1. Mechanistic modeling of microbial interactions at pore to profile scales resolve methane emission dynamics from permafrost soil
Ali Ebrahimi and Dani Or
Department of Environmental Systems Science, ETH Zurich, Switzerland
Introduction
Boundary conditions over active layer
Results and discussions: seasonal CH4 fluxes
The depth of the active soil permafrost is systematically quantified as function of soil surface temperature and soil moisture
The sensitivity of polar regions to raising temperature is reflected in rapidly changing hydrological processes with pronounced seasonal thawing of permafrost soil and increased biological activity
Soil anoxic microsites forming in the presence of vertical wetness profiles and in soil aggregates (or similar hotspots) promote methane production by methanogens
Methane emission rates are affected by seasonal variations in environmental conditions (e.g., temperature, water table and thaw depth)
Methane fluxes during the fall season shows unusual peaks contributing significantly to the annual methane emissions
Soil permafrost is characterized by soil aggregates and bulk soil with 3D and 2D networks
For thawed zone:
For frozen zone:
Figure 3. The theoretical and boundary conditions for modeling thaw penetrations in permafrost soil and deriving water saturation profiles in the active layer.
Figure 6. CH4 emission dynamics during the growing season and variations in boundary conditions (water table level, thaw depth and soil temperature)
Figure 1. Schematic representation of soil permafrost with characterization of aggregates and bulk soils with 3D and 2D networks, respectively.
Abiotic factors affecting CH4 emissions
Figure 7. Simulated seasonal variations of methane concentration and production within the soil depth
Lowering the water table level reduces the net methane emissions
Higher C:N ratio reduces methane emissions
Objectives:
To develop a mechanistic model for the response of microbial communities to changes in thermal and hydrologic conditions in thawing permafrost soil
To quantify methane production and consumption rates under different hydration and thermal conditions at profile scale
To evaluate model performance for estimation of seasonal patterns and magnitude of methane emissions using field data and offer insights concerning observed late season spikes in CH4 emissions
Soil structure effects on CH4 emissions:
Soil aggregates, due to 3D structure, provide simultaneous oxic and anoxic microsites
Aggregates provide favorable conditions for methanogenic activity under unsaturated conditions
Summary and conclusions
A physically based model that combines interactions of individual microbial cells with hydration and thermal conditions were developed
Fluxes were upscaled from individual aggregates to quantification of methane production/consumption at the soil profile scale
The simulations highlighted the crucial role of water table position within the active layer of permafrost on promoting CH4 emissions
The study improves ability to estimate future CH4 emissions budgets by systematic representation of biological and hydrological processes
Figure 4. The relation between the CH4 emission rates from columns of three peat types, as a function of the static water table position
Microbial activity in Pore Networks
Complex pore spaces are abstracted using a novel 3-D angular pore network model
Individual-based model : Microbial cells are simulated individually that:
1) consume nutrients, 2) grow, 3)divide (or die)
4) chemotactically move, and 5) Produce enzymes
Figure 5. (a) Simulation results of methane production and (b) consumption rates over the soil profile as a function of water table positions. The results separate processes in the bulk vs. aggregated fractions of a soil layer.
References: Ebrahimi A, Or D (2014) Microbial dispersal in unsaturated porous media: Characteristics of motile bacterial cell motions in unsaturated angular pore networks. Water Resources Research, 50, 7406–7429.
Ebrahimi A, Or D (2015) Hydration and diffusion processes shape microbial community organization and function in model soil aggregates. Water Resources Research, 51, 9804–9827.
Ebrahimi A, Or D (2016) Microbial community dynamics in soil aggregates shape biogeochemical gas fluxes from soil profiles., Glob. Chang. Biol., 22, 3141–3156.
Ebrahimi A, Or D (2017) Mechanistic modeling of microbial interactions at pore to profile scales resolve methane emission dynamics from permafrost soil, under review JGR: Biogeosciences.
Enzyme activity:
Depolymerization rate
of soil organic carbon:
Figure 2. Schematic of an angular node of pore network model connected by triangular bonds
And nitrogen:
Acknowledgments: We acknowledge financial support of the European Research Council Advanced Grant SoilLife and SystemsX.ch (MicroScapes).
“C:N ratio controls the level of enzyme and, in turn, local microbial activity in permafrost soil”