Natural pipes in blanket peatlands: major point sources for the release of carbon to the aquatic system

Natural soil pipes, which have been widely reported in peatlands, have been shown to contribute significantly to total stream flow. Here, using measurements from eight pipe outlets, we consider the role of natural pipes in the transport of fluvial carbon within a 17.4‐ha blanket‐peat‐covered catchment. Concentrations of dissolved and particulate organic carbon (DOC and POC) from pipe waters varied greatly between pipes and over time, ranging between 5.3 and 180.6 mg L^?1 for DOC and 0.08 and 220 mg L^?1 for POC. Pipes were important pathways for peatland fluvial carbon export, with fluxes varying between 0.6 and 67.8 kg yr^?1 (DOC) and 0.1 and 14.4 kg yr^?1 (POC) for individual pipes. Pipe DOC flux was equivalent to 20% of the annual DOC flux from the stream outlet while the POC flux from pipes was equivalent to 56% of the annual stream POC flux. The proportion of different forms of aquatic carbon to total aquatic carbon flux varied between pipes, with DOC ranging between 80.0% and 91.2%, POC from 3.6% to 17.1%, dissolved CO₂‐C from 2.4% to 11.1% and dissolved CH₄‐C from 0.004% to 1.3%. The total flux of dissolved CO₂‐C and CH₄‐C scaled up to all pipe outlets in the study catchment was estimated to be 89.4 and 3.6 kg yr^?1 respectively. Overall, pipe outlets produced discharge equivalent to 14% of the discharge in the stream but delivered an amount of aquatic carbon equivalent to 22% of the aquatic carbon flux at the catchment outlet. Pipe densities in blanket peatlands are known to increase when peat is affected by drainage or drying. Hence, environmental change in many peatlands may lead to an increase in aquatic carbon fluxes from natural pipes, thereby influencing the peatland carbon balance and downstream ecological processes.     

Evasion of CO2 from streams – The dominant component of the carbon export through the aquatic conduit in a boreal landscape

Evasion of gaseous carbon (C) from streams is often poorly quantified in landscape C budgets. Even though the potential importance of the capillary network of streams as C conduits across the land–water–atmosphere interfaces is sometimes mentioned, low‐order streams are often left out of budget estimates due to being poorly characterized in terms of gas exchange and even areal surface coverage. We show that evasion of C is greater than all the total dissolved C (both organic and inorganic) exported downstream in the waters of a boreal landscape. In this study evasion of carbon dioxide (CO₂) from running waters within a 67 km² boreal catchment was studied. During a 4 year period (2006–2009) 13 streams were sampled on 104 different occasions for dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). From a locally determined model of gas exchange properties, we estimated the daily CO₂ evasion with a high‐resolution (5 × 5 m) grid‐based stream evasion model comprising the entire ~100 km stream network. Despite the low areal coverage of stream surface, the evasion of CO₂ from the stream network constituted 53% (5.0 (±1.8) g C m^?2 yr^?1) of the entire stream C flux (9.6 (±2.4) g C m^?2 yr^?1) (lateral as DIC, DOC, and vertical as CO₂). In addition, 72% of the total CO₂ loss took place already in the first‐ and second‐order streams. This study demonstrates the importance of including CO₂ evasion from low‐order boreal streams into landscape C budgets as it more than doubled the magnitude of the aquatic conduit for C from this landscape. Neglecting this term will consequently result in an overestimation of the terrestrial C sink strength in the boreal landscape.     

Role of the aquatic pathway in the carbon and greenhouse gas budgets of a peatland catchment

Peatland streams have repeatedly been shown to be highly supersaturated in both CO₂ and CH₄ with respect to the atmosphere, and in combination with dissolved (DOC) and particulate organic carbon (POC) represent a potentially important pathway for catchment greenhouse gas (GHG) and carbon (C) losses. The aim of this study was to create a complete C and GHG (CO₂, CH₄, N₂O) budget for Auchencorth Moss, an ombrotrophic peatland in southern Scotland, by combining flux tower, static chamber and aquatic flux measurements from 2 consecutive years. The sink/source strength of the catchment in terms of both C and GHGs was compared to assess the relative importance of the aquatic pathway. During the study period (2007–2008) the catchment functioned as a net sink for GHGs (352 g CO₂‐Eq m^?2 yr^?1) and C (69.5 g C m^?2 yr^?1). The greatest flux in both the GHG and C budget was net ecosystem exchange (NEE). Terrestrial emissions of CH₄ and N₂O combined returned only 4% of CO₂ equivalents captured by NEE to the atmosphere, whereas evasion of GHGs from the stream surface returned 12%. DOC represented a loss of 24% of NEE C uptake, which if processed and evaded downstream, outside of the catchment, may lead to a significant underestimation of the actual catchment‐derived GHG losses. The budgets clearly show the importance of aquatic fluxes at Auchencorth Moss and highlight the need to consider both the C and GHG budgets simultaneously.     

Aquatic export of young dissolved and gaseous carbon from a pristine boreal fen: Implications for peat carbon stock stability

The stability of northern peatland's carbon (C) store under changing climate is of major concern for the global C cycle. The aquatic export of C from boreal peatlands is recognized as both a critical pathway for the remobilization of peat C stocks as well as a major component of the net ecosystem C balance (NECB). Here, we present a full year characterization of radiocarbon content (¹⁴C) of dissolved organic carbon (DOC), carbon dioxide (CO₂), and methane (CH₄) exported from a boreal peatland catchment coupled with ¹⁴C characterization of the catchment's peat profile of the same C species. The age of aquatic C in runoff varied little throughout the year and appeared to be sustained by recently fixed C from the atmosphere (<60 years), despite stream DOC, CO₂, and CH₄ primarily being sourced from deep peat horizons (2–4 m) near the mire's outlet. In fact, the ¹⁴C content of DOC, CO₂, and CH₄ across the entire peat profile was considerably enriched with postbomb C compared with the solid peat material. Overall, our results demonstrate little to no mobilization of ancient C stocks from this boreal peatland and a relatively large resilience of the source of aquatic C export to forecasted hydroclimatic changes.     

Biogeochemical effects of simulated sea level rise on carbon loss in an Everglades mangrove peat soil

Saltwater intrusion and inundation can affect soil microbial activity, which regulates the carbon (C) balance in mangroves and helps to determine if these coastal forests can keep pace with sea level rise (SLR). This study evaluated the effects of increased salinity (+15 ppt), increased inundation (?8 cm), and their combination, on soil organic C loss from a mangrove peat soil (Everglades, Florida, USA) under simulated tides. Soil respiration (CO₂ flux), methane (CH₄) flux, dissolved organic carbon (DOC) production, and porewater nutrient concentrations were quantified. Soil respiration was the major pathway of soil organic C loss (94–98%) and was approximately 90% higher in the control water level than the inundated treatment under elevated salinity. Respiration rate increased with water temperature, but depended upon salinity and tidal range. CH₄ flux was minimal, while porewater DOC increased with a concomitant, significant decline in soil bulk density under increased inundation. Porewater ammonium increased (73%) with inundation and soluble reactive phosphorus increased (32%) with salinity. Overall, the decline in soil organic C mineralization from combined saltwater intrusion and prolonged inundation was not significant, but results suggest SLR could increase this soil’s susceptibility to peat collapse and accelerate nutrient and DOC export to adjacent Florida Bay.     

Temperature and precipitation drive temporal variability in aquatic carbon and GHG concentrations and fluxes in a peatland catchment

The aquatic pathway is increasingly being recognized as an important component of catchment carbon and greenhouse gas (GHG) budgets, particularly in peatland systems due to their large carbon store and strong hydrological connectivity. In this study, we present a complete 5‐year data set of all aquatic carbon and GHG species from an ombrotrophic Scottish peatland. Measured species include particulate and dissolved forms of organic carbon (POC, DOC), dissolved inorganic carbon (DIC), CO₂, CH₄ and N₂O. We show that short‐term variability in concentrations exists across all species and this is strongly linked to discharge. Seasonal cyclicity was only evident in DOC, CO₂ and CH₄ concentration; however, temperature correlated with monthly means in all species except DIC. Although the temperature correlation with monthly DOC and POC concentrations appeared to be related to biological productivity in the terrestrial system, we suggest the temperature correlation with CO₂ and CH₄ was primarily due to in‐stream temperature‐dependent solubility. Interannual variability in total aquatic carbon concentration was strongly correlated with catchment gross primary productivity (GPP) indicating a strong potential terrestrial aquatic linkage. DOC represented the largest aquatic carbon flux term (19.3 ± 4.59 g C m^?2 yr^?1), followed by CO₂ evasion (10.0 g C m^?2 yr^?1). Despite an estimated contribution to the total aquatic carbon flux of between 8 and 48%, evasion estimates had the greatest uncertainty. Interannual variability in total aquatic carbon export was low in comparison with variability in terrestrial biosphere–atmosphere exchange, and could be explained primarily by temperature and precipitation. Our results therefore suggest that climatic change is likely to have a significant impact on annual carbon losses through the aquatic pathway, and as such, aquatic exports are fundamental to the understanding of whole catchment responses to climate change.     

Identifying DOC gains and losses during a 20-year record in the Trout Beck catchment,Moor House,UK

The turnover of organic carbon in rivers could represent a large source of greenhouse gases to the atmosphere and studies have suggested that of the order of 70% of the dissolved organic carbon exported from soils could be lost in rivers before it flows to continental seas. The Environmental Change Network (ECN) monitoring of the dominantly peat-covered Trout Beck catchment within the Moor House site enabled the amount of dissolved organic carbon (DOC) lost within a stream over a 20-year period to be estimated. The study compared DOC concentrations of precipitation, shallow and deep soil waters with those at the catchment outlet. The mass balance between source and outlet was reconstructed by two methods: a single conservative tracer; and based upon a principal component analysis (PCA) using multiple tracers. The study showed the two methods had different outcomes, with the PCA showing a DOC gain and the single tracer showing a DOC loss. The DOC gain was attributed to an unmeasured groundwater contribution that dominates when the river discharge is lower. The DOC loss was related to the in-stream residence time, the soil temperature and month of the year, with longer in-stream residence times, warmer soils and summer months having larger DOC losses. The single tracer study suggested a 10 year average loss of 8.77 g C m⁻² year⁻¹, which is 33.1 g CO_2eq m⁻² year⁻¹, or 29% of the DOC flux from the source over a mean in-stream residence time of 4.33 h.     

How strong is the current carbon sequestration of an Atlantic blanket bog?

Although northern peatlands cover only 3% of the land surface, their thick peat deposits contain an estimated one‐third of the world's soil organic carbon (SOC). Under a changing climate the potential of peatlands to continue sequestering carbon is unknown. This paper presents an analysis of 6 years of total carbon balance of an almost intact Atlantic blanket bog in Glencar, County Kerry, Ireland. The three components of the measured carbon balance were: the land‐atmosphere fluxes of carbon dioxide (CO₂) and methane (CH₄) and the flux of dissolved organic carbon (DOC) exported in a stream draining the peatland. The 6 years C balance was computed from 6 years (2003–2008) of measurements of meteorological and eddy‐covariance CO₂ fluxes, periodic chamber measurements of CH₄ fluxes over 3.5 years, and 2 years of continuous DOC flux measurements. Over the 6 years, the mean annual carbon was ?29.7±30.6 (±1 SD) g C m^?2 yr^?1 with its components as follows: carbon in CO₂ was a sink of ?47.8±30.0 g C m^?2 yr^?1; carbon in CH₄ was a source of 4.1±0.5 g C m^?2 yr^?1 and the carbon exported as stream DOC was a source of 14.0±1.6 g C m^?2 yr^?1. For 2 out of the 6 years, the site was a source of carbon with the sum of CH₄ and DOC flux exceeding the carbon sequestered as CO₂. The average C balance for the 6 years corresponds to an average annual growth rate of the peatland surface of 1.3 mm yr^?1.     

Diurnal variations in the carbon chemistry of two acidic peatland streams in north-east Scotland

1. Two acidic peatland upland streams in north‐east Scotland draining catchments of 1.3 and 41.4 km² were sampled each season for 2 years to investigate diurnal variations in dissolved and gaseous forms of carbon. Stream metabolism, alkalinity, discharge, pH, air and water temperatures were measured to aid data interpretation. 2. Free CO₂ showed marked diurnal variation with lowest concentrations during the period from late morning to early afternoon and highest during the hours of darkness. Although alkalinity and pH also showed some diurnal fluctuations, in comparison with other more productive alkaline systems, variation was small. Dissolved organic carbon (DOC) showed no significant diurnal pattern. However, significant changes in stream discharge influenced DOC concentrations, as well as over‐riding diurnal patterns of free CO₂, alkalinity and pH. 3. The highest diurnal ratios (maximum concentration/minimum concentration) in CO₂, gross primary productivity (GPP) and community respiration (CR) occurred in spring and summer and the lowest in autumn and winter. Variation in biotic in‐stream processes caused changes in CO₂ concentrations and temperature affected both the solubility of CO₂ and changes in up‐stream CO₂ inputs. There was no significant difference in diurnal fluctuations between the two orders of stream studied. 4. The mean GPP (as CO₂) was 0.81 g CO₂ m^?2 day^?1 and mean CR 2.67 g CO₂ m^?2 day^?1. The mean primary production/respiration (P/R) ratio was 0.26 ± 0.09 and 0.33 ± 0.15 in the first and second order streams, respectively. These values are low compared with published data because these heterotrophic headwater streams are dominated by benthic respiration and upstream allochthonous inputs with little autotrophic metabolism, particularly during the colder autumn and winter months. 5. The results have implications for the calculation of dissolved inorganic carbon (DIC) fluxes in streamwater. Samples taken during daylight hours tend to have lower concentrations of free CO₂ and HCO₃^? than samples taken during darkness. During spring, concentrations of free CO₂ were measured up to 2.4 (annual mean 1.8) times higher at night than during the day at a similar discharge. It is suggested that fluxes based on daytime measurements alone will under‐estimate the annual flux of these determinands in streamwater by as much as 40%.     

Topographically regulated traps of dissolved organic carbon create hotspots of soil carbon dioxide efflux in forests

Soil carbon pools are an essential but poorly understood factor in heterotrophic soil respiration on forested landscapes. We hypothesized that the topographically regulated distribution of dissolved organic carbon (DOC) is the dominant factor contributing to soil CO₂ efflux. We tested this hypothesis by monitoring soil CO₂ efflux and sampling particulate and dissolved substrates (both mobile DOC in soil solution and DOC potentially sorbed onto Fe and Al oxyhydroxides) in surface (freshly fallen leaves (FFL) and forest floor) and near-surface (A-horizon or top 10 cm of peat) soils along three hillslope transects (15°, 25° and 35° slopes) that included upland (crest, shoulder, backslope, footslope, and toeslope) and wetland (periphery and central) topographic features during the snowfree season within a sugar maple forest. We observed that median snowfree season soil CO₂ efflux ranged from <1 to >5 μmol CO₂ m^?2 s^?1. Substrates in the near-surface mineral soil were most strongly related to median soil CO₂ efflux, and when combined mobile DOC and sorbed DOC together explained 78% of the heterogeneity in median soil CO₂ efflux (p < 0.001). When the carbon pool in FFL (an important source of DOC to the forest soils) was included, the explanation of variance increased to 81% (p < 0.001). Topographically regulated processes created high concentrations of mobile DOC at the footslope, and high concentrations of sorbed DOC further downslope at the toeslope, forming distinct traps of DOC that can become hotspots for soil CO₂ production. A reduction in the uncertainty of forest carbon budgets can be achieved by taking into consideration the topographic regulation of the substrates contributing to soil CO₂ efflux.     

Effect of catchment characteristics on aquatic carbon export from a boreal catchment and its importance in regional carbon cycling

Inland waters transport and emit into the atmosphere large amounts of carbon (C), which originates from terrestrial ecosystems. The effect of land cover and land‐use practises on C export from terrestrial ecosystems to inland waters is not fully understood, especially in heterogeneous landscapes under human influence. We sampled for dissolved C species in five tributaries with well‐determined subcatchments (total size 174.5 km²), as well as in various points of two of the subcatchments draining to a boreal lake in southern Finland over a full year. Our aim was to find out how land cover and land‐use affect C export from the catchments, as well as CH₄ and CO₂ concentrations of the streams, and if the origin of C in stream water can be determined from proxies for quality of dissolved organic matter (DOM). We further estimated the gas evasion from stream surfaces and the role of aquatic fluxes in regional C cycling. The export rate of C from the terrestrial system through an aquatic conduit was 19.3 g C m^?2(catchment) yr^?1, which corresponds to 19% of the estimated terrestrial net ecosystem exchange of the catchment. Most of the C load to the recipient lake consisted of dissolved organic carbon (DOC, 6.1 ± 1.0 g C m^?2 yr^?1); the share of dissolved inorganic carbon (DIC) was much smaller (1.0 ± 0.2 g C m^?2 yr^?1). CO₂ and CH₄ emissions from stream and ditch surfaces were 7.0 ± 2.4 g C m^?2 yr^?1 and 0.1 ± 0.04 g C m^?2 yr^?1, respectively, C emissions being thus equal with C load to the lake. The proportion of peatland in the catchment and the drainage density of peatland increased DOC in streams, whereas the proportion of agricultural land in the catchment decreased it. The opposite was true for DIC. Drained peatlands were an important CH₄ source for streams.     

Full carbon and greenhouse gas balances of fertilized and nonfertilized reed canary grass cultivations on an abandoned peat extraction area in a dry year

Bioenergy crop cultivation on former peat extraction areas is a potential after‐use option that provides a source of renewable energy while mitigating climate change through enhanced carbon (C) sequestration. This study investigated the full C and greenhouse gas (GHG) balances of fertilized (RCG‐F) and nonfertilized (RCG‐C) reed canary grass (RCG; Phalaris arundinacea) cultivation compared to bare peat (BP) soil within an abandoned peat extraction area in western Estonia during a dry year. Vegetation sampling, static chamber and lysimeter measurements were carried out to estimate above‐ and belowground biomass production and allocation, fluxes of carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O) in cultivated strips and drainage ditches as well as the dissolved organic carbon (DOC) export, respectively. Heterotrophic respiration was determined from vegetation‐free trenched plots. Fertilization increased the above‐ to belowground biomass production ratio and the autotrophic to heterotrophic respiration ratio. The full C balance (incl. CO₂, CH₄ and DOC fluxes from strips and ditches) was 96, 215 and 180 g C m^?2 yr^?1 in RCG‐F, RCG‐C and BP, respectively, suggesting that all treatments acted as C sources during the dry year. The C balance was driven by variations in the net CO₂ exchange, whereas the combined contribution of CH₄ and DOC fluxes was <5%. The GHG balances were 3.6, 7.9 and 6.6 t CO₂ eq ha^?1 yr^?1 in RCG‐F, RCG‐C and BP, respectively. The CO₂ exchange was also the dominant component of the GHG balance, while the contributions of CH₄ and N₂O were <1% and 1–6%, respectively. Overall, this study suggests that maximizing plant growth and the associated CO₂ uptake through adequate water and nutrient supply is a key prerequisite for ensuring sustainable high yields and climate benefits in RCG cultivations established on organic soils following drainage and peat extraction.     

Influence of drought-induced acidification on the mobility of dissolved organic carbon in peat soils

A strong relationship between dissolved organic carbon (DOC) and sulphate (SO₄^2?) dynamics under drought conditions has been revealed from analysis of a 10‐year time series (1993–2002). Soil solution from a blanket peat at 10 cm depth and stream water were collected at biweekly and weekly intervals, respectively, by the Environmental Change Network at Moor House‐Upper Teesdale National Nature Reserve in the North Pennine uplands of Britain. DOC concentrations in soil solution and stream water were closely coupled, displaying a strong seasonal cycle with lowest concentrations in early spring and highest in late summer/early autumn. Soil solution DOC correlated strongly with seasonal variations in soil temperature at the same depth 4‐weeks prior to sampling. Deviation from this relationship was seen, however, in years with significant water table drawdown (>?25 cm), such that DOC concentrations were up to 60% lower than expected. Periods of drought also resulted in the release of SO₄^2?, because of the oxidation of inorganic/organic sulphur stored in the peat, which was accompanied by a decrease in pH and increase in ionic strength. As both pH and ionic strength are known to control the solubility of DOC, inclusion of a function to account for DOC suppression because of drought‐induced acidification accounted for more of the variability of DOC in soil solution (R²=0.81) than temperature alone (R²=0.58). This statistical model of peat soil solution DOC at 10 cm depth was extended to reproduce 74% of the variation in stream DOC over this period. Analysis of annual budgets showed that the soil was the main source of SO₄^2? during droughts, while atmospheric deposition was the main source in other years. Mass balance calculations also showed that most of the DOC originated from the peat. The DOC flux was also lower in the drought years of 1994 and 1995, reflecting low DOC concentrations in soil and stream water. The analysis presented in this paper suggests that lower concentrations of DOC in both soil and stream waters during drought years can be explained in terms of drought‐induced acidification. As future climate change scenarios suggest an increase in the magnitude and frequency of drought events, these results imply potential for a related increase in DOC suppression by episodic acidification.     

Carbon Dioxide Variation in a Hardwood Forest Stream: An Integrative Measure of Whole Catchment Soil Respiration

The concentration of CO₂ in stream water is a product of not only instream metabolism but also upland, riparian, and groundwater processes and as such can provide an integrative measure of whole catchment soil respiration. Using a 5-year dataset of pH, alkalinity, Ca²⁺, and Mg²⁺ in surface water of the West Fork of Walker Branch in eastern Tennessee in conjunction with a hydrological flowpath chemistry model, we investigated how CO₂ concentrations and respiration rates in stream, bedrock, and soil environments vary seasonally and interannually. Dissolved inorganic carbon concentration was highest in summer and autumn (P < 0.05) although the proportion as free CO₂ (pCO₂) did not vary seasonally (P > 0.05). Over the 5 years, pCO₂ was always supersaturated with respect to the atmosphere ranging from 374 to 3626 ppmv (1.0- to 10.1-fold greater than atmospheric equilibrium), and CO₂ evasion from the stream to the atmosphere ranged from 146 to 353 mmol m⁻² d⁻¹. Whereas pCO₂ in surface water exhibited little intra-annual or interannual variation, distinct seasonal patterns in soil and bedrock pCO₂ were revealed by the catchment CO₂ model. Seasonally, soil pCO₂ increased from a winter low of 8167 ppmv to a summer high of 27,068 ppmv. Driven by the seasonal variation in gas levels, evasion of CO₂ from soils to the atmosphere ranged from 83 mmol m⁻² d⁻¹ in winter to 287 mmol m⁻² d⁻¹ in summer. The seasonal variation in soil CO₂ tracked soil temperature (r ²= 0.46, P < 0.001) and model-derived estimates of CO₂ evasion rate from soils agreed with previously reported fluxes measured using chambers (Pearson correlation coefficient = 0.62, P < 0.05) supporting the model assumptions. Although rates of CO₂ evasion were similar between the stream and soils, the overall rate of evasion from the channel was only 0.4% of the 70,752 mol/d that evaded from soils due to the vastly different areas of the two subsystems. Our model provides a means to assess whole catchment CO₂ dynamics from easily collected and measured stream-water samples and an approach to study catchment scale variation in soil ecosystem respiration. Received 24 July 1997; accepted 14 November 1997.     

Gaseous and fluvial carbon export from an Amazon forest watershed

The transfer of carbon (C) from Amazon forests to aquatic ecosystems as CO₂ supersaturated in groundwater that outgases to the atmosphere after it reaches small streams has been postulated to be an important component of terrestrial ecosystem C budgets. We measured C losses as soil respiration and methane (CH₄) flux, direct CO₂ and CH₄ fluxes from the stream surface and fluvial export of dissolved inorganic C (DIC), dissolved organic C (DOC), and particulate C over an annual hydrologic cycle from a 1,319-ha forested Amazon perennial first-order headwater watershed at Tanguro Ranch in the southern Amazon state of Mato Grosso. Stream pCO₂ concentrations ranged from 6,491 to 14,976 ??atm and directly-measured stream CO₂ outgassing flux was 5,994 ± 677 g C m^?2 y^?1 of stream surface. Stream pCH₄ concentrations ranged from 291 to 438 ??atm and measured stream CH₄ outgassing flux was 987 ± 221 g C m^?2 y^?1. Despite high flux rates from the stream surface, the small area of stream itself (970 m², or 0.007% of watershed area) led to small directly-measured annual fluxes of CO₂ (0.44 ± 0.05 g C m² y^?1) and CH₄ (0.07 ± 0.02 g C m² y^?1) per unit watershed land area. Measured fluvial export of DIC (0.78 ± 0.04 g C m^?2 y^?1), DOC (0.16 ± 0.03 g C m^?2 y^?1) and coarse plus fine particulate C (0.001 ± 0.001 g C m^?2 y^?1) per unit watershed land area were also small. However, stream discharge accounted for only 12% of the modeled annual watershed water output because deep groundwater flows dominated total runoff from the watershed. When C in this bypassing groundwater was included, total watershed export was 10.83 g C m^?2 y^?1 as CO₂ outgassing, 11.29 g C m^?2 y^?1 as fluvial DIC and 0.64 g C m^?2 y^?1 as fluvial DOC. Outgassing fluxes were somewhat lower than the 40?C50 g C m^?2 y^?1 reported from other Amazon watersheds and may result in part from lower annual rainfall at Tanguro. Total stream-associated gaseous C losses were two orders of magnitude less than soil respiration (696 ± 147 g C m^?2 y^?1), but total losses of C transported by water comprised up to about 20% of the ± 150 g C m^?2 (±1.5 Mg C ha^?1) that is exchanged annually across Amazon tropical forest canopies.     

The flux of carbon from rivers: the case for flux from England and Wales

This study uses the extensive monitoring datasets of the Environment Agency of England and Wales to calculate the flux of dissolved organic carbon (DOC); particulate organic carbon (POC); and excess dissolved CO₂ through English and Welsh rivers. The innovation of this study’s approach is to account for the losses of carbon within the fluvial system as well as fluxes at the catchment outlet. In order to make this assessment this study considers: the biochemical oxygen demand (BOD) as a measure of the degradation of DOC; and the dissolved CO₂ concentration of groundwater as calculated and apportioned into surface waters on the basis of Ca concentrations. The study shows that the best estimate of carbon export, via rivers, from England and Wales is 10.34 Mg C/km²/year, with 4.19 Mg C/km²/year of this going to the atmosphere. The mapping of the carbon export shows that there are regional hotspots of carbon export and in a small number of cases rivers could be net sinks of carbon due to their low dissolved CO₂ content relative to the atmosphere. The flux calculated by this approach is probably still an underestimate of the carbon flux through fluvial systems but the scale of the export is greater than that previously reported and there is evidence that the fluvial flux of carbon is increasing on a decadal scale.     

Age and source of different forms of carbon released from boreal peatland streams during spring snowmelt in E. Finland

Isotopic data are increasingly being used to quantify and understand the processes that control the release of carbon (C) from northern peatlands. We used δ¹³C and ¹⁴C measurements to investigate the source and age of different forms of aquatic C (DOC, POC and evasion CO₂) released from 2 contrasting (undrained v drained) forested peatland catchments at the end of the winter snowmelt period in boreal E Finland. The δ¹³C_VPDB values of DOC (range ?28.3 to ?28.8 ‰) were generally more ¹³C depleted than evasion CO₂ (range ?22.7 to ?31.5 ‰) and showed no clear differences between the pre-flood, flood and post-flood periods. Both forms of C had evidence of bomb-¹⁴C (i.e. >100%modern), indicating that they contained substantial quantities of C fixed since the mid AD 1950s. However, DOC was ¹⁴C enriched compared to evasion CO₂, with ¹⁴C concentrations suggesting that, on average, DOC-C was ~5–6 years younger than evasion CO₂–C, with the most recently fixed C being released when temperatures were highest. POC was significantly depleted in ¹⁴C with conventional (uncalibrated) radiocarbon ages of 805–1135 BP. In contrast to other studies, the isotopic compositions of DOC and evasion CO₂ were very similar, suggesting a predominantly single and consistent C source (decomposition of soil organic matter; SOM) during the snowmelt period. Whilst we found no evidence to suggest that old (pre-bomb) C was being released at the end of the winter period, the drained site was associated with more ¹⁴C depleted and ¹³C enriched evasion CO₂, suggesting a closer link to the atmospheric CO₂ pool. Our isotopic data suggest that the various forms of C released to the aquatic system from these forested Finnish peatlands are closely related, largely unaffected by drainage and (at least in the case of evasion CO₂ and DOC) indicate strong connectivity between C cycling in the soil–plant–water system.     

Carbon turnover in peatland mesocosms exposed to different water table levels

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.     

Evolution of carbon fluxes during initial soil formation along the forefield of Damma glacier, Switzerland

Soil carbon (C) fluxes, soil respiration and dissolved organic carbon (DOC) leaching were explored along the young Damma glacier forefield chronosequence (7–128 years) over a three-year period. To gain insight into the sources of soil CO₂ effluxes, radiocarbon signatures of respired CO₂ were measured and a vegetation-clipping experiment was performed. Our results showed a clear increase in soil CO₂ effluxes with increasing site age from 9 ± 1 to 160 ± 67 g CO₂–C m^?2 year^?1, which was linked to soil C accumulation and development of vegetation cover. Seasonal variations of soil respiration were mainly driven by temperature; between 62 and 70 % of annual CO₂ effluxes were respired during the 4-month long summer season. Sources of soil CO₂ effluxes changed along the glacier forefield. For most recently deglaciated sites, radiocarbon-based age estimates indicated ancient C to be the dominant source of soil-respired CO₂. At intermediate site age (58–78 years), the contribution of new plant-fixed C via rhizosphere respiration amounted up to 90 %, while with further soil formation, heterotrophically respired C probably from accumulated ‘older’ soil organic carbon (SOC) became increasingly important. In comparison with soil respiration, DOC leaching at 10 cm depth was small, but increased similarly from 0.4 ± 0.02 to 7.4 ± 1.6 g DOC m^?2 year^?1 over the chronosequence. A strong rise of the ratio of SOC to secondary iron and aluminium oxides strongly suggests that increasing DOC leaching with site age results from a faster increase of the DOC source, SOC, than of the DOC sink, reactive mineral surfaces. Overall, C losses from soil by soil respiration and DOC leaching increased from 9 ± 1 to 70 ± 17 and further to 168 ± 68 g C m^?2 year^?1 at the <10, 58–78, and 110–128 year old sites. By comparison, total ecosystem C stocks increased from 0.2 to 1.1 and to 3.1 kg C m^?2 from the young to intermediate and old sites. Therefore, the ecosystem evolved from a dominance of C accumulation in the initial phase to a high throughput system. We suggest that the relatively strong increase in soil C stocks compared to C fluxes is a characteristic feature of initial soil formation on freshly exposed rocks.     

Peatland carbon efflux partitioning reveals that Sphagnum photosynthate contributes to the DOC pool

Over half of the world's peat originated from Sphagnum, representing 10–15% of the terrestrial carbon stock. However, information regarding the release and exudation of organic carbon by living Sphagnum plants into the surface peat is scarce. In this study, we examined the contribution of recent Sphagnum subnitens (Russ. and Warnst.) photosynthate carbon to the peatland dissolved organic carbon (DOC) pool. This was done using a ¹³CO₂ pulse-chase experimental approach during the growing season. Despite the importance of Sphagnum in long-term carbon accumulation, results showed that the Sphagnum community rapidly contributes recently synthesized carbon to the peatland DOC pool. We estimate that by 4 h up to 4% of the total DOC in peat leachate was derived from ¹³CO₂ pulse labelling at ambient CO₂ concentrations. Nonetheless, a huge 64% of the ¹³C initially assimilated by photosynthesis was retained in Sphagnum subnitens for 23 days after labelling, consistent with the role of Sphagnum in peatland carbon accumulation. The majority of ¹³C loss as respired CO₂ came within the few days post ¹³CO₂ labelling, suggesting that it was derived from plant respiration of photosynthates.