Quantifying Footprint-Weighted Methane Fluxes and the Annual Radiative Balance of a Humid Wet Grassland

Natural and managed ecosystems are greenhouse gas sources and sinks, and large uncertainties remain about changes in their magnitude and direction with climate change. Natural greenhouse gas exchange is poorly constrained for ecosystems intermediate along the terrestrial-aquatic continuum, including seasonally wet grasslands, and potential exists both for high productivity and methane production and oxidation. In this study we used eddy covariance to measure fluxes of carbon dioxide and methane from June 2022 to March 2023 over a seasonally wet humid Midwest grassland at Argonne National Laboratory. We posed two main questions: What is the annual carbon, greenhouse gas, and radiative balance of a seasonally wet humid grassland in the Chicagoland region, and how does each compare to global grasslands? How do the spatial and temporal dynamics of greenhouse gas exchange relate and respond to patterns and fluctuations in soil conditions? Using machine learning methods to gap-fill eddy covariance time series, we estimate that the humid grassland site was a sink of both carbon dioxide and methane over the study, whereas we find most other grasslands in a global eddy covariance dataset are carbon dioxide sinks and methane sources. We relate spatio-temporal patterns in methane fluxes to continuous, spatially gridded soil moisture and temperature within the flux footprint, finding generally weak explanatory power for both. We consider the limitations of eddy covariance for measuring grassland methane fluxes where small fluxes may be close to limits of detection. This study demonstrates that relatively wet soil conditions do not necessarily lead to methane emissions, even in humid, productive grasslands, and that further grassland carbon, greenhouse gas, and radiative balance research is needed to constrain drivers of variability across and within different grassland ecosystems.