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1. Net Methane Production Predicted by Patch Characteristics in a Freshwater Wetland

   https://doi.org/10.1029/2023JG007814  

   Sharp, Sean J.; Maietta, Christine E.; Stewart, Graham A.; Taylor, Aileen K.; Williams, Michael R.; Palmer, Margaret A. (December 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Methane (CH₄) dynamics in wetlands are spatially variable and difficult to estimate at ecosystem scales. Patches with different plant functional types (PFT) represent discrete units within wetlands that may help characterize patterns in CH₄variability. We investigate dissolved porewater CH₄concentrations, a representation of net CH₄production and potential source of atmospheric flux, in five wetland patches characterized by a dominant PFT or lack of plants. Using soil, porewater, and plant variables we hypothesized to influence CH₄, we used three modeling approaches—Classification and regression tree, AIC model selection, and Structural Equation Modeling—to identify direct and indirect influences on porewater CH₄dynamics. Across all three models, dissolved porewater CO₂concentration was the dominant driver of CH₄concentrations, partly through the influence of PFT patches. Plants in each patch type likely had variable influence on CH₄via root exudates (a substrate for methanogens), capacity to transport gas (both O₂from and CH₄to the atmosphere), and plant litter quality which impacted soil respiration and production of CO₂in the porewater. We attribute the importance of CO₂to the dominant methanogenic pathway we identified, which uses CO₂as a terminal electron acceptor. We propose a mechanistic relationship between PFT patches and porewater CH₄dynamics which, when combined with sources of CH₄loss including methanotrophy, oxidation, or plant‐mediated transport, can provide patch‐scale estimates of CH₄flux. Combining these estimates with the distribution of PFTs can improve ecosystem CH₄flux estimates in heterogenous wetlands and improve global CH₄budgets. 

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2. FLUXNET-CH₄: a global, multi-ecosystem dataset and analysis of methane seasonality from freshwater wetlands

   https://doi.org/10.5194/essd-13-3607-2021  

   Delwiche, Kyle B.; Knox, Sara Helen; Malhotra, Avni; Fluet-Chouinard, Etienne; McNicol, Gavin; Feron, Sarah; Ouyang, Zutao; Papale, Dario; Trotta, Carlo; Canfora, Eleonora; et al (January 2021, Earth System Science Data)

   null (Ed.)

   Abstract. Methane (CH4) emissions from natural landscapes constituteroughly half of global CH4 contributions to the atmosphere, yet largeuncertainties remain in the absolute magnitude and the seasonality ofemission quantities and drivers. Eddy covariance (EC) measurements ofCH4 flux are ideal for constraining ecosystem-scale CH4emissions due to quasi-continuous and high-temporal-resolution CH4flux measurements, coincident carbon dioxide, water, and energy fluxmeasurements, lack of ecosystem disturbance, and increased availability ofdatasets over the last decade. Here, we (1) describe the newly publisheddataset, FLUXNET-CH4 Version 1.0, the first open-source global dataset ofCH4 EC measurements (available athttps://fluxnet.org/data/fluxnet-ch4-community-product/, last access: 7 April 2021). FLUXNET-CH4includes half-hourly and daily gap-filled and non-gap-filled aggregatedCH4 fluxes and meteorological data from 79 sites globally: 42freshwater wetlands, 6 brackish and saline wetlands, 7 formerly drainedecosystems, 7 rice paddy sites, 2 lakes, and 15 uplands. Then, we (2) evaluate FLUXNET-CH4 representativeness for freshwater wetland coverageglobally because the majority of sites in FLUXNET-CH4 Version 1.0 arefreshwater wetlands which are a substantial source of total atmosphericCH4 emissions; and (3) we provide the first global estimates of theseasonal variability and seasonality predictors of freshwater wetlandCH4 fluxes. Our representativeness analysis suggests that thefreshwater wetland sites in the dataset cover global wetland bioclimaticattributes (encompassing energy, moisture, and vegetation-relatedparameters) in arctic, boreal, and temperate regions but only sparselycover humid tropical regions. Seasonality metrics of wetland CH4emissions vary considerably across latitudinal bands. In freshwater wetlands(except those between 20∘ S to 20∘ N) the spring onsetof elevated CH4 emissions starts 3 d earlier, and the CH4emission season lasts 4 d longer, for each degree Celsius increase in meanannual air temperature. On average, the spring onset of increasing CH4emissions lags behind soil warming by 1 month, with very few sites experiencingincreased CH4 emissions prior to the onset of soil warming. Incontrast, roughly half of these sites experience the spring onset of risingCH4 emissions prior to the spring increase in gross primaryproductivity (GPP). The timing of peak summer CH4 emissions does notcorrelate with the timing for either peak summer temperature or peak GPP.Our results provide seasonality parameters for CH4 modeling andhighlight seasonality metrics that cannot be predicted by temperature or GPP(i.e., seasonality of CH4 peak). FLUXNET-CH4 is a powerful new resourcefor diagnosing and understanding the role of terrestrial ecosystems andclimate drivers in the global CH4 cycle, and future additions of sitesin tropical ecosystems and site years of data collection will provide addedvalue to this database. All seasonality parameters are available athttps://doi.org/10.5281/zenodo.4672601 (Delwiche et al., 2021).Additionally, raw FLUXNET-CH4 data used to extract seasonality parameterscan be downloaded from https://fluxnet.org/data/fluxnet-ch4-community-product/ (last access: 7 April 2021), and a completelist of the 79 individual site data DOIs is provided in Table 2 of this paper. 

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3. Lagged Wetland CH ₄ Flux Response in a Historically Wet Year

   https://doi.org/10.1029/2021JG006458  

   Turner, J.; Desai, A. R.; Thom, J.; Wickland, K. P. (November 2021, Journal of Geophysical Research: Biogeosciences)

   Abstract While a stimulating effect of plant primary productivity on soil carbon dioxide (CO₂) emissions has been well documented, links between gross primary productivity (GPP) and wetland methane (CH₄) emissions are less well investigated. Determination of the influence of primary productivity on wetland CH₄emissions (FCH₄) is complicated by confounding influences of water table level and temperature on CH₄production, which also vary seasonally. Here, we evaluate the link between preceding GPP and subsequent FCH₄at two fens in Wisconsin using eddy covariance flux towers, Lost Creek (US‐Los) and Allequash Creek (US‐ALQ). Both wetlands are mosaics of forested and shrub wetlands, with US‐Los being larger in scale and having a more open canopy. Co‐located sites with multi‐year observations of flux, hydrology, and meteorology provide an opportunity to measure and compare lag effects on FCH₄without interference due to differing climate. Daily average FCH₄from US‐Los reached a maximum of 47.7 ηmol CH₄m⁻² s⁻¹during the study period, while US‐ALQ was more than double at 117.9 ηmol CH₄ m⁻² s⁻¹. The lagged influence of GPP on temperature‐normalized FCH₄(T_air‐FCH₄) was weaker and more delayed in a year with anomalously high precipitation than a following drier year at both sites. FCH₄at US‐ALQ was lower coincident with higher stream discharge in the wet year (2019), potentially due to soil gas flushing during high precipitation events and lower water temperatures. Better understanding of the lagged influence of GPP on FCH₄due to this study has implications for climate modeling and more accurate carbon budgeting. 

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4. Methane fluxes in tidal marshes of the conterminous United States

   https://doi.org/10.1111/gcb.17462  

   Arias‐Ortiz, Ariane; Wolfe, Jaxine; Bridgham, Scott_D; Knox, Sara; McNicol, Gavin; Needelman, Brian_A; Shahan, Julie; Stuart‐Haëntjens, Ellen_J; Windham‐Myers, Lisamarie; Oikawa, Patty_Y; et al (September 2024, Global Change Biology)

   Abstract Methane (CH₄) is a potent greenhouse gas (GHG) with atmospheric concentrations that have nearly tripled since pre‐industrial times. Wetlands account for a large share of global CH₄emissions, yet the magnitude and factors controlling CH₄fluxes in tidal wetlands remain uncertain. We synthesized CH₄flux data from 100 chamber and 9 eddy covariance (EC) sites across tidal marshes in the conterminous United States to assess controlling factors and improve predictions of CH₄emissions. This effort included creating an open‐source database of chamber‐based GHG fluxes (https://doi.org/10.25573/serc.14227085). Annual fluxes across chamber and EC sites averaged 26 ± 53 g CH₄m⁻² year⁻¹, with a median of 3.9 g CH₄m⁻² year⁻¹, and only 25% of sites exceeding 18 g CH₄m⁻² year⁻¹. The highest fluxes were observed at fresh‐oligohaline sites with daily maximum temperature normals (MATmax) above 25.6°C. These were followed by frequently inundated low and mid‐fresh‐oligohaline marshes with MATmax ≤25.6°C, and mesohaline sites with MATmax >19°C. Quantile regressions of paired chamber CH₄flux and porewater biogeochemistry revealed that the 90th percentile of fluxes fell below 5 ± 3 nmol m⁻² s⁻¹at sulfate concentrations >4.7 ± 0.6 mM, porewater salinity >21 ± 2 psu, or surface water salinity >15 ± 3 psu. Across sites, salinity was the dominant predictor of annual CH₄fluxes, while within sites, temperature, gross primary productivity (GPP), and tidal height controlled variability at diel and seasonal scales. At the diel scale, GPP preceded temperature in importance for predicting CH₄flux changes, while the opposite was observed at the seasonal scale. Water levels influenced the timing and pathway of diel CH₄fluxes, with pulsed releases of stored CH₄at low to rising tide. This study provides data and methods to improve tidal marsh CH₄emission estimates, support blue carbon assessments, and refine national and global GHG inventories. 

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5. Methane and Carbon Dioxide Fluxes in a Temperate Tidal Salt Marsh: Comparisons Between Plot and Ecosystem Measurements

   https://doi.org/10.1029/2022JG006943  

   Hill, Andrew C.; Vargas, Rodrigo (July 2022, Journal of Geophysical Research: Biogeosciences)

   Abstract Tidal wetlands are comprised of complex interdependent pathways where measurements of carbon exchange are often scale dependent. Common data collection methods (i.e., chambers and eddy covariance) are inherently constrained to different spatial and temporal scales which could generate biased information for applications of carbon accounting, identifying functional relationships and predicting future responses to climate change. Consequently, it is needed to systematically evaluate measurements derived from multiple approaches to identify differences and how techniques complement each other to reconcile interpretations. To accomplish this, we tested ecosystem‐scale eddy covariance with plot‐scale chamber measurements within a temperate salt marsh. We found good agreement (R² = 0.71–0.95) when comparing measurements of CH₄emissions and CO₂exchange but this agreement was dependent upon canopy phenology with discrepancies mainly arising during senescence and dormancy phenophases. The environmental drivers for CH₄and CO₂fluxes were mostly preserved across different measurement techniques, but the number of drivers increases while their individual strength decreases at the ecosystem scale. Empirical upscaling models parameterized with chamber measurements overestimated annual net ecosystem exchange (NEE; 108%) and gross primary production (GPP; 12%) while underestimating ecosystem respiration (Reco; 14%) and CH₄emissions (69%) compared to eddy covariance measurements. Our results suggest that the environmental complexity of CH₄and CO₂fluxes in salt marshes may be underestimated by chamber‐based measurements, and highlights how different techniques are complementary while considering limitations at each level of measurement. 

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