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 Ma. 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 , 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 , Jurasinski Gerald , Kang Minseok , Kasak Kuno , King John , Klatt Janina , Koebsch Franziska , Krauss Ken W. , Lai Derrick Y. F. , Lohila Annalea , Mammarella Ivan , Belelli Marchesini Luca , Manca Giovanni , Matthes Jaclyn Hatala , Maximov 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: July 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 due to quasi-continuous and high-temporal-resolution CH4
flux measurements, coincident carbon dioxide, water, and energy flux
measurements, 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 open-source global dataset of
CH4 EC measurements (available at
https://fluxnet.org/data/fluxnet-ch4-community-product/, last access: 7 April 2021). 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 which are a substantial source of total atmospheric
CH4 emissions; and (3) we 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 3 d earlier, and the CH4
emission season lasts 4 d longer, for each degree Celsius increase in mean
annual air temperature. On average, the spring onset of increasing CH4
emissions lags behind soil warming by 1 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). FLUXNET-CH4 is a powerful new resource
for diagnosing and understanding the role of terrestrial ecosystems and
climate drivers in the global CH4 cycle, and future additions of sites
in tropical ecosystems and site years of data collection will provide added
value to this database. All seasonality parameters are available at
https://doi.org/10.5281/zenodo.4672601 (Delwiche et al., 2021).
Additionally, raw FLUXNET-CH4 data used to extract seasonality parameters
can be downloaded from https://fluxnet.org/data/fluxnet-ch4-community-product/ (last access: 7 April 2021), and a complete
list of the 79 individual site data DOIs is provided in Table 2 of this paper.