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.     

CO2 supersaturation along the aquatic conduit in Swedish watersheds as constrained by terrestrial respiration,aquatic respiration and weathering

We tested the hypothesis that CO₂ supersaturation along the aquatic conduit over Sweden can be explained by processes other than aquatic respiration. A first generalized‐additive model (GAM) analysis evaluating the relationships between single water chemistry variables and pCO₂ in lakes and streams revealed that water chemistry variables typical for groundwater input, e.g., dissolved silicate (DSi) and Mg²⁺ had explanatory power similar to total organic carbon (TOC). Further GAM analyses on various lake size classes and stream orders corroborated the slightly higher explanatory power for DSi in lakes and Mg²⁺ for streams compared with TOC. Both DSi and TOC explained 22–46% of the pCO₂ variability in various lake classes (0.01–>100 km²) and Mg²⁺ and TOC explained 11–41% of the pCO₂ variability in the various stream orders. This suggests that aquatic pCO₂ has a strong groundwater signature. Terrestrial respiration is a significant source of the observed supersaturation and we may assume that both terrestrial respiration and aquatic respiration contributed equally to pCO₂ efflux. pCO₂ and TOC concentrations decreased with lake size suggesting that the longer water residence time allow greater equilibration of CO₂ with the atmosphere and in‐lake mineralization of TOC. For streams, we observed a decreasing trend in pCO₂ with stream orders between 3 and 6. We calculated the total CO₂ efflux from all Swedish lakes and streams to be 2.58 Tg C yr^?1. Our analyses also demonstrated that 0.70 Tg C yr^?1 are exported to the ocean by Swedish watersheds as HCO₃^? and CO₃^2? of which about 0.56 Tg C yr^?1 is also a residual from terrestrial respiration and constitute a long‐term sink for atmospheric CO₂. Taking all dissolved inorganic carbon (DIC) fluxes along the aquatic conduit into account will lower the estimated net ecosystem C exchange (NEE) by 2.02 Tg C yr^?1, which corresponds to 10% of the NEE in Sweden.     

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.     

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%.     

Fluxes of dissolved carbon dioxide and inorganic carbon from an upland peat catchment: implications for soil respiration

This study uses long-term water chemistry records for a circum-neutral peat stream to reconstruct a 7-year record of dissolved CO₂ and DIC flux from the catchment. Combining catchment flux with a knowledge of in-stream metabolism and gas evasion from the stream surface enables an estimate of the dissolved CO₂ content of water emerging from the peat profile to be made; furthermore, these can be used to estimate soil CO₂ respiration. In this way multi-annual records of CO₂ production can be reconstructed, and therefore inter-annual controls on production examined. The results suggest that:(i) Stream evasion of CO₂ within the catchment varied between 80 and 220 g C/m of stream/yr, while in-stream metabolism produces between 1.0 and 2.9 g C/m of stream/yr;Export of dissolved CO₂ emerging from the soil profile, above that expected at equilibrium with the atmosphere, varies between 9.6 and 25.6 tonnes,C/km²/yr; andThe export of dissolved CO₂ implies a soil respiration rate of between 64.2 and 94.9 tonnes C/km²/yr.The inter-annual variation in both dissolved CO₂ flux and soil CO₂ respiration suggests that severe drought has no long-term effect on CO₂ production and that temperature-based models of soil CO₂ respiration will be adequate in all but the severest of summer droughts. The inter-annual variation in CO₂ flux shows that CO₂ production is decoupled from dissolved organic carbon (DOC) production. The decoupling of DOC and dissolved CO₂ production shows that enzymatic-latch production of DOC is an anaerobic process and will not increase soil CO₂ respiration.     

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.     

Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

The flux and composition of carbon (C) from land to rivers represents a critical component of the global C cycle as well as a powerful integrator of landscape‐level processes. In the Congo Basin, an expansive network of streams and rivers transport and cycle terrigenous C sourced from the largest swathe of pristine tropical forest on Earth. Increasing rates of deforestation and conversion to agriculture in the Basin are altering the current regime of terrestrial‐to‐aquatic biogeochemical cycling of C. To investigate the role of deforestation on dissolved organic and inorganic C (DOC and DIC, respectively) biogeochemistry in the Congo Basin, six lowland streams that drain catchments of varying forest proportion (12%–77%) were sampled monthly for 1 year. Annual mean concentrations of DOC exhibited an asymptotic response to forest loss, while DIC concentrations increased continuously with forest loss. The isotopic signature of DIC became significantly more enriched with deforestation, indicating a shift in source and processes controlling DIC production. The composition of dissolved organic matter (DOM), as revealed by ultra‐high‐resolution mass spectrometry, indicated that deforested catchments export relatively more aliphatic and heteroatomic DOM sourced from microbial biomass in soils. The DOM compositional results imply that DOM from the deforested sites is more biolabile than DOM from the forest, consistent with the corresponding elevated stream CO₂ concentrations. In short, forest loss results in significant and comprehensive shifts in the C biogeochemistry of the associated streams. It is apparent that land‐use conversion has the potential to dramatically affect the C cycle in the Congo Basin by reducing the downstream flux of stable, vascular‐plant derived DOC while increasing the transfer of biolabile soil C to the atmosphere.     

Trophic‐level dependent effects on CO2 emissions from experimental stream ecosystems

Concern over accelerating rates of species invasions and losses have initiated investigations into how local and global changes to predator abundance mediate trophic cascades that influence CO₂ fluxes of aquatic ecosystems. However, to date, no studies have investigated how species additions or losses at other consumer trophic levels influence the CO₂ flux of aquatic ecosystems. In this study, we added a large predatory stonefly, detritivorous stonefly, or grazer tadpole to experimental stream food webs and over a 70‐day period quantified their effects on community composition, leaf litter decomposition, chlorophyll‐a concentrations, and stream CO₂ emissions. In general, streams where the large grazer or large detritivore were added showed no change in total invertebrate biomass, leaf litter loss, chlorophyll‐a concentrations, or stream CO₂ emissions compared with controls; although we did observe a spike in CO₂ emissions in the large grazer treatment following a substantial reduction in chlorophyll‐a concentrations on day 28. However, the large grazer and large detritivore altered the community composition of streams by reducing the densities of other grazer and detritivore taxa, respectively, compared with controls. Conversely, the addition of the large predator created trophic cascades that reduced total invertebrate biomass and increased primary producer biomass. The cascading effects of the predator additions on the food web ultimately led to decreased CO₂ emissions from stream channels by up to 95%. Our results suggest that stream ecosystem processes were more influenced by changes in large predator abundance than large grazer or detritivore abundance, because of a lack of functionally similar large predators. Our study demonstrates that the presence/absence of species with unique functional roles may have consequences for the exchange of CO₂ between the ecosystem and the atmosphere.     

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.     

A large proportion of North American net ecosystem production is offset by emissions from harvested products,river/stream evasion,and biomass burning

Diagnostic carbon cycle models produce estimates of net ecosystem production (NEP, the balance of net primary production and heterotrophic respiration) by integrating information from (i) satellite‐based observations of land surface vegetation characteristics; (ii) distributed meteorological data; and (iii) eddy covariance flux tower observations of net ecosystem exchange (NEE) (used in model parameterization). However, a full bottom‐up accounting of NEE (the vertical carbon flux) that is suitable for integration with atmosphere‐based inversion modeling also includes emissions from decomposition/respiration of harvested forest and agricultural products, CO₂ evasion from streams and rivers, and biomass burning. Here, we produce a daily time step NEE for North America for the year 2004 that includes NEP as well as the additional emissions. This NEE product was run in the forward mode through the CarbonTracker inversion setup to evaluate its consistency with CO₂ concentration observations. The year 2004 was climatologically favorable for NEP over North America and the continental total was estimated at 1730 ± 370 TgC yr^?1 (a carbon sink). Harvested product emissions (316 ± 80 TgC yr^?1), river/stream evasion (158 ± 50 TgC yr^?1), and fire emissions (142 ± 45 TgC yr^?1) counteracted a large proportion (35%) of the NEP sink. Geographic areas with strong carbon sinks included Midwest US croplands, and forested regions of the Northeast, Southeast, and Pacific Northwest. The forward mode run with CarbonTracker produced good agreement between observed and simulated wintertime CO₂ concentrations aggregated over eight measurement sites around North America, but overestimates of summertime concentrations that suggested an underestimation of summertime carbon uptake. As terrestrial NEP is the dominant offset to fossil fuel emission over North America, a good understanding of its spatial and temporal variation – as well as the fate of the carbon it sequesters ─ is needed for a comprehensive view of the carbon cycle.     

Aquatic carbon fluxes dampen the overall variation of net ecosystem productivity in the Amazon basin: An analysis of the interannual variability in the boundless carbon cycle

The river–floodplain network plays an important role in the carbon (C) cycle of the Amazon basin, as it transports and processes a significant fraction of the C fixed by terrestrial vegetation, most of which evades as CO₂ from rivers and floodplains back to the atmosphere. There is empirical evidence that exceptionally dry or wet years have an impact on the net C balance in the Amazon. While seasonal and interannual variations in hydrology have a direct impact on the amounts of C transferred through the river–floodplain system, it is not known how far the variation of these fluxes affects the overall Amazon C balance. Here, we introduce a new wetland forcing file for the ORCHILEAK model, which improves the representation of floodplain dynamics and allows us to closely reproduce data‐driven estimates of net C exports through the river–floodplain network. Based on this new wetland forcing and two climate forcing datasets, we show that across the Amazon, the percentage of net primary productivity lost to the river–floodplain system is highly variable at the interannual timescale, and wet years fuel aquatic CO₂ evasion. However, at the same time overall net ecosystem productivity (NEP) and C sequestration are highest during wet years, partly due to reduced decomposition rates in water‐logged floodplain soils. It is years with the lowest discharge and floodplain inundation, often associated with El Nino events, that have the lowest NEP and the highest total (terrestrial plus aquatic) CO₂ emissions back to atmosphere. Furthermore, we find that aquatic C fluxes display greater variation than terrestrial C fluxes, and that this variation significantly dampens the interannual variability in NEP of the Amazon basin. These results call for a more integrative view of the C fluxes through the vegetation‐soil‐river‐floodplain continuum, which directly places aquatic C fluxes into the overall C budget of the Amazon basin.     

CO2 dynamics along Danish lowland streams: water–air gradients, piston velocities and evasion rates

We measured CO₂ concentration and determined evasion rate and piston velocity across the water–air interface in flow-through chambers at eight stations along two 20 km long streams in agricultural landscapes in Zealand, Denmark. Both streams were 9–18-fold supersaturated in CO₂ with daily means of 240 and 340 μM in January–March and 130 and 180 μM in June–August. Annual CO₂ medians were 212 μM in six other streams and 460 μM in four groundwater wells, while seven lakes were weakly supersaturated (29 μM). Air concentrations immediately above stream surfaces were close to mean atmospheric conditions except during calm summer nights. Piston velocity from 0.4 to 21.6 cm h^?1 was closely related to current velocity permitting calculation of evasion rates for entire streams. CO₂ evasion rates were highest in midstream reaches (170–1,200 mmol m^?2 day^?1) where CO₂-rich soil water entered fast stream flow, while rates were tenfold lower (25–100 mmol m^?2 day^?1) in slow-flowing lower reaches. CO₂ evasion mainly derived from the input of CO₂ in soil water. The variability of CO₂ evasion along the two lowland streams covered much of the range in sub-Arctic and temperate streams reported previously. In budgets for the two stream catchments, loss of carbon from soils via the hydrological cycle was substantial (3.2–5.7 mmol m^?2 day^?1) and dominated by CO₂ consumed to form HCO₃ ^? by mineral dissolution (69–76%) and export of organic carbon (15–23%) relative to dissolved CO₂ export (7–9%).     

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.     

Carbon respired by terrestrial ecosystems – recent progress and challenges

Net ecosystem production is the residual of two much larger fluxes: photosynthesis and respiration. While photosynthesis is a single process with a well‐established theoretical underpinning, respiration integrates the variety of plant and microbial processes by which CO₂ returns from ecosystems to the atmosphere. Limits to current capacity for predicting ecosystem respiration fluxes across biomes or years result from the mismatch between what is usually measured – bulk CO₂ fluxes – and what process‐based models can predict – fluxes of CO₂ from plant (autotrophic) or microbial (heterotrophic) respiration. Papers in this Thematic Issue and in the recent literature, document advances in methods for separating respiration into autotrophic and heterotrophic components using three approaches: (1) continuous measurements of CO₂ fluxes and assimilation of these data into process‐based models; (2) application of isotope measurements, particularly radiocarbon; and (3) manipulation experiments. They highlight the role of allocation of C fixed by plants to respiration, storage, growth or transfer to other organisms as a control of seasonal and interannual variability in soil respiration and the oxidation state of C in the terrestrial biosphere. A second theme is the potential for comparing C isotope signatures in organic matter, CO₂ evolved in incubations and microbial biomarkers to elucidate the pathways (respiration, recycling, or transformation) of C during decomposition. Together, these factors determine the continuum of timescales over which C is returned to the atmosphere by respiration and enable testing of theories of plant and microbial respiration that go beyond empirical models and allow predictions of future respiration responses to future change in climate, pollution and land use.     

Fluvial CO2 and CH4 patterns across wildfire‐disturbed ecozones of subarctic Canada: Current status and implications for future change

Despite occupying a small fraction of the landscape, fluvial networks are disproportionately large emitters of CO₂ and CH₄, with the potential to offset terrestrial carbon sinks. Yet the extent of this offset remains uncertain, because current estimates of fluvial emissions often do not integrate beyond individual river reaches and over the entire fluvial network in complex landscapes. Here we studied broad patterns of concentrations and isotopic signatures of CO₂ and CH₄ in 50 streams in the western boreal biome of Canada, across an area of 250,000 km². Our study watersheds differ starkly in their geology (sedimentary and shield), permafrost extent (sporadic to extensive discontinuous) and land cover (large variability in lake and wetland extents). We also investigated the effect of wildfire, as half of our study streams drained watersheds affected by megafires that occurred 3 years prior. Similar to other boreal regions, we found that stream CO₂ concentrations were primarily associated with greater terrestrial productivity and warmer climates, and decreased downstream within the fluvial network. No effects of recent wildfire, bedrock geology or land cover composition were found. The isotopic signatures suggested dominance of biogenic CO₂ sources, despite dominant carbonate bedrock in parts of the study region. Fluvial CH₄ concentrations had a high variability which could not be explained by any landscape factors. Estimated fluvial CO₂ emissions were 0.63 (0.09–6.06, 95% CI) and 0.29 (0.17–0.44, 95% CI) g C m^?2 year^?1 at the landscape scale using a stream network modelling and a mass balance approach, respectively, a small but potentially important component of the landscape C balance. These fluvial CO₂ emissions are lower than in other northern regions, likely due to a drier climate. Overall, our study suggests that fluvial CO₂ emissions are unlikely to be sensitive to altered fire regimes, but that warming and permafrost thaw will increase emissions significantly.     

Terrestrial higher-plant response to increasing atmospheric [CO2] in relation to the global carbon cycle

Terrestrial higher plants exchange large amounts of CO₂ with the atmosphere each year; c. 15% of the atmospheric pool of C is assimilated in terrestrial-plant photosynthesis each year, with an about equal amount returned to the atmosphere as CO₂ in plant respiration and the decomposition of soil organic matter and plant litter. Any global change in plant C metabolism can potentially affect atmospheric CO₂ content during the course of years to decades. In particular, plant responses to the presently increasing atmospheric CO₂ concentration might influence the rate of atmospheric CO₂ increase through various biotic feedbacks. Climatic changes caused by increasing atmospheric CO₂ concentration may modulate plant and ecosystem responses to CO₂ concentration. Climatic changes and increases in pollution associated with increasing atmospheric CO₂ concentration may be as significant to plant and ecosystem C balance as CO₂ concentration itself. Moreover, human activities such as deforestation and livestock grazing can have impacts on the C balance and structure of individual terrestrial ecosystems that far outweigh effects of increasing CO₂ concentration and climatic change. In short-term experiments, which in this case means on the order of 10 years or less, elevated atmospheric CO₂ concentration affects terrestrial higher plants in several ways. Elevated CO₂ can stimulate photosynthesis, but plants may acclimate and (or) adapt to a change in atmospheric CO₂ concentration. Acclimation and adaptation of photosynthesis to increasing CO₂ concentration is unlikely to be complete, however. Plant water use efficiency is positively related to CO₂ concentration, implying the potential for more plant growth per unit of precipitation or soil moisture with increasing atmospheric CO₂ concentration. Plant respiration may be inhibited by elevated CO₂ concentration, and although a naive C balance perspective would count this as a benefit to a plant, because respiration is essential for plant growth and health, an inhibition of respiration can be detrimental. The net effect on terrestrial plants of elevated atmospheric CO₂ concentration is generally an increase in growth and C accumulation in phytomass. Published estimations, and speculations about, the magnitude of global terrestrial-plant growth responses to increasing atmospheric CO₂ concentration range from negligible to fantastic. Well-reasoned analyses point to moderate global plant responses to CO₂ concentration. Transfer of C from plants to soils is likely to increase with elevated CO₂ concentrations because of greater plant growth, but quantitative effects of those increased inputs to soils on soil C pool sizes are unknown. Whether increases in leaf-level photosynthesis and short-term plant growth stimulations caused by elevated atmospheric CO₂ concentration will have, by themselves, significant long-term (tens to hundreds of years) effects on ecosystem C storage and atmospheric CO₂ concentration is a matter for speculation, not firm conclusion. Long-term field studies of plant responses to elevated atmospheric CO₂ are needed. These will be expensive, difficult, and by definition, results will not be forthcoming for at least decades. Analyses of plants and ecosystems surrounding natural geological CO₂ degassing vents may provide the best surrogates for long-term controlled experiments, and therefore the most relevant information pertaining to long-term terrestrial-plant responses to elevated CO₂ concentration, but pollutants associated with the vents are a concern in some cases, and quantitative knowledge of the history of atmospheric CO₂ concentrations near vents is limited. On the whole, terrestrial higher-plant responses to increasing atmospheric CO₂ concentration probably act as negative feedbacks on atmospheric CO₂ concentration increases, but they cannot by themselves stop the fossil-fuel-oxidation-driven increase in atmospheric CO₂ concentration. And, in the very long-term, atmospheric CO₂ concentration is controlled by atmosphere-ocean C equilibrium rather than by terrestrial plant and ecosystem responses to atmospheric CO₂ concentration.     

Nutrient Constraints on Metabolism Affect the Temperature Regulation of Aquatic Bacterial Growth Efficiency

Inorganic nutrient availability and temperature are recognized as major regulators of organic carbon processing by aquatic bacteria, but little is known about how these two factors interact to control bacterial metabolic processes. We manipulated the temperature of boreal humic stream water samples within 0–25°C and measured bacterial production (BP) and respiration (BR) with and without inorganic nitrogen?+?phosphorus addition. Both BP and BR increased exponentially with temperature in all experiments, with Q ₁₀ values varying between 1.2 and 2.4. The bacterial growth efficiency (BGE) showed strong negative relationships with temperature in nutrient-enriched samples and in natural stream water where community-level BP and BR were not limited by nutrients. However, there were no relationships between BGE and temperature in samples where BP and BR were significantly constrained by the inorganic nutrient availability. The results suggest that metabolic responses of aquatic bacterial communities to temperature variations can be strongly dependent on whether the bacterial metabolism is limited by inorganic nutrients or not. Such responses can have consequences for both the carbon flux through aquatic food webs and for the flux of CO₂ from aquatic systems to the atmosphere.     

Increasing CO2 from subambient to elevated concentrations increases grassland respiration per unit of net carbon fixation

Respiration (carbon efflux) by terrestrial ecosystems is a major component of the global carbon (C) cycle, but the response of C efflux to atmospheric CO₂ enrichment remains uncertain. Respiration may respond directly to an increase in the availability of C substrates at high CO₂, but also may be affected indirectly by a CO₂‐mediated alteration in the amount by which respiration changes per unit of change in temperature or C uptake (sensitivity of respiration to temperature or C uptake). We measured CO₂ fluxes continuously during the final 2 years of a 4‐year experiment on C₃/C₄ grassland that was exposed to a 200–560 μmol mol^?1 CO₂ gradient. Flux measurements were used to determine whether CO₂ treatment affected nighttime respiration rates and the response of ecosystem respiration to seasonal changes in net C uptake and air temperature. Increasing CO₂ from subambient to elevated concentrations stimulated grassland respiration at night by increasing the net amount of C fixed during daylight and by increasing either the sensitivity of C efflux to daily changes in C fixation or the respiration rate in the absence of C uptake (basal ecosystem respiration rate). These latter two changes contributed to a 30–47% increase in the ratio of nighttime respiration to daytime net C influx as CO₂ increased from subamient to elevated concentrations. Daily changes in net C uptake were highly correlated with variation in temperature, meaning that the shared contribution of C uptake and temperature in explaining variance in respiration rates was large. Statistically controlling for collinearity between temperature and C uptake reduced the effect of a given change in C influx on respiration. Conversely, CO₂ treatment did not affect the response of grassland respiration to seasonal variation in temperature. Elevating CO₂ concentration increased grassland respiration rates by increasing both net C input and respiration per unit of C input. A better understanding of how C efflux varies with substrate supply thus may be required to accurately assess the C balance of terrestrial ecosystems.     

CO2 evasion from boreal lakes: Revised estimate,drivers of spatial variability,and future projections

Lakes (including reservoirs) are an important component of the global carbon (C) cycle, as acknowledged by the fifth assessment report of the IPCC. In the context of lakes, the boreal region is disproportionately important contributing to 27% of the worldwide lake area, despite representing just 14% of global land surface area. In this study, we used a statistical approach to derive a prediction equation for the partial pressure of CO₂ (pCO₂) in lakes as a function of lake area, terrestrial net primary productivity (NPP), and precipitation (r² = .56), and to create the first high‐resolution, circumboreal map (0.5°) of lake pCO₂. The map of pCO₂ was combined with lake area from the recently published GLOWABO database and three different estimates of the gas transfer velocity k to produce a resulting map of CO₂ evasion (FCO₂). For the boreal region, we estimate an average, lake area weighted, pCO₂ of 966 (678–1,325) μatm and a total FCO₂ of 189 (74–347) Tg C year^?1, and evaluate the corresponding uncertainties based on Monte Carlo simulation. Our estimate of FCO₂ is approximately twofold greater than previous estimates, as a result of methodological and data source differences. We use our results along with published estimates of the other C fluxes through inland waters to derive a C budget for the boreal region, and find that FCO₂ from lakes is the most significant flux of the land‐ocean aquatic continuum, and of a similar magnitude as emissions from forest fires. Using the model and applying it to spatially resolved projections of terrestrial NPP and precipitation while keeping everything else constant, we predict a 107% increase in boreal lake FCO₂ under emission scenario RCP8.5 by 2100. Our projections are largely driven by increases in terrestrial NPP over the same period, showing the very close connection between the terrestrial and aquatic C cycle.     

Metabolic balance of streams draining urban and agricultural watersheds in central Japan

Empirical data that describe the metabolic balance of stream ecosystems in human-dominated watersheds are scarce. We measured ecosystem metabolism in 23 open-canopied lowland streams draining urban and agricultural areas in the Fuji River Basin, central Japan. Gross primary production (GPP) and community respiration (CR) were estimated using the diurnal dissolved oxygen (DO) change technique, with the reaeration coefficient (K ₂) determined from seven empirical depth-velocity equations. Because the predicted values of K ₂ showed variation among the depth-velocity equations, the estimates of stream metabolism also varied according to the equations. However, CR was almost always greater than GPP, resulting in negative net ecosystem production (NEP) and GPP/CR ratios below unity for most of the study reaches. Highly heterotrophic streams were found in intensively farmed watersheds, suggesting that organic matter loading from agricultural lands is likely to be a source of allochthonous carbon fueling excess respiration in the study streams. In contrast, streams draining more urbanized areas were less heterotrophic. The present results suggest that lowland streams in agriculturally developed watersheds are associated strongly with terrestrial ecosystems as a source of organic carbon. The resultant strong respiration might become the dominant process in ecosystem metabolism, as reported for headwater streams, large downstream rivers, and estuaries.