Methane emissions from natural wetlands tend to increase with temperature and therefore may lead to a positive feedback under future climate change. However, their temperature response includes ...confounding factors and appears to differ on different time scales. Observed methane emissions depend strongly on temperature on a seasonal basis, but if the annual mean emissions are compared between sites, there is only a small temperature effect. We hypothesize that microbial dynamics are a major driver of the seasonal cycle and that they can explain this apparent discrepancy. We introduce a relatively simple model of methanogenic growth and dormancy into a wetland methane scheme that is used in an Earth system model. We show that this addition is sufficient to reproduce the observed seasonal dynamics of methane emissions in fully saturated wetland sites, at the same time as reproducing the annual mean emissions. We find that a more complex scheme used in recent Earth system models does not add predictive power. The sites used span a range of climatic conditions, with the majority in high latitudes. The difference in apparent temperature sensitivity seasonally versus spatially cannot be recreated by the non‐microbial schemes tested. We therefore conclude that microbial dynamics are a strong candidate to be driving the seasonal cycle of wetland methane emissions. We quantify longer‐term temperature sensitivity using this scheme and show that it gives approximately a 12% increase in emissions per degree of warming globally. This is in addition to any hydrological changes, which could also impact future methane emissions.
Plain Language Summary
Wet soils such as bogs, fens, and other wetlands emit methane gas. Methane is a powerful greenhouse gas that adds to climate warming. It is important to understand its net production and also how this might change as the Earth warms. Generally, scientists have found that warmer soils emit more methane. However, there is a discrepancy between comparing warm versus cold sites—where the effect of the temperature difference is relatively small—and comparing warmer and colder seasons of the year, where the effect of temperature is much stronger. Since methane emissions are caused by microbes, we investigated whether their behavior might provide an explanation for this discrepancy. We carefully constructed a computer model to simulate the microbes and found that the model could indeed explain the apparent discrepancy in the seasonal and location effects of temperature that was measured. We found that two global climate models did not recreate these seasonal and location effects of temperature until we included the behavior of the soil microbes. Using our new global microbial model, we estimate that there will be around 12% extra methane emission from global wetlands for each degree of global warming, assuming other factors do not change.
Key Points
Current Earth system models generally reproduce the observed spatial pattern of wetland methane emissions but not the seasonal dynamics
Modeling microbes reproduces observed methane emissions and resolves the discrepancy between the seasonal versus spatial temperature response
The modeled long‐term wetland methane emissions increase by 12% per degree of global warming
Arctic tundra ecosystems are currently facing amplified rates of climate warming. Since these ecosystems store significant amounts of soil organic carbon, which can be mineralized to carbon dioxide ...(CO2) and methane (CH4), rising temperatures may cause increasing greenhouse gas fluxes to the atmosphere. To understand how net the ecosystem exchange (NEE) of CO2 will respond to changing climatic and environmental conditions, it is necessary to understand the individual responses of the processes contributing to NEE. Therefore, this study aimed to partition NEE at the soil–plant–atmosphere interface in an arctic tundra ecosystem and to identify the main environmental drivers of these fluxes. NEE was partitioned into gross primary productivity (GPP) and ecosystem respiration (Reco) and further into autotrophic (RA) and heterotrophic respiration (RH). The study examined CO2 flux data collected during the growing season in 2015 using closed-chamber measurements in a polygonal tundra landscape in the Lena River Delta, northeastern Siberia. To capture the influence of soil hydrology on CO2 fluxes, measurements were conducted at a water-saturated polygon center and a well-drained polygon rim. These chamber-measured fluxes were used to model NEE, GPP, Reco, RH, RA, and net primary production (NPP) at the pedon scale (1–10 m) and to determine cumulative growing season fluxes. Here, the response of in situ measured RA and RH fluxes from permafrost-affected soils of the polygonal tundra to hydrological conditions have been examined. Although changes in the water table depth at the polygon center sites did not affect CO2 fluxes from RH, rising water tables were linked to reduced CO2 fluxes from RA. Furthermore, this work found the polygonal tundra in the Lena River Delta to be a net sink for atmospheric CO2 during the growing season. The NEE at the wet, depressed polygon center was more than twice that at the drier polygon rim. These differences between the two sites were caused by higher GPP fluxes due to a higher vascular plant density and lower Reco fluxes due to oxygen limitation under water-saturated conditions at the polygon center in comparison to the rim. Hence, soil hydrological conditions were one of the key drivers for the different CO2 fluxes across this highly heterogeneous tundra landscape.
With respect to their role in the global carbon cycle, natural peatlands are characterized by their ability to sequester atmospheric carbon. This trait is strongly connected to the water regime of ...these ecosystems. Large parts of the soil profile in natural peatlands are water saturated, leading to anoxic conditions and to a diminished decomposition of plant litter. In functioning peatlands, the rate of carbon fixation by plant photosynthesis is larger than the decomposition rate of dead organic material. Over time, the amount of carbon that remains in the soil and is not converted back to carbon dioxide grows. Land use of peatlands often goes along with water level manipulations and thereby with alterations of carbon flux dynamics. In this study, carbon dioxide (CO2) and methane (CH4) flux measurements from a bog site in northwestern Germany that has been heavily degraded by peat mining are presented. Two contrasting types of management have been implemented at the site: (1) drainage during ongoing peat harvesting on one half of the central bog area and (2) rewetting on the other half that had been taken out of use shortly before measurements commenced. The presented 2-year data set was collected with an eddy covariance (EC) system set up on a central railroad dam that divides the two halves of the (former) peat harvesting area. We used footprint analysis to split the obtained CO2 and CH4 flux time series into data characterizing the gas exchange dynamics of both contrasting land use types individually. The time series gaps resulting from data division were filled using the response of artificial neural networks (ANNs) to environmental variables, footprint variability, and fuzzy transformations of seasonal and diurnal cyclicity. We used the gap-filled gas flux time series from 2 consecutive years to evaluate the impact of rewetting on the annual vertical carbon balances of the cutover bog. Rewetting had a considerable effect on the annual carbon fluxes and led to increased CH4 and decreased CO2 release. The larger relative difference between cumulative CO2 fluxes from the rewetted (13±6 mol m−2 a−1) and drained (22±7 mol m−2 a−1) section occurred in the second observed year when rewetting apparently reduced CO2 emissions by 40 %. The absolute difference in annual CH4 flux sums was more similar between both years, while the relative difference of CH4 release between the rewetted (0.83±0.15 mol m−2 a−1) and drained (0.45±0.11 mol m−2 a−1) section was larger in the first observed year, indicating a maximum increase in annual CH4 release of 84 % caused by rewetting at this particular site during the study period.
With respect to their role in the global carbon cycle, natural peatlands are characterized by their ability to sequester atmospheric carbon. This trait is strongly connected to the water regime of ...these ecosystems. Large parts of the soil profile in natural peatlands are water saturated, leading to anoxic conditions and to a diminished decomposition of plant litter. In functioning peatlands, the rate of carbon fixation by plant photosynthesis is larger than the decomposition rate of dead organic material. Over time, the amount of carbon that remains in the soil and is not converted back to carbon dioxide grows. Land use of peatlands often goes along with water level manipulations and thereby with alterations of carbon flux dynamics. In this study, carbon dioxide (CO.sub.2) and methane (CH.sub.4) flux measurements from a bog site in northwestern Germany that has been heavily degraded by peat mining are presented. Two contrasting types of management have been implemented at the site: (1) drainage during ongoing peat harvesting on one half of the central bog area and (2) rewetting on the other half that had been taken out of use shortly before measurements commenced. The presented 2-year data set was collected with an eddy covariance (EC) system set up on a central railroad dam that divides the two halves of the (former) peat harvesting area. We used footprint analysis to split the obtained CO.sub.2 and CH.sub.4 flux time series into data characterizing the gas exchange dynamics of both contrasting land use types individually. The time series gaps resulting from data division were filled using the response of artificial neural networks (ANNs) to environmental variables, footprint variability, and fuzzy transformations of seasonal and diurnal cyclicity. We used the gap-filled gas flux time series from 2 consecutive years to evaluate the impact of rewetting on the annual vertical carbon balances of the cutover bog. Rewetting had a considerable effect on the annual carbon fluxes and led to increased CH.sub.4 and decreased CO.sub.2 release.
Arctic permafrost landscapes have functioned as a global carbon sink for millennia. These landscapes are very heterogeneous, and the omnipresent water bodies within them act as a carbon source. Yet, ...few studies have focused on the impact of these water bodies on the landscape carbon budget. We deepen our understanding of carbon emissions from thermokarst ponds and constrain their impact by comparing carbon dioxide and methane fluxes from these ponds to fluxes from the surrounding tundra. We use eddy covariance measurements from a tower located at the border between a large pond and semi-terrestrial tundra.
Biogeochemical cycling in permafrost‐affected ecosystems remains associated with large uncertainties, which could impact the Earth's greenhouse gas budget and future climate policies. In particular, ...increased nutrient availability following permafrost thaw could perturb the greenhouse gas exchange in these systems, an effect largely unexplored until now. Here, we enhance the terrestrial ecosystem model QUINCY (QUantifying Interactions between terrestrial Nutrient CYcles and the climate system), which simulates fully coupled carbon (C), nitrogen (N) and phosphorus (P) cycles in vegetation and soil, with processes relevant in high latitudes (e.g., soil freezing and snow dynamics). In combination with site‐level and satellite‐based observations, we use the model to investigate impacts of increased nutrient availability from permafrost thawing in comparison to other climate‐induced effects and CO2 fertilization over 1960 to 2018 across the high Arctic. Our simulations show that enhanced availability of nutrients following permafrost thaw account for less than 15% of the total Gross primary productivity increase over the time period, despite simulated N limitation over the high Arctic scale. As an explanation for this weak fertilization effect, observational and model data indicate a mismatch between the timing of peak vegetative growth (week 26–27 of the year, corresponding to the beginning of July) and peak thaw depth (week 32–35, mid‐to‐late August), resulting in incomplete plant use of nutrients near the permafrost table. The resulting increasing N availability approaching the permafrost table enhances N loss pathways, which leads to rising nitrous oxide (N2O) emissions in our model. Site‐level emission trends of 2 mg N m−2 year−1 on average over the historical time period could therefore predict an emerging increasing source of N2O emissions following future permafrost thaw in the high Arctic.
Our model and observation‐based approach shows an increase of deep‐soil nitrogen availability following permafrost thaw over the 1960 to 2018 time period. However, due to the disconnect of nitrogen release from thawed soil and uptake by vegetation, nitrogen loss pathways are opened, particularly leading to increased nitrous oxide emissions to the atmosphere. Thus, our results suggest that future thaw of the permafrost and associated increase in nitrogen release could cause an increasing disproportionate source of nitrous oxide to the atmosphere.
With respect to their role in the global carbon cycle, natural peatlands are characterized by their ability to sequester atmospheric carbon. This trait is strongly connected to the water regime of ...these ecosystems. Large parts of the soil profile in natural peatlands are water saturated, leading to anoxic conditions and to a diminished decomposition of plant litter. In functioning peatlands, the rate of carbon fixation by plant photosynthesis is larger than the decomposition rate of dead organic material. Over time, the amount of carbon that remains in the soil and is not converted back to carbon dioxide grows. Land use of peatlands often goes along with water level manipulations and thereby with alterations of carbon flux dynamics. In this study, carbon dioxide (CO2) and methane (CH4) flux measurements from a bog site in northwestern Germany that has been heavily degraded by peat mining are presented. Two contrasting types of management have been implemented at the site: (1) drainage during ongoing peat harvesting on one half of the central bog area and (2) rewetting on the other half that had been taken out of use shortly before measurements commenced. The presented 2-year data set was collected with an eddy covariance (EC) system set up on a central railroad dam that divides the two halves of the (former) peat harvesting area. We used footprint analysis to split the obtained CO2 and CH4 flux time series into data characterizing the gas exchange dynamics of both contrasting land use types individually. The time series gaps resulting from data division were filled using the response of artificial neural networks (ANNs) to environmental variables, footprint variability, and fuzzy transformations of seasonal and diurnal cyclicity. We used the gap-filled gas flux time series from 2 consecutive years to evaluate the impact of rewetting on the annual vertical carbon balances of the cutover bog. Rewetting had a considerable effect on the annual carbon fluxes and led to increased CH4 and decreased CO2 release.The larger relative difference between cumulative CO2 fluxes from the rewetted (13±6 mol m-2 a-1) and drained (22±7 mol m-2 a-1) section occurred in the second observed year when rewetting apparently reduced CO2 emissions by 40 %. The absolute difference in annual CH4 flux sums was more similar between both years, while the relative difference of CH4 release between the rewetted (0.83±0.15 mol m-2 a-1) and drained (0.45±0.11 mol m-2 a-1) section was larger in the first observed year, indicating a maximum increase in annual CH4 release of 84 % caused by rewetting at this particular site during the study period.
Abstract
Floodplain ecosystems play a significant role in the global carbon (C) cycle, particularly due to their C sink potential in hardwood floodplain forests. However, in these forests, ...interactions between a heterogeneous micro‐relief and anthropogenic landscape changes make estimating C loss through soil CO
2
efflux difficult. To determine the drivers of soil CO
2
efflux, we selected six hardwood floodplain forests at the lower middle Elbe River, which were distributed among different relief positions (low‐lying or high‐elevated) in the active and former flooding zone. We measured soil CO
2
effluxes over a full year using the closed‐chamber method. Based on the response of soil CO
2
efflux to soil moisture and temperature, annual efflux rates were determined, which were then related to soil properties, such as pH, texture, soil organic carbon (SOC) and nitrogen (N) content. Soil CO
2
efflux ranged between 1006 (±99) and 2214 (±118) gC m
−2
year
−1
. Maximum efflux occurred in a former floodplain forest that was disconnected from Elbe River water table fluctuations. SOC‐specific soil CO
2
efflux (gC gSOC
−1
year
−1
) was smallest in low‐lying forests of the active flooding zone and reflected by the appearance of redoximorphic mottling close to the soil surface. Fine texture (<6.3 μm), SOC and N were related positively and electric conductivity, C/N and pH negatively to total soil CO
2
efflux. Soil pH and fine texture were the strongest univariate predictors for total soil CO
2
efflux (both
R
2
= 0.59). Fine texture, pH and C/N ratio explained 66% of the variance in total soil CO
2
efflux according to multiple linear regression. We conclude that, in hardwood floodplain forests, soil CO
2
efflux is mainly controlled by fine texture and soil pH. Fine texture can be related to soil moisture and nutrient availability and may have a positive effect on the activity of microorganisms.
Highlights
Soil CO
2
efflux ranged between 1006 (±99) and 2214 (±118) gC m
2
year
−1
, whereby the maximum was measured in a disconnected floodplain forest
Maximum soil CO
2
efflux was measured in a disconnected floodplain forest
SOC‐specific soil CO
2
efflux was smallest in forests where redoximorphic mottling occurred close to the surface
Fine texture (<6.3 μm) and pH were the strongest predictors for annual soil CO
2
efflux
The regional variability in tundra and boreal carbon dioxide (CO2) fluxes can be high, complicating efforts to quantify sink‐source patterns across the entire region. Statistical models are ...increasingly used to predict (i.e., upscale) CO2 fluxes across large spatial domains, but the reliability of different modeling techniques, each with different specifications and assumptions, has not been assessed in detail. Here, we compile eddy covariance and chamber measurements of annual and growing season CO2 fluxes of gross primary productivity (GPP), ecosystem respiration (ER), and net ecosystem exchange (NEE) during 1990–2015 from 148 terrestrial high‐latitude (i.e., tundra and boreal) sites to analyze the spatial patterns and drivers of CO2 fluxes and test the accuracy and uncertainty of different statistical models. CO2 fluxes were upscaled at relatively high spatial resolution (1 km2) across the high‐latitude region using five commonly used statistical models and their ensemble, that is, the median of all five models, using climatic, vegetation, and soil predictors. We found the performance of machine learning and ensemble predictions to outperform traditional regression methods. We also found the predictive performance of NEE‐focused models to be low, relative to models predicting GPP and ER. Our data compilation and ensemble predictions showed that CO2 sink strength was larger in the boreal biome (observed and predicted average annual NEE −46 and −29 g C m−2 yr−1, respectively) compared to tundra (average annual NEE +10 and −2 g C m−2 yr−1). This pattern was associated with large spatial variability, reflecting local heterogeneity in soil organic carbon stocks, climate, and vegetation productivity. The terrestrial ecosystem CO2 budget, estimated using the annual NEE ensemble prediction, suggests the high‐latitude region was on average an annual CO2 sink during 1990–2015, although uncertainty remains high.
We synthesized eddy covariance and chamber measurements of annual and growing season carbon dioxide (CO2) fluxes from 148 terrestrial high‐latitude (i.e., tundra and boreal) sites. We used statistical models to predict terrestrial ecosystem CO2 fluxes across the region over 1990–2015. Average annual net ecosystem CO2 sink strength was generally strong in the boreal biome but decreased with increasing latitude towards the tundra which was nearly neutral. Uncertainties remained high, but our results suggest that the high‐latitude region was on average an annual terrestrial ecosystem CO2 sink.
Vascular plant-dominated cushion bogs, which are exclusive to the Southern Hemisphere, are highly productive and constitute large sinks for atmospheric carbon dioxide compared to their moss-dominated ...counterparts around the globe. In this study, we experimentally investigated how a cushion bog plant community responded to elevated surface temperature conditions as they are predicted to occur in a future climate. We conducted the study in a cushion bog dominated by Astelia pumila on Tierra del Fuego, Argentina. We installed a year-round passive warming experiment using semicircular plastic walls that raised average near-surface air temperatures by between 0.4 and 0.7 ∘C (at the 3 of the 10 treatment plots which were equipped with temperature sensors). We focused on characterizing differences in morphological cushion plant traits and in carbon dioxide exchange dynamics using chamber gas flux measurements. We used a mechanistic modeling approach to quantify physiological plant traits and to partition the net carbon dioxide flux into its two components of photosynthesis and total ecosystem respiration. We found that A. pumila reduced its photosynthetic activity under elevated temperatures. At the same time, we observed enhanced respiration which we largely attribute, due to the limited effect of our passive warming on soil temperatures, to an increase in autotrophic respiration. Passively warmed A. pumila cushions sequestered between 55 % and 85 % less carbon dioxide than untreated control cushions over the main growing season. Our results suggest that even moderate future warming under the SSP1-2.6 scenario could decrease the carbon sink function of austral cushion bogs.
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