Agricultural peatland restoration: effects of land-use change on greenhouse gas (CO2 and CH4) fluxes in the Sacramento-San Joaquin Delta

Multiple microbial guilds mediate soil methane cycling along a wetland salinity gradient

Estuarine wetlands harbor considerable carbon stocks, but rising sea levels could affect their ability to sequester soil carbon as well as their potential to emit methane (CH4). While sulfate loading from seawater intrusion may reduce CH4 production due to the higher energy yield of microbial sulfate reduction, existing studies suggest other factors are likely at play. Our study of 11 wetland complexes spanning a natural salinity and productivity gradient across the San Francisco Bay and Delta found that while CH4 fluxes generally declined with salinity, they were highest in oligohaline wetlands (ca. 3-ppt salinity). Methanogens and methanogenesis genes were weakly correlated with CH4 fluxes but alone did not explain the highest rates observed. Taxonomic and functional gene data suggested that other microbial guilds that influence carbon and nitrogen cycling need to be accounted for to better predict CH4 fluxes at landscape scales. Higher methane production occurring near the freshwater boundary with slight salinization (and sulfate incursion) might result from increased sulfate-reducing fermenter and syntrophic populations, which can produce substrates used by methanogens. Moreover, higher salinities can solubilize ionically bound ammonium abundant in the lower salinity wetland soils examined here, which could inhibit methanotrophs and potentially contribute to greater CH4 fluxes observed in oligohaline sediments.IMPORTANCELow-level salinity intrusion could increase CH4 flux in tidal freshwater wetlands, while higher levels of salinization might instead decrease CH4 fluxes. High CH4 emissions in oligohaline sites are concerning because seawater intrusion will cause tidal freshwater wetlands to become oligohaline. Methanogenesis genes alone did not account for landscape patterns of CH4 fluxes, suggesting mechanisms altering methanogenesis, methanotrophy, nitrogen cycling, and ammonium release, and increasing decomposition and syntrophic bacterial populations could contribute to increases in net CH4 flux at oligohaline salinities. Improved understanding of these influences on net CH4 emissions could improve restoration efforts and accounting of carbon sequestration in estuarine wetlands. More pristine reference sites may have older and more abundant organic matter with higher carbon:nitrogen compared to wetlands impacted by agricultural activity and may present different interactions between salinity and CH4. This distinction might be critical for modeling efforts to scale up biogeochemical process interactions in estuarine wetlands.

Peatland restoration pathways to mitigate greenhouse gas emissions and retain peat carbon

Peatlands play a crucial role in the global carbon (C) cycle, making their restoration a key strategy for mitigating greenhouse gas (GHG) emissions and retaining C. This study analyses the most common restoration pathways employed in boreal and temperate peatlands, potentially applicable in tropical peat swamp forests. Our analysis focuses on the GHG emissions and C retention potential of the restoration measures. To assess the C stock change in restored (rewetted) peatlands and afforested peatlands with continuous drainage, we adopt a conceptual approach that considers short-term C capture (GHG exchange between the atmosphere and the peatland ecosystem) and long-term C sequestration in peat. The primary criterion of our conceptual model is the capacity of restoration measures to capture C and reduce GHG emissions. Our findings indicate that carbon dioxide (CO2) is the most influential part of long-term climate impact of restored peatlands, whereas moderate methane (CH4) emissions and low N2O fluxes are relatively unimportant. However, lateral losses of dissolved and particulate C in water can account up to a half of the total C stock change. Among the restored peatland types, Sphagnum paludiculture showed the highest CO2 capture, followed by shallow lakes and reed/grass paludiculture. Shallow lakeshore vegetation in restored peatlands can reduce CO2 emissions and sequester C but still emit CH4, particularly during the first 20 years after restoration. Our conceptual modelling approach reveals that over a 300-year period, under stable climate conditions, drained bog forests can lose up to 50% of initial C content. In managed (regularly harvested) and continuously drained peatland forests, C accumulation in biomass and litter input does not compensate C losses from peat. In contrast, rewetted unmanaged peatland forests are turning into a persistent C sink. The modelling results emphasized the importance of long-term C balance analysis which considers soil C accumulation, moving beyond the short-term C cycling between vegetation and the atmosphere.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10533-023-01103-1.

AmeriFlux BASE data pipeline to support network growth and data sharing

AmeriFlux is a network of research sites that measure carbon, water, and energy fluxes between ecosystems and the atmosphere using the eddy covariance technique to study a variety of Earth science questions. AmeriFlux's diversity of ecosystems, instruments, and data-processing routines create challenges for data standardization, quality assurance, and sharing across the network. To address these challenges, the AmeriFlux Management Project (AMP) designed and implemented the BASE data-processing pipeline. The pipeline begins with data uploaded by the site teams, followed by the AMP team's quality assurance and quality control (QA/QC), ingestion of site metadata, and publication of the BASE data product. The semi-automated pipeline enables us to keep pace with the rapid growth of the network. As of 2022, the AmeriFlux BASE data product contains 3,130 site years of data from 444 sites, with standardized units and variable names of more than 60 common variables, representing the largest long-term data repository for flux-met data in the world. The standardized, quality-ensured data product facilitates multisite comparisons, model evaluations, and data syntheses.

Influencing factors and prediction of net carbon sink in the primary sector of the coastal city in China

To achieve the goal of urban carbon dioxide emission reduction, how to increase carbon sequestration has become a top priority. The biological sink is mainly divided into green carbon sink and blue carbon sink. Coastal cities have two kinds of carbon sinks. There, the study of carbon sinks in coastal cities is the primary choice to cope with climate change. Therefore, this study chooses coastal cities with primary industries including agriculture, fishery, and forestry as the study subjects. The LMDI (Log-Mean Divisia Index) method and multiple regression prediction models were used to explore the low-carbon countermeasures which increase urban net carbon sink from the perspective of influencing factors and future potential. The study found that the average output value of employees in the primary industry is the main driving factor, and the change in the purchasing power of unit carbon sinks and the change in the proportion of employees in the primary industry have inhibited the increase in net carbon sinks. Projections based on the primary industry's output and afforestation area as independent variables show an overall upward trend in net carbon sinks, reaching 15.70 million tons of net carbon sinks in 2060, offsetting 10-20% of total carbon emissions in the same year. Based on the calculation results, this paper puts forward some corresponding countermeasures to increase carbon sinks. This paper provides a theoretical reference for the low-carbon development of coastal cities in China, and the strategies can be also expanded to other cities with similar resources around the world.

A comprehensive review of greenhouse gas based on subject categories

Greenhouse gas (GHG) concentrations have continued to increase in the atmosphere and unequivocally warmed the climate system, and human activities contribute significantly to the growth impact. Various research puzzles and potential mitigation evidence involving GHG related research (GRR) need to be explored and deciphered from large-scale literature records to provide a whole picture and clear scientific view on the current state of GRR to promoting action on climate change. By combining Bibexcel-based bibliometrics with Pajek's social network analysis, we explore the literature statistics and interdisciplinary characteristics of GRR, and identify frequently debated topics in interdisciplinary by going deep into the texts of those classical literature. We found the trend of GRR's publications in the Environmental/Ecology group increased exponentially with an annual growth rate of 47.3 % and continue to expand in 13 subject categories. There are four types of relationships in the author cooperation, which gradually promote the cross-study of GHG in different subject categories, and the regional cooperation relations are relatively stable involving North America, Asia, Europe, Oceania, and South America. Those classical literature are widely distributed in six interdisciplinary categories, specifically 'Agronomy, Forestry and Zoology', 'Biodiversity Conservation and Ecology', 'Engineering, Environmental and Green & Sustainable Science & Technology', 'Geography and Remote Sensing', 'Limnology, Marine & Freshwater Biology and Water Resources', and 'Public, Environmental & Occupational Health'.

Restoring function: Positive responses of carbon and nitrogen to 20 years of hydrologic restoration in montane meadows

Montane meadows are highly productive ecosystems that contain high densities of soil carbon (C) and nitrogen (N). However, anthropogenic disturbances that have led to channel incision and disconnected floodplain hydrology have altered the C balance of many meadows, converting them from net C sinks to net sources of C to the atmosphere. Restoration efforts designed to reconnect floodplain hydrology may slow rates of soil C loss from degraded meadows and restore the conditions for C sequestration and N immobilization, yet questions remain about the long-term impact of such efforts. Here, we used a 22-year meadow restoration chronosequence to measure the decadal impact of hydrologic restoration on aboveground and belowground C and N stocks and concentrations. Increases in herbaceous vegetation biomass preceded changes in soil C stocks, with the largest gains occurring belowground. Root biomass (0-15 cm) increased at a rate of 270.3 g m^-2 year^-1 and soil C stocks (0-15 cm) increased by 232.9 g C m^-2 year^-1 across the chronosequence. Increases in soil C concentration (2.99 g C kg^-1 year^-1 ) were tightly coupled with increases in soil N concentration (0.21 g N kg^-1 year^-1 ) and soil C:N did not vary with time since restoration. Fourier transform infrared spectroscopy results showed that the fraction of labile aliphatic C-H and carboxylate C-O (COO) compounds in the soil increased with the age of restoration and were positively correlated with soil C and N concentrations. Our results demonstrate that restoration of floodplain hydrology in montane meadows has significant impacts on belowground C and N stocks, soil C and N concentration, and soil C chemistry within the first two decades following restoration.

Detecting Hot Spots of Methane Flux Using Footprint-Weighted Flux Maps

In this study, we propose a new technique for mapping the spatial heterogeneity in gas exchange around flux towers using flux footprint modeling and focusing on detecting hot spots of methane (CH4) flux. In the first part of the study, we used a CH4 release experiment to evaluate three common flux footprint models: the Hsieh model (Hsieh et al., 2000), the Kljun model (Kljun et al., 2015), and the K & M model (Kormann and Meixner, 2001), finding that the K & M model was the most accurate under these conditions. In the second part of the study, we introduce the Footprint-Weighted Flux Map, a new technique to map spatial heterogeneity in fluxes. Using artificial CH4 release experiments, natural tracer approaches and flux chambers we mapped the spatial flux heterogeneity, and detected and validated a hot spot of CH4 flux in a oligohaline restored marsh. Through chamber measurements during the months of April and May, we found that fluxes at the hot spot were on average as high as 6589 ± 7889 nmol m-2 s-1 whereas background flux from the open water were on average 15.2 ± 7.5 nmol m-2 s-1. This study provides a novel tool to evaluate the spatial heterogeneity of fluxes around eddy-covariance towers and creates important insights for the interpretation of hot spots of CH4 flux, paving the way for future studies aiming to understand subsurface biogeochemical processes and the microbiological conditions that lead to the occurrence of hot spots and hot moments of CH4 flux.

Typha for paludiculture-Suitable water table and nutrient conditions for potential biomass utilization explored in mesocosm gradient experiments

Drainage has turned 650,000 km2 of peatlands worldwide into greenhouse gas sources. To counteract climate change, large-scale rewetting is necessary while agricultural use of rewetted areas, termed paludiculture, is still possible. However, more information is required on the performance of suitable species, such as cattail, in the range of environmental conditions after rewetting. We investigated productivity and biomass quality (morphological traits and tissue chemical composition) of Typha angustifolia and Typha latifolia along gradients of water table depth (-45 to +40 cm) and nutrient addition (3.6-400 kg N ha-1 a-1) in a six-month mesocosm experiment with an emphasis on their high-value utilization, e.g., as building material, paper, or biodegradable packaging. Over a wide range of investigated conditions, T. latifolia was more productive than T. angustifolia. Productivity was remarkably tolerant of low nutrient addition, suggesting that long-term productive paludiculture is possible. Low water tables were beneficial for T. latifolia productivity and high water tables for T. angustifolia biomass quality. Rewetting will likely create a mosaic of different water table depths. Our findings that the yield of T. angustifolia and tissue chemical composition of T. latifolia were largely unaffected by water table depth are therefore promising. Depending on intended utilization, optimal cultivation conditions and preferable species differ. Considering yield or diameter, e.g., for building materials, T. latifolia is generally preferable over T. angustifolia. A low N, P, K content, high Si content and high C/N-ratio can be beneficial for processing into disposable tableware, charcoal, or building material. For these utilizations, T. angustifolia is preferable at high water tables, and both species should be cultivated at a low nutrient supply. When cellulose and lignin contents are relevant, e.g., for paper and biodegradable packaging, T. angustifolia is preferable at high water tables and both species should be cultivated at nutrient additions of about 20 kg N ha-1 a-1.

Nexus between Agricultural Land Use, Economic Growth and N2O Emissions in Canada: Is There an Environmental Kuznets Curve?

The present study investigates the relationship between nitrous oxide emissions and economic growth using the ARDL bounds testing approach in Canada over the period of 1970–2020. The agricultural land use and exports are included in the estimated models as additional control variables. The empirical findings confirmed the environmental Kuznets curve hypothesis when total N2O emissions are used as a dependent variable in the case of Canada, and similar results are found when we used agricultural induced N2O emissions as a dependent variable. The results also indicate that Canada is already in the decreasing segment of the Kuznets curve, and the turning point of GDP per capita for the total N2O emissions is $41,718, while for agricultural induced N2O emissions, it is $38,825. Our empirical evidence confirms that agricultural land use had a positive and significant effect on total N2O emissions, while a negative but insignificant effect in the case of agricultural induced N2O emissions. However, Canadian exports are negatively associated with total N2O emissions as well as agricultural induced N2O emissions, but it requires more stringent laws to curb N2O emissions-oriented exports to keep the ecosystem in balance in the short-run and intends to meet its long-term target of reducing emissions as it progresses towards Canada’s 2050 net-zero ambition.

Responsible agriculture must adapt to the wetland character of mid-latitude peatlands

Drained, lowland agricultural peatlands are greenhouse gas (GHG) emission hotspots and a large but vulnerable store of irrecoverable carbon. They exhibit soil loss rates of ~2.0 cm yr^-1 and are estimated to account for 32% of global cropland emissions while producing only 1.1% of crop kilocalories. Carbon dioxide emissions account for >80% of their terrestrial GHG emissions and are largely controlled by water table depth. Reducing drainage depths is, therefore, essential for responsible peatland management. Peatland restoration can substantially reduce emissions. However, this may conflict with societal needs to maintain productive use, to protect food security and livelihoods. Wetland agriculture strategies will, therefore, be required to adapt agriculture to the wetland character of peatlands, and balance GHG mitigation against productivity, where halting emissions is not immediately possible. Paludiculture may substantially reduce GHG emissions but will not always be viable in the current economic landscape. Reduced drainage intensity systems may deliver partial reductions in the rate of emissions, with smaller modifications to existing systems. These compromise systems may face fewer hurdles to adoption and minimize environmental harm until societal conditions favour strategies that can halt emissions. Wetland agriculture will face agronomic, socio-economic and water management challenges, and careful implementation will be required. Diversity of values and priorities among stakeholders creates the potential for conflict. Successful implementation will require participatory research approaches and co-creation of workable solutions. Policymakers, private sector funders and researchers have key roles to play but adoption risks would fall predominantly on land managers. Development of a robust wetland agriculture paradigm is essential to deliver resilient production systems and wider environmental benefits. The challenge of responsible use presents an opportunity to rethink peatland management and create thriving, innovative and green wetland landscapes for everyone's future benefit, while making a vital contribution to global climate change mitigation.

A novel approach to partitioning evapotranspiration into evaporation and transpiration in flooded ecosystems

Reliable partitioning of micrometeorologically measured evapotranspiration (ET) into evaporation (E) and transpiration (T) would greatly enhance our understanding of the water cycle and its response to climate change related shifts in local-to-regional climate conditions and rising global levels of vapor pressure deficit (VPD). While some methods on ET partitioning have been developed, their underlying assumptions make them difficult to apply more generally, especially in sites with large contributions of E. Here, we report a novel ET partitioning method using artificial neural networks (ANNs) in combination with a range of environmental input variables to predict daytime E from nighttime ET measurements. The study uses eddy covariance data from four restored wetlands in the Sacramento-San Joaquin Delta, California, USA, as well as leaf-level T data for validation. The four wetlands vary in their vegetation make-up and structure, representing a range of ET conditions. The ANNs were built with increasing complexity by adding the input variable that resulted in the next highest average value of model testing R² across all sites. The order of variable inclusion (and importance) was: VPD > gap-filled sensible heat flux (H_gf) > air temperature (T_air ) > friction velocity (u_∗ ) > other variables. The model using VPD, H_gf, T_air , and u_∗ showed the best performance during validation with independent data and had a mean testing R²  value of 0.853 (averaged across all sites, range from 0.728 to 0.910). In comparison to other methods, our ANN method generated T/ET partitioning results which were more consistent with CO₂ exchange data especially for more heterogeneous sites with large E contributions. Our method improves the understanding of T/ET partitioning. While it may be particularly suited to flooded ecosystems, it can also improve T/ET partitioning in other systems, increasing our knowledge of the global water cycle and ecosystem functioning.

Quantification of Ecosystem-Scale Methane Sinks Observed in a Tropical Rainforest in Hainan, China

Tropical rainforest ecosystems are important when considering the global methane (CH4) budget and in climate change mitigation. However, there is a lack of direct and year-round observations of ecosystem-scale CH4 fluxes from tropical rainforest ecosystems. In this study, we examined the temporal variations in CH4 flux at the ecosystem scale and its annual budget and environmental controlling factors in a tropical rainforest of Hainan Island, China, using 3 years of continuous eddy covariance measurements from 2016 to 2018. Our results show that CH4 uptake generally occurred in this tropical rainforest, where strong CH4 uptake occurred in the daytime, and a weak CH4 uptake occurred at night with a mean daily CH4 flux of −4.5 nmol m−2 s−1. In this rainforest, the mean annual budget of CH4 for the 3 years was −1260 mg CH4 m−2 year−1. Furthermore, the daily averaged CH4 flux was not distinctly different between the dry season and wet season. Sixty-nine percent of the total variance in the daily CH4 flux could be explained by the artificial neural network (ANN) model, with a combination of air temperature (Tair), latent heat flux (LE), soil volumetric water content (VWC), atmospheric pressure (Pa), and soil temperature at −10 cm (Tsoil), although the linear correlation between the daily CH4 flux and any of these individual variables was relatively low. This indicates that CH4 uptake in tropical rainforests is controlled by multiple environmental factors and that their relationships are nonlinear. Our findings also suggest that tropical rainforests in China acted as a CH4 sink during 2016–2018, helping to counteract global warming.

Soil carbon loss from drained agricultural peatland after coverage with mineral soil

Drainage for agriculture has turned peatlands from a net sink to a net source of carbon (C). In order to reduce the environmental footprint of agricultural peatland drainage, and to counteract soil subsidence, mineral soil coverage is becoming an increasingly used practice in Switzerland. To explore the effect of mineral soil coverage on soil C loss and the source of CO₂ from peatland drained for agriculture, we utilized the radiocarbon signature (F¹⁴C) of soil C and emitted CO₂ in the field. The experiment, located in the Swiss Rhine Valley, was carried out on two adjacent drained organic soils, either without mineral soil cover (reference 'Ref'), or covered with mineral soil (thickness ~ 40 cm) (coverage 'Cov') 13 years ago. Drainage already commenced 130 years ago and the site was managed as meadow since the 1970ies. Drainage induced 41-75 kg C m^-2 loss, which is equivalent to annual C loss rates of 0.49-0.58 kg C m^-2 yr^-1 and 0.31-0.63 kg C m^-2 yr^-1 for Cov and Ref, respectively. Mineral soil coverage had no significant effect on the amount of heterotrophic respiration, however, at Cov, the radiocarbon signature of heterotrophic CO₂ was significantly (p<0.01) younger than at Ref, indicating that mineral soil coverage moved the source of decomposition of soil organic carbon (SOC) from a higher share of old peat towards a higher share of relatively younger material. In summary, our study lends support to the hypothesis that mineral soil coverage might reduce the decomposition of old peat underneath, and may therefore be a promising peatland management technique for the future use of drained peatland for agriculture.

Hot moments drive extreme nitrous oxide and methane emissions from agricultural peatlands

Agricultural peatlands are estimated to emit approximately one third of global greenhouse gas (GHG) emissions from croplands, but the temporal dynamics and controls of these emissions are poorly understood, particularly for nitrous oxide (N₂ O). We used cavity ring-down spectroscopy and automated chambers in a drained agricultural peatland to measure over 70,000 individual N₂ O, methane (CH₄ ), and carbon dioxide (CO₂ ) fluxes over 3 years. Our results showed that N₂ O fluxes were high, contributing 26% (annual range: 16%-35%) of annual CO₂ e emissions. Total N₂ O fluxes averaged 26 ± 0.5 kg N₂ O-N ha^-1 y^-1 and exhibited significant inter- and intra-annual variability with a maximum annual flux of 42 ± 1.8 kg N₂ O-N ha^-1 y^-1 . Hot moments of N₂ O and CH₄ emissions represented 1.1 ± 0.2 and 1.3 ± 0.2% of measurements, respectively, but contributed to 45 ± 1% of mean annual N₂ O fluxes and to 140 ± 9% of mean annual CH₄  fluxes. Soil moisture, soil temperature, and bulk soil oxygen (O₂ ) concentrations were strongly correlated with soil N₂ O and CH₄ emissions; soil nitrate ( NO3- ) concentrations were also significantly correlated with soil N₂ O emissions. These results suggest that IPCC benchmarks underestimate N₂ O emissions from these high emitting agricultural peatlands by up to 70%. Scaling to regional agricultural peatlands with similar management suggests these ecosystems could emit up to 1.86 Tg CO₂ e y^-1 (range: 1.58-2.21 Tg CO₂ e y^-1 ). Data suggest that these agricultural peatlands are large sources of GHGs, and that short-term hot moments of N₂ O and CH₄ are a significant fraction of total greenhouse budgets.

Temporal trends in methane emissions from a small eutrophic reservoir: the key role of a spring burst

Waters impounded behind dams (i.e., reservoirs) are important sources of greenhouses gases (GHGs), especially methane (CH₄), but emission estimates are not well constrained due to high spatial and temporal variability, limitations in monitoring methods to characterize hot spot and hot moment emissions, and the limited number of studies that investigate diurnal, seasonal, and interannual patterns in emissions. In this study, we investigate the temporal patterns and biophysical drivers of CH₄ emissions from Acton Lake, a small eutrophic reservoir, using a combination of methods: eddy covariance monitoring, continuous warm-season ebullition measurements, spatial emission surveys, and measurements of key drivers of CH₄ production and emission. We used an artificial neural network to gap fill the eddy covariance time series and to explore the relative importance of biophysical drivers on the interannual timescale. We combined spatial and temporal monitoring information to estimate annual whole-reservoir emissions. Acton Lake had cumulative areal emission rates of 45.6 ± 8.3 and 51.4 ± 4.3 g CH₄ m^-2 in 2017 and 2018, respectively, or 109 ± 14 and 123 ± 10 Mg CH₄ in 2017 and 2018 across the whole 2.4 km² area of the lake. The main difference between years was a period of elevated emissions lasting less than 2 weeks in the spring of 2018, which contributed 17 % of the annual emissions in the shallow region of the reservoir. The spring burst coincided with a phytoplankton bloom, which was likely driven by favorable precipitation and temperature conditions in 2018 compared to 2017. Combining spatially extensive measurements with temporally continuous monitoring enabled us to quantify aspects of the spatial and temporal variability in CH₄ emission. We found that the relationships between CH₄ emissions and sediment temperature depended on location within the reservoir, and we observed a clear spatiotemporal offset in maximum CH₄ emissions as a function of reservoir depth. These findings suggest a strong spatial pattern in CH₄ biogeochemistry within this relatively small (2.4 km²) reservoir. In addressing the need for a better understanding of GHG emissions from reservoirs, there is a trade-off in intensive measurements of one water body vs. short-term and/or spatially limited measurements in many water bodies. The insights from multi-year, continuous, spatially extensive studies like this one can be used to inform both the study design and emission upscaling from spatially or temporally limited results, specifically the importance of trophic status and intra-reservoir variability in assumptions about upscaling CH₄ emissions.

Using Artificial Neural Network Algorithm and Remote Sensing Vegetation Index Improves the Accuracy of the Penman-Monteith Equation to Estimate Cropland Evapotranspiration

Accurate estimation of evapotranspiration (ET) can provide useful information for water management and sustainable agricultural development. However, most of the existing studies used physical models, which are not accurate enough due to our limited ability to represent the ET process accurately or rarely focused on cropland. In this study, we trained two models of estimating croplands ET. The first is Medlyn-Penman-Monteith (Medlyn-PM) model. It uses artificial neural network (ANN)-derived gross primary production along with Medlyn’s stomatal conductance to compute surface conductance (Gs), and the computed Gs is used to estimate ET using the PM equation. The second model, termed ANN-PM, directly uses ANN to construct Gs and simulate ET using the PM equation. The results showed that the two models can reasonably reproduce ET with ANN-PM showing a better performance, as indicated by the lower error and higher determination coefficients. The results also showed that the performances of ANN-PM without the facilitation of any remote sensing (RS) factors degraded significantly compared to the versions that used RS factors. We also evidenced that ANN-PM can reasonably characterize the time-series changes of ET at sites having a dry climate. The ANN-PM method can reasonably estimate the ET of croplands under different environmental conditions.

Quantifying the effects of multiple land management practices, land cover change, and wildfire on the California landscape carbon budget with an empirical model

The effectiveness of land-based climate mitigation strategies is generally estimated on a case-by-case basis without considering interactions with other strategies or influencing factors. Here we evaluate a new, comprehensive approach that incorporates interactions among multiple management strategies, land use/cover change, wildfire, and climate, although the potential effects of climate change are not evaluated in this study. The California natural and working lands carbon and greenhouse gas model (CALAND) indicates that summing individual practice estimates of greenhouse gas impacts may underestimate emission reduction benefits in comparison with an integrated estimate. Annual per-area estimates of the potential impact of specific management practices on landscape emissions can vary based on the estimation period, which can be problematic for extrapolating such estimates over space and time. Furthermore, the actual area of implementation is a primary factor in determining potential impacts of management on landscape emissions. Nonetheless, less intensive forest management, avoided conversion to urban land, and urban forest expansion generally create the largest annual per-area reductions, while meadow restoration and forest fuel reduction and harvest practices generally create the largest increases with respect to no management. CALAND also shows that data uncertainty is too high to determine whether California land is a source or a sink of carbon emissions, but that estimating effects of management with respect to a baseline provides valid results. Important sources of this uncertainty are initial carbon density, net ecosystem carbon accumulation rates, and land use/cover change data. The appropriate choice of baseline is critical for generating valid results.

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.

Productive wetlands restored for carbon sequestration quickly become net CO2 sinks with site-level factors driving uptake variability

Inundated wetlands can potentially sequester substantial amounts of soil carbon (C) over the long-term because of slow decomposition and high primary productivity, particularly in climates with long growing seasons. Restoring such wetlands may provide one of several effective negative emission technologies to remove atmospheric CO2 and mitigate climate change. However, there remains considerable uncertainty whether these heterogeneous ecotones are consistent net C sinks and to what degree restoration and management methods affect C sequestration. Since wetland C dynamics are largely driven by climate, it is difficult to draw comparisons across regions. With many restored wetlands having different functional outcomes, we need to better understand the importance of site-specific conditions and how they change over time. We report on 21 site-years of C fluxes using eddy covariance measurements from five restored fresh to brackish wetlands in a Mediterranean climate. The wetlands ranged from 3 to 23 years after restoration and showed that several factors related to restoration methods and site conditions altered the magnitude of C sequestration by affecting vegetation cover and structure. Vegetation established within two years of re-flooding but followed different trajectories depending on design aspects, such as bathymetry-determined water levels, planting methods, and soil nutrients. A minimum of 55% vegetation cover was needed to become a net C sink, which most wetlands achieved once vegetation was established. Established wetlands had a high C sequestration efficiency (i.e. the ratio of net to gross ecosystem productivity) comparable to upland ecosystems but varied between years undergoing boom-bust growth cycles and C uptake strength was susceptible to disturbance events. We highlight the large C sequestration potential of productive inundated marshes, aided by restoration design and management targeted to maximise vegetation extent and minimise disturbance. These findings have important implications for wetland restoration, policy, and management practitioners.

Multiple processes contribute to methane emission in a riparian cottonwood forest ecosystem

Methane emission from trees may partially or completely offset the methane sink in upland soils, the only process that has been regularly included in methane budgets for forest ecosystems. Our objective was to analyze multiple biogeochemical processes that influence the production, oxidation and transport of methane in a riparian cottonwood ecosystem and its adjacent river. We combined chamber flux measurements on tree stems, forest soil and the river surface with eddy covariance measurements of methane net ecosystem exchange. In addition, we tested whether methanogens were present in cottonwood stems, shallow soil layers and alluvial groundwater. Average midday peak in net methane emission measured by eddy covariance was c. 12 nmol m^-2  s^-1 . The average uptake of methane by soils (0.87 nmol m^-2  s^-1 ) was largely offset by tree stem methane emission (0.75 nmol m^-2  s^-1 ). There was evidence of methanogens in tree stems but not in shallow soil. Growing season (May-September) cumulative net methane emission (17.4 mmol CH₄  m^-2 ) included methane produced in cottonwood stems and methane input to the nocturnal boundary layer from the forest and the adjacent river. The multiple processes contributing to methane emission illustrated the linked nature of these adjacent terrestrial and aquatic ecosystems.

Nutrient removal potential and biomass production by Phragmites australis and Typha latifolia on European rewetted peat and mineral soils

Paludiculture, sustainable and climate-smart land use of formerly drained, rewetted organic soils, can produce significant biomass in peatlands whilst potentially restoring several additional wetland services. However, the site conditions that allow maximum biomass production and nutrient removal by paludiculture crops have rarely been studied. We studied the relationship between soil characteristics, including plant-available nutrients, peak biomass, stand age, harvest period, and nutrient removal potential for two important paludiculture species, Typha latifolia and Phragmites australis, on rewetted peat and mineral soils in a large-scale European survey. T. latifolia and P. australis were able to produce an aboveground peak biomass of 10-30 t dry matter ha^-1 y^-1 and absorbed significant amounts of carbon, nitrogen, phosphorus, and potassium in stands older than 3 years. They were able to grow in a wide range of abiotic soil conditions. Low N:P ratios (5-9) and low N content (< 2%) in T. latifolia tissue suggest N limitation, but P uptake was still surprisingly high. P. australis had higher N:P ratios (8-25) and was less responsive to nutrients, suggesting a higher nutrient use efficiency. However, both species could still produce significant biomass at lower nutrient loads and in winter, when water content was low and nutrient removal still reasonable. Based on this European wetland survey, paludiculture holds a great potential to combine peat preservation, water purification, nutrient removal, and a high biomass production. Paludicrops take up substantial amounts of nutrients, and both summer and winter harvests provide an effective way to sequester carbon in a range of high-valued biomass products and to control nutrient effluxes from rewetted sites at the landscape scale.

The Response of Microbial Communities to Peatland Drainage and Rewetting. A Review

Peatlands are significant global carbon stores and play an important role in mediating the flux of greenhouse gasses into the atmosphere. During the 20th century substantial areas of northern peatlands were drained to repurpose the land for industrial or agricultural use. Drained peatlands have dysfunctional microbial communities, which can lead to net carbon emissions. Rewetting of drained peatlands is therefore an environmental priority, yet our understanding of the effects of peatland drainage and rewetting on microbial communities is still incomplete. Here we summarize the last decade of research into the response of the wider microbial community, methane-cycling microorganisms, and micro-fauna to drainage and rewetting in fens and bogs in Europe and North America. Emphasis is placed on current research methodologies and their limitations. We propose targets for future work including: accounting for timescale of drainage and rewetting events; better vertical and lateral coverage of samples across a peatland; the integration of proteomic and metabolomic datasets into functional community analysis; the use of RNA sequencing to differentiate the active community from legacy DNA; and further study into the response of the viral and micro-faunal communities to peatland drainage and rewetting. This review should benefit researchers embarking on studies in wetland microbiology and non-microbiologists working on peatland drainage and rewetting in general.

Mineralogical associations with soil carbon in managed wetland soils

Carbon (C)-rich wetland soils are often drained for agriculture due to their capacity to support high net primary productivity. Increased drainage is expected this century to meet the agricultural demands of a growing population. Wetland drainage can result in large soil C losses and the concentration of residual soil minerals such as iron (Fe) and aluminum (Al). In upland soils, reactive Fe and Al minerals can contribute to soil C accumulation through sorption to poorly crystalline minerals and coprecipitation of organo-metal complexes, as well as C loss via anaerobic respiration by Fe-reducing bacteria. The role of these minerals in soil C dynamics is often overlooked in managed wetland soils and may be particularly important in both drained and reflooded systems with elevated mineral concentrations. Reflooding drained soils have been proposed as a means to sequester C for climate change mitigation, yet little is known about how reactive Fe and Al minerals affect C cycling in restored wetlands. We explored the interactions among soil C and reactive Fe and Al minerals in drained and reflooded wetland soils. In reflooded soils, soil C was negatively associated with reactive Fe and reduced Fe(II), a proxy for anaerobic conditions (reactive Fe: R²  = .54-.79; Fe(II): R²  = .59-.89). In drained soils, organo-Al complexes were positively associated with soil C and Fe(II) (Al R²  = .91; Fe(II): R²  = .54-.60). Soil moisture, organo-Al, and reactive Fe explained most of the variation observed in soil C concentrations across all sites (p < .01). Reactive Fe was negatively correlated to soil C concentrations across sites, suggesting these Fe pools may drive additional C losses in drained soils and limit C sequestration with reflooding. In contrast, reactive organo-Al in drained soils facilitates C storage via aggregation and/or formation of anaerobic (micro)sites that protect residual soil C from oxidation and may at least partially offset C losses.

Towards a global-scale soil climate mitigation strategy

Sustainable soil carbon sequestration practices need to be rapidly scaled up and implemented to contribute to climate change mitigation. We highlight that the major potential for carbon sequestration is in cropland soils, especially those with large yield gaps and/or large historic soil organic carbon losses. The implementation of soil carbon sequestration measures requires a diverse set of options, each adapted to local soil conditions and management opportunities, and accounting for site-specific trade-offs. We propose the establishment of a soil information system containing localised information on soil group, degradation status, crop yield gap, and the associated carbon-sequestration potentials, as well as the provision of incentives and policies to translate management options into region- and soil-specific practices.

Reducing soil degradation and improving soil management could make an important contribute to climate change mitigation. Here the authors discuss opportunities and challenges towards implementing a global climate mitigation strategy focused on carbon sequestration in agricultural soils, and propose a framework for guiding region- and soil-specific management options.

Reduced magnitude and shifted seasonality of CO2 sink by experimental warming in a coastal wetland

Coastal wetlands have the highest carbon sequestration rate per unit area among all unmanaged natural ecosystems. However, how the magnitude and seasonality of the CO₂ sink in coastal wetlands will respond to future climate warming remains unclear. Here, based on measurements of ecosystem CO₂ fluxes in a field experiment in the Yellow River Delta, we found that experimental warming (i.e., a 2.4°C increase in soil temperature) reduced net ecosystem productivity (NEP) by 23.7% across two growing seasons of 2017-2018. Such a reduction in NEP resulted from the greater decrease in gross primary productivity (GPP) than ecosystem respiration (ER) under warming. The negative warming effect on NEP mainly occurred in summer (-43.9%) but not in autumn (+61.3%), leading to a shifted NEP seasonality under warming. Further analyses showed that the warming effects on ecosystem CO₂ exchange were mainly controlled by soil salinity and its corresponding impacts on species composition. For example, warming increased soil salinity (+35.0%), reduced total aboveground biomass (-9.9%), and benefited the growth of plant species with high salt tolerance and late peak growth. To the best of our knowledge, this study provides the first experimental evidence on the reduced magnitude and shifted seasonality of CO₂ exchange under climate warming in coastal wetlands. These findings underscore the high vulnerability of wetland CO₂ sink in coastal regions under future climate change.

Global nitrogen input on wetland ecosystem: The driving mechanism of soil labile carbon and nitrogen on greenhouse gas emissions

Greenhouse gas emissions from wetlands are significantly promoted by global nitrogen input for changing the rate of soil carbon and nitrogen cycling, and are substantially affected by soil labile carbon and nitrogen conversely. However, the driving mechanism by which soil labile carbon and nitrogen affect greenhouse gas emissions from wetland ecosystems under global nitrogen input is not well understood. Working out the driving factor of nitrogen input on greenhouse gas emissions from wetlands is critical to reducing global warming from nitrogen input. Thus, we synthesized 72 published studies (2144 paired observations) of greenhouse gas fluxes and soil labile compounds of carbon and nitrogen (ammonium, nitrate, dissolved organic carbon, soil microbial biomass nitrogen and carbon), to understand the effects of labile carbon and nitrogen on greenhouse gas emissions under global nitrogen input. Across the data set, nitrogen input significantly promoted carbon dioxide, methane and nitrous oxide emissions from wetlands. In particular, at lower nitrogen rates (<100 kg ha^-1·yr^-1) and with added ammonium compounds, freshwater wetland significantly promoted carbon dioxide and methane emissions. Peatland was the largest nitrous oxide source under these conditions. This meta-analysis also revealed that nitrogen input stimulated dissolved organic carbon, ammonium, nitrate, microbial biomass carbon and microbial biomass nitrogen accumulation in the wetland ecosystem. The variation-partitioning analysis and structural equation model were used to analyze the relationship between the greenhouse gas and labile carbon and nitrogen further. These results revealed that dissolved organic carbon (DOC) is the primary factor driving greenhouse gas emission from wetlands under global nitrogen input, whereas microbial biomass carbon (MBC) more directly affects greenhouse gas emission than other labile carbon and nitrogen.

Methane emissions reduce the radiative cooling effect of a subtropical estuarine mangrove wetland by half

The role of coastal mangrove wetlands in sequestering atmospheric carbon dioxide (CO₂ ) and mitigating climate change has received increasing attention in recent years. While recent studies have shown that methane (CH₄ ) emissions can potentially offset the carbon burial rates in low-salinity coastal wetlands, there is hitherto a paucity of direct and year-round measurements of ecosystem-scale CH₄ flux (F_CH4 ) from mangrove ecosystems. In this study, we examined the temporal variations and biophysical drivers of ecosystem-scale F_CH4 in a subtropical estuarine mangrove wetland based on 3 years of eddy covariance measurements. Our results showed that daily mangrove F_CH4 reached a peak of over 0.1 g CH₄ -C m^-2  day^-1 during the summertime owing to a combination of high temperature and low salinity, while the wintertime F_CH4 was negligible. In this mangrove, the mean annual CH₄ emission was 11.7 ± 0.4 g CH₄ -C m^-2  year^-1 while the annual net ecosystem CO₂ exchange ranged between -891 and -690 g CO₂ -C m^-2  year^-1 , indicating a net cooling effect on climate over decadal to centurial timescales. Meanwhile, we showed that mangrove F_CH4 could offset the negative radiative forcing caused by CO₂ uptake by 52% and 24% over a time horizon of 20 and 100 years, respectively, based on the corresponding sustained-flux global warming potentials. Moreover, we found that 87% and 69% of the total variance of daily F_CH4 could be explained by the random forest machine learning algorithm and traditional linear regression model, respectively, with soil temperature and salinity being the most dominant controls. This study was the first of its kind to characterize ecosystem-scale F_CH4 in a mangrove wetland with long-term eddy covariance measurements. Our findings implied that future environmental changes such as climate warming and increasing river discharge might increase CH₄ emissions and hence reduce the net radiative cooling effect of estuarine mangrove forests.

Forgotten peatlands of eastern Australia: An unaccounted carbon capture and storage system

In a carbon-constrained world, global peatlands are vital carbon capture and storage systems. Here we calculate regional carbon stocks, sequestration rates and potential carbon emissions of Temperate Highland Peat Swamps on Sandstone (THPSS) found in low order headwater streams in eastern Australia. We find that total carbon stocks within THPSS in two regions are 25 Mt CO₂ eq. with annual carbon sequestration rates at 60.5 kt CO₂ eq. A risk assessment model, based on anthropogenic activities known to impair the carbon storage functions of THPSS is used to identify swamps most at risk of carbon loss. Potential CO₂ emissions from at risk swamps could be up to 8.6 Mt CO₂ eq. When carbon stock is valued at the current carbon abatement price of $AUD16.10 t^-1 CO₂ eq, the total value of THPSS is over AUD$404 million dollars (US$281 million). This makes a strong economic case for the implementation of sustainable swamp conservation and restoration activities.

Duration and frequency of drainage and flooding events interactively affect soil biogeochemistry and N flux in subtropical peat soils

With the demand for restoration and future prediction of climate change effects, subtropical peatlands are expected to be subjected to hydrologic regimes with variable duration and frequency of drained and flooded conditions, but knowledge of their interactive effects on soil biogeochemistry and emission of greenhouse gases including nitrous oxide (N₂O) is largely limited. The objective of this study was to investigate how the duration and frequency of drainage and flooding events interactively influence soil biogeochemical properties and denitrification and related net N₂O production rates following rewetting. Surface soils are susceptible to different hydrologic regimes. Significantly higher pH, extractable organic carbon (ext. OC), ammonium (NH₄⁺-N), denitrification enzyme activity (DEA), but lower nitrate (NO₃^--N), microbial biomass C and N were observed when the peat soils were under flooded conditions compared to drained conditions. Two-week and four-week drainage or flooding duration did not result in statistically significant differences in soil biogeochemical properties. A 24-week prolonged drainage led to an accumulation of NO₃^--N and a significantly lower pH. Soil microbial biomass and fungal:bacterial abundance likely increased with the frequency of drainage-flooding cycles. Significant differences in denitrification and net N₂O production rates following reflooding were mainly found in the surface soils. Structural equation modeling indicated that hydroperiod and water-filled pore space (WFPS) prior to reflooding is likely to control denitrification and net N₂O production through its regulation of NO₃^--N and activity of microorganisms involved in denitrification while higher drainage-flooding frequency decreases the availability of organic C and NO₃^--N for denitrification. Our results also suggest high NO₃^--N and low pH within peat soils caused by prolonged drainage likely leads to a significant N₂O emission pulse following reflooding. For peat soils subjected to frequent drainage-flooding cycles, N₂O emission pulses following reflooding would decrease with time, attributing to the loss of substrates for denitrification.

Experimental harvesting of wetland plants to evaluate trade-offs between reducing methane emissions and removing nutrients accumulated to the biomass in constructed wetlands

Constructed wetlands built for water treatment often need biomass harvesting to remove nutrients from the system. Usually harvesting is done during the peak growing season to maximize the amount of nutrients removed from the system. This, however, can create huge methane fluxes that escape from plant tissues to the atmosphere. We used manual chambers and eddy covariance measurements to analyze the increase in methane emissions due to the harvesting of two common wetland species, Typha spp. and Schoenoplectus spp., in two climatically different constructed wetlands in Estonia and California. In addition, we determined the biomass nutrient and carbon concentrations from harvested biomass. We found that harvesting during the summer season, e.g. June and August, resulted in a significant release of methane at both sites. At the California site, baseline median methane emissions were 217.6 nmol m^-2 s^-1, and harvesting resulted in increases to 395.4 nmol m^-2 s^-1 that decreased to baseline emission within three days. Footprint modeling demonstrated that the emission increases measured by eddy covariance were dominated by contributions from the cut area to the total footprint signal. At the Estonian site, harvesting resulted in methane increases of 15.9 nmol m^-2 s^-1 to 110.4 nmol m^-2 s^-1 in August. However, in September and October the emission was significantly lower. Plant biomass analyses showed clear temporal dynamics in terms of nutrient concentration, being highest in summer and lowest in winter. Our experiments indicate that the optimal time for aboveground biomass harvesting is at the end of the growing season before nutrient translocation to belowground plant structures begins coinciding with lowest methane emissions. Therefore, strategic planning of the harvest timing may help reduce greenhouse gas emissions from managed wetlands and thus improve their multi-faceted ecological benefit.

Potential greenhouse gas reductions from Natural Climate Solutions in Oregon, USA

Increasing concentrations of greenhouse gases (GHGs) are causing global climate change and decreasing the stability of the climate system. Long-term solutions to climate change will require reduction in GHG emissions as well as the removal of large quantities of GHGs from the atmosphere. Natural climate solutions (NCS), i.e., changes in land management, ecosystem restoration, and avoided conversion of habitats, have substantial potential to meet global and national greenhouse gas (GHG) reduction targets and contribute to the global drawdown of GHGs. However, the relative role of NCS to contribute to GHG reduction at subnational scales is not well known. We examined the potential for 12 NCS activities on natural and working lands in Oregon, USA to reduce GHG emissions in the context of the state's climate mitigation goals. We evaluated three alternative scenarios wherein NCS implementation increased across the applicable private or public land base, depending on the activity, and estimated the annual GHG reduction in carbon dioxide equivalents (CO2e) attributable to NCS from 2020 to 2050. We found that NCS within Oregon could contribute annual GHG emission reductions of 2.7 to 8.3 MMT CO2e by 2035 and 2.9 to 9.8 MMT CO2e by 2050. Changes in forest-based activities including deferred timber harvest, riparian reforestation, and replanting after wildfires contributed most to potential GHG reductions (76 to 94% of the overall annual reductions), followed by changes to agricultural management through no-till, cover crops, and nitrogen management (3 to 15% of overall annual reductions). GHG reduction benefits are relatively high per unit area for avoided conversion of forests (125-400 MT CO2e ha-1). However, the existing land use policy in Oregon limits the current geographic extent of active conversion of natural lands and thus, avoided conversions results in modest overall potential GHG reduction benefits (i.e., less than 5% of the overall annual reductions). Tidal wetland restoration, which has high per unit area carbon sequestration benefits (8.8 MT CO2e ha-1 yr-1), also has limited possible geographic extent resulting in low potential (< 1%) of state-level GHG reduction contributions. However, co-benefits such as improved habitat and water quality delivered by restoration NCS pathways are substantial. Ultimately, reducing GHG emissions and increasing carbon sequestration to combat climate change will require actions across multiple sectors. We demonstrate that the adoption of alternative land management practices on working lands and avoided conversion and restoration of native habitats can achieve meaningful state-level GHG reductions.

Recovery of fen peatland microbiomes and predicted functional profiles after rewetting

Many of the world's peatlands have been affected by water table drawdown and subsequent loss of organic matter. Rewetting has been proposed as a measure to restore peatland functioning and to halt carbon loss, but its effectiveness is subject to debate. An important prerequisite for peatland recovery is a return of typical microbial communities, which drive key processes. To evaluate the effect of rewetting, we investigated 13 fen peatland areas across a wide (>1500 km) longitudinal gradient in Europe, in which we compared microbial communities between drained, undrained, and rewetted sites. There was a clear difference in microbial communities between drained and undrained fens, regardless of location. Community recovery upon rewetting was substantial in the majority of sites, and predictive functional profiling suggested a concomitant recovery of biogeochemical peatland functioning. However, communities in rewetted sites were only similar to those of undrained sites when soil organic matter quality (as expressed by cellulose fractions) and quantity were still sufficiently high. We estimate that a minimum organic matter content of ca. 70% is required to enable microbial recovery. We conclude that peatland recovery after rewetting is conditional on the level of drainage-induced degradation: severely altered physicochemical peat properties may preclude complete recovery for decades.

Investigation of the climatological impacts of agricultural management and abandonment on a boreal bog in western Newfoundland, Canada

We compared greenhouse gas (GHG) fluxes and albedo of a pristine boreal bog and an adjacent abandoned peatland pasture in western Newfoundland, Canada to estimate the magnitude of radiative forcing (RF) created by agricultural drainage and abandonment. Our results indicated that these anthropogenic activities induced a climate cooling effect (negative RF), with the magnitude of the RF caused by the albedo change comparable to that induced by altered GHGs. Although the albedo-induced RF was positive in winter and negative in summer, the summer effect dominated because of greater solar radiation received. The climate cooling effect of GHGs change was due to an increase in the carbon dioxide sink capacity and a reduction in methane emissions under lower water table levels following agricultural drainage and abandonment. Calculation of sustained-flux global warming/cooling potentials also supported this finding. Our results show that the overall increase in albedo resulting from agricultural drainage and abandonment contributes significantly to the negative RF, strengthening the cooling effect due to the changing GHG fluxes. Therefore, changes in albedo due to altered vegetation coverage and hydrology and GHG fluxes should be considered when assessing the climatic impacts from land-use change in northern peatland.

From Understanding to Sustainable Use of Peatlands: The WETSCAPES Approach

Of all terrestrial ecosystems, peatlands store carbon most effectively in long-term scales of millennia. However, many peatlands have been drained for peat extraction or agricultural use. This converts peatlands from sinks to sources of carbon, causing approx. 5% of the anthropogenic greenhouse effect and additional negative effects on other ecosystem services. Rewetting peatlands can mitigate climate change and may be combined with management in the form of paludiculture. Rewetted peatlands, however, do not equal their pristine ancestors and their ecological functioning is not understood. This holds true especially for groundwater-fed fens. Their functioning results from manifold interactions and can only be understood following an integrative approach of many relevant fields of science, which we merge in the interdisciplinary project WETSCAPES. Here, we address interactions among water transport and chemistry, primary production, peat formation, matter transformation and transport, microbial community, and greenhouse gas exchange using state of the art methods. We record data on six study sites spread across three common fen types (Alder forest, percolation fen, and coastal fen), each in drained and rewetted states. First results revealed that indicators reflecting more long-term effects like vegetation and soil chemistry showed a stronger differentiation between drained and rewetted states than variables with a more immediate reaction to environmental change, like greenhouse gas (GHG) emissions. Variations in microbial community composition explained differences in soil chemical data as well as vegetation composition and GHG exchange. We show the importance of developing an integrative understanding of managed fen peatlands and their ecosystem functioning.

Conversion of coastal wetlands, riparian wetlands, and peatlands increases greenhouse gas emissions: A global meta-analysis

Land-use/land-cover change (LULCC) often results in degradation of natural wetlands and affects the dynamics of greenhouse gases (GHGs). However, the magnitude of changes in GHG emissions from wetlands undergoing various LULCC types remains unclear. We conducted a global meta-analysis with a database of 209 sites to examine the effects of LULCC types of constructed wetlands (CWs), croplands (CLs), aquaculture ponds (APs), drained wetlands (DWs), and pastures (PASs) on the variability in CO₂ , CH₄ , and N₂ O emissions from the natural coastal wetlands, riparian wetlands, and peatlands. Our results showed that the natural wetlands were net sinks of atmospheric CO₂ and net sources of CH₄ and N₂ O, exhibiting the capacity to mitigate greenhouse effects due to negative comprehensive global warming potentials (GWPs; -0.9 to -8.7 t CO₂ -eq ha^-1  year^-1 ). Relative to the natural wetlands, all LULCC types (except CWs from coastal wetlands) decreased the net CO₂ uptake by 69.7%-456.6%, due to a higher increase in ecosystem respiration relative to slight changes in gross primary production. The CWs and APs significantly increased the CH₄ emissions compared to those of the coastal wetlands. All LULCC types associated with the riparian wetlands significantly decreased the CH₄ emissions. When the peatlands were converted to the PASs, the CH₄ emissions significantly increased. The CLs, as well as DWs from peatlands, significantly increased the N₂ O emissions in the natural wetlands. As a result, all LULCC types (except PASs from riparian wetlands) led to remarkably higher GWPs by 65.4%-2,948.8%, compared to those of the natural wetlands. The variability in GHG fluxes with LULCC was mainly sensitive to changes in soil water content, water table, salinity, soil nitrogen content, soil pH, and bulk density. This study highlights the significant role of LULCC in increasing comprehensive GHG emissions from global natural wetlands, and our results are useful for improving future models and manipulative experiments.

Gap-filling approaches for eddy covariance methane fluxes: A comparison of three machine learning algorithms and a traditional method with principal component analysis

Methane flux (FCH₄ ) measurements using the eddy covariance technique have increased over the past decade. FCH₄ measurements commonly include data gaps, as is the case with CO₂ and energy fluxes. However, gap-filling FCH₄ data are more challenging than other fluxes due to its unique characteristics including multidriver dependency, variabilities across multiple timescales, nonstationarity, spatial heterogeneity of flux footprints, and lagged influence of biophysical drivers. Some researchers have applied a marginal distribution sampling (MDS) algorithm, a standard gap-filling method for other fluxes, to FCH₄ datasets, and others have applied artificial neural networks (ANN) to resolve the challenging characteristics of FCH₄ . However, there is still no consensus regarding FCH₄ gap-filling methods due to limited comparative research. We are not aware of the applications of machine learning (ML) algorithms beyond ANN to FCH₄ datasets. Here, we compare the performance of MDS and three ML algorithms (ANN, random forest [RF], and support vector machine [SVM]) using multiple combinations of ancillary variables. In addition, we applied principal component analysis (PCA) as an input to the algorithms to address multidriver dependency of FCH₄ and reduce the internal complexity of the algorithmic structures. We applied this approach to five benchmark FCH₄ datasets from both natural and managed systems located in temperate and tropical wetlands and rice paddies. Results indicate that PCA improved the performance of MDS compared to traditional inputs. ML algorithms performed better when using all available biophysical variables compared to using PCA-derived inputs. Overall, RF was found to outperform other techniques for all sites. We found gap-filling uncertainty is much larger than measurement uncertainty in accumulated CH₄ budget. Therefore, the approach used for FCH₄ gap filling can have important implications for characterizing annual ecosystem-scale methane budgets, the accuracy of which is important for evaluating natural and managed systems and their interactions with global change processes.

Assessing passive rehabilitation for carbon gains in rain-filled agricultural wetlands

Wetland ecosystems have a disproportionally large influence on the global carbon cycle. They can act as carbon sinks or sources depending upon their location, type, and condition. Rehabilitation of wetlands is gaining popularity as a nature-based approach to helping mitigate climate change; however, few studies have empirically tested the carbon benefits of wetland restoration, especially in freshwater environments. Here we investigated the effects of passive rehabilitation (i.e. fencing and agricultural release) of 16 semi-arid rain-filled freshwater wetlands in southeastern Australia. Eight control sites were compared with older (>10 year) or newer (2-5 year) rehabilitated sites, dominated by graminoids or eucalypts. Carbon stocks (soils and plant biomass), and emissions (carbon dioxide - CO₂; and methane - CH₄) were sampled across three seasons, representing natural filling and drawdown, and soil microbial communities were sampled in spring. We found no significant difference in soil carbon or greenhouse gas emissions between rehabilitated and control sites, however, plant biomass was significantly higher in older rehabilitated sites. Wetland carbon stocks were 19.21 t C_org ha^-1 and 2.84 t C_org ha^-1 for soils (top 20 cm; n = 137) and plant biomass (n = 288), respectively. Hydrology was a strong driver of wetland greenhouse gas emissions. Diffusive fluxes (n = 356) averaged 117.63 mmol CO₂ m² d^-1 and 2.98 mmol CH₄ m² d^-1 when wet, and 124.01 mmol CO₂ m² d^-1 and -0.41 mmol CH₄ m² d^-1 when dry. Soil microbial community richness was nearly 2-fold higher during the wet phase than the dry phase, including relative increases in Nitrososphaerales, Myxococcales and Koribacteraceae and methanogens Methanobacteriales. Vegetation type significantly influenced soil carbon, aboveground carbon, and greenhouse gas emissions. Overall, our results suggest that passive rehabilitation of rain-filled wetlands, while valuable for biodiversity and habitat provisioning, is ineffective for increasing carbon gains within 20 years. Carbon offsetting opportunities may be better in systems with faster sediment accretion. Active rehabilitation methods, particularly that reinstate the natural hydrology of drained wetlands, should also be considered.

Where old meets new: An ecosystem study of methanogenesis in a reflooded agricultural peatland

Reflooding formerly drained peatlands has been proposed as a means to reduce losses of organic matter and sequester soil carbon for climate change mitigation, but a renewal of high methane emissions has been reported for these ecosystems, offsetting mitigation potential. Our ability to interpret observed methane fluxes in reflooded peatlands and make predictions about future flux trends is limited due to a lack of detailed studies of methanogenic processes. In this study we investigate methanogenesis in a reflooded agricultural peatland in the Sacramento Delta, California. We use the stable-and radio-carbon isotopic signatures of wetland sediment methane, ecosystem-scale eddy covariance flux observations, and laboratory incubation experiments, to identify which carbon sources and methanogenic production pathways fuel methanogenesis and how these processes are affected by vegetation and seasonality. We found that the old peat contribution to annual methane emissions was large (~30%) compared to intact wetlands, indicating a biogeochemical legacy of drainage. However, fresh carbon and the acetoclastic pathway still accounted for the majority of methanogenesis throughout the year. Although temperature sensitivities for bulk peat methanogenesis were similar between open-water (Q₁₀  = 2.1) and vegetated (Q₁₀  = 2.3) soils, methane production from both fresh and old carbon sources showed pronounced seasonality in vegetated zones. We conclude that high methane emissions in restored wetlands constitute a biogeochemical trade-off with contemporary carbon uptake, given that methane efflux is fueled primarily by fresh carbon inputs.

Greenhouse gas emissions resulting from conversion of peat swamp forest to oil palm plantation

Conversion of tropical peat swamp forest to drainage-based agriculture alters greenhouse gas (GHG) production, but the magnitude of these changes remains highly uncertain. Current emissions factors for oil palm grown on drained peat do not account for temporal variation over the plantation cycle and only consider CO₂ emissions. Here, we present direct measurements of GHGs emitted during the conversion from peat swamp forest to oil palm plantation, accounting for CH₄ and N₂O as well as CO₂. Our results demonstrate that emissions factors for converted peat swamp forest is in the range 70–117 t CO₂ eq ha⁻¹ yr⁻¹ (95% confidence interval, CI), with CO₂ and N₂O responsible for ca. 60 and ca. 40% of this value, respectively. These GHG emissions suggest that conversion of Southeast Asian peat swamp forest is contributing between 16.6 and 27.9% (95% CI) of combined total national GHG emissions from Malaysia and Indonesia or 0.44 and 0.74% (95% CI) of annual global emissions.

The magnitude of greenhouse gas emissions from land use change on tropical peatlands is unclear. Here, the authors measure greenhouse gas fluxes throughout the conversion from peat swamp forest to oil palm plantation, and estimate the contribution to regional and global emissions.

Error characterization of methane fluxes and budgets derived from a long-term comparison of open- and closed-path eddy covariance systems

Wetlands represent the dominant natural source of methane (CH₄) to the atmosphere. Thus, substantial effort has been spent examining the CH₄ budgets of global wetlands via continuous ecosystem-scale measurements using the eddy covariance (EC) technique. Robust error characterization for such measurements, however, remains a major challenge. Here, we quantify systematic, random and gap-filling errors and the resulting uncertainty in CH₄ fluxes using a 3.5 year time series of simultaneous open- and closed path CH₄ flux measurements over a sub-boreal wetland. After correcting for high- and low frequency flux attenuation, the magnitude of systematic frequency response errors were negligible relative to other uncertainties. Based on three different random flux error estimations, we found that errors of the CH₄ flux measurement systems were smaller in magnitude than errors associated with the turbulent transport and flux footprint heterogeneity. Errors on individual half-hourly CH₄ fluxes were typically 6%-41%, but not normally distributed (leptokurtic), and thus need to be appropriately characterized when fluxes are compared to chamber-derived or modeled CH₄ fluxes. Integrated annual fluxes were only moderately sensitive to gap-filling, based on an evaluation of 4 different methods. Calculated budgets agreed on average to within 7% (≤ 1.5 g - CH₄ m^-2 yr^-1). Marginal distribution sampling using open source code was among the best-performing of all the evaluated gap-filling approaches and it is therefore recommended given its transparency and reproducibility. Overall, estimates of annual CH₄ emissions for both EC systems were in excellent agreement (within 0.6 g - CH₄ m^-2 yr^-1) and averaged 18 g - CH₄ m^-2 yr^-1. Total uncertainties on the annual fluxes were larger than the uncertainty of the flux measurement systems and estimated between 7-17%. Identifying trends and differences among sites or site years requires that the observed variability exceeds these uncertainties.

Variations and drivers of methane fluxes from a rice-wheat rotation agroecosystem in eastern China at seasonal and diurnal scales

The paddy rice fields act as an important anthropogenic source of methane (CH₄) to the atmosphere. The study of pattern, magnitude, and environmental controls of CH₄ emissions are still insufficient due to limited measurements and understand of underlying drivers for variations of CH₄ fluxes at different temporal scales. In this study, CH₄ fluxes from a rice-wheat rotation agroecosystem in eastern China were continuously measured using the eddy covariance technique. The diurnal and seasonal variations of CH₄ flux and potential controlling factors in 2016 were analyzed using wavelet coherence, conditional Granger causality, correlation analysis and path analysis methods. CH₄ fluxes showed distinguishable diurnal variations with single peaks during 13: 00-16: 00 local time. At the diurnal timescale, gross primary productivity (GPP) regulates CH₄ fluxes after accounting for the effects of latent heat flux (LE), air temperature (TA), and soil temperature (TS) on CH₄ fluxes. LE mirrored the diurnal pattern of CH₄ fluxes when the effects of TA and TS on CH₄ fluxes were considered. Daily CH₄ fluxes exhibited large seasonal variations, with the largest daily CH₄ flux of 1191.78 mg C-CH₄ m^-2 d^-1 on 29 July 2016. The daily CH₄ fluxes were continuously low in the growing season of wheat, and sharply increased from very low values in late June to peaks in late July and early August, and then gradually decreased to low values at the end of the rice growing season in late November and early December. Correlation analysis and path analysis showed that seasonal variations of soil temperature, air temperature, and GPP had strong effects on daily CH₄ fluxes during pre-panicle initiation of the rice growing season, while soil temperature and leaf area index (LAI) had very strong effects on daily CH₄ fluxes during the post-panicle initiation stage. The total of CH₄ fluxes from the rice-wheat rotation agroecosystem into the atmosphere amounted to 58.08 ± 9.87 g C m^-2 in 2016, and the annual net carbon (C) budget and greenhouse gas (GHG) budget were 163.50 ± 9.87 g C m^-2 and 2322.53 ± 329.00 g CO₂eq m^-2, respectively. This study represents a comprehensive assessment of fluxes and drivers of CH₄ from a rice-wheat rotation agroecosystem at different timescales. Additionally, the consecutive data of CH₄ emission in this region will also useful for model calibration and validation.

Methane flux dynamics in a submerged aquatic vegetation zone in a subtropical lake

Submerged macrophytes are important primary producers for shallow lake systems. So far, their overall role in regulating lake methane flux is a subject of debate because the oxygen produced by their roots can promote methane oxidation in the sediment but they can also enhance methanogenesis through organic substrate production. In this study, we used the eddy covariance method to investigate the temporal dynamics of the CH₄ flux in a habitat of submerged macrophytes in Lake Taihu. The results show that the nighttime CH₄ flux is on average 33% higher than the daytime flux, although a clear diurnal pattern is evident only in the spring. At the daily to the seasonal time scale, the sediment temperature is the main driver of the CH₄ flux variations, implying higher methane production in the sediment at higher temperatures. The annual CH₄ emission (6.12 g C m^-2 yr^-1) is much higher than the published whole-lake mean flux (1.12 g C m^-2 yr^-1) and that reported previously in the eutrophic phytoplankton zone of the lake (1.35 g C m^-2 yr^-1), indicating that the net effect of the submerged macrophytes is to enhance methane emission. At the annual time scale, 3.5% of the carbon gained by the net ecosystem production is lost to the atmosphere in the form of CH₄.

The physics and ecology of mining carbon dioxide from the atmosphere by ecosystems

Reforesting and managing ecosystems have been proposed as ways to mitigate global warming and offset anthropogenic carbon emissions. The intent of our opinion piece is to provide a perspective on how well plants and ecosystems sequester carbon. The ability of individual plants and ecosystems to mine carbon dioxide from the atmosphere, as defined by rates and cumulative amounts, is limited by laws of physics and ecological principles. Consequently, the rates and amount of net carbon uptake are slow and low compared to the rates and amounts of carbon dioxide we release by fossil fuels combustion. Managing ecosystems to sequester carbon can also cause unintended consequences to arise. In this paper, we articulate a series of key take-home points. First, the potential amount of carbon an ecosystem can assimilate on an annual basis scales with absorbed sunlight, which varies with latitude, leaf area index and available water. Second, efforts to improve photosynthesis will come with the cost of more respiration. Third, the rates and amount of net carbon uptake are relatively slow and low, compared to the rates and amounts and rates of carbon dioxide we release by fossil fuels combustion. Fourth, huge amounts of land area for ecosystems will be needed to be an effective carbon sink to mitigate anthropogenic carbon emissions. Fifth, the effectiveness of using this land as a carbon sink will depend on its ability to remain as a permanent carbon sink. Sixth, converting land to forests or wetlands may have unintended costs that warm the local climate, such as changing albedo, increasing surface roughness or releasing other greenhouse gases. We based our analysis on 1,163 site-years of direct eddy covariance measurements of gross and net carbon fluxes from 155 sites across the globe.

Blue carbon potential of coastal wetland restoration varies with inundation and rainfall

There is a growing interest in how the management of 'blue carbon' sequestered by coastal wetlands can influence global greenhouse gas (GHG) budgets. A promising intervention is through restoring tidal exchange to impounded coastal wetlands for reduced methane (CH₄) emissions. We monitored an impounded wetland's GHG flux (CO₂ and CH₄) prior to and following tidal reinstatement. We found that biogeochemical responses varied across an elevation gradient. The low elevation zone experienced a greater increase in water level and an associated greater marine transition in the sediment microbial community (16 S rRNA) than the high elevation zone. The low elevation zone's GHG emissions had a reduced sustained global warming potential of 264 g m^-2 yr^-1 CO₂-e over 100 years, and it increased to 351 g m^-2 yr^-1 with the removal of extreme rain events. However, emission benefits were achieved through a reduction in CO₂ emissions, not CH₄ emissions. Overall, the wetland shifted from a prior CH₄ sink (-0.07 to -1.74 g C m^-2 yr^-1) to a variable sink or source depending on the elevation site and rainfall. This highlights the need to consider a wetland's initial GHG emissions, elevation and future rainfall trends when assessing the efficacy of tidal reinstatement for GHG emission control.