Abstract
Abstract. Wetlands are one of the most significant natural sources of methane (CH4) to the atmosphere. They emit CH4 because decomposition of soil organic matter in waterlogged anoxic conditions produces CH4, in addition to carbon dioxide (CO2). Production of CH4 and how much of it escapes to the atmosphere depend on a multitude of environmental drivers. Models simulating the processes leading to CH4 emissions are thus needed for upscaling observations to estimate present CH4 emissions and for producing scenarios of future atmospheric CH4 concentrations. Aiming at a CH4 model that can be added to models describing peatland carbon cycling, we composed a model called HIMMELI that describes CH4 build-up in and emissions from peatland soils. It is not a full peatland carbon cycle model but it requires the rate of anoxic soil respiration as input. Driven by soil temperature, leaf area index (LAI) of aerenchymatous peatland vegetation, and water table depth (WTD), it simulates the concentrations and transport of CH4, CO2, and oxygen (O2) in a layered one-dimensional peat column. Here, we present the HIMMELI model structure and results of tests on the model sensitivity to the input data and to the description of the peat column (peat depth and layer thickness), and demonstrate that HIMMELI outputs realistic fluxes by comparing modeled and measured fluxes at two peatland sites. As HIMMELI describes only the CH4-related processes, not the full carbon cycle, our analysis revealed mechanisms and dependencies that may remain hidden when testing CH4 models connected to complete peatland carbon models, which is usually the case. Our results indicated that (1) the model is flexible and robust and thus suitable for different environments; (2) the simulated CH4 emissions largely depend on the prescribed rate of anoxic respiration; (3) the sensitivity of the total CH4 emission to other input variables is mainly mediated via the concentrations of dissolved gases, in particular, the O2 concentrations that affect the CH4 production and oxidation rates; (4) with given input respiration, the peat column description does not significantly affect the simulated CH4 emissions in this model version.
Highlights
Methane (CH4) is an important greenhouse gas, atmospheric concentrations of which have increased by more than 250 % since preindustrial times, inducing the second largest radiative forcing among well-mixed greenhouse gases (Myhre et al, 2013)
We forced the model with daily averages of water table depth (WTD), peat temperature profile, leaf area index (LAI), and anoxic respiration rate, and compared the results with daily medians of CH4 flux data from the years 2005 to 2011 from Siikaneva and daily averages of CH4 fluxes from the years 2006 to 2010 from Lompolojänkkä
The net primary production (NPP) model was driven with the WTD, photosynthetically active radiation (PAR), and air temperature (Tair)
Summary
Methane (CH4) is an important greenhouse gas, atmospheric concentrations of which have increased by more than 250 % since preindustrial times, inducing the second largest radiative forcing among well-mixed greenhouse gases (Myhre et al, 2013). In order to upscale observed CH4 fluxes and to produce realistic scenarios for the future atmospheric greenhouse gas concentrations, it is essential to know how wetland CH4 emissions respond to climatic variables Modeling these responses has been active in recent years (e.g., Wania et al, 2010; Riley et al, 2011; Melton et al, 2013; Schuldt et al, 2013; Grant et al, 2015). For this kind of use, we composed HIMMELI, the HelsinkI Model of MEthane buiLd-up and emIssion, which is a module that simulates only the processes related to transport and oxidation of CH4 It takes the rate of anoxic peat respiration as input, defined here as the rate of anoxic decomposition of organic compounds in peatland soil, and computes the subsequent CH4 emission by simulating the transport and build-up of CH4, O2, and CO2 in the soil, as well as the CH4 oxidation rate that depends on the prevailing O2 concentrations. We (a) define key factors for CH4 transport and oxidation, (b) describe the model, (c) analyze its dynamics and sensitivity of output fluxes to input data in steady-state tests, (d) analyze the model sensitivity to the description of the peat column by running the model for a Finnish peatland flux measurement site (Siikaneva) (Rinne et al, 2007), and (e) demonstrate with data from Siikaneva and another site (Lompolojänkkä) (Aurela et al, 2009) that, combined with realistic input, HIMMELI output CH4 fluxes are realistic compared to measurements, which is not so evident if looking only at the mechanistic sensitivity tests
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