Detecting Hot Spots of Methane Flux Using Footprint‐Weighted Flux Maps

• Published in: Journal of geophysical research. Biogeosciences 2022-08, Vol.127 (8), p.e2022JG006977-n/a
• Main Authors: Rey‐Sanchez, Camilo, Arias‐Ortiz, Ariane, Kasak, Kuno, Chu, Housen, Szutu, Daphne, Verfaillie, Joseph, Baldocchi, Dennis
• Format: Article
• Language: English
• Subjects: Atmosphere; Atmospheric models; Chambers; Covariance; eddy covariance; Emissions; ENVIRONMENTAL SCIENCES; Evaluation; Fluctuations; flux footprint; Fluxes; Gas exchange; Gases; Greenhouse effect; Greenhouse gases; Heterogeneity; Hot spots; Methane; Patchiness; Sequestering; Soils; Spatial heterogeneity; Towers; Tracers; Vortices; Wetlands
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2022JG006977

In this study, we propose a new technique for mapping the spatial heterogeneity in gas exchange around flux towers using flux footprint modeling and focusing on detecting hot spots of methane (CH4) flux. In the first part of the study, we used a CH4 release experiment to evaluate three common flux footprint models: the Hsieh model (Hsieh et al., 2000), the Kljun model (Kljun et al., 2015), and the K & M model (Kormann and Meixner, 2001), finding that the K & M model was the most accurate under these conditions. In the second part of the study, we introduce the Footprint‐Weighted Flux Map, a new technique to map spatial heterogeneity in fluxes. Using artificial CH4 release experiments, natural tracer approaches and flux chambers we mapped the spatial flux heterogeneity, and detected and validated a hot spot of CH4 flux in a oligohaline restored marsh. Through chamber measurements during the months of April and May, we found that fluxes at the hot spot were on average as high as 6589 ± 7889 nmol m−2 s−1 whereas background flux from the open water were on average 15.2 ± 7.5 nmol m−2 s−1. This study provides a novel tool to evaluate the spatial heterogeneity of fluxes around eddy‐covariance towers and creates important insights for the interpretation of hot spots of CH4 flux, paving the way for future studies aiming to understand subsurface biogeochemical processes and the microbiological conditions that lead to the occurrence of hot spots and hot moments of CH4 flux. Plain Language Summary Wetlands are capable of sequestering large amounts of carbon in their soils but they also emit about a third of all the methane emissions to the atmosphere. These methane emissions vary significantly in space, with some places becoming hot spots of methane flux that so far remain understudied. In this paper, we present a new method to map the spatial heterogeneity in methane fluxes in wetlands as measured by flux towers called eddy covariance towers. We find that this new technique can be used to map the spatial heterogeneity in fluxes of multiple greenhouse gases, with a special ability to map hot spots of methane flux. The presence and the magnitude of these hot spots are validated using chamber measurements finding satisfactory results. This technique paves the way for future studies whose goal is to understand what are the chemical and microbiological processes in the soils leading to these high methane emissions, and thus create strategies to better model and mitigate