Should aquatic CO2 evasion be included in contemporary carbon budgets for peatland ecosystems?

Quantifying the sink strength of northern hemisphere peatlands requires measurements or realistic estimates of all major C flux terms. Whilst assessments of the net ecosystem carbon balance (NECB) routinely include annual measurements of net ecosystem exchange and lateral fluxes of dissolved organic carbon (DOC), they rarely include estimates of evasion (degassing) of CO2 and CH4 from the water surface to the atmosphere, despite supersaturation being a consistent feature of peatland streams. Instantaneous gas exchange measurements from temperate UK peatland streams suggest that the CO2 evasion fluxes scaled to the whole catchment are a significant component of the aquatic C flux (23.3±6.9 g C m-2 catchment yr-1) and comparable in magnitude to the downstream DOC flux (29.1±12.9 g C m-2 catchment yr-1). Inclusion of the evasion flux term in the NECB would be justified if evaded CO2 and CH4 were isotopically “young” and derived from a “within-ecosystem” source, such as peat or in-stream processing of DOC. Derivation from “old” biogenic or geogenic sources would indicate a separate origin and age of C fixation, disconnected from the ecosystem accumulation rate that the NECB definition implies. Dual isotope analysis (δ13C and 14C) of evasion CO2 and DOC strongly suggest that the source and age of both are different and that evasion CO2 is largely derived from allochthonous (non-stream) sources. Whilst evasion is an important flux term relative to the other components of the NECB, isotopic data suggest that its source and age are peatland-specific. Evidence suggests that a component of the CO2-C evading from stream surfaces was originally fixed from the atmosphere at a significantly earlier time (pre-AD1955) than modern (post-AD1955) C fixation by photosynthesis

Drivers of long-term variability in CO2 net ecosystem exchange in a temperate peatland

Land–atmosphere exchange of carbon dioxide (CO2) in peatlands exhibits marked seasonal and inter-annual variability, which subsequently affects the carbon (C) sink strength of catchments across multiple temporal scales. Long-term studies are needed to fully capture the natural variability and therefore identify the key hydrometeorological drivers in the net ecosystem exchange (NEE) of CO2. Since 2002, NEE has been measured continuously by eddy-covariance at Auchencorth Moss, a temperate lowland peatland in central Scotland. Hence this is one of the longest peatland NEE studies to date. For 11 years, the site was a consistent, yet variable, atmospheric CO2 sink ranging from −5.2 to −135.9 g CO2-C m−2 yr−1 (mean of −64.1 ± 33.6 g CO2-C m−2 yr−1). Inter-annual variability in NEE was positively correlated to the length of the growing season. Mean winter air temperature explained 87% of the inter-annual variability in the sink strength of the following summer, indicating an effect of winter climate on local phenology. Ecosystem respiration (Reco) was enhanced by drought, which also depressed gross primary productivity (GPP). The CO2 uptake rate during the growing season was comparable to three other sites with long-term NEE records; however, the emission rate during the dormant season was significantly higher. To summarise, the NEE of the peatland studied is modulated by two dominant factors: - phenology of the plant community, which is driven by winter air temperature and impacts photosynthetic potential and net CO2 uptake during the growing season (colder winters are linked to lower summer NEE), - water table level, which enhanced soil respiration and decreased GPP during dry spells. Although summer dry spells were sporadic during the study period, the positive effects of the current climatic trend towards milder winters on the site's CO2 sink strength could be offset by changes in precipitation patterns especially during the growing season

Biogeochemistry of “pristine” freshwater stream and lake systems in the western Canadian Arctic

Climate change poses a substantial threat to the stability of the Arctic terrestrial carbon (C) pool as warmer air temperatures thaw permafrost and deepen the seasonally-thawed active layer of soils and sediments. Enhanced water flow through this layer may accelerate the transport of C and major cations and anions to streams and lakes. These act as important conduits and reactors for dissolved C within the terrestrial C cycle. It is important for studies to consider these processes in small headwater catchments, which have been identified as hotspots of rapid mineralisation of C sourced from ancient permafrost thaw. In order to better understand the role of inland waters in terrestrial C cycling we characterised the biogeochemistry of the freshwater systems in a c. 14 km2 study area in the western Canadian Arctic. Sampling took place during the snow-free seasons of 2013 and 2014 for major inorganic solutes, dissolved organic and inorganic C (DOC and DIC, respectively), carbon dioxide (CO2) and methane (CH4) concentrations from three water type groups: lakes, polygonal pools and streams. These groups displayed differing biogeochemical signatures, indicative of contrasting biogeochemical controls. However, none of the groups showed strong signals of enhanced permafrost thaw during the study seasons. The mean annual air temperature in the region has increased by more than 2.5 °C since 1970, and continued warming will likely affect the aquatic biogeochemistry. This study provides important baseline data for comparison with future studies in a warming Arctic

CO2 fluxes and ecosystem dynamics at five European treeless peatlands – merging data and process oriented modeling

The carbon dioxide (CO2) exchange of five different peatland systems across Europe with a wide gradient in land use intensity, water table depth, soil fertility and climate was simulated with the process oriented CoupModel. The aim of the study was to find out whether CO2 fluxes, measured at different sites, can be explained by common processes and parameters or to what extend a site specific configuration is needed. The model was calibrated to fit measured CO2 fluxes, soil temperature, snow depth and leaf area index (LAI) and resulting differences in model parameters were analyzed. Finding site independent model parameters would mean that differences in the measured fluxes could be explained solely by model input data: water table, meteorological data, management and soil inventory data. Seasonal variability in the major fluxes was well captured, when a site independent configuration was utilized for most of the parameters. Parameters that differed between sites included the rate of soil organic decomposition, photosynthetic efficiency, and regulation of the mobile carbon (C) pool from senescence to shooting in the next year. The largest difference between sites was the rate coefficient for heterotrophic respiration. Setting it to a common value would lead to underestimation of mean total respiration by a factor of 2.8 up to an overestimation by a factor of 4. Despite testing a wide range of different responses to soil water and temperature, rate coefficients for heterotrophic respiration were consistently the lowest on formerly drained sites and the highest on the managed sites. Substrate decomposability, pH and vegetation characteristics are possible explanations for the differences in decomposition rates. Specific parameter values for the timing of plant shooting and senescence, the photosynthesis response to temperature, litter fall and plant respiration rates, leaf morphology and allocation fractions of new assimilates, were not needed, even though the gradient in site latitude ranged from 48° N (southern Germany) to 68° N (northern Finland) differed largely in their vegetation. This was also true for common parameters defining the moisture and temperature response for decomposition, leading to the conclusion that a site specific interpretation of these processes is not necessary. In contrast, the rate of soil organic decomposition, photosynthetic efficiency, and the regulation of the mobile carbon pool need to be estimated from available information on specific soil conditions, vegetation and management of the ecosystems, to be able to describe CO2 fluxes under different condition

Peatland pools are tightly coupled to the contemporary carbon cycle

Peatlands are globally important stores of soil carbon (C) formed over millennial timescales but are at risk of destabilization by human and climate disturbance. Pools are ubiquitous features of many peatlands and can contain very high concentrations of C mobilized in dissolved and particulate organic form and as the greenhouses gases carbon dioxide (CO2) and methane (CH4). The radiocarbon content (14C) of these aquatic C forms tells us whether pool C is generated by contemporary primary production or from destabilized C released from deep peat layers where it was previously stored for millennia. We present novel 14C and stable C (δ13C) isotope data from 97 aquatic samples across six peatland pool locations in the United Kingdom with a focus on dissolved and particulate organic C and dissolved CO2. Our observations cover two distinct pool types: natural peatland pools and those formed by ditch blocking efforts to rewet peatlands (restoration pools). The pools were dominated by contemporary C, with the majority of C (~50%–75%) in all forms being younger than 300 years old. Both pool types readily transform and decompose organic C in the water column and emit CO2 to the atmosphere, though mixing with the atmosphere and subsequent CO2 emissions was more evident in natural pools. Our results show little evidence of destabilization of deep, old C in natural or restoration pools, despite the presence of substantial millennial-aged C in the surrounding peat. One possible exception is CH4 ebullition (bubbling), with our observations showing that millennial-aged C can be emitted from peatland pools via this pathway. Our results suggest that restoration pools formed by ditch blocking are effective at preventing the release of deep, old C from rewetted peatlands via aquatic export

Utilising conservative tracers and spatial surveys to identify controls on pathways and DOC exports in an Arctic catchment

Dissolved organic carbon (DOC) is typically the predominant form of carbon exported from headwater streams, it therefore represents a major carbon export from Arctic catchments. The projected deepening of thaw depth in permafrost regions, due to an increase in air temperature, may have a significant effect on the amount of DOC exported from these systems. However, quantification of the impacts of climate driven changes on DOC export are still highly uncertain. Understanding the processes controlling DOC export is therefore crucial in predicting the potential impact of projected environmental changes. The controls of DOC production and transport are heavily influenced by soil and vegetation, which are highly variable across the landscape. To completely understand these systems information regarding spatial variability of plants, soils and thaw depths must be taken into account. In this study sub-weekly sampling of DOC was undertaken throughout 2014 in a headwater (<1 km2) catchment in the Northwest Territories, Canada. Spatial surveys of soil properties, active thaw depth and normalised difference vegetation index (NDVI) were collected and used in conjunction with conservative stable water isotopes tracers and major ions to understand sources, flow pathways and timing of DOC exports from the catchment. Stable isotope tracers act as fingerprints of water allowing sources and pathways to be assessed. Observations reveal changing DOC concentrations throughout the season as the active layer deepens and the connectivity of the soils to the stream network throughout the catchment increases. Linking the DOC data with the conservative tracer response improves the identification of carbon pathways and fluxes from the soils; preliminary analysis indicates DOC is being delivered via deeper more mineral soils later in the season. The results indicate that the active layer depth has a strong influence on the amount of DOC exported from the system, independent of the amount of carbon stored in these deeper soils

Aquatic carbon and GHG export from a permafrost catchment; identifying source areas and primary flow paths

The aquatic pathway is increasingly being recognized as an important component of landscape scale greenhouse gas (GHG) budgets. Due to low temperatures and short residence times limiting in-stream production in northern headwater catchments, much of the exported carbon is likely to be allochthonous, transported via throughflow to the surface drainage system. Identifying sources and primary flow pathways is therefore essential in understanding and predicting changes in the aquatic flux magnitude. Arctic landscapes are now widely recognised as being particularly vulnerable to climate driven changes. The HYDRA project (“Permafrost catchments in transition: hydrological controls on carbon cycling and greenhouse gas budgets”) aims to understand the fundamental role that hydrological processes play in regulating landscape-scale carbon fluxes, and predict how changes in vegetation and active layer depth in permafrost environments influence the delivery and export of aquatic carbon. In this study we present aquatic concentrations and fluxes of carbon and GHG species collected across two field seasons (2013, 2014) from an arctic headwater catchment in northern Canada. Measured species include dissolved organic (DOC) and inorganic carbon (DIC), CO2, CH4 and N2O. Measurements were made across a range of freshwater types within the tundra landscape, including lakes, ice-wedge polygons, and the ‘Siksik’ stream which drains the (c.a. 1 km2) primary study catchment. A nested sub-catchment approach was used along the ‘Siksik’ stream; ‘snapshot’ sampling of eight points along the stream length allowed specific vegetation communities to be targeted to assess individually their contribution to aquatic export. A combination of stable isotopes and major ion concentrations measured at each sampling point provide additional information to trace source areas and flow paths within the main study catchment. Catchment scale evasion and downstream export were calculated and an initial comparison between the relative importance of different water body types presented

Contrasting CO2 concentration discharge dynamics in headwater streams: a multi-catchment comparison

Aquatic CO2 concentrations are highly variable and strongly linked to discharge but until recently measurements have been largely restricted to low-frequency manual sampling. Using new in-situ CO2 sensors we present concurrent, high-frequency (<30-min resolution) CO2 concentration and discharge data collected from five catchments across Canada, UK and Fennoscandinavia to explore concentration-discharge dynamics; we also consider the relative importance of high flows to lateral aquatic CO2 export. The catchments encompassed a wide range of mean CO2 concentrations (0.73 – 3.05 mg C L-1) and hydrological flow regimes from flashy peatland streams to muted outflows within a Finnish lake-system. In three of the catchments CO2 concentrations displayed clear bimodal distributions indicating distinct CO2 sources. Concentration-discharge relationships were not consistent across sites with three of the catchments displaying a negative relationship and two catchments displaying a positive relationship. When individual high flow events were considered, we found a strong correlation between both the average magnitude of the hydrological and CO2 response peaks, and the average response lag times. An analysis of lateral CO2 export showed that in three of the catchments the top 30% of flow (i.e. flow that was exceeded only 30% of the time) had the greatest influence on total annual load. This indicates that an increase in precipitation extremes (greater high-flow contributions) may have a greater influence on the flushing of CO2 from soils to surface waters than a long-term increase in mean annual precipitation, assuming source limitation does not occur

Role of high-flow extremes in aquatic carbon export from peatlands

Peatland streams have repeatedly been shown to be highly supersaturated in gaseous carbon and export significant loads of both dissolved (DOC) and particulate (POC) organic carbon. Previous studies have shown that aquatic carbon export is strongly bias towards high flow events, which may become more frequent under predicted climate change scenarios. However, due to technical limitations and the lack of high flow representation in many regular spot sampling regimes, our understanding of high flow concentration dynamics is limited. Here we bring together 2 separate analyses of (i) the role of high-flow ‘extremes’ on DOC export based on long term (1993-2007) weekly spot samples across 7 UK upland streams, and (ii) stormflow CO2 dynamics across 5 headwater streams (UK, Sweden, Finland and Canada) using continuous, in-situ CO2 sensors. Catchment weighted DOC exports from the 3 peatland streams included in analysis (i) ranged from 16.9 to 28.0 g C m-2. Results showed 38.4%-44.9% of this DOC was exported during ‘extreme’ highs, which represented only 5% of time and 38.4%-40.6% of runoff. Although DOC export was greater from peatland streams, the proportion exported during ‘extreme’ events was similar across all 7 catchments. A comparison between the effects of storm intensity and duration on annual DOC export, and a seasonal breakdown of storm contributions, will also be presented. As well as quantifying the downstream export of CO2 during storm events (6%-33% of total CO2 export in 5% time), analysis (ii) will also consider high-resolution concentration responses across individual storms

Greenhouse gas losses from peatland pipes: a major pathway for loss to the atmosphere?

Peatland pipes are large natural macropores which contribute significantly to streamflow and represent a potentially important transport pathway between terrestrial and aquatic/atmospheric systems. Our study aimed to estimate the contribution of pipeflow to catchment-scale greenhouse gas (GHG) losses (CO2, CH4 and N2O) in a UK peatland using a combination of fortnightly spot and continuous sensor measurements. Inter-pipe variability was high for all GHGs. Mean pipe water concentrations ranged from 0.70 to 6.51 mg C L-1, 0.90 to 897 μg C L-1, and 0.36 to 1.36 μg N L-1 for CO2, CH4 and N2O respectively. High-resolution CO2 data showed temporal changes in the connectivity between pipes and the surrounding peat, with connectivity greatest when water table was high and lowest at low water table depths when discharge was associated with deeper, CO2-enriched sources. Total downstream export from the eight studied pipes represented 3%, 38% and 3% of CO2, CH4 and N2O export at the catchment outlet, whilst contributing only ~2% of total catchment runoff. Direct degassing of CO2 and CH4 to the atmosphere was evident from an intensively monitored pipe outlet. Upscaling evasion estimates from the pipe outlets gave conservative catchment-scale emission rates of 7.08 g CO2-eq m-2 yr-1 and 50.2 g CO2-eq m-2 yr-1 for CO2 and CH4, respectively. Although the catchment-scale estimates contain significant uncertainty, they highlight the potential importance of pipes as a pathway for the release of terrestrially-produced GHGs to the atmosphere