title: Ecosystem fluxes of hydrogen in a mid-latitude forest driven by soil microorganisms and plants.
abstract: Molecular hydrogen (H2) is an atmospheric trace gas with a large microbe-mediated soil sink, yet cycling of this compound throughout ecosystems is poorly understood. Measurements of the sources and sinks of H2in various ecosystems are sparse, resulting in large uncertainties in the global H2budget. Constraining the H2cycle is critical to understanding its role in atmospheric chemistry and climate. We measured H2fluxes at high frequency in a temperate mixed deciduous forest for 15 months using a tower-based flux-gradient approach to determine both the soil-atmosphere and the net ecosystem flux of H2. We found that Harvard Forest is a net H2sink (-1.4 ± 1.1 kg H2ha-1) with soils as the dominant H2sink (-2.0 ± 1.0 kg H2ha-1) and aboveground canopy emissions as the dominant H2source (+0.6 ± 0.8 kg H2ha-1). Aboveground emissions of H2were an unexpected and substantial component of the ecosystem H2flux, reducing net ecosystem uptake by 30% of that calculated from soil uptake alone. Soil uptake was highly seasonal (July maximum, February minimum), positively correlated with soil temperature and negatively correlated with environmental variables relevant to diffusion into soils (i.e., soil moisture, snow depth, snow density). Soil microbial H2uptake was correlated with rhizosphere respiration rates (r = 0.8, P < 0.001), and H2metabolism yielded up to 2% of the energy gleaned by microbes from carbon substrate respiration. Here, we elucidate key processes controlling the biosphere-atmosphere exchange of H2and raise new questions regarding the role of aboveground biomass as a source of atmospheric H2and mechanisms linking soil H2and carbon cycling. Results from this study should be incorporated into modeling efforts to predict the response of the H2soil sink to changes in anthropogenic H2emissions and shifting soil conditions with climate and land-use change.
authors: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ, USA.|Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA, USA.
authors: Laura K
authors: 0000-0003-4244-4366
authors: LK
authors: Meredith
authors: Department of Earth and Planetary Science, School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
authors: Róisín
authors: R
authors: Commane
authors: Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA.
authors: Trevor F
authors: TF
authors: Keenan
authors: Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, USA.
authors: Stephen T
authors: ST
authors: Klosterman
authors: Department of Earth and Planetary Science, School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
authors: J William
authors: 0000-0002-1042-8452
authors: JW
authors: Munger
authors: Department of Biology, Boston University, Boston, MA, USA.
authors: Pamela H
authors: PH
authors: Templer
authors: Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA, USA.
authors: Jianwu
authors: J
authors: Tang
authors: Department of Earth and Planetary Science, School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
authors: Steven C
authors: SC
authors: Wofsy
authors: Department of Earth, Atmospheric and Planetary Science, Massachusetts Institute of Technology, Cambridge, MA, USA.
authors: Ronald G
authors: RG
authors: Prinn
mesh_terms: D002244:Carbon; D002245:Carbon Dioxide; D017753:Ecosystem; D065928:Forests; D006859:Hydrogen / Q000737:chemistry*; D010944:Plants; D012987:Soil; D012988:Soil Microbiology; D014197:Trees
chemical_list: D012987:Soil; D002245:Carbon Dioxide; D002244:Carbon; D006859:Hydrogen
keywords: atmosphere; carbon cycle; flux; hydrogen; microbe; phenology; snow; soil