Predicting Hot Spots and Hot Moments of Biogenic Gas Accumulation and Release in a Subtropical Ecosystem Using Airborne Ground-Penetrating Radar
Authors
Xavier Comas1* (xcomas@fau.edu), Neil Terry2, Caiyun Zhang3, Rajeun Islam1
Institutions
1Department of Geosciences, Florida Atlantic University–Davie, FL; 2U.S. Geological Survey, New York Water Science Center, Troy, NY; 3Department of Geosciences, Florida Atlantic University–Boca Raton, FL
Abstract
Peatlands are major terrestrial carbon (C) stores and large natural producers of biogenic greenhouse gases (e.g., methane; CH4 and carbon dioxide; CO2), which accumulate in the soil matrix and subsequently release to the atmosphere. While several conceptual models have been developed over the last 2 decades, identifying hot spots and hot moments for the accumulation and release of biogenic gases are limited in peatlands, likely due to the lack of effective noninvasive methods available to be deployed at reasonable temporal and spatial scales for such an identification. This project has been testing a small unoccupied aircraft system (sUAS)-based ground-penetrating radar (GPR) prototype for imaging hot spots of gas accumulation in the Everglades, as representative peat soils in subtropical wetland ecosystems. After several delays related to changes in drone legislation in Florida, field campaigns in 2023 provided (1) a seasonal GPR time-lapse dataset collected over a 140 m by 140 m grid in two project sites that show gas content distribution ranging between 2 to 18% during the dry season, shifting to 6 to 28% during the wet season; (2) CH4 fluxes directly inferred from gas traps ranging between 16 to 80 mg CH4 m-2 d-1 with values almost three times larger in identified hot spots characterized by higher GPR inferred soil gas content; and (3) coincident optical sUAS based multispectral imagery products with five spectral channels (red, green, blue, near infrared, and red edge) and a resolution of 1 foot that will be combined with GPR and field measurements to identify the relationship between the remotely sensed surface reflectance and gas fluxes.
This study also further tested airborne GPR measurements in monoliths under controlled laboratory conditions. Overall GPR gas content distribution in the soil was strikingly similar to field conditions and ranged between 12 to 34% gas content that resulted in gas flux releases up to 82 mg CH4 m-2 d-1 (as directly inferred from gas traps), and higher fluxes once again coinciding with areas of higher GPR inferred gas content. Measurements will be expanded both in the field and laboratory to monitor gas content dynamics in the identified hot spots. Researchers will further analyze (1) gas samples for CH4 metabolic pathways; and (2) how soil matrix may influence gas distribution via X-ray computed tomography (via a limited scope proposal submitted to the Environmental Molecular Sciences Laboratory).