Biogeochemical modeling of CO<sub>2</sub> and CH<sub>4</sub> production in anoxic Arctic soil microcosms

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.

Mechanistic modeling of microbial interactions at pore to profile scale resolve methane emission dynamics from permafrost soil

“…The model results are consistent with the field observations in showing the importance of late season emissions that could account for about 50% of the annual emissions. The role of late season emission in arctic regions remains unclear requiring better and more systematic understanding for improving current process‐based models [ Tang et al , ; Xu et al , ].…”

Microbial community structure and soil pH correspond to methane production in Arctic Alaska soils

“…The proportional abundance of FeRB also correlated with PMP rates through a positive relationship, which was unexpected, given that ironreduction can compete with methanogenesis (Bond and Lovley, 2002). However, reduction of Fe 31 to Fe 21 can increase soil pH, thereby alleviating inhibition of methanogenesis associated with soil acidity (Tang et al, 2016), and possibly negating the effect of competitive inhibition of the methanogens. There is not a good universal marker that encompasses all FeRB, as there are many different mechanisms for iron-reduction (Weber et al, 2006).…”

“…This study also demonstrates the importance of rare soil microbial communities, and validates the importance of 'the rare biosphere' (Lynch and Neufeld, 2015) in microbial biogeochemical cycling. Finally, this work begins to addresses the complex relationship between soil chemistry and functional microbial groups, the understanding of which will be critical to informing the next generation of landscape-level and global CH 4 models (Xu et al, 2015Tang et al, 2016). This study is limited in that it tries to tease apart the role of specific microbial groups in the complex process that leads to CH 4 production in Arctic soils.…”

“…However, there is evidence that ironreduction may competitively inhibit methanogenesis (Teh et al, 2008;Lipson et al, 2010;Miller et al, 2015), and, therefore, the iron-reducing bacteria (FeRB) are similarly investigated as well. Methanogen and methanotroph abundance data, and biogeochemical data relating to ironreduction, have been successfully incorporated into landscape methane modelling (Xu et al, 2015;Tang et al, 2016), justifying the need to investigate these groups with regard to methane production. Through the lens of these key microbial groups, this research investigates the relatively under-studied interior western portion of the Alaska North Slope, broadening our understanding of soil microbial ecology and CH 4 cycling across Arctic tundra ecosystems.…”

Modeling anaerobic soil organic carbon decomposition in Arctic polygon tundra: insights into soil geochemical influences on carbon mineralization

“…The northern permafrost region contains 1400-1800 Pg soil carbon (C), which is more than twice as much C as is currently contained in the atmosphere (Tarnocai et al, 2009;McGuire et al, 2012). Persistent cold and saturated soil conditions have limited C decomposition in this reservoir.…”