Subsurface Redox Interactions Regulate Ebullitive Methane Flux in Heterogeneous Mississippi River Deltaic Wetland

Abstract

As interfaces connecting terrestrial and ocean ecosystems, coastal wetlands develop temporally and spatially complex redox conditions, which drive uncertainties in greenhouse gas emission as well as the total carbon budget of the coastal ecosystem. To evaluate the role of complex redox reactions in methane emission from coastal wetlands, a coupled reactive‐transport model was configured to represent subsurface biogeochemical cycles of carbon, nitrogen, and sulfur, along with production and transport of multiple gas species through diffusion and ebullition. This model study was conducted at multiple sites along a salinity gradient in the Barataria Basin at the Mississippi River Deltaic Plain. Over a freshwater to saline gradient, simulated total flux of methane was primarily controlled by its subsurface production and consumption, which were determined by redox reactions directly (e.g., methanogenesis, methanotrophy) and indirectly (e.g., competition with sulfate reduction) under aerobic and/or anaerobic conditions. At fine spatiotemporal scales, surface methane fluxes were also strongly dependent on transport processes, with episodic ebullitive fluxes leading to higher spatial and temporal variability compared to the gradient‐driven diffusion flux. Ebullitive methane fluxes were determined by methane fraction in total ebullitive gas and the frequency of ebullitive events, both of which varied with subsurface methane concentrations and other gas species. Although ebullition thresholds are constrained by local physical factors, this study indicates that redox interactions not only determine gas composition in ebullitive fluxes but can also regulate ebullition frequency through gas production.

Developing a Redox Network for Coastal Saltmarsh Systems in the PFLOTRAN Reaction Model

“…PFLOTRAN is not currently capable of simulating gas diffusion through porewater (Lichtner et al, 2019) Therefore, methane builds up in the subsurface and does not off-gas by diffusion or bubble ebullition to be subjected to oxidation. While the methods we deployed to mimic plant-transport and ebullition are similarly represented in other models (Walter & Heimann, 2000;Wania et al, 2010), improvements could be developed to incorporate additional factors (Peltola et al, 2018) to make the representation more mechanistic such as pressure gradients (Wang et al, 2023). these well-studied sites often focus on a particular process or subset of processes, report net production/consumption rates, and/or ignore interactions between redox species or environmental effects (Capone et al, 1983;Ensign et al, 2013;Rich et al, 2008;Sela-Adler et al, 2017).…”

“…Due to the strong vertical gradients in soil and sediment porewater redox state in these systems, it is important to characterize variation in vegetation and microbial processes with soil depth to accurately predict fluxes of greenhouse gases and availability of nutrients (Yu et al, 2006). The representation of competition between plants and microbes for available nutrients has been hindered in previous implementations of ELM by ignoring constraints on nutrient uptake by plants related to rooting depth, with plants modeled as having access to available nutrients from anywhere within the active layer of the soil column (Wang et al, 2023). Using smaller scale simulations in PFLOTRAN allows for the initial tests of plant-water-soil connections to identify and prioritize which processes are important for scaling representation of redox processes from pore to ecosystem scale using ELM-PFLOTRAN.…”

“…While this simplification is often used for purely terrestrial ecosystems, it is not effective for coastal wetland ecosystems. Plant-mediated transport through roots, bioturbation and bioirrigation, wind or wave action, and pressure gradients caused by tidal flux can all alter oxygen penetration depth and the flow of nutrients across the sediment-water interface independently of water saturation (Kauppi et al, 2023;Noyce et al, 2023;Taillefert et al, 2007;Wang et al, 2023). In addition, constantly shifting environmental conditions and regular inundation increase the importance of additional redox-active elements such as sulfur that can function as terminal electron acceptors and interact directly with the carbon and nutrient cycles.…”

Three Decades of Wetland Methane Surface Flux Modeling by Earth System Models‐Advances, Applications, and Challenges

“…Detailed reaction networks are only very recently being incorporated into LSMs (Ricciuto et al, 2021;J. Wang et al, 2024).…”

Integrating Tide‐Driven Wetland Soil Redox and Biogeochemical Interactions Into a Land Surface Model

“…Ebullition is included as a transport pathway for dissolved gases, following (Wang et al, 2024). Pressure in each layer is calculated using the weight of water in layers above, including atmospheric pressure.…”