Biogeochemical modeling of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and CH&amp;lt;sub&amp;gt;4&amp;lt;/sub&amp;gt; production in anoxic Arctic soil microcosms

Guoping Tang,Jianqiu Zheng,Baohua Gu,Xiaofeng Xu,Ziming Yang,David E Graham,Peter E Thornton,Scott L Painter

doi:10.5194/bg-13-5021-2016

Biogeochemical modeling of CO&lt;sub&gt;2&lt;/sub&gt; and CH&lt;sub&gt;4&lt;/sub&gt; production in anoxic Arctic soil microcosms

Guoping Tang, Jianqiu Zheng + Show 6 more

Open Access

https://doi.org/10.5194/bg-13-5021-2016

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Abstract

Abstract. Soil organic carbon turnover to CO2 and CH4 is sensitive to soil redox potential and pH conditions. However, land surface models do not consider redox and pH in the aqueous phase explicitly, thereby limiting their use for making predictions in anoxic environments. Using recent data from incubations of Arctic soils, we extend the Community Land Model with coupled carbon and nitrogen (CLM-CN) decomposition cascade to include simple organic substrate turnover, fermentation, Fe(III) reduction, and methanogenesis reactions, and assess the efficacy of various temperature and pH response functions. Incorporating the Windermere Humic Aqueous Model (WHAM) enables us to approximately describe the observed pH evolution without additional parameterization. Although Fe(III) reduction is normally assumed to compete with methanogenesis, the model predicts that Fe(III) reduction raises the pH from acidic to neutral, thereby reducing environmental stress to methanogens and accelerating methane production when substrates are not limiting. The equilibrium speciation predicts a substantial increase in CO2 solubility as pH increases, and taking into account CO2 adsorption to surface sites of metal oxides further decreases the predicted headspace gas-phase fraction at low pH. Without adequate representation of these speciation reactions, as well as the impacts of pH, temperature, and pressure, the CO2 production from closed microcosms can be substantially underestimated based on headspace CO2 measurements only. Our results demonstrate the efficacy of geochemical models for simulating soil biogeochemistry and provide predictive understanding and mechanistic representations that can be incorporated into land surface models to improve climate predictions.

Highlights

Global warming is expected to accelerate permafrost thaw, which may trigger the release of the large amount of frozen soil organic matter (SOM) stored in the Arctic as carbon dioxide (CO2) and methane (CH4) into the atmosphere, possibly forming a positive feedback to climate change (Treat et al, 2015; Knoblauch et al, 2013; Elberling et al, 2013)
With the accumulation of new data on metabolic intermediates, electron acceptors, greenhouse gases, and pH from incubations with Arctic soils at various temperatures (Drake et al, 2015; Herndon et al, 2015a, b; Yang et al, 2016; Mann et al, 2015), our objectives are to integrate these new data into geochemical models to (1) extend the CLM-CN decomposition cascade to include simple substrates such as sugars and organic acids and add Fe(III) reduction and methanogenesis processes; (2) account for gas, aqueous, and adsorbed-phase speciation; (3) describe pH mechanistically; and (4) assess the existing temperature and pH response functions
Soil samples from the organic and mineral horizons of the three cores were analyzed for gravimetric water content, pH, Fe(II), waterextractable organic carbon (WEOC), organic acids, and total organic carbon content (TOTC)

Summary

Introduction

Global warming is expected to accelerate permafrost thaw, which may trigger the release of the large amount of frozen soil organic matter (SOM) stored in the Arctic as carbon dioxide (CO2) and methane (CH4) into the atmosphere, possibly forming a positive feedback to climate change (Treat et al, 2015; Knoblauch et al, 2013; Elberling et al, 2013). Permafrost thawing leads to significant changes in soil water saturation, creating favorable conditions for anaerobic respiration and methanogenesis (Lawrence et al, 2015). Current biogeochemical models predominantly represent SOM decomposition under aerobic conditions (Manzoni and Porporato, 2009). They are modified for use under anaerobic conditions. The Community Land Model with coupled carbon and nitrogen (CLM-CN) decomposition cascade is used to implicitly represent anaerobic decomposition with a moisture response function that approaches unity at saturation and an oxygen scalar that has a large unresolved uncertainty (Oleson et al, 2013). In a recent permafrost carbon–climate feedback modeling study, the carbon release rate from permafrost soils after thawing under aerobic conditions was assumed to be 3.4 times higher than the release rate under anaerobic conditions

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