Understanding and Modelling Current and Future Coastal Wetland Methane Dynamics

The coastal terrestrial-aquatic interface (TAI) is a highly dynamic component of the Earth system that plays a critical role in biogeochemical cycling. Due to its dynamic nature, the processes that regulate decomposition and methane (CH4) emissions are of greater significance at the TAI than in upland systems. Despite this, we have limited mechanistic understanding of how climate stressors interact to regulate the electron acceptors and donors that determine decomposition pathways within TAIs, including the generation of hot spots and hot moments. Accurately modeling these processes is critical for incorporating the coastal TAI into Earth systems models, such as DOE's E3SM. With previous DOE support, we adapted an aerobic terrestrial representation of decomposition using PFLOTRAN, a reactive flow and transport model, and added anerobic decomposition pathways, salinity, and oxygen (O2) that fluctuates independently of water table level. However, because this model (PFLOTRANTAI) is based on decomposition rates and organic matter carbon to nitrogen ratios from terrestrial systems, its performance in TAI systems is currently limited by the lack of empirical data to properly parameterize variables. In addition, while PFLOTRANTAI can simulate movement of O2 into sediments, it is not currently capable of tracking the movement of CH4 gas through plant tissues due to both current model structure and lack of available data.

Our overall objective of this proposal is to improve the representation of anaerobic decomposition biogeochemistry in PFLOTRANTAI. Based on knowledge gaps identified during PFLOTRANTAI development, we have pinpointed three specific objectives that will increase our mechanistic understanding of CH4 dynamics in response to environmental change and improve our model representation:

• Objective 1: Quantify rates of multiple anaerobic decomposition pathways that regulate CH4 emissions, including hot moments, across varying time scales.

• Objective 2: Determine the individual and interactive effects of warming, salinity, and inundation on CH4 cycling pathways and other anaerobic decomposition processes.

• Objective 3: Using an iterative process, connect field work and model development to

i) parameterize anaerobic decomposition in PFLOTRANTAI, focusing on CH4 emissions, and ii) identify future areas of focus for model improvement.

We have identified the five key field measurements required to update PFLOTRANTAI as: 1) relative rates of redox processes, 2) effects of global change stressors, 3) soil C quality, 4) plant-mediated transport of O2 and CH4, and 5) sediment heterogeneity. To accomplish this, we will integrate both field-based (natural gradient and manipulative experiments) and lab-based (redox incubations) measurements into our research plan. We propose to conduct spatial (space-for-time) monitoring at coastal sites that span a natural gradient of salinity, as well as temporal and manipulative process-level studies, looking at effects of warming, salinity, and inundation. These manipulations will include installing automated flux chambers in a novel soil heating experiment and setting up marsh organs with varying levels of warming, inundation, and salinity. Our proposed efforts will result in the collection of chamber-level CH4 flux measurements across multiple sites, ecological conditions, and timeframes, as well as corresponding measurements on porewater chemistry, soil carbon quality, plant biomass, and redox reaction rates. Using these new datasets, we will first improve PFLOTRANTAI to accurately model CH4 emissions and then conduct sensitivity analyses to identify areas for future field studies. The updated PFLOTRANTAI will be linked to the land model of E3SM and vastly improve the representation of coastal systems within a larger framework.