No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Origin and fate of dissolved inorganic carbon in a karst groundwater fed peatland using δ13CDIC
Abstract
Continental hydrosystems and in particular peatlands play an important role in the carbon cycle of the Critical Zone (CZ). Peatlands are important sinks for organic carbon and have therefore been extensively studied. However, peatlands are not only important for the fate of organic carbon, but they also affect the cycle of Dissolved Inorganic Carbon (DIC) of the peatland and the surrounding watershed. The fate of DIC is particularly complex in peatlands in limestone-dominated regions, because bicarbonate concentrations in surface and groundwater are high and the interaction between peatlands and surrounding hydrosystems are facilitated by the presence of highly permeable karst aquifers. In the present paper we study the origin and the fractionation of DIC in a peatland located on top of a karst aquifer. The study is based on hydrochemical and isotopic (δ¹³CDIC) data from samples recovered during 2 campaigns (low flow, high flow) at various depths within the Forbonnet peatland (Jura Mountains, eastern France), at the peatland outlet and at adjacent karst springs representing the underlying aquifer. In order to evaluate secondary fractionation processes, the measured δ¹³CDIC compositions were compared to modeled values considering the origin of DIC and potentially associated fractionation and speciation processes. The main results are: (1) DIC is lost at the bog surface by CO2 outgassing. (2) The δ¹³CDIC compositions of deep catotelm pore waters from the bog were much heavier than the modeled values. We relate this discrepancy to methanogenesis and show that this process is favored by reduced conditions at pH ~ 6 and a HCO3⁻ content of ~1 mmol/L, most probably due to punctual groundwater inflows at the base of the bog. Finally, contrasted δ¹³CDIC compositions between the bog and the fen of the peatland reveal an additional ecohydrological control on DIC speciation.
... The karst streams showed seasonal variations (Figure 3), with the lowest values when the surface of the peatland was frozen or covered with snow (late autumn, winter, and spring) and the highest in the summer. The melting of the ice released the lighter 16 O and therefore yielded lower 18 O values. In the summer, higher temperatures caused the preferential evaporation of the lighter oxygen isotope ( 16 O), causing the water to be enriched with 18 O. ...
... A difference in the EC values was also observed among the locations, as the average values for location P1 remained similar with depth (75-85 µS/cm), while in location P2, the values increased from 46 µS/cm (30 cm) to 115 µS/cm (90 cm). The consistent increase in pH and EC with depth at location P2 could be due to an additional water source that influenced the deeper layer of the peat profile, as shown in Lhosmot et al. [16]. The results of the Eh measurements were relatively high and indicated oxidizing conditions in all samples, due to the sampling method. ...
... Following a rapid increase in the dissolved oxygen content, the curve gradually flattened and stabilized. . The consistent increase in pH and EC with depth at location P2 could be due to an additional water source that influenced the deeper layer of the peat profile, as shown in Lhosmot et al. [16]. The results of the Eh measurements were relatively high and indicated oxidizing conditions in all samples, due to the sampling method. ...
Peatland hydrology plays an important role in preserving or changing the record in any consideration of past atmospheric deposition records in peat bogs. The Šijec bog, located on the Pokljuka plateau in Slovenia, is one of the largest ombrotrophic peatlands. We sampled the surface pools, pore water, drainage from the peatland, and karst streams not connected to the peatland. Additionally, we sampled the precipitation, as ombrotrophic peatlands receive mineral matter solely from the atmosphere. The results of the evaluation of the chemical and isotopic composition indicated different origins of dissolved mineral matter in different water types. The components originating from the bedrock and surrounding soils (Ca, Mg, Al, Si, Sr) predominated in the streams. The chemical composition of the peatland drainage water revealed the significant removal of major components from the peatland, particularly elements like Al, Fe, and REE, and metals that are readily dissolved in an acidic environment or mobile in their reduced state. Despite their solubility, concentrations of metals (As, Cr, Cu, Fe, Ni, Pb, Ti) and REE in surface pools remained higher than in the drainage due to incomplete elimination from the peatland. The composition of pore water reflects variations among the W and E parts of the peatland, indicating a heterogenous hydrological structure with different dynamics, such as an additional source of water at approximately 90 cm depth in the NW part. The chemical composition and isotope signature (¹⁸O and ²H) of pore water additionally indicated a heterogeneous recharge with residence times of less than a year. The overall analysis indicated a predominantly ombrotrophic type and a small part in the NW area of the peatland as a minerotrophic type of peat.
... Most of the time their pH is between 7.6 and 8.2 as in many other streams of the area, but at one time (April 2023) we measured a 6.8 pH. This very slight acidity of the water entering the network is probably due to sphagnum activity in the surrounding peatland, favoring low pH (Lhosmot et al., 2023) in water exported from the peat masses during rainy periods. This would favor the dissolution of the calcite drumlin matrix, acting together with mechanical piping process to create the suspected conduit in this upstream part of the cave network. ...
... Most of the time their pH is between 7.6 and 8.2 as in many other streams of the area, but at one time (April 2023) we measured a 6.8 pH. This very slight acidity of the water entering the network is probably due to sphagnum activity in the surrounding peatland, favoring low pH (Lhosmot et al., 2023) in water exported from the peat masses during rainy periods. This would favor the dissolution of the calcite drumlin matrix, acting together with mechanical piping process to create the suspected conduit in this upstream part of the cave network. ...
Wetlands, as crucial terrestrial carbon reservoirs, have recently suffered severe degradation due to intense human activities. Lacustrine sediments serve as vital indicators for understanding wetland environmental changes. In the current paper, porewater samples were extracted from lacustrine sediment in three boreholes with a depth of ~75 cm in the Huixian karst wetland, southwest China, to study the chemical and dissolved inorganic carbon (DIC) evolution under anthropogenic influence. Two boreholes are situated beneath the Mudong Lake, while the other one is in the degraded wetland area. The results show that porewater in the central region of Mudong Lake is natural HCO 3 –Ca type water and recharged by karst groundwater as evidenced by depleted ² H ‐ ¹⁸ O isotopes. Methanogenesis prevails in this area, suggested by positive δ ¹³ C values ranging from 4.29‰ to 7.05‰. However, shallow porewater at the western edge of Mudong Lake and porewater in the degraded wetland exhibit significantly higher concentrations of NO 3 ⁻ and SO 4 ²⁻ , resulting from the agricultural input and recharged groundwater influenced by oxidation of pyrite. These processes lead to a decrease in methane production and generate DIC through degradation of organic fertilizer and carbonate weathering by sulfuric acid, thereby significantly altering porewater δ ¹³ C values. Two types of DIC mixing processes were observed based on the increasing δ ¹³ C values with depth, which can be attributed to the unique karst groundwater subsystems. This work highlights the potential impact of human‐induced porewater chemical variations on the fate of DIC, particularly in karst wetland environments.
Peatlands provide a large panel of socio-ecosystemic services such as biodiversity, water and carbon storage, and amenities. Hydrological and geochemical interactions between peats and their surroundings are expected to be favored in mountainous areas, which are nowadays increasingly sensitive to climate changes. In order to provide an integrated scheme of potential interactions, this study evaluates spatio-temporal patterns of environmental tracers (⁸⁷Sr/⁸⁶Sr, δ¹⁸O/δ²H, elemental ratios) during high- and low-flow periods in the largest peatland complex of the Jura Mountains (France). Systematically depleted δ¹⁸O/δ²H values in the deepest peat pore waters suggest contrasted dynamics and origins, both compatible with either preferential winter recharge or supply from adjacent high-elevation areas. Combined with strontium isotopes (⁸⁷Sr/⁸⁶Sr), we show that these water fluxes are purveyors of solutes derived from water-rock interactions, modified by mixing, evapotranspiration and dilution with local meteoric inputs. An end member mixing analysis of the peat pore water solute composition is consistent with a major contribution of carbonates from the regional Cretaceous limestone formations, located beneath fluvio-glacial Quaternary deposits underlying the peat. This contribution implies a significant upward water flux from the underlying syncline that could reach a sufficient hydraulic head thanks to recharge from an adjacent regional anticline. These multiscale (anticline-syncline, syncline-peatland, peatland-surface) constraints allow us to propose a relevant scheme for the hydrogeochemical functioning of the peatland, enabling an improved understanding of the current high socio-ecosystemic value of the area, and the potential future evolution of the related services.
This article is protected by copyright. All rights reserved.
To understand the variability of methane (CH4) fluxes between a temperate mid-altitude Sphagnum-dominated peatland and the atmosphere, we monitored simultaneously eddy covariance, hydrometeorological and physical parameters between April 2019 and December 2021. The site was a CH4 source for the atmosphere, with a cumulative emission of 23.9 ± 0.6 g C m⁻² year⁻¹. At the interannual scale, deeper water table during vegetation growth periods resulted in lower CH4 fluxes (FCH4), and reciprocally. Furthermore, the seasonal temperature variation in the anaerobic peat layer was a good predictor for FCH4. However, while the lowest temperatures occurred between December and February, the lowest FCH4 were observed between March and May, with around 30% of negative FCH4. Indeed, the fastest increase in temperature of the aerobic layer likely stimulated methanotrophy at the expense of methanogenesis. Negative FCH4, systematically observed at midday, were concurrent with high photon flux densities, latent heat fluxes and net negative ecosystem CO2 exchanges, suggesting the control of photosynthesis over CH4 oxidation. Moreover, our results highlighted marked diurnal cycles with FCH4 maximal at night and minimal at midday for all seasons. This diurnal cyclicity is in opposition to what is typically known for peatlands dominated by vascular plants. Physical parameters, such as soil surface temperature and sensible heat fluxes, likely contribute to this diurnal FCH4 cyclicity and require further investigation. Our study thus demonstrates that diurnal variations in FCH4 must be considered before upscaling to seasonal or annual cycles, along with the effect of vegetation on CH4 transfer and oxidation processes.
Peatlands and associated ecosystem services are sensitive to climate changes and anthropogenic pressures such as drainage. This study illustrates these effects on the Forbonnet bog (7 ha), belonging to the Frasne peatland complex (~300 ha, French Jura Mountain) and shows how they can inform about the ecohydrological functioning of peatlands. The southern part of the Forbonnet bog was restored in 2015‐2016 by backfilling of artificial drains dating from the end of the 19th century. Piezometric data from 2014 to 2018 allow to evaluate the restoration effect on the Water Table Depth (WTD) and highlight the reactivation of lateral inflows from the surrounding raised peatland complex. Vertical EC profiles permit to identify 3 main peat compartments depending on different water supplies arguing for a nested hydrological functioning. This involves: (i) One‐off karst groundwater inputs at the substratum/peat interface supplying the deepest peat layer, (ii) lateral seepage inputs from the neighboring raised wooded peatlands sustaining the intermediate peat level, and (iii) direct rainfall infiltrating the most superficial peat layer. This nested multi‐reservoir model operates at various spatio‐temporal scales and is consistent with the complex seasonal hydrological and physico‐chemical response at the bog outlet, which will be increasingly affected by climate change in the coming decades.
Peatlands are a highly effective natural carbon sink. However, the future of the carbon stored in these ecosystems is still uncertain because of the pressure they undergo. An estimation of the peatland carbon balance shows whether the system functions as a carbon sink or a carbon source. La Guette peatland is a temperate Sphagnum-dominated peatland invaded by vascular plants. The studied site was hydrologically disturbed for years by a road crossing its southern part and draining water out of the system. Our aim was to estimate the main carbon fluxes and to calculate the carbon balance at the ecosystem scale. To reach this goal, CO2 and CH4 fluxes, DOC content as well as environmental variables were measured monthly for 2 years on 20 plots distributed across the site to take into account spatial variability. The peatland carbon balance was estimated using empirical models. Results showed that the CO2 fluxes were above 1000 gC m−2 yr−1. In 2013 and 2014 the peatland was a net C source to the atmosphere with an emission of 220 ± 33 gC m−2 yr−1. These results provide evidence that restoration should be performed in order to reduce the water losses and favour the Sphagnum-dominance of this peatland.
In this “Grand Challenges” paper, we review how the carbon isotopic composition of atmospheric CO2 has changed since the Industrial Revolution due to human activities and their influence on the natural carbon cycle, and we provide new estimates of possible future changes for a range of scenarios. Emissions of CO2 from fossil fuel combustion and land use change reduce the ratio of ¹³C/¹²C in atmospheric CO2 (δ¹³CO2). This is because ¹²C is preferentially assimilated during photosynthesis and δ¹³C in plant‐derived carbon in terrestrial ecosystems and fossil fuels is lower than atmospheric δ¹³CO2. Emissions of CO2 from fossil fuel combustion also reduce the ratio of ¹⁴C/C in atmospheric CO2 (Δ¹⁴CO2) because ¹⁴C is absent in million‐year‐old fossil fuels, which have been stored for much longer than the radioactive decay time of ¹⁴C. Atmospheric Δ¹⁴CO2 rapidly increased in the 1950s to 1960s because of ¹⁴C produced during nuclear bomb testing. The resulting trends in δ¹³C and Δ¹⁴C in atmospheric CO2 are influenced not only by these human emissions but also by natural carbon exchanges that mix carbon between the atmosphere and ocean and terrestrial ecosystems. This mixing caused Δ¹⁴CO2 to return toward preindustrial levels in the first few decades after the spike from nuclear testing. More recently, as the bomb ¹⁴C excess is now mostly well mixed with the decadally overturning carbon reservoirs, fossil fuel emissions have become the main factor driving further decreases in atmospheric Δ¹⁴CO2. For δ¹³CO2, in addition to exchanges between reservoirs, the extent to which ¹²C is preferentially assimilated during photosynthesis appears to have increased, slowing down the recent δ¹³CO2 trend slightly. A new compilation of ice core and flask δ¹³CO2 observations indicates that the decline in δ¹³CO2 since the preindustrial period is less than some prior estimates, which may have incorporated artifacts owing to offsets from different laboratories' measurements. Atmospheric observations of δ¹³CO2 have been used to investigate carbon fluxes and the functioning of plants, and they are used for comparison with δ¹³C in other materials such as tree rings. Atmospheric observations of Δ¹⁴CO2 have been used to quantify the rate of air‐sea gas exchange and ocean circulation, and the rate of net primary production and the turnover time of carbon in plant material and soils. Atmospheric observations of Δ¹⁴CO2 are also used for comparison with Δ¹⁴C in other materials in many fields such as archaeology, forensics, and physiology. Another major application is the assessment of regional emissions of CO2 from fossil fuel combustion using Δ¹⁴CO2 observations and models. In the future, δ¹³CO2 and Δ¹⁴CO2 will continue to change. The sign and magnitude of the changes are mainly determined by global fossil fuel emissions. We present here simulations of future δ¹³CO2 and Δ¹⁴CO2 for six scenarios based on the shared socioeconomic pathways (SSPs) from the 6th Coupled Model Intercomparison Project (CMIP6). Applications using atmospheric δ¹³CO2 and Δ¹⁴CO2 observations in carbon cycle science and many other fields will be affected by these future changes. We recommend an increased effort toward making coordinated measurements of δ¹³C and Δ¹⁴C across the Earth System and for further development of isotopic modeling and model‐data analysis tools.
The carbon balance of peatlands is predicted to shift from a sink to a source this century. However, peatland ecosystems are still omitted from the main Earth system models that are used for future climate change projections, and they are not considered in integrated assessment models that are used in impact and mitigation studies. By using evidence synthesized from the literature and an expert elicitation, we define and quantify the leading drivers of change that have impacted peatland carbon stocks during the Holocene and predict their effect during this century and in the far future. We also identify uncertainties and knowledge gaps in the scientific community and provide insight towards better integration of peatlands into modelling frameworks. Given the importance of the contribution by peatlands to the global carbon cycle, this study shows that peatland science is a critical research area and that we still have a long way to go to fully understand the peatland–carbon–climate nexus.
Globally, peatlands play an important role in the carbon (C) cycle. High water level is a key factor in maintaining C storage in peatlands, but water levels are vulnerable to climate change and anthropogenic disturbance. This review examines literature related to the effects of water level alteration on C cycling in peatlands to summarize new ideas and uncertainties emerging in this field. Peatland ecosystems maintain their function by altering plant community structure to adapt to changing water levels. Regarding primary production, woody plants are more productive in unflooded, well-aerated conditions, while Sphagnum mosses are more productive in wetter conditions. The responses of sedges to water level alteration are species-specific. While peat decomposition is faster in unflooded, well aerated conditions, increased plant production may counteract the C loss induced by increased ecosystem respiration (ER) for a period of time. In contrast, rising water table maintains anaerobic conditions and enhances the role of the peatland as a C sink. Nevertheless, changes in DOC flux during water level fluctuation are complicated and depend on the interactions of flooding with environment. Notably, vegetation also plays a role in C flux but particular species vary in their ability to sequester and transport C. Bog ecosystems have a greater resilience to water level alteration than fens, due to differences in biogeochemical responses to hydrology. The full understanding of the role of peatlands in global C cycling deserves much more study due to uncertainties of vegetation feedbacks, peat–water interactions, microbial mediation of vegetation, wildfire, and functional responses after hydrologic restoration.
Peatlands store ∼ 20 %–30 % of the global soil organic carbon stock and are an important source of dissolved organic carbon (DOC) for inland waters. Recent improvements for in situ optical monitoring revealed that the DOC concentration in streams draining peatlands is highly variable, showing seasonal variation and short and intense DOC concentration peaks. This study aimed to statistically determine the variables driving stream DOC concentration variations at seasonal and event scales. Two mountainous peatlands (one fen and one bog) were monitored in the French Pyrenees to capture their outlet DOC concentration variability at a high-frequency rate (30 min). Abiotic variables including precipitation, stream temperature and water level, water table depth, and peat water temperature were also monitored at high frequency and used as potential predictors to explain DOC concentration variability. Results show that at both sites DOC concentration time series can be decomposed into a seasonal baseline interrupted by many short and intense peaks of higher concentrations. The DOC concentration baseline is driven, at the seasonal scale, by peat water temperature. At the event scale, DOC concentration increases are mostly driven by a rise in the water table within the peat at both sites. Univariate linear models between DOC concentration and peat water temperature or water table increases show greater efficiency at the fen site. Water recession times were derived from water level time series using master recession curve coefficients. They vary greatly between the two sites but also within one peatland site. They partly explain the differences between DOC dynamics in the studied peatlands, including peat porewater DOC concentrations and the links between stream DOC concentration and water table rise within the peatlands. This highlights that peatland complexes are composed of a mosaic of heterogeneous peat units distinctively producing or transferring DOC to streams.
Karst regions offer a variety of natural resources such as freshwater and biodiversity, and many cultural resources. The World Karst Aquifer Map (WOKAM) is the first detailed and complete global geodatabase concerning the distribution of karstifiable rocks (carbonates and evaporites) representing potential karst aquifers. This study presents a statistical evaluation of WOKAM, focusing entirely on karst in carbonate rocks and addressing four main aspects: (1) global occurrence and geographic distribution of karst; (2) karst in various topographic settings and coastal areas; (3) karst in different climatic zones; and (4) populations living on karst. According to the analysis, 15.2% of the global ice-free continental surface is characterized by the presence of karstifiable carbonate rock. The largest percentage is in Europe (21.8%); the largest absolute area occurs in Asia (8.35 million km2). Globally, 31.1% of all surface exposures of carbonate rocks occur in plains, 28.1% in hills and 40.8% in mountains, and 151,400 km or 15.7% of marine coastlines are characterized by carbonate rocks. About 34.2% of all carbonate rocks occur in arid climates, followed by 28.2% in cold and 15.9% in temperate climates, whereas only 13.1 and 8.6% occur in tropical and polar climates, respectively. Globally, 1.18 billion people (16.5% of the global population) live on karst. The highest absolute number occurs in Asia (661.7 million), whereas the highest percentages are in Europe (25.3%) and North America (23.5%). These results demonstrate the global importance of karst and serve as a basis for further research and international water management strategies.
A 4 m core was extracted from the center of a peatland located in the Drugeon valley (France). Thirteen radiocarbon dates were used to build a robust age model. Testate amoebae were used for reconstructing mire surface wetness. High-resolution pollen analysis of the sequence reconstructed 9 millennia of development of the peatland and its surrounding vegetation. During the early/middle Holocene (9500 to 5800 cal BP), warm conditions led to high evapotranspiration and low water levels. The vegetation history is characterized by the development of a Pinus and a mixed Quercus forest. From 5800 cal BP, testate amoebae show wetter conditions, indicating the onset of the cooler Neoglacial period. The cooling is also evidenced by the development of Abies and Fagus trees, replacing the oak forest. The first indicators of human impact appear at about 4800 cal BP, and indicators of farming activity remains very rare until ca. 2600 cal BP, at the beginning of the Iron Age. The development of the peatland responded to climatic fluctuation until 2600 cal BP, after which human impact became the main driver. The last millennium has been characterized by sudden drying and the spread of pine on the peatland.
Mountains contain many small and fragmented peatlands within watersheds. As they are difficult to monitor, their role in the water and carbon cycle is often disregarded. This study aims to assess the stream organic carbon exports from a montane peatland and characterizes its contribution to the water chemistry in a headstream watershed. High frequency in situ monitoring of turbidity and fDOM were used to quantify respectively particulate organic carbon (POC) and dissolved organic carbon (DOC) exports at the inlet and outlet of a peatland over three years in a French Pyrenean watershed (1,343 m.a.s.l.). The DOC and POC signals are both highly dynamic, characterized by numerous short peaks lasting from a few hours to a few days. Forty‐six percent of the exports occurred during 9% of the time corresponding to the highest flows monitored at the outlet. Despite its small area (3%) within the watershed, the peatland contributes at least 63% of the DOC export at the outlet. The specific DOC flux ranges from 16.1 ± 0.4 to 34.6 ± 1.5 g m² year⁻¹. POC contributes 17% of the total stream organic carbon exports from the watershed. As the frequency of extreme climatic events is expected to increase in the context of climate change, further studies should be conducted to understand the evolution of underestimated mountainous peatland carbon fluxes and their implication in the carbon cycle of headwaters.
Peatlands store about 20 % of the global soil organic carbon stock and are an important source of dissolved organic carbon (DOC) for inland waters. Recent improvements for in situ optical monitoring revealed that the DOC concentration in streams draining peatlands is highly variable, showing seasonal variation and short and intense DOC concentration peak periods. This study aimed to determine the variables driving stream DOC concentration variations at seasonal and event scales. Two mountainous peatlands (one fen and one bog) were monitored in the French Pyrenees to capture their outlet DOC concentration variability at a high frequency rate (30 min). Abiotic variables including precipitation, stream temperature and water level, water table depth and peat water temperature were also monitored at high frequency and used as potential predictors to explain DOC concentration variability. Results show that at both sites, DOC concentration time series can be decomposed into a seasonal baseline interrupted by many short and intense peaks of higher concentrations. The DOC concentration baseline is driven, at the seasonal scale, by peat water temperature. At the event scale, DOC concentration increases are mostly driven by water table increases within the peat at both sites. Univariate linear models between DOC concentration and peat water temperature or water table increases show greater efficiency at the fen site. Water recession times were derived from water level time series using master recession curve coefficients. They vary greatly between bog and fen but also within one peatland site. They partly explain the differences between DOC dynamics in the studied peatlands, including porewater DOC concentrations and the links between stream DOC concentration and water table rise. This highlights that peatland complexes are composed of a mosaic of heterogeneous peat units distinctively producing or transferring DOC to streams.
Dissolved organic carbon (DOC) originating in peatlands can be mineralized to carbon dioxide (CO2) and methane (CH4), two potent greenhouse gases. Knowledge of the dynamics of DOC export via runoff is needed for a more robust quantification of C cycling in peatland ecosystems, a prerequisite for realistic predictions of future climate change. We studied dispersion pathways of DOC in a mountain‐top peat bog in the Czech Republic (Central Europe), using a dual isotope approach. While δ13CDOC values made it possible to link exported DOC with its within‐bog source, δ18OH2O values of precipitation and runoff helped to understand runoff generation. Our two‐year DOC‐H2O isotope monitoring was complemented by a laboratory peat incubation study generating an experimental time‐series of δ13CDOC values. DOC concentrations in runoff during high‐flow periods were 20‐30 mg L‐1. The top 2 cm of the peat profile, composed of decaying green moss, contained isotopically lighter C than deeper peat, and this isotopically light C was present in runoff in high‐flow periods. In contrast, baseflow contained only 2‐10 mg DOC L‐1, and its more variable C isotope composition intermittently fingerprinted deeper peat. DOC in runoff occasionally contained isotopically extremely light C whose source in solid peat substrate was not identified. Pre‐event water made up on average 60 % of the water runoff flux, while direct precipitation contributed 40 %. Runoff response to precipitation was relatively fast. A highly leached horizon was identified in shallow catotelm. This peat layer was likely affected by a lateral influx of precipitation. Within 36 days of laboratory incubation, isotopically heavy DOC that had been initially released from the peat was replaced by isotopically lighter DOC, whose δ13C values converged to the solid substrate and natural runoff. We suggest that δ13C systematics can be useful in identification of vertically stratified within‐bog DOC sources for peatland runoff.
In natural coastal wetlands, high supplies of marine sulfate suppress methanogenesis. Coastal wetlands are, however, often subject to disturbance by diking and drainage for agricultural use and can turn to potent methane sources when rewetted for remediation. This suggests that preceding land use measures can suspend the sulfate-related methane suppressing mechanisms. Here, we unravel the hydrological relocation and biogeochemical S and C transformation processes that induced high methane emissions in a disturbed and rewetted peatland despite former brackish impact. The underlying processes were investigated along a transect of increasing distance to the coastline using a combination of concentration patterns, stable isotope partitioning, and analysis of the microbial community structure. We found that diking and freshwater rewetting caused a distinct freshening and an efficient depletion of the brackish sulfate reservoir by dissimilatory sulfate reduction (DSR). Despite some legacy effects of brackish impact expressed as high amounts of sedimentary S and elevated electrical conductivities, contemporary metabolic processes operated mainly under sulfate-limited conditions. This opened up favorable conditions for the establishment of a prospering methanogenic community in the top 30–40 cm of peat, the structure and physiology of which resemble those of terrestrial organic-rich environments. Locally, high amounts of sulfate persisted in deeper peat layers through the inhibition of DSR, probably by competitive electron acceptors of terrestrial origin, for example Fe(III). However, as sulfate occurred only in peat layers below 30–40 cm, it did not interfere with high methane emissions on an ecosystem scale. Our results indicate that the climate effect of disturbed and remediated coastal wetlands cannot simply be derived by analogy with their natural counterparts. From a greenhouse gas perspective, the re-exposure of diked wetlands to natural coastal dynamics would literally open up the floodgates for a replenishment of the marine sulfate pool and therefore constitute an efficient measure to reduce methane emissions.
Core Ideas
OZCAR is a network of sites studying the critical zone.
OZCAR covers various disciplines.
OZCAR will help disciplines to work together for a better representation and modeling of the critical zone.
The French critical zone initiative, called OZCAR (Observatoires de la Zone Critique–Application et Recherche or Critical Zone Observatories–Application and Research) is a National Research Infrastructure (RI). OZCAR‐RI is a network of instrumented sites, bringing together 21 pre‐existing research observatories monitoring different compartments of the zone situated between “the rock and the sky,” the Earth's skin or critical zone (CZ), over the long term. These observatories are regionally based and have specific initial scientific questions, monitoring strategies, databases, and modeling activities. The diversity of OZCAR‐RI observatories and sites is well representative of the heterogeneity of the CZ and of the scientific communities studying it. Despite this diversity, all OZCAR‐RI sites share a main overarching mandate, which is to monitor, understand, and predict (“earthcast”) the fluxes of water and matter of the Earth's near surface and how they will change in response to the “new climatic regime.” The vision for OZCAR strategic development aims at designing an open infrastructure, building a national CZ community able to share a systemic representation of the CZ, and educating a new generation of scientists more apt to tackle the wicked problem of the Anthropocene. OZCAR articulates around: (i) a set of common scientific questions and cross‐cutting scientific activities using the wealth of OZCAR‐RI observatories, (ii) an ambitious instrumental development program, and (iii) a better interaction between data and models to integrate the different time and spatial scales. Internationally, OZCAR‐RI aims at strengthening the CZ community by providing a model of organization for pre‐existing observatories and by offering CZ instrumented sites. OZCAR is one of two French mirrors of the European Strategy Forum on Research Infrastructure (eLTER‐ESFRI) project.
Hydrological disturbances could increase dissolved organic carbon (DOC) exports through changes in runoff and leaching, which reduces the potential carbon sink function of peatlands. The objective of this study was to assess the impact of hydrological restoration on hydrological processes and DOC dynamics in a rehabilitated Sphagnum-dominated peatland. A conceptual hydrological model calibrated on the water table and coupled with a biogeochemical module was applied to La Guette peatland (France), which experienced a rewetting initiative on February 2014. The model (eight calibrated parameters) reproduced water-table (0.1
Considerable CO2 evasion to the atmosphere occurs as DIC is transported from soils to streams. While this physical process has been the focus of multiple studies, less is known about the underlying biogeochemical transformations that accompany this transfer of C from soils to streams. Here, we used patterns in streamwater and groundwater δ13C-DIC values within three headwater catchments with contrasting land-cover to identify the sources and processes regulating DIC during its transport. We found that although considerable CO2 evasion occurs as DIC is transported from soils to streams, there were also other processes affecting the DIC pool. Methane production and mixing of C sources, associated with different types and spatial distribution of peat-rich areas within each catchment, had a considerable influence on the δ13C-DIC values in both soils and streams. These processes represent an additional control on δ13C-DIC values and the catchment-scale cycling of DIC across different northern landscape types. The results from this study demonstrate that the transport of DIC from soils to streams results in more than just rapid CO2 evasion to the atmosphere, but also represents a channel of C transformation, which questions some of our current conceptualizations of C cycling at the landscape scale.
It is well established that stream dissolved inorganic carbon (DIC) fluxes play a central role in the global C cycle, yet the sources of stream DIC remain to a large extent unresolved. Here, we explore large-scale patterns in δ¹³C-DIC from streams across Sweden to separate and further quantify the sources and sinks of stream DIC. We found that stream DIC is governed by a variety of sources and sinks including biogenic and geogenic sources, CO2 evasion, as well as in-stream processes. Although soil respiration was the main source of DIC across all streams, a geogenic DIC influence was identified in the northernmost region. All streams were affected by various degrees of atmospheric CO2 evasion, but residual variance in δ¹³C-DIC also indicated a significant influence of in-stream metabolism and anaerobic processes. Due to those multiple sources and sinks, we emphasize that simply quantifying aquatic DIC fluxes will not be sufficient to characterise their role in the global C cycle.
The degradation of organic matter to CH4 and CO2 was investigated in three different boreal peatland systems in Finland, a mesotrophic fen (MES), an oligotrophic fen (OLI), and an ombrotrophic peat (OMB). MES had similar production rates of CO2 and CH4, but the two nutrient-poor peatlands (OLI and OMB) produced in general more CO2 than CH4. δ 13C analysis of CH4 and CO2 in the presence and absence methyl fluoride (CH3F), an inhibitor of acetoclastic methanogenesis, showed that CH4 was predominantly produced by hydrogenotrophic methanogenesis and that acetoclastic methanogenesis only played an important role in MES. These results, together with our observations concerning the collective inhibition of CH4 and CO2 production rates by CH3F, indicate that organic matter was degraded through different paths in the mesotrophic and the nutrient-poor peatlands. In the mesotrophic fen, the major process is canonical fermentation followed by acetoclastic and hydrogenotrophic methanogenesis, while in the nutrient-poor peat, organic matter was apparently degraded to a large extent by a different path which finally involved hydrogenotrophic methanogenesis. Our data suggest that degradation of organic substances in the oligotrophic environments was incomplete and involved the use of organic compounds as oxidants.
Quantifying rates of microbial carbon transformation in peatlands is essential for gaining mechanistic understanding of the factors that influence methane emissions from these systems, and for predicting how emissions will respond to climate change and other disturbances. In this study, we used porewater stable isotopes collected from both the edge and center of a thermokarst bog in Interior Alaska to estimate in situ microbial reaction rates. We expected that near the edge of the thaw feature, actively thawing permafrost and greater abundance of sedges would increase carbon, oxygen and nutrient availability, enabling faster microbial rates relative to the center of the thaw feature. We developed three different conceptual reaction networks that explained the temporal change in porewater CO2, CH4, δ
13C–CO2 and δ
13C–CH4. All three reaction-network models included methane production, methane oxidation and CO2 production, and two of the models included homoacetogenesis—a reaction not previously included in isotope-based porewater models. All three models fit the data equally well, but rates resulting from the models differed. Most notably, inclusion of homoacetogenesis altered the modeled pathways of methane production when the reaction was directly coupled to methanogenesis, and it decreased gross methane production rates by up to a factor of five when it remained decoupled from methanogenesis. The ability of all three conceptual reaction networks to successfully match the measured data indicate that this technique for estimating in situ reaction rates requires other data and information from the site to confirm the considered set of microbial reactions. Despite these differences, all models indicated that, as expected, rates were greater at the edge than in the center of the thaw bog, that rates at the edge increased more during the growing season than did rates in the center, and that the ratio of acetoclastic to hydrogenotrophic methanogenesis was greater at the edge than in the center. In both locations, modeled rates (excluding methane oxidation) increased with depth. A puzzling outcome from the effort was that none of the models could fit the porewater dataset without generating “fugitive” carbon (i.e., methane or acetate generated by the models but not detected at the field site), indicating that either our conceptualization of the reactions occurring at the site remains incomplete or our site measurements are missing important carbon transformations and/or carbon fluxes. This model–data discrepancy will motivate and inform future research efforts focused on improving our understanding of carbon cycling in permafrost wetlands.
River systems connect the terrestrial biosphere, the atmosphere
and the ocean in the global carbon cycle1. A recent estimate suggests
that up to 3 petagrams of carbon per year could be emitted as
carbon dioxide (CO2) from global inland waters, offsetting the carbon
uptake by terrestrial ecosystems2. It is generally assumed that
inland waters emit carbon that has been previously fixed upstream
by land plant photosynthesis, then transferred to soils, and subsequently
transported downstreamin run-off. But at the scale of entire
drainage basins, the lateral carbon fluxes carried by small rivers
upstreamdo not account for all of the CO2 emitted frominundated
areas downstream3,4. Three-quarters of the world’s flooded land consists
of temporary wetlands5, but the contribution of these productive
ecosystems6 to the inland water carbon budget has been largely
overlooked. Here we show that wetlands pump large amounts of
atmospheric CO2 into river waters in the floodplains of the central
Amazon. Flooded forests and floating vegetation export large amounts
of carbon to river waters and the dissolved CO2 can be transported
dozens to hundreds of kilometres downstream before being emitted.
Weestimate thatAmazonian wetlands export half of their gross
primary production to river waters as dissolved CO2 and organic
carbon, compared with only a few per cent of gross primary production
exported in upland (not flooded) ecosystems1,7. Moreover,
we suggest that wetland carbon export is potentially large enough to
account for at least the 0.21 petagrams of carbon emitted per year as
CO2 from the central Amazon River and its floodplains8. Global
carbon budgets should explicitly address temporary or vegetated
flooded areas, because these ecosystems combine high aerial primary
production with large, fast carbon export, potentially supporting
a substantial fraction of CO2 evasion from inland waters.
Mineralization of groundwater in volcanic aquifers is partly acquired through silicates weathering. This alteration depends on the dissolution of atmospheric, biogenic, or mantellic gaseous CO2 whose contributions may depend on substratum geology, surface features, and lava flow hydrological functionings. Investigations of
and δ13CTDIC (total dissolved inorganic carbon) on various spatiotemporal scales in the unsaturated and saturated zones of volcanic flows of the Argnat basin (French Massif Central) have been carried out to identify the carbon sources in the system. Mantellic sources are related to faults promoting CO2 uplift from the mantle to the saturated zone. The contribution of this source is counterbalanced by infiltration of water through the unsaturated zone, accompanied by dissolution of soil CO2 or even atmospheric CO2 during cold periods. Monitoring and modeling of δ13CTDIC in the unsaturated zone shows that both
and δ13CTDIC are controlled by air temperature which influences soil respiration and soil-atmosphere CO2 exchanges. The internal geometry of volcanic lava flows controls water patterns from the unsaturated zone to saturated zone and thus may explain δ13C heterogeneity in the saturated zone at the basin scale.
The global distribution of C3 and C4 plants is required for accurately simulating exchanges of CO2, water, and energy between the land surface and atmosphere. It is also important to know the C3/C4 distribution for simulations of the carbon isotope composition of atmospheric CO2 owing to the distinct fractionations displayed by each photosynthetic type. Large areas of the land surface are spatial and temporal mosaics of both photosynthetic types. We developed an approach for capturing this heterogeneity by combining remote sensing products, physiological modeling, a spatial distribution of global crop fractions, and national harvest area data for major crop types. Our C3/C4 distribution predicts the global coverage of C4 vegetation to be 18.8 million km2, while C3 vegetation covers 87.4 million km2. We incorporated our distribution into the SiB2 model and simulated carbon fluxes for each photosynthetic type. The gross primary production (GPP) of C4 plants is 35.3 Pg C yr-1, or ~23% of total GPP, while that of C3 plants is 114.7 Pg C yr-1. The assimilation-weighted terrestrial discrimination against 13CO2 is -16.50/00. If the terrestrial component of the carbon sink is proportional to GPP, this implies a net uptake of 2.4 Pg C yr-1 on land and 1.4 Pg C yr-1 in the ocean using a 13C budgeting approach and average carbon cycle parameter values for the 1990s. We also simulated the biomass of each photosynthetic type using the CASA model. The simulated biomass values of C3 and C4 vegetation are 389.3 and 18.6 Pg C, respectively.
Time tracers (NO3−, TOC, δ13CTDIC, Mg2+) have been used to define the hydrodynamic behavior of a karst system: high values in NO3− and TOC reflect rapid infiltration and consequently a short residence time within the aquifer, whereas enriched δ13CTDIC and high Mg2+ are expected for “old water”. 9 Springs and 5 boreholes have been sampled during three field campaigns in the Doubs valley karst aquifer: low water, flood and recession periods. A clear differentiation can be highlighted between boreholes, characterized by a long residence time, and springs that show a rapid infiltration. Considering only the springs values, it appears that TOC and δ13CTDIC contents can easily be correlated to the sampling period. We show then the contribution of the unsaturated zone to the discharge during the low-water period, and the existence of reserves that seem badly connected to the drainage network, and that contribute poorly to the minimal flow.
The chemical components in water, and changes thereof, can control the ecology of peatlands and impact the accumulation of peat; while at the same time ecological characteristics, hydrology and interactions with the atmosphere and geosphere control peatland water chemistry. This review summarizes results from Canadian and northern European research published over the past two decades for major and minor, inorganic and organic constituents dissolved in surface or pore waters associated with bogs and fens. First those studies that describe natural peatlands are discussed, followed by those that involve peatlands that are disturbed by various direct or indirect anthropogenic influences. Finally several recommendations are made regarding future studies and how they can assist in modelling efforts that can inform peatland management and further scientific inquiry.
At the Earth's surface, a complex suite of chemical, biological, and physical processes combines to create the engine that transforms bedrock into soil (Figure 1). Earth's weathering engine provides nutrients to nourish ecosystems and human society mediates the transport of toxic components within the biosphere, creates water flow paths that carve and weaken bedrock, and contributes to the evolution of landscapes at all temporal and spatial scales. At the longest time scales, the weathering engine sequesters CO 2 , thereby influencing long‐term climate change.
Despite the importance of soil, our knowledge of the rate of soil formation is limited because the weathering zone forms a complex, ever‐changing interface, and because scientific approaches and funding paradigms have not promoted integrated research agendas to investigate such complex interactions. No national initiative has promoted a systems approach to investigation of weathering science across the broad array of geology, soil science, ecology and hydrology. Such a program is certainly needed, and this article describes a platform on which to build the initiative to answer the following question: How does the Earth weathering engine break down rock to nourish ecosystems, carve errestrial landscapes, and control carbon dioxide in the global atmosphere?
Studies of Earth’s critical zone have largely focused on areas underlain by silicate bedrock, leaving gaps in our understanding of widespread and vital carbonate-dominated landscapes.
Microbial methanogenesis is a major source of the greenhouse gas methane (CH4). It is the final step in the anaerobic degradation of organic matter when inorganic electron acceptors such as nitrate, ferric iron, or sulfate have been depleted. Knowledge of this degradation pathway is important for the creation of mechanistic models, prediction of future CH4 emission scenarios, and development of mitigation strategies. In most anoxic environments, CH4 is produced from either acetate (aceticlastic methanogenesis) or hydrogen (H2) plus carbon dioxide (CO2) (hydrogenotrophic methanogenesis). Hydrogen can be replaced by other CO2-type methanogenesis, using formate, carbon monoxide (CO), or alcohols as substrates. The ratio of these two pathways is tightly constrained by the stoichiometry of conversion processes. If the degradation of organic matter is complete (e.g., degradation of straw in rice paddies), then fermentation eventually results in production of acetate and H2 at a ratio of > 67% aceticlastic and < 33% hydrogenotrophic methanogensis. However, acetate production can be favored when heterotrophic or chemolithotrophic acetogenesis is enhanced, and H2 production can be favored when syntrophic acetate oxidation is enhanced. This typically occurs at low and elevated temperatures, respectively. Thus, temperature can strongly influence the methanogenic pathway, which may range from 100% aceticlastic methanogenesis at low temperatures to 100% hydrogenotrophic methanogenesis at high temperatures. However, if the degradation of organic matter is not complete (e.g., degradation of soil organic matter), the stoichiometry of fermentation is not tightly constrained, resulting, for example, in the preferential production of H2, followed by hydrogenotrophic methanogenesis. Preferential production of CH4 by either aceticlastic or hydrogenotrophic methanogenesis can also happen if one of the methanogenic substrates is not consumed by methanogens but is, instead, accumulated, volatilized, or utilized otherwise. Methylotrophic methanogens, which can use methanol as a substrate, are widespread, but it is unlikely that methanol is produced in similar quantities as acetate, CO2, and H2. Methylotrophic methanogenesis is important in saline environments, where compatible solutes are degraded to methyl compounds (trimethyl amine and dimethyl sulfide) and then serve as non-competitive substrates, while acetate and hydrogen are degraded by non-methanogenic processes, e.g., sulfate reduction.
Peatlands are habitats for a range of fragile flora and fauna species. Their eco-physicochemical characteristics make them as outstanding global carbon and water storage systems. These ecosystems occupy 3% of the worldwide emerged land surface but represent 30% of the global organic soil carbon and 10% of the global fresh water volumes. In such systems, carbon speciation depends to a large extent on specific redox conditions which are mainly governed by the depth of the water table. Hence, understanding their hydrological variability, that conditions both their ecological and biogeochemical functions, is crucial for their management, especially when anticipating their future evolution under climate change. This study illustrates how long-term monitoring of basic hydro-meteorological parameters combined with statistical modeling can be used as a tool to evaluate i) the horizontal (type of peat), ii) vertical (acrotelm/catotelm continuum) and iii) future hydrological variability. Using cross-correlations between meteorological data (precipitation, potential evapotranspiration) and water table depth (WTD), we primarily highlight the spatial heterogeneity of hydrological reactivity across the Sphagnum-dominated Frasne peatland (French Jura Mountain). Then, a multiple linear regression model allows performing hydrological projections until 2100, according to regionalized IPCC RCP4.5 and 8.5 scenarios. Although WTD remains stable during the first half of 21th century, seasonal trends beyond 2050 show lower WTD in winter and markedly greater WTD in summer. In particular, after 2050, more frequent droughts in summer and autumn should occur, increasing WTD. These projections are completed with risk evaluations for peatland droughts until 2100 that appear to be increasing especially for transition seasons, i.e. May-June and September-October. Comparing these trends with previous evaluations of phenol concentrations in water throughout the vegetative period, considered as a proxy of plant functioning intensity, highlights that these hydrological modifications during transitional seasons could be a great ecological perturbation, especially by affecting Sphagnum metabolism.
Carbon dioxide (CO2) emissions from rivers are a vital part of the global carbon budget. However, data from subtropical areas, especially in the headwater streams, are scare. Spatiotemporal dynamics and drives of partial pressure of CO2 (pCO2) and water-air CO2 fluxes (FCO2) in a monsoonal headwater stream Jinshui River of the Yangtze were unraveled. Our findings suggested that natural process (i.e., hydrology, CO2 outgassing and lithology) and human activities were recognized as significant players in regulating the variability of riverine pCO2. The mean pCO2 was significantly higher in the dry season (1562 ± 975 µatm) than the wet season (834 ± 639 µatm), seasonal trends of pCO2 were controlled by in situ biogenic activities and rainfall events. The slight fluctuations of pCO2 from upstream to downstream along main stem implied the mixed influences of distinct water environments and anthropogenic disturbance. Correlation analysis showed that environmental factors, i.e., temperature, pH, TN and DOC were relevant to pronounced spatial and seasonal variability of pCO2. We highlighted that high water-air CO2 flux was estimated at 343 ± 413 mmol/m2/d (dry: 542 ± 477 mmol/m2/d vs wet: 192 ± 278 mmol/m2/d) in the monsoonal headwater stream, and the watershed-scale carbon budget demonstrated carbon loss via atmospheric exchange was 1.2 times the riverine dissolved carbon export. Our results would fill a large gap in the headwater stream of the Yangtze, and help to accurately estimate the regional to global CO2 outgassing from rivers to the atmosphere.
Carbonate terrains (CT) underlie one-fifth of terrestrial, ice-free land and are an important supply of potable water to the world's population, and yet processes endemic to CT critical zones (CZ) and responses of these processes to climatic and anthropogenic pressures are not well understood. Given the rapid dissolution rates and ability to generate well-developed networks of secondary porosity these landscapes can be highly sensitive to impacts from climate change (e.g., modifications of temperature, precipitation, sea level) and human disturbance (e.g., water withdrawal/diversions, changes in land use/land cover). This special issue includes 16 papers focused on CT-CZ processes and potential responses to climatic and human perturbations. Five major themes emerge from these papers, namely: (1) anthropogenic climate and land use changes alter CT-CZ weathering rate and diagenesis, (2) metal and carbon fluxes in CT-CZ will respond to increasing hydrologic variance caused by climate change, (3) endogenous and exogenous processes operating over short time periods (<10,000 yrs) form landscape patterns in carbonate terrains, (4) rates of carbonate mineral dissolution depend on vadose zone and soil thickness, and (5) open systems may not always promote greater carbonate weathering rates in CT-CZ. These findings reflect the importance of carbonate minerals in Earth's CZ, both in terms of processes unique to carbonate minerals, as well as a predictor of future responses to anthropogenic and environmental changes.
Streams and rivers emit significant amounts of CO2 and constitute a preferential pathway of carbon transport from terrestrial ecosystems to the atmosphere. However, the estimation of CO2 degassing based on the water-air CO2 gradient, gas transfer velocity and stream surface area is subject to large uncertainties. Furthermore, the stable isotope signature of dissolved inorganic carbon (δ13C-DIC) in streams is strongly impacted by gas exchange, which makes it a useful tracer of CO2 degassing under specific conditions. For this study, we characterized the annual transfers of dissolved inorganic carbon (DIC) along the groundwater-stream-river continuum based on dissolved inorganic carbon (DIC) concentrations, stable isotope composition and measurements of stream discharges. We selected a homogeneous, forested and sandy lowland watershed (Leyre River) as a study site, where the hydrology occurs almost exclusively through drainage of shallow groundwater (no surface runoff). We observed the first general spatial pattern of decreases in pCO2 and DIC and an increase in δ13C-DIC from groundwater to stream orders 1 and 2, which was due to the experimentally verified faster degassing of groundwater 12C-DIC compared to 13C-DIC. This downstream enrichment in 13C-DIC could be modelled by simply considering the isotopic equilibration of groundwater-derived DIC with the atmosphere during CO2 degassing. A second spatial pattern occurred between stream orders 2 and 4, consisting of an increase in the proportion of carbonate alkalinity to the DIC accompanied by the enrichment of 13C in the stream DIC, which was due to the occurrence of carbonate rock weathering downstream. We could separate the contribution of these two processes (gas exchange and carbonate weathering) in the stable isotope budget of the river network. Thereafter, we built a hydrological mass balance based on drainages and the relative contribution of groundwater in streams of increasing order. After combining with the dissolved CO2 concentrations, we quantified CO2 degassing for each stream order for the whole watershed. Approximately 75% of the total CO2 degassing from the watershed occurred in first- and second-order streams. Furthermore, from stream order 2 to 4, our CO2 degassing fluxes compared well with those based on stream hydraulic geometry, water pCO2, gas transfer velocity, and stream surface area. In first-order streams, however, our approach showed CO2 fluxes that were twice as large, suggesting that a fraction of degassing occurred as hotspots in the vicinity of groundwater resurgence and was missed by conventional stream sampling.
Peatlands play important ecological, economic and cultural roles in human well-being. Although considered sensitive to climate change and anthropogenic pressures, the spatial extent of peatlands is poorly constrained. We report the development of an improved global peatland map, PEATMAP, based on a meta-analysis of geospatial information collated from a variety of sources at global, regional and national levels. We estimate total global peatland area to be 4.23 million km 2 , approximately 2.84% of the world land area. Our results suggest that previous global peatland inventories are likely to underestimate peat extent in the tropics, and to overestimate it in parts of mid-and high-latitudes of the Northern Hemisphere. Global wetland and soil datasets are poorly suited to estimating peatland distribution. For instance, tropical peatland extents are overestimated by Global Lakes and Wetlands Database – Level 3 (GLWD-3) due to the lack of ground-truthing data; and underestimated by the use of histosols to represent peatlands in the Harmonized World Soil Database (HWSD) v1.2, as large areas of swamp forest peat in the humid tropics are omitted. PEATMAP and its underlying data are freely available as a potentially useful tool for scientists and policy makers with interests in peatlands or wetlands. PEATMAP's data format and file structure are intended to allow it to be readily updated when previously undocumented peatlands are found and mapped, and when regional or national land cover maps are updated and refined.
La France, à l'instar de nombreux pays européens, connaît un recul assez net de sa population paysanne et par la même occasion de son potentiel agricole. Cette baisse sensible, enregistrée sur les trente dernières années est la conséquence directe de l'exode massif des populations vers les grandes agglomérations. Cette mutation s'est accompagnée de la fermeture progressive des paysages, marqués le plus souvent par une colonisation rapide par des espèces forestières. Ces évolutions sont également dues au changement climatique qui favorise certaines espèces par rapport à d'autres. Du point de vue de la gestion du bassin versant, il est très important de savoir comment ces changements pourraient affecter les ressources en eau. La mise en œuvre de la directive-cadre sur l'eau du 24 octobre 2012, définit des structures de gestion de l'eau sur des unités hydrologiques (bassin versant, nappe d'eau souterraine) pour un bon état des eaux en 2015, et le respect des objectifs pour 2027 dans le cadre d'une nouvelle directive-cadre sur l'eau (DCE). Cette mise en œuvre nécessite de prendre en compte la diversité spatiale et thématique des données sur l'unité hydrologique considérée. La télédétection et les Systèmes d'Information Géographiques (SIG) sont des outils utiles permettant de représenter cette diversité. Ils servent à la fois à l'organisation, à l'actualisation et à l'analyse des données spatiales. Ils sont utiles également pour le paramétrage de modèles hydrogéochimiques afin de modéliser la variabilité spatio-temporelle des ressources en eau. L'intérêt de la télédétection et des SIG couplés au modèle hydrogéochimique WARMF, est mis en évidence dans l'étude du cas d'un grand bassin versant du Massif Jura : l'Ain (4780 km²). Une telle étude répond aux problèmes qui se posent lors d'une DCE : estimation des quantités des eaux sur des secteurs non-mesurés, prévision des quantités en fonction de scénarii de l'occupation des sols et de scénarii météorologiques, vulnérabilité des ressources en eau superficielles, effet de la matière organique sur la qualité des eaux. Cette étude fournit une base scientifique pour la formulation de stratégies pour la gestion de la ressource en eau. Le couplage de la télédétection et des SIG au modèle hydrogéochimique est une nouvelle approche offrant de grands avantages en matière de disponibilité des données, de construction des scénarii, et d'interprétation des résultats. Cette approche sera un outil efficace d'aide à la décision pour la gestion intégrée de la ressource en eau des lacs et plus largement du bassin versant (oxygène, pH, etc.).
The total dissolved inorganic carbon (TDIC) and 13CTDIC have been used as chemical and isotopic tracers to evaluate the contribution of different water components discharging at the Fontaine de Vaucluse karst spring near Avignon. At the same time they have been used to separate its flood hydrograph. Waters flowing from unsaturated zone (UZ) and saturated zone (SZ) show similar concentration in TDIC. In UZ and SZ water rock interactions do not obey to the same kinetic. The mixing rate between water coming from the UZ characterised by a short residence time and water from the SZ with a longer residence time has been evaluated in the spring discharge. In a hydrodynamic system, which is rather complex as it is open to the soil CO2 in UZ and closed to the same CO2 in the SZ, 13CTDIC has excellent characteristics as an environmental tracer.In order to better describe the inwardness of mass movements within the aquifer, the apparent contrasting information obtained using two different isotopes (18O of water molecules and 13C of TDIC) must be combined. 18O informs whether the hydrodynamic system acts as piston flow (PF) or follows a well mixing model (WMM). Conversely, 13C gives more complete information on the UZ contributes to the total discharge.
The acrotelm–catotelm model of peatland hydrological and biogeochemical processes posits that the permeability of raised bogs is largely homogenous laterally but varies strongly with depth through the soil profile; uppermost peat layers are highly permeable while deeper layers are, effectively, impermeable.
We measured down‐core changes in peat permeability, plant macrofossil assemblages, dry bulk density and degree of humification beneath two types of characteristic peatland microform – ridges and hollows – at a raised bog in Wales. Six ¹⁴ C dates were also collected for one hollow and an adjacent ridge.
Contrary to the acrotelm–catotelm model, we found that deeper peat can be as highly permeable as near‐surface peat and that its permeability can vary by more than an order of magnitude between microforms over horizontal distances of 1–5 m.
Our palaeoecological data paint a complicated picture of microform persistence. Some microforms can remain in the same position on a bog for millennia, growing vertically upwards as the bog grows. However, adjacent areas on the bog (< 10 m distant) show switches between microform type over time, indicating a lack of persistence.
Synthesis . We suggest that the acrotelm–catotelm model should be used cautiously; spatial variations in peatland permeability do not fit the simple patterns suggested by the model. To understand how peatlands as a whole function both hydrologically and ecologically, it is necessary to understand how patterns of peat physical properties and peatland vegetation develop and persist.
Arctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem-scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) towards hydrogenotropic (September) methanogenesis at sites located towards the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape-scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant-mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to < 0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e- acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale, and suggests broad scale investigations are required to build better regional or pan-Arctic representations of CH4 and CO2 production.
Accurately estimating methane (CH4) flux in terrestrial ecosystems is critically important for investigating and predicting biogeochemistry-climate feedbacks. Improved simulations of CH4 flux require explicit representations of the microbial processes that account for CH4 dynamics. A microbial functional group-based module was developed, building on the decomposition subroutine of the Community Land Model 4.5. This module considers four key mechanisms for CH4 production and consumption: methanogenesis from acetate or from single-carbon compounds and CH4 oxidation using molecular oxygen or other inorganic electron acceptors. Four microbial functional groups perform these processes: acetoclastic methanogens, hydrogenotrophic methanogens, aerobic methanotrophs, and anaerobic methanotrophs. This module was used to simulate dynamics of carbon dioxide (CO2) and CH4 concentrations from an incubation experiment with permafrost soils. The results show that the model captures the dynamics of CO2 and CH4 concentrations in microcosms with top soils, mineral layer soils, and permafrost soils under natural and saturated moisture conditions and three temperature conditions of À2°C, 3°C, and 5°C (R2 > 0.67; P < 0.001). The biases for modeled results are less than 30% across the soil samples and moisture and temperature conditions. Sensitivity analysis confirmed the importance of acetic acid's direct contribution as substrate and indirect effects through pH feedback on CO2 and CH4 production and consumption. This study suggests that representing the microbial mechanisms is critical for modeling CH 4 production and consumption; it is urgent to incorporate microbial mechanisms into Earth system models for better predicting trace gas dynamics and the behavior of the climate system.
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 y−1) and comparable in magnitude to the downstream DOC flux (29.1 ± 12.9 g C m−2 catchment y−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.
While it is widely recognized that peatlands are important in the global carbon cycle, there is limited information on belowground gas production in tropical peatlands. We measured porewater methane (CH4) and carbon dioxide (CO2) concentrations and δ13C isotopic composition and CH4 and CO2 production rates in peat incubations from the Changuinola wetland in Panama. Our most striking finding was that CH4 was depleted in 13C (-94‰ in porewater and produced at -107‰ in incubated peat) relative to CH4 found in most temperate and northern wetlands, potentially impacting the accuracy of approaches that use carbon isotopes to constrain global mass balance estimates. Fractionation factors between CH4 and CO2 showed that hydrogenotrophic methanogenesis was the dominant CH4 production pathway, with up to 100% of the CH4 produced via this route. Far more CO2 than CH4 (7 to 100 X) was measured in porewater, due in part to loss of CH4 through ebullition or oxidation and to the production of CO2 from pathways other than methanogenesis. We analyzed data on 58 wetlands from the literature to determine the dominant factors influencing the relative proportions of CH4 produced by hydrogenotrophic and acetoclastic methanogenesis and found that a combination of environmental parameters including pH, vegetation type, nutrient status and latitude are correlated to the dominant methanogenic pathway. Methane production pathways in tropical peatlands do not correlate with these variables in the same way as their more northerly counterparts and thus may be differently affected by climate change.
By using 13C as a tracer in karstic aquifers, one can
distinguish between water from the unsaturated and the saturated zones,
because the system behaves as one open to the biogenic CO 2
in the unsaturated zone. This method shows that a significant reserve
remains in the unsaturated zone at the end of the minimal flow. It is
therefore likely that the unsaturated zone contributes to the outflow
during this period.
Although methanogenic pathways generally produce equimolar amounts of carbon dioxide and methane, CO2 concentrations are often reported to be higher than CH4 concentrations in both field and laboratory incubation studies of peat decomposition. In field settings, higher pore water concentrations of CO2 may result from the loss of methane by: (1) ebullition due to the low solubility of methane in pore water and (2) vascular-plant transport. Higher CO2 concentrations may also be caused by: (1) production of additional CO2 by high-molecular weight (HMW) organic matter (OM) fermentation and/or (2) respiration from non-methanogenic pathways. In this study of a peatland where advection and transverse dispersion were the dominant pore water solute transport mechanisms, an isotope-mass balance approach was used to determine the proportions of CO2 formed from non-fractionating OM respiration and HMW fermentation relative to CO2 production from methanogenesis. This approach also allowed us to estimate the loss of CH4 from the belowground system. The pathways of CO2 production varied with depth and surface vegetation type. In a Carex-dominated fen, methane production initially produced 40 % of the total CO2 and then increased to 90–100 % with increasing depth. In a Sphagnum-dominated bog, methanogenesis resulted in 60 % of total CO2 production which increased to 100 % at depth. Both bogs and fens showed 85–100 % of methane loss from pore waters. Our results indicate that the isotopic composition of dissolved CO2 is a powerful indicator to allow partitioning of the processes affecting peat remineralization and methane production.
To elucidate the roles of hydrology and vegetation in belowground carbon cycling within peatlands, radiocarbon values were obtained for pore water dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), CH4, and peat from the Glacial Lake Agassiz peatland. The major implication of this work is that the rate of microbial respiration within a peat column is greater than the peat decomposition rate. The radiocarbon content of DOC at both bog and fen was enriched relative to solid-phase peat by ~150-300% consistent with the advection of recently photosynthesized DOC downward into the peat column. Fen Delta14C values for DIC and CH4 closely track the Delta14C of pore water DOC at depth, indicating that this recent plant production was the predominant substrate for microbial respiration. Aceticlastic methanogenesis apparently dominated the upper third of the peat column (alpha=1.05), shifting toward CO2 reduction with depth (1.05
This paper presents a theoretical treatment of the evolution of the carbon isotopes C13 and C14 in natural waters and in precipitates which derive from such waters. The effects of an arbitrary number of sources (such as dissolution of carbonate minerals and oxidation of organic material) and sinks (such as mineral precipitation, CO2 degassing and production of methane), and of equilibrium fractionation between solid, gas and aqueous phases are considered. The results are expressed as equations relating changes in isotopic composition to changes in conventional carbonate chemistry. One implication of the equations is that the isotopic composition of an aqueous phase may approach a limiting value whenever there are simultaneous inputs and outputs of carbonate. In order to unambiguously interpret isotopic data from carbonate precipitates and identify reactants and products in reacting natural waters, it is essential that isotopic changes are determined chiefly by reactant and product stoichiometry, independent of reaction path. We demonstrate that this is so by means of quantitative examples. The evolution equations are applied to: 1.(1) carbon-14 dating of groundwaters;2.(2) interpretation of the isotopic composition of carbonate precipitates, carbonate cements and diagenetically altered carbonates; and3.(3) the identification of chemical reaction stoichiometry. These applications are illustrated by examples which show the variation of δC13 in solutions and in precipitates formed under a variety of conditions involving incongruent dissolution, CO2 degassing, methane production and mineral precipitation.