the current rate, to compensate the potential peat loss of the former years. In contrast, the natural bog-pine ecosystem has likely been a small but stable carbon sink for decades, which our results suggest is very robust regarding short-term changes of environmental factors

A new approach for estimating northern peatland gross primary productivity using a satellite-sensor-derived chlorophyll index

Carbon flux models that are largely driven by remotely sensed data can be used to estimate gross primary productivity (GPP) over large areas, but despite the importance of peatland ecosystems in the global carbon cycle, relatively little attention has been given to determining their success in these ecosystems. This paper is the first to explore the potential of chlorophyll-based vegetation index models for estimating peatland GPP from satellite data. Using several years of carbon flux data from contrasting peatlands, we explored the relationships between the MERIS terrestrial chlorophyll index (MTCI) and GPP, and determined whether the inclusion of environmental variables such as PAR and temperature, thought to be important determinants of peatland carbon flux, improved upon direct relationships. To place our results in context, we compared the newly developed GPP models with the MODIS (Moderate Resolution Imaging Spectrometer) GPP product. Our results show that simple MTCI-based models can be used for estimates of interannual and intra-annual variability in peatland GPP. The MTCI is a good indicator of GPP and compares favorably with more complex products derived from the MODIS sensor on a site-specific basis. The incorporation of MTCI into a light use efficiency type model, by means of partitioning the fraction of photosynthetic material within a plant canopy, shows most promise for peatland GPP estimation, outperforming all other models. Our results demonstrate that satellite data specifically related to vegetation chlorophyll content may ultimately facilitate improved quantification of peatland carbon flux dynamics

Mineral dust as a driver of carbon accumulation in northern latitudes

Peatlands in northern latitudes sequester one third of the world\u27s soil organic carbon. Mineral dusts can affect the primary productivity of terrestrial systems through nutrient transport but this process has not yet been documented in these peat-rich regions. Here we analysed organic and inorganic fractions of an 8900-year-old sequence from Store Mosse (the "Great Bog") in southern Sweden. Between 5420 and 4550 cal yr BP, we observe a seven-fold increase in net peat-accumulation rates corresponding to a maximum carbon-burial rate of 150 g C m(-2) yr(-1) -more than six times the global average. This high peat accumulation event occurs in parallel with a distinct change in the character of the dust deposited on the bog, which moves from being dominated by clay minerals to less weathered, phosphate and feldspar minerals. We hypothesize that this shift boosted nutrient input to the bog and stimulated ecosystem productivity. This study shows that diffuse sources and dust dynamics in northern temperate latitudes, often overlooked by the dust community in favour of arid and semi-arid regions, can be important drivers of peatland carbon accumulation and by extension, global climate, warranting further consideration in predictions of future climate variability

Isoprene emission from Sphagnum species occupying different growth positions above the water table

Isoprene emission from Sphagnum species naturally growing at different positions above the water table were measured in a subarctic peatland and at monoliths from a temperate bog. Our objectives were to investigate (1) whether emission rates were species and/or moisture dependent, and (2) whether short-term temperature history had an influence on emission capacity. We expected greater emission capacities in moist than dry growing conditions, and from species adapted to wet habitats. We also expected that higher emission capacities would be found in response to elevated temperatures. Average peak growing season isoprene emission capacities (standardized to 20 degrees C and PAR 1000 mu mol m(-2) s(-1)) at the subarctic site were 106 and 74 mu g C m(-2) h(-1) from a S. balticum wet lawn and a S. balticum dry hummock/palsa, respectively. Emission capacities correlated strongly with gross primary productivity (GPP) and the average air temperature of the 48 hours prior to measurement (T-48), but the effect of T-48 seemed to be partly masked by the influence of GPP when moisture was not limiting. The laboratory experiments suggested that a typical hummock species, S. rubellum had higher capacity for isoprene emission than a typical lawn species S. magellanicum. Instantaneous emission rates increased with temperature, but no effect of temperature history was discernible. Sphagnum mosses are known to emit substantial amounts of isoprene, but in this study we also showed significant inter-species differences in emission capacity. The results imply that climate change induced alterations of peatland hydrology may change the total ecosystem isoprene source strength, as individual species adapt to new growth conditions or as a consequence of species succession

Determining the effects of peatland restoration on carbon dioxide exchange and potential for climate change mitigation

Over the last millennium peatlands have accumulated significant carbon stores. Drainage for agricultural use has been widespread and has altered the functioning of these mires: shifting them towards carbon release. Recently, in recognition of the range of ecosystem services derived from these landscapes peatland restoration projects have been initiated. Carbon storage is often cited amongst the aims of these projects, especially since the inclusion of rewetting wetlands in the Kyoto Protocol. However, little is known about the effects of ditch blocking on CO2 fluxes, particularly in Molinia caerulea dominated peatlands, a species common on degraded peatlands which tolerates a range of water table depths. This thesis aims firstly to quantify CO2 fluxes from a drained Molinia caerulea dominated blanket bog and to improve understanding of the temporal and spatial controls on these fluxes and secondly, to quantify the immediate effects of ditch blocking. Closed chamber measurements of net ecosystem exchange and partitioned below-ground respiration from control-restored paired sites were collected over the growing seasons immediately pre- (2012) and post-restoration (2013/2014). These flux data were coupled with remotely sensed data quantifying vegetation phenology and structure with a fine resolution (daily/cm) over large extents (annual/catchment). Although temporal variation in water table depth was not related to CO2 fluxes, the seasonal average related to vegetation composition suggesting raising water tables may promote a change in vegetation composition within these species-poor ecosystems. The distribution of water table depths, vegetation composition and CO2 fluxes did not vary with proximity to drainage ditches despite their prominence. An empirical model suggests in a drained state these peatlands are CO2 sources, indicating carbon previously accumulated is gradually being lost. Data suggest restoration does not always significantly affect water tables and consequently CO2 fluxes in the short-term. Where shallower water tables were maintained during dry conditions photosynthesis decreased and heterotrophic respiration increased: enhancing carbon release. Research undertaken during atypical weather has been unable to determine if restoration will be able to raise water tables sufficiently to protect the existing peat store and promote the vegetation change required to reinstate CO2 sequestration in the longer-term.University of ExeterSouth West Wate