Environmental Drivers of Coastal Wetland Biogeochemical Cycling
Authors
Alia Al-Haj1*, Roy Rich1, Teri O’Meara2, Genevieve Noyce1 (noyceg@si.edu)
Institutions
1Smithsonian Environmental Research Center, Edgewater, MD; 2Oak Ridge National Laboratory, Oak Ridge, TN
Abstract
The dynamic nature of the coastal terrestrial-aquatic interface (TAI) means that biogeochemical cycling can be highly variable in response to continued or episodic changes in environmental drivers. Incorporating the coastal TAI into Earth systems models requires a better mechanistic understanding of the biogeochemical processes that regulate greenhouse gas (GHG) emissions. In response, researchers started a model experiment (ModEx) project in 2021 in which they use automated measurements of wetland GHG emissions combined with mesocosm global change experiments to gain a predictive understanding of the mechanisms controlling anaerobic decomposition and GHG emissions in the coastal TAI so that these processes and their casual factors can be incorporated into process-oriented biogeochemical models.
Researchers have been using autochambers to measure continuous methane (CH4) and carbon dioxide (CO2) emissions since April 2021 and nitrous oxide (N2O) emissions since Nov 2023. In January 2024, the chambers successfully collected data while floodwaters were 95 cm above the soil surface, the third-highest flooding event on record. Initially, the team observed strong diurnal patterns where CH4 emissions were lower and more variable during the day than at night, but there was surprisingly no diurnal pattern in 2023. Though soil temperature is a strong driver of the magnitude of CH4 fluxes, researchers also found that CH4 emissions are suppressed during high salinity periods. Thus far, N2O emissions are highly variable, but overall quite low during winter months.
The mesocosm experiments are designed to understand the biogeochemical mechanisms underlying the field observations so that they can be incorporated into process models. From June 2022 to October 2023, the team conducted a sea level rise (SLR) experiment using mesocosms with and without vegetation at two salinity levels. Overall, researchers found that SLR increases CH4 emissions from both brackish and freshwater marshes while warming has a larger impact on CH4 emissions from brackish marshes. In incubation experiments, methanol was the best source of carbon for CH4 production across both salinities, followed by methylamine, whereas acetate was only a major substrate for methanogenesis in freshwater wetlands. In June 2024, the team will start a new mesocosm experiment focused on GHG responses to pulses of inundation, salinity, temperature, and nutrient availability.