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

Abstract

Abstract. Rapid warming of Arctic ecosystems exposes soil organic matter (SOM) to accelerated microbial decomposition, potentially leading to increased emissions of carbon dioxide (CO2) and methane (CH4) that have a positive feedback on global warming. Current estimates of the magnitude and form of carbon emissions from Earth system models include significant uncertainties, partially due to the oversimplified representation of geochemical constraints on microbial decomposition. Here, we coupled modeling principles developed in different disciplines, including a thermodynamically based microbial growth model for methanogenesis and iron reduction, a pool-based model to represent upstream carbon transformations, and a humic ion-binding model for dynamic pH simulation to build a more versatile carbon decomposition model framework that can be applied to soils under varying redox conditions. This new model framework was parameterized and validated using synthesized anaerobic incubation data from permafrost-affected soils along a gradient of fine-scale thermal and hydrological variabilities across Arctic polygonal tundra. The model accurately simulated anaerobic CO2 production and its temperature sensitivity using data on labile carbon pools and fermentation rates as model constraints. CH4 production is strongly influenced by water content, pH, methanogen biomass, and presence of competing electron acceptors, resulting in high variability in its temperature sensitivity. This work provides new insights into the interactions of SOM pools, temperature increase, soil geochemical feedbacks, and resulting CO2 and CH4 production. The proposed anaerobic carbon decomposition framework presented here builds a mechanistic link between soil geochemistry and carbon mineralization, making it applicable over a wide range of soils under different environmental settings.

Increasing Organic Carbon Biolability With Depth in Yedoma Permafrost: Ramifications for Future Climate Change

“…These bacteria are known to ferment complex organic matter into acetate, H 2 , and CO 2 (Rojas et al, 2018), and have been previously shown to co-occur with methanogens in environments with degraded OM (Baldwin et al, 2015). Given that this fermentation of complex OM into substrate for methanogenesis is the rate-limiting step in Arctic anaerobic C mineralization (Zheng et al, 2019), it is possible that the degradation of unsaturated high oxygen compounds we observed in our anaerobic incubation correlates to fermentation of OM by Bacteroidales and Clostridales into substrate for methanogenesis.…”

Authors' responses to interactive comments

“…The model is archived at https://doi.org/10.5440/1430703 (Zheng et al, 2018c), with a detailed description of model implementation, input files, and various sensitivity analyses described in this paper.…”

“…How quickly frozen soil organic matter (SOM) will be mineralized, and how much permafrost C will be released to the atmosphere following thaw, is highly uncertain. Earth system models project 27-508 Pg carbon release from the permafrost zone by 2100 under current climate forcing (Zhuang et al, 2006;Koven et al, 2015;MacDougall et al, 2012;Schaefer et al, 2014). Understanding environmental dependencies of SOM decomposition is therefore essential for reducing model uncertainties and improving predictions of future climate change.…”

Theoretical Constraints on Fe Reduction Rates in Upland Soils as a Function of Hydroclimatic Conditions

“…Accounting for all the factors affecting Fe redox dynamics discussed above, for example, detailed soil biogeochemistry (Zheng et al, 2019), microbial growth and decay, C cycling, vegetation dynamics, and soil water balance (e.g., Calabrese & Porporato, 2019), would make it hard to analyze the interaction between internal and external controls, because these many interacting variables could overshadow the role of Journal of Geophysical Research: Biogeosciences 10.1029/2020JG005894 external forcings. Therefore, here we focus on potential Fe reduction rates, which are achieved under nonlimiting availability of labile C and microbial populations and their upper bound.…”