FLUXNET-CH4: A global, multi-ecosystem dataset and analysis
of methane seasonality from freshwater wetlands
Authors: Delwiche Kyle B. , Knox Sara Helen , Malhotra Avni , Fluet-Chouinard Etienne , McNicol Gavin , Feron Sarah , Ouyang Zutao , Papale Dario , Trotta Carlo , Canfora Eleonora , Cheah You-Wei , Christianson Danielle , Alberto M. Carmelita R. , Alekseychik Pavel , Aurela Mika , Baldocchi Dennis , Bansal Sheel , Billesbach David P. , Bohrer Gil , Bracho Rosvel , Buchmann Nina , Campbell David I. , Celis Gerardo , Chen Jiquan , Chen Weinan , Chu Housen , Dalmagro Higo J. , Dengel Sigrid , Desai Ankur R. , Detto Matteo , Dolman Han , Eichelmann Elke , Euskirchen Eugenie , Famulari Daniela , Friborg Thomas , Fuchs Kathrin , Goeckede Mathias , Gogo Sébastien , Gondwe Mangaliso J. , Goodrich Jordan P. , Gottschalk Pia , Graham Scott L. , Heimann Martin , Helbig Manuel , Helfter Carole , Hemes Kyle S. , Hirano Takashi , Hollinger David , Hörtnagl Lukas , Iwata Hiroki , Jacotot Adrien , Jansen Joachim , Jurasinski Gerald , Kang Minseok , Kasak Kuno , King John , Klatt Janina , Koebsch Franziska , Krauss Ken W. , Lai Derrick Y. F. , Mammarella Ivan , Manca Giovanni , Marchesini Luca Belelli , Matthes Jaclyn Hatala , Maximon Trofim , Merbold Lutz , Mitra Bhaskar , Morin Timothy H. , Nemitz Eiko , Nilsson Mats B. , Niu Shuli , Oechel Walter C. , Oikawa Patricia Y. , Ono Keisuke , Peichl Matthias , Peltola Olli , Reba Michele L. , Richardson Andrew D. , Riley William , Runkle Benjamin R. K. , Ryu Youngryel , Sachs Torsten , Sakabe Ayaka , Sanchez Camilo Rey , Schuur Edward A. , Schäfer Karina V. R. , Sonnentag Oliver , Sparks Jed P. , Stuart-Haëntjens Ellen , Sturtevant Cove , Sullivan Ryan C. , Szutu Daphne J. , Thom Jonathan E. , Torn Margaret S. , Tuittila Eeva-Stiina , Turner Jessica , Ueyama Masahito , Valach Alex C. , Vargas Rodrigo , Varlagin Andrej , Vazquez-Lule Alma , Verfaillie Joseph G. , Vesala Timo , Vourlitis George L. , Ward Eric J. , Wille Christian , Wohlfahrt Georg , Wong Guan Xhuan , Zhang Zhen , Zona Donatella , Windham-Myers Lisamarie , Poulter Benjamin , Jackson Robert B. 
Published: January 2021

Abstract. Methane (CH4) emissions from natural landscapes constitute roughly half of global CH4 contributions to the atmosphere, yet large uncertainties remain in the absolute magnitude and the seasonality of emission quantities and drivers. Eddy covariance (EC) measurements of CH4 flux are ideal for constraining ecosystem-scale CH4 emissions, including their seasonality, due to quasi-continuous and high temporal resolution of flux measurements, coincident measurements of carbon, water, and energy fluxes, lack of ecosystem disturbance, and increased availability of datasets over the last decade. Here, we 1) describe the newly published dataset, FLUXNET-CH4 Version 1.0, the first global dataset of CH4 EC measurements (available at https://fluxnet.org/data/fluxnet-ch4- community-product/). FLUXNET-CH4 includes half-hourly and daily gap-filled and non gap-filled aggregated CH4 fluxes and meteorological data from 79 sites globally: 42 freshwater wetlands, 6 brackish and saline wetlands, 7 formerly drained ecosystems, 7 rice paddy sites, 2 lakes, and 15 uplands. Then, we 2) evaluate FLUXNET-CH4 representativeness for freshwater wetland coverage globally, because the majority of sites in FLUXNET-CH4 Version 1.0 are freshwater wetlands and because freshwater wetlands are a substantial source of total atmospheric CH4 emissions; and 3) provide the first global estimates of the seasonal variability and seasonality predictors of freshwater wetland CH4 fluxes. Our representativeness analysis suggests that the freshwater wetland sites in the dataset cover global wetland bioclimatic attributes (encompassing energy, moisture, and vegetation-related parameters) in arctic, boreal, and temperate regions, but only sparsely cover humid tropical regions. Seasonality metrics of wetland CH4 emissions vary considerably across latitudinal bands. In freshwater wetlands (except those between 20° S to 20° N) the spring onset of elevated CH4 emissions starts three days earlier, and the CH4 emission season lasts 4 days longer, for each degree C increase in mean annual air temperature. On average, the onset of increasing CH4 emissions lags soil warming by one month, with very few sites experiencing increased CH4 emissions prior to the onset of soil warming. In contrast, roughly half of these sites experience the spring onset of rising CH4 emissions prior to the spring increase in gross primary productivity (GPP). The timing of peak summer CH4 emissions does not correlate with the timing for either peak summer temperature or peak GPP. Our results provide seasonality parameters for CH4 modeling, and highlight seasonality metrics that cannot be predicted by temperature or GPP (i.e., seasonality of CH4 peak). The FLUXNET-CH4 dataset provides an open-access resource for CH4 flux synthesis, has a range of applications, and is unique in that it includes coupled measurements of important CH4 drivers such as GPP and temperature. Although FLUXNET-CH4 could certainly be improved by adding more sites in tropical ecosystems and by increasing the number of site-years at existing sites, it is a powerful new resource for diagnosing and understanding the role of terrestrial ecosystems and climate drivers in the global CH4 cycle. All seasonality parameters are available at https://doi.org/10.5281/zenodo.4408468. Additionally, raw FLUXNET-CH4 data used to extract seasonality parameters can be downloaded from https://fluxnet.org/data/fluxnet-ch4-community-product/, and a complete list of the 79 individual site data DOIs is provided in Table 2 in the Data Availability section of this document.