Patterns and processes governing greenhouse gas emissions from tidal salt marsh soils
Date
2022
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Coastal vegetated ecosystems, such as tidal salt marshes, mangrove forests, and seagrass beds, store large amounts of carbon and thereby have been referred to as “blue carbon” ecosystems. These ecosystems also release carbon in the form of carbon dioxide (CO2) and methane (CH4), as well as other climate-active trace gases such as nitrous oxide (N2O), carbon disulfide (CS2), and dimethylsulfide (DMS). There is high uncertainity about the production and emissions of these gases from salt marsh soils, as well as their spatiotemporal variability. Knowledge about soil trace gas fluxes is important for calculating budgets, calibrating models, and assessing the viability of marshes as natural climate solutions. This dissertation focuses on better understanding the patterns and processes that govern greenhouse (GHG; CO2, CH4, N2O) and sulfur-based gas (CS2, DMS) fluxes from soils in a Mid-Atlantic temperate tidal salt marsh. Fluxes were measured using a variety of chamber techniques, coupled with biophysical and biogeochemical measurements. ☐ The first study presented in this dissertation investigates the effect of storm-surge salinity changes on GHG fluxes from tidal salt marsh soils, with the goal of understanding how fluxes respond to and recover from salinity changes. A flow-through mesocosm experiment was coupled with automated GHG flux and pore water chemistry measurements. Decreases in salinity contributed to an increase in GHG fluxes. Throughout the experiment, the role of different biogeochemical processes in producing GHG fluxes changed over time. This underscores the complex nature of the production and emission of GHG, particularly during extreme events. Once salinity returned to the initial conditions, CH4 and N2O fluxes returned to baseline within 15 days, illustrating that tidal salt marshes are resilient ecosystems. ☐ The second study sought to better quantify CO2 fluxes, as well as identify its main biophysical drivers using long-term, continuous data collected in the field. Hourly averages of CO2 flux were collected at two sites for ~20 months, as well as manual CO2 flux data to assess spatial variability. This study comprises of the first long-term datasets of soil CO2 flux measurements from a salt marsh. Although seasonal patterns of CO2 fluxes were found, there were no consistent diel patterns. The main biophysical driver of CO2 flux was air temperature, but other drivers such as water level, salinity, PAR, and NDVI played roles. Manual measurements collected every two weeks underestimated the annual flux, highlighting the need for high-frequency data to calculate annual budgets more accurately. ☐ The third study built upon the questions about biophysical drivers and measurement techniques posed in the second chapter to include CH4, N2O, CS2, and DMS. Continuous, automated chambers were deployed for ~72 hours throughout the year to obtain high-temporal frequency data to assess how gas fluxes changed throughout the day and over tidal cycles. No consistent diel patterns were found, but rather CH4, N2O, CS2, and DMS fluxes were highly variable with frequent pulse emissions. Likewise, when continuous measurements were compared to discrete (during daytime, at low tide) measurements for these four gases, discrepancies arose due to high temporal variability. However, both continuous and discrete measurements of CO2 provided similar information regarding the mean and distribution of CO2 fluxes, providing support for the use of discrete measurements of CO2 for budgets. ☐ The fourth study sought to better understand CH4 production and fate in tidal salt marsh soils. The continuous, automated measurements of CH4 and CO2 performed in the third study were coupled with measurements of soil CH4 and CO2 gas concentrations, stable and radioisotopes, pore water and organic carbon chemistry, and microbial community composition. CH4 was found to be produced by two pathways: hydrogenotrophic and methylotrophic methanogenesis, the latter of which can produce CH4 in the presence of sulfate reduction. Once produced, data showed that CH4 can take a variety of pathways: diffusion into the atmosphere, CH4 oxidation, and lateral transport to the tidal creek. The findings showed that CH4 production and fate is biogeochemically heterogeneous and that each process involved varied in importance over the growing season. ☐ Overall, this dissertation provided key insights into the spatiotemporal variability of greenhouse and sulfur-based fluxes from tidal salt marsh soils, as well as the processes that produce these gases. The findings from these studies will provide better insights for scientists and policymakers on the role salt marshes have in the carbon cycle as well as provide better GHG estimates for evaluating whether salt marshes are a net carbon sink.
Description
Keywords
Aquatic-terrestrial interface, Biogeochemistry, Carbon cycle, Greenhouse gas, Salt marsh, Soil respiration