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Abstract. Tidal salt marsh soils can be a dynamic source of greenhouse gases such ascarbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O),as well as sulfur-based trace gases such as carbon disulfide (CS2) anddimethylsulfide (DMS) which play roles in global climate and carbon–sulfurbiogeochemistry. Due to the difficulty in measuring trace gases in coastalecosystems (e.g., flooding, salinity), our current understanding is based onsnapshot instantaneous measurements (e.g., performed during daytime lowtide) which complicates our ability to assess the role of these ecosystemsfor natural climate solutions. We performed continuous, automatedmeasurements of soil trace gas fluxes throughout the growing season toobtain high-temporal frequency data and to provide insights into magnitudesand temporal variability across rapidly changing conditions such as tidalcycles. We found that soil CO2 fluxes did not show a consistent dielpattern, CH4, N2O, and CS2 fluxes were highly variable withfrequent pulse emissions (> 2500 %, > 10 000 %,and > 4500 % change, respectively), and DMS fluxes onlyoccurred midday with changes > 185 000 %. When we comparedcontinuous measurements with discrete temporal measurements (during daytime,at low tide), discrete measurements of soil CO2 fluxes were comparablewith those from continuous measurements but misrepresent the temporalvariability and magnitudes of CH4, N2O, DMS, and CS2.Discrepancies between the continuous and discrete measurement data result indifferences for calculating the sustained global warming potential (SGWP),mainly by an overestimation of CH4 fluxes when using discretemeasurements. The high temporal variability of trace gas fluxes complicatesthe accurate calculation of budgets for use in blue carbon accounting andearth system models. 

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.

 

The exchange of multiple greenhouse gases (i.e., CO₂ and CH₄) between tree stems and the atmosphere represents a knowledge gap in the global carbon cycle. Stem CO₂ and CH₄ fluxes vary across time and space and is unclear which are their individual or shared drivers. This dataset contains information of CO₂ and CH₄ fluxes at different stem heights combining manual (biweekly; n=678) and automated (hourly; n>38,000) measurements in a temperate upland forest. 

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)) 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.  

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)) 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. We measured GHG concentrations in water, water quality, meteorological parameters, sediment CO₂efflux, ecosystem‐scale GHG fluxes, and plant phenology; all at half‐hour intervals over 1 year. Manual creek GHG flux measurements were used to calculate gas transfer velocity (k) and parameterize a model of water‐to‐atmosphere GHG fluxes. The creek was a source of GHGs to the atmosphere where tidal patterns controlled diel variability. Dissolved oxygen and wind speed were negatively correlated with creek CH₄efflux. Despite lacking a seasonal pattern, creek CO₂efflux was correlated with drivers such as turbidity across phenological phases. Overall, nighttime creek CO₂efflux (3.6 ± 0.63 μmol/m²/s) was at least 2 times higher than nighttime marsh sediment CO₂efflux (1.5 ± 1.23 μmol/m²/s). Creek CH₄efflux (17.5 ± 6.9 nmol/m²/s) was 4 times lower than ecosystem‐scale CH₄fluxes (68.1 ± 52.3 nmol/m²/s) across the year. These results suggest that tidal creeks are potential hotspots for CO₂emissions and could contribute to lateral transport of CH₄to the coastal ocean due to supersaturation of CH₄(>6,000 μmol/mol) in water. This study provides insights for modeling GHG efflux from tidal creeks and suggests that changes in tide stage overshadow water temperature in determining magnitudes of fluxes.