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1. Carbon Dioxide and Methane Emissions from a Temperate Salt Marsh Tidal Creek: Dataset

   https://doi.org/10.6084/m9.figshare.12340580  

   Trifunovic, Branimir; Vazquez-Lule, Alma; Capooci, Margaret; Seyfferth, Angelia; Moffat, Carlos; Vargas, Rodrigo (January 2020, figshare)

   Coastal salt marshes store large amounts of carbon but the magnitude and patterns of greenhouse gas (GHG; i.e., carbon dioxide (CO₂) and methane (CH4</sub>)) fluxes are unclear. Information about GHG fluxes from these ecosystems comes from studies of sediments or at the ecosystem-scale (eddy covariance) but fluxes from tidal creeks are unknown. </div>This dataset includes GHG concentrations in water, water quality, meteorology, sediment CO2</sub> efflux, ecosystem-scale GHG fluxes, and plant phenology; all at half-hour time-steps over one year.</div></div>This study was carried out in the St. Jones Reserve, a component of the Delaware National Estuarine Research Reserve in Dover, Delaware, U.S.A. The study site is part of the following networks:</div></div>- AmeriFlux (https://ameriflux.lbl.gov/sites/siteinfo/US-StJ) </div>- Phenocam (https://phenocam.sr.unh.edu/webcam/sites/stjones/) </div></div>The GHG concentration and efflux sampling point was located at Aspen Landing within a microtidal (mean tide range of 1.5 m), mesohaline (typical salinity of 5-18 ppt) salt marsh (Delaware Department of Natural Resources and Environmental Control, 2006) tidal creek.</div></div>Main reference:</div> Trifunovic, B., Vázquez‐Lule, A., Capooci, M., Seyfferth, A. L., Moffat, C., & Vargas, R. (2020). Carbon dioxide and methane emissions from a temperate salt marsh tidal creek. Journal of Geophysical Research: Biogeosciences, 125, e2019JG005558. https://doi.org/ 10.1029/2019JG005558 </p> </div> </div> </div></div></sub> 

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2. High methane concentrations in tidal salt marsh soils: Where does the methane go?

   https://doi.org/10.1111/gcb.17050  

   Capooci, Margaret; Seyfferth, Angelia_L; Tobias, Craig; Wozniak, Andrew_S; Hedgpeth, Alexandra; Bowen, Malique; Biddle, Jennifer_F; McFarlane, Karis_J; Vargas, Rodrigo (November 2023, Global Change Biology)

   Abstract Tidal salt marshes produce and emit CH₄. Therefore, it is critical to understand the biogeochemical controls that regulate CH₄spatial and temporal dynamics in wetlands. The prevailing paradigm assumes that acetoclastic methanogenesis is the dominant pathway for CH₄production, and higher salinity concentrations inhibit CH₄production in salt marshes. Recent evidence shows that CH₄is produced within salt marshes via methylotrophic methanogenesis, a process not inhibited by sulfate reduction. To further explore this conundrum, we performed measurements of soil–atmosphere CH₄and CO₂fluxes coupled with depth profiles of soil CH₄and CO₂pore water gas concentrations, stable and radioisotopes, pore water chemistry, and microbial community composition to assess CH₄production and fate within a temperate tidal salt marsh. We found unexpectedly high CH₄concentrations up to 145,000 μmol mol⁻¹positively correlated with S²⁻(salinity range: 6.6–14.5 ppt). Despite large CH₄production within the soil, soil–atmosphere CH₄fluxes were low but with higher emissions and extreme variability during plant senescence (84.3 ± 684.4 nmol m⁻² s⁻¹). CH₄and CO₂within the soil pore water were produced from young carbon, with most Δ¹⁴C‐CH₄and Δ¹⁴C‐CO₂values at or above modern. We found evidence that CH₄within soils was produced by methylotrophic and hydrogenotrophic methanogenesis. Several pathways exist after CH₄is produced, including diffusion into the atmosphere, CH₄oxidation, and lateral export to adjacent tidal creeks; the latter being the most likely dominant flux. Our findings demonstrate that CH₄production and fluxes are biogeochemically heterogeneous, with multiple processes and pathways that can co‐occur and vary in importance over the year. This study highlights the potential for high CH₄production, the need to understand the underlying biogeochemical controls, and the challenges of evaluating CH₄budgets and blue carbon in salt marshes. 

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3. Automated measurements of greenhouse gases fluxes from tree stems and soils: magnitudes, patterns and drivers

   https://doi.org/10.1038/s41598-019-39663-8  

   Barba, Josep; Poyatos, Rafael; Vargas, Rodrigo (March 2019, Scientific Reports)

   Abstract Tree stems exchange CO₂, CH₄and N₂O with the atmosphere but the magnitudes, patterns and drivers of these greenhouse gas (GHG) fluxes remain poorly understood. Our understanding mainly comes from static-manual measurements, which provide limited information on the temporal variability and magnitude of these fluxes. We measured hourly CO₂, CH₄and N₂O fluxes at two stem heights and adjacent soils within an upland temperate forest. We analyzed diurnal and seasonal variability of fluxes and biophysical drivers (i.e., temperature, soil moisture, sap flux). Tree stems were a net source of CO₂(3.80 ± 0.18 µmol m⁻²s⁻¹; mean ± 95% CI) and CH₄(0.37 ± 0.18 nmol m⁻²s⁻¹), but a sink for N₂O (−0.016 ± 0.008 nmol m⁻²s⁻¹). Time series analysis showed diurnal temporal correlations between these gases with temperature or sap flux for certain days. CO₂and CH₄showed a clear seasonal pattern explained by temperature, soil water content and sap flux. Relationships between stem, soil fluxes and their drivers suggest that CH₄for stem emissions could be partially produced belowground. High-frequency measurements demonstrate that: a) tree stems exchange GHGs with the atmosphere at multiple time scales; and b) are needed to better estimate fluxes magnitudes and understand underlying mechanisms of GHG stem emissions. 

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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. Eddy Covariance Data Reveal That a Small Freshwater Reservoir Emits a Substantial Amount of Carbon Dioxide and Methane

   https://doi.org/10.1029/2022JG007091  

   Hounshell, Alexandria G.; D’Acunha, Brenda M.; Breef‐Pilz, Adrienne; Johnson, Mark S.; Thomas, R. Quinn; Carey, Cayelan C. (March 2023, Journal of Geophysical Research: Biogeosciences)

   Abstract Small freshwater reservoirs are ubiquitous and likely play an important role in global greenhouse gas (GHG) budgets relative to their limited water surface area. However, constraining annual GHG fluxes in small freshwater reservoirs is challenging given their footprint area and spatially and temporally variable emissions. To quantify the GHG budget of a small (0.1 km²) reservoir, we deployed an Eddy covariance (EC) system in a small reservoir located in southwestern Virginia, USA over 2 years to measure carbon dioxide (CO₂) and methane (CH₄) fluxes near‐continuously. Fluxes were coupled with in situ sensors measuring multiple environmental parameters. Over both years, we found the reservoir to be a large source of CO₂(633–731 g CO₂‐C m⁻² yr⁻¹) and CH₄(1.02–1.29 g CH₄‐C m⁻² yr⁻¹) to the atmosphere, with substantial sub‐daily, daily, weekly, and seasonal timescales of variability. For example, fluxes were substantially greater during the summer thermally stratified season as compared to the winter. In addition, we observed significantly greater GHG fluxes during winter intermittent ice‐on conditions as compared to continuous ice‐on conditions, suggesting GHG emissions from lakes and reservoirs may increase with predicted decreases in winter ice‐cover. Finally, we identified several key environmental variables that may be driving reservoir GHG fluxes at multiple timescales, including, surface water temperature and thermocline depth followed by fluorescent dissolved organic matter. Overall, our novel year‐round EC data from a small reservoir indicate that these freshwater ecosystems likely contribute a substantial amount of CO₂and CH₄to global GHG budgets, relative to their surface area. 

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