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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-222
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-222
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  29 Jul 2020

29 Jul 2020

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This preprint is currently under review for the journal BG.

Representing methane emissions from wet tropical forest soils using microbial functional groups constrained by soil diffusivity

Debjani Sihi1, Xiaofeng Xu2, Mónica Salazar Ortiz3, Christine S. O'Connell4,5, Whendee L. Silver5, Carla López-Lloreda6, Julia M. Brenner1, Ryan K. Quinn1,7, Jana R. Phillips1, Brent D. Newman8, and Melanie A. Mayes1 Debjani Sihi et al.
  • 1Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831, USA
  • 2Department of Biology, San Diego State University, San Diego, CA, 92182-4614, USA
  • 3Institute of Plant Science and Microbiology, University of Hamburg, Hamburg, 20148, Germany
  • 4Department of Environmental Studies, Macalester College, St. Paul, MN, 55105-1899, USA
  • 5Department of Environmental Science, Policy and Management, University of California, Berkeley, CA, 94720-3114, USA
  • 6Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH, 03824, USA
  • 7Department of Biology, Boston University, Boston, MA, 02215, USA
  • 8Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM, 87545, USA

Abstract. Tropical ecosystems contribute significantly to global emissions of methane (CH4) and landscape topography influences the rate of CH4 emissions from wet tropical forest soils. However, extreme events such as drought can alter normal topographic patterns of emissions. Here we explain the dynamics of CH4 emissions during normal and drought conditions across a catena in the Luquillo Experimental Forest, Puerto Rico. Valley soils served as the major source of CH4 emissions in a normal precipitation year (2016), but drought recovery in 2015 resulted in dramatic pulses in CH4 emissions from all topographic positions. Geochemical parameters including dissolved organic carbon (C) (ridge ≫ slope ≫ valley), acetate (ridge ≥ slope > valley), and soil pH (valley ≫ slope ≫ ridge), and meteorological parameters like soil moisture (valley > slope = ridge) and oxygen (O2) concentrations (slope = ridge > valley) varied across the catena. During the drought, soil moisture decreased in the slope and ridge and O2 concentrations increased in the valley. We simulated the dynamics of CH4 emissions with the Microbial Model for Methane Dynamics-Dual Arrhenius and Michaelis Menten (M3D-DAMM) which couples a microbial functional group CH4 model with a diffusivity module for solute and gas transport within soil microsites. Contrasting patterns of soil moisture, O2, acetate, and associated changes in soil pH with topography regulated simulated CH4 emissions, but emissions were also altered by rate-limited diffusion in soil microsites. Changes in simulated available substrate for CH4 production (acetate, CO2, and H2) and oxidation (O2 and CH4) increased the predicted biomass of methanotrophs during the drought event and methanogens during drought recovery, which in turn affected net emissions of CH4. A variance-based sensitivity analysis suggested that parameters related to acetotrophic methanogenesis and methanotrophy were most critical to simulate net CH4 emissions. This study enhanced the predictive capability for CH4 emissions associated with complex topography and drought in wet tropical forest soils.

Debjani Sihi et al.

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Short summary
Humid tropical soils are important sources and sinks of methane. We used model simulation to understand how different kinds of microbes and observed soil moisture and oxygen dynamics contribute to production and consumption of methane along a wet tropical hillslope during normal and drought conditions. Drought alters the diffusion of oxygen and microbial substrates into and out of soil microsites, resulting in enhanced methane release from the entire hillslope during drought recovery.
Humid tropical soils are important sources and sinks of methane. We used model simulation to...
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