Water table fluctuations control CO2 exchange in wet and dry bogs through different mechanisms

Nitrous oxide respiration in acidophilic methanotrophs

Aerobic methanotrophic bacteria are considered strict aerobes but are often highly abundant in hypoxic and even anoxic environments. Despite possessing denitrification genes, it remains to be verified whether denitrification contributes to their growth. Here, we show that acidophilic methanotrophs can respire nitrous oxide (N2O) and grow anaerobically on diverse non-methane substrates, including methanol, C-C substrates, and hydrogen. We study two strains that possess N2O reductase genes: Methylocella tundrae T4 and Methylacidiphilum caldifontis IT6. We show that N2O respiration supports growth of Methylacidiphilum caldifontis at an extremely acidic pH of 2.0, exceeding the known physiological pH limits for microbial N2O consumption. Methylocella tundrae simultaneously consumes N2O and CH4 in suboxic conditions, indicating robustness of its N2O reductase activity in the presence of O2. Furthermore, in O2-limiting conditions, the amount of CH4 oxidized per O2 reduced increases when N2O is added, indicating that Methylocella tundrae can direct more O2 towards methane monooxygenase. Thus, our results demonstrate that some methanotrophs can respire N2O independently or simultaneously with O2, which may facilitate their growth and survival in dynamic environments. Such metabolic capability enables these bacteria to simultaneously reduce the release of the key greenhouse gases CO2, CH4, and N2O.

A drained nutrient-poor peatland forest in boreal Sweden constitutes a net carbon sink after integrating terrestrial and aquatic fluxes

Northern peatlands provide a globally important carbon (C) store. Since the beginning of the 20th century, however, large areas of natural peatlands have been drained for biomass production across Fennoscandia. Today, drained peatland forests constitute a common feature of the managed boreal landscape, yet their ecosystem C balance and associated climate impact are not well understood, particularly within the nutrient-poor boreal region. In this study, we estimated the net ecosystem carbon balance (NECB) from a nutrient-poor drained peatland forest and an adjacent natural mire in northern Sweden by integrating terrestrial carbon dioxide (CO2 ) and methane (CH4 ) fluxes with aquatic losses of dissolved organic C (DOC) and inorganic C based on eddy covariance and stream discharge measurements, respectively, over two hydrological years. Since the forest included a dense spruce-birch area and a sparse pine area, we were able to further evaluate the effect of contrasting forest structure on the NECB and component fluxes. We found that the drained peatland forest was a net C sink with a 2-year mean NECB of -115 ± 5 g C m-2  year-1 while the adjacent mire was close to C neutral with 14.6 ± 1.7 g C m-2  year-1 . The NECB of the drained peatland forest was dominated by the net CO2 exchange (net ecosystem exchange [NEE]), whereas NEE and DOC export fluxes contributed equally to the mire NECB. We further found that the C sink strength in the sparse pine forest area (-153 ± 8 g C m-2  year-1 ) was about 1.5 times as high as in the dense spruce-birch forest area (-95 ± 8 g C m-2  year-1 ) due to enhanced C uptake by ground vegetation and lower DOC export. Our study suggests that historically drained peatland forests in nutrient-poor boreal regions may provide a significant net ecosystem C sink and associated climate benefits.

Carbon stock stability in drained peatland after simulated plant carbon addition: Strong dependence on deeper soil

Peatlands are vital soil carbon sinks, yet this function is jeopardized by plant carbon which could change the decomposition rate of soil organic carbon, knowing as "priming effect". How the priming effect depends on depth is a critical question in drained peatland given the heterogeneity of soil layers defined by the water table, which include the surface acrotelm, inter-mesotelm and deep catotelm. Here, through incubation, we quantified the response of these three soil layers to addition of ¹³C-labeled oxalate, glucose, cellulose, or cinnamic acid under anoxic or oxic conditions on the Zoige Plateau in Tibet. Soil carbon in the inter-mesotelm showed the greatest decomposition, with the highest humification index and lowest microbial biomass carbon, while the soil carbon at the surface acrotelm was least decomposed. Under anoxic conditions, exogenous carbon addition reduced CO₂ emission by 12.2% at the surface acrotelm but increased by 59.8% in the inter-mesotelm and 23.5% in the deep catotelm. In the inter-mesotelm, oxalate addition significantly increased CO₂ emission by 63.9%, while cinnamic acid significantly increased it by 92.9%. In the deep catotelm, cinnamic acid significantly increased CO₂ emission by 55.3%. These results suggested that deeper soil organic carbon was more sensitive to plant carbon, particularly complex or recalcitrant carbon, than surface acrotelm soil. Under oxic conditions, carbon addition increased surface soil CO₂ emission by 18.9%, and triggered even greater increase at inter-mesotelm and deep catotelm soil, with proportions of 48.3% and 32.0%, respectively. Under both conditions, peat profile CO₂ release increased by 17.2-31.4% after exogenous carbon addition, and more than 77.8% of the increase came from the deeper two layers. These findings highlighted the need to take full account of priming effect of deeper soil in order to assess and predict the stability of carbon stocks in drained peatland.

Methane Emissions from Wetlands in China and Their Climate Feedbacks in the 21st Century

Wetlands are large sinks of carbon dioxide (CO₂) and sources of methane (CH₄). Both fluxes can be altered by wetland management (e.g., restoration), leading to changes in the climate system. Here, we use multiple models to assess CH₄ emissions and CO₂ sequestration from the wetlands in China and the impacts on climate under three climate scenarios and four wetland management scenarios with various levels of wetland restoration in the 21st century. We find that wetland restoration leads to increased CH₄ emissions with a national total of 0.32-11.31 Tg yr^-1. These emissions induce an additional radiative forcing of 0.0005-0.0075 W m^-2 yr^-1 and global annual mean air temperature rise of 0.0003-0.0053 °C yr^-1, across all future climate and management scenarios. However, wetland restoration also resulted in net CO₂ sequestration, leading to a combined net greenhouse gas sink in all climate management scenarios, except in the highest restoration level combined with the hottest climate scenario. The highest climate cooling was achieved under medium restoration, with the climate scenario consistent with the Paris agreement target of below 2 °C, with a cumulative global warming potential of -3.2 Pg CO₂-eq (2020-2100). Wetland restoration in the Qinghai-Tibet Plateau offers the greatest cooling effect.

Bridge to the future: Important lessons from 20 years of ecosystem observations made by the OzFlux network

In 2020, the Australian and New Zealand flux research and monitoring network, OzFlux, celebrated its 20th anniversary by reflecting on the lessons learned through two decades of ecosystem studies on global change biology. OzFlux is a network not only for ecosystem researchers, but also for those 'next users' of the knowledge, information and data that such networks provide. Here, we focus on eight lessons across topics of climate change and variability, disturbance and resilience, drought and heat stress and synergies with remote sensing and modelling. In distilling the key lessons learned, we also identify where further research is needed to fill knowledge gaps and improve the utility and relevance of the outputs from OzFlux. Extreme climate variability across Australia and New Zealand (droughts and flooding rains) provides a natural laboratory for a global understanding of ecosystems in this time of accelerating climate change. As evidence of worsening global fire risk emerges, the natural ability of these ecosystems to recover from disturbances, such as fire and cyclones, provides lessons on adaptation and resilience to disturbance. Drought and heatwaves are common occurrences across large parts of the region and can tip an ecosystem's carbon budget from a net CO2 sink to a net CO2 source. Despite such responses to stress, ecosystems at OzFlux sites show their resilience to climate variability by rapidly pivoting back to a strong carbon sink upon the return of favourable conditions. Located in under-represented areas, OzFlux data have the potential for reducing uncertainties in global remote sensing products, and these data provide several opportunities to develop new theories and improve our ecosystem models. The accumulated impacts of these lessons over the last 20 years highlights the value of long-term flux observations for natural and managed systems. A future vision for OzFlux includes ongoing and newly developed synergies with ecophysiologists, ecologists, geologists, remote sensors and modellers.

Integrating McGill Wetland Model (MWM) with peat cohort tracking and microbial controls

Peatlands store a large amount of organic carbon and are vulnerable to climate change and human disturbances. However, ecosystem-scale peatland models often do not explicitly simulate the decrease in peat substrate quality, i.e., decomposability or the dynamics of decomposers during peat decomposition, which are key controls in determining peat carbon's response to a changing environment. In this paper, we incorporated the tracking of each year's litter input (a cohort) and controls of microbial processes into the McGill Wetland Model (MWMmic) to address this discrepancy. Three major modifications were made: (1) the simple acrotelm-catotelm decomposition model in MWM was changed into a time-aggregated cohort model, to track the decrease in peat quality with decomposition age; (2) microbial dynamics: growth, respiration and death were incorporated into the model and decomposition rates are regulated by microbial biomass; and (3) vertical and horizontal transport of the dissolved organic carbon (DOC) were added and used to regulate the growth of microbial biomass. MWMmic was evaluated against measurements from the Mer Bleue peatland, a raised ombrotrophic bog located in southern Ontario, Canada. The model was able to replicate microbial and DOC dynamics, while at the same time reproduce the ecosystem-level CO₂ and DOC fluxes. Sensitivity analysis with MWMmic showed increased peatland resilience to perturbations compared to the original MWM, because of the tracking of peat substrate quality. The analysis revealed the most important parameters in the model to be microbial carbon use efficiency (CUE) and turnover rate. Simulated microbial adaptation with those two physiological parameters less sensitive to disturbances leads to a significantly larger peat C loss in response to warming and water table drawdown. Thus, the rarely explored peatland microbial physiological traits merit further research. This work paves the way for further model development to examine important microbial controls on peatland's biogeochemical cycling.

Response of the peatland carbon dioxide sink function to future climate change scenarios and water level management

Stress factors such as climate change and drought may switch the role of temperate peatlands from carbon dioxide (CO₂ ) sinks to sources, leading to positive feedback to global climate change. Water level management has been regarded as an important climate change mitigation strategy as it can sustain the natural net CO₂ sink function of a peatland. Little is known about how resilient peatlands are in the face of future climate change scenarios, as well as how effectively water level management can sustain the CO₂ sink function to mitigate global warming. The authors assess the effect of climate change on CO₂ exchange of south Swedish temperate peatlands, which were either unmanaged or subject to water level regulation. Climate chamber simulations were conducted using experimental peatland mesocosms exposed to current and future representative concentration pathway (RCP) climate scenarios (RCP 2.6, 4.5 and 8.5). The results showed that all managed and unmanaged systems under future climate scenarios could serve as CO₂ sinks throughout the experimental period. However, the 2018 extreme drought caused the unmanaged mesocosms under the RCP 4.5 and RCP 8.5 switch from a net CO₂ sink to a source during summer. Surprisingly, the unmanaged mesocosms under RCP 2.6 benefited from the warmer climate, and served as the best sink among the other unmanaged systems. Water level management had the greatest effect on the CO₂ sink function under RCP 8.5 and RCP 4.5, which improved their CO₂ sink capability up to six and two times, respectively. Under the current climate scenario, water level management had a negative effect on the CO₂ sink function, and it had almost no effect under RCP 2.6. Therefore, the researchers conclude that water level management is necessary for RCP 8.5, beneficial for RCP 4.5 and unimportant for RCP 2.6 and the current climate.

Overriding water table control on managed peatland greenhouse gas emissions

Global peatlands store more carbon than is naturally present in the atmosphere^1,2. However, many peatlands are under pressure from drainage-based agriculture, plantation development and fire, with the equivalent of around 3 per cent of all anthropogenic greenhouse gases emitted from drained peatland^3-5. Efforts to curb such emissions are intensifying through the conservation of undrained peatlands and re-wetting of drained systems⁶. Here we report eddy covariance data for carbon dioxide from 16 locations and static chamber measurements for methane from 41 locations in the UK and Ireland. We combine these with published data from sites across all major peatland biomes. We find that the mean annual effective water table depth (WTD_e; that is, the average depth of the aerated peat layer) overrides all other ecosystem- and management-related controls on greenhouse gas fluxes. We estimate that every 10 centimetres of reduction in WTD_e could reduce the net warming impact of CO₂ and CH₄ emissions (100-year global warming potentials) by the equivalent of at least 3 tonnes of CO₂ per hectare per year, until WTD_e is less than 30 centimetres. Raising water levels further would continue to have a net cooling effect until WTD_e is within 10 centimetres of the surface. Our results suggest that greenhouse gas emissions from peatlands drained for agriculture could be greatly reduced without necessarily halting their productive use. Halving WTD_e in all drained agricultural peatlands, for example, could reduce emissions by the equivalent of over 1 per cent of global anthropogenic emissions.

Temperate mire fluctuations from carbon sink to carbon source following changes in water table

The generally-accepted paradigm of wetland response to climate change is that water table drawdown and higher temperatures will cause wetlands to switch from a sink to a source of atmospheric carbon. However, it is hard to find a multi-year, ecosystem scale dataset representative of an undisturbed wetland that clearly demonstrates this paradigm on an annual total basis. Here we provide strong empirical confirmation of the above scenario based on six years of continuous eddy-covariance CO₂ and CH₄ flux measurements in Biebrza Valley, north-eastern Poland. In wet years the mire was a significant sink of atmospheric carbon (down to -270 ± 70 gC-CO₂ m^-2 yr^-1 against +21.8 ± 3.4 gC-CH₄ m^-2 yr^-1 in 2013) whereas in dry years it constituted a substantial carbon source (releasing up to +130 ± 70 gC-CO₂ m^-2 yr^-1 and +2.6 ± 1.4 gC-CH₄ m^-2 yr^-1 in 2015). Our findings demonstrate that the scenario of positive feedback between wetland carbon release and the present climate change trajectory is realistic and support the need of natural wetland preservation or rewetting. Our findings also indicate that conclusions drawn regarding a wetland's response to changing climate can depend strongly on the chosen period of analysis.

Recovery of the CO2 sink in a remnant peatland following water table lowering

Peatland biological, physical and chemical properties change over time in response to alterations in long-term water table position. Such changes complicate our ability to predict the response of peatland carbon stocks to sustained drying. In order to better understand the effect of sustained lowering of the water table on peatland carbon dynamics, we re-visited a drainage-affected bog, repeating eddy covariance measurements of CO₂ flux after a 16-year interval. We found the ecosystem CO₂ sink to have strengthened across the intervening period, despite a deep and fluctuating water table. This was mostly due to an increase in CO₂ uptake through photosynthesis associated with increased shrub growth. We also observed a decline in CO₂ loss through ecosystem respiration. These changes could not be attributed to environmental conditions. Air temperature was the only significant contemporaneous driver of monthly anomalies in CO₂ fluxes, with higher temperatures decreasing the net CO₂ sink via increased ecosystem respiration. However, the effect of air temperature was weak in comparison to the underlying differences between time periods. Therefore, we demonstrate that for drying peatlands, long-term changes within the ecosystem can be of primary importance as drivers of CO₂ exchange. In this peatland, the ecosystem carbon sink has shown resilience to water table drawdown, with internal feedbacks leading to a recovery of the CO₂ sink after a 16-year interval.