Dynamic soil columns simulate Arctic redox biogeochemistry and carbon release during changes in water saturation

Abstract

Thawing Arctic permafrost can induce hydrologic change and alter redox conditions, shifting the balance of soil organic matter (SOM) decomposition. There remains uncertainty about how soil saturation and redox transitions impact dissolved and gas phase carbon fluxes, and efforts to link hydrobiogeochemical processes to ecosystem-scale models are limited. This study evaluates SOM decomposition of Arctic tundra soils using column experiments, water chemistry measurements, microbial community analysis, and a PFLOTRAN reactive transport model. Soil columns from a thermokarst channel (TC) and an upland tundra (UC) were exposed to cycles of saturation and drainage, which controlled carbon emissions. During saturation, an outflow of dissolved organic carbon from the UC soil correlated with elevated reduced iron and decreased pH; during drainage, UC carbon dioxide fluxes were 70% higher than TC fluxes. Intermittent methane release was observed for TC, consistent with higher methanogen abundance. Slower drainage in the TC soil correlated with more subtle biogeochemical changes. PFLOTRAN simulations captured experimental trends in soil carbon fluxes, oxygen concentrations, and water contents. The model was then used to evaluate additional soil water drainage rates. This study emphasizes the importance of considering hydrologic change when evaluating and simulating SOM decomposition in dynamic Arctic tundra environments.

Fig. 1

Soil core depth profiles for in situ thermokarst soil (blue squares and triangles) and upland soil (green circles and diamonds) show gravimetric water content (a), pH extracted with 2 M KCl (b), ferrous iron concentration from 2 M KCl extractions (c), total carbon: nitrogen ratios (d), and soil carbon composition (e). Based on the geochemical depth characterization, soil depths were assigned to be organic soils (darkest marker colors), mineral/transition soils, or permafrost/deep soils (lightest marker colors). Depth profiles were characterized prior to processing for experimental setup. Thermokarst 2 was shifted down 10 cm to account for assumed soil loss in the field during sampling. Triplicate soil samples at each depth for each core were evaluated. Shading in the table indicates depth increments of cores that were later homogenized for soil column experiments.

Fig. 2

Experimental timeseries over the three-month column experiments with data from the upland column shown in green and the thermokarst in blue. Color bars across the top indicate periods of draining and saturation for each experiment. Outflow volume represents the cumulative volume of water leaving the base of the columns during draining phases, and decreases occur during water addition in saturation phases (a). Oxygen concentrations (% volume) measured with an optical sensor at 8.5 cm depth are presented as an example of oxygen concentration variability (b), with data from other depths presented in the Supporting Information Figure S9. Discrete headspace gas samples were used to determine fluxes of CO₂ (c) and CH₄ (d) during the experiments.

Fig. 3

Experimental timeseries of discrete aqueous samples from Rhizons in column sample ports over the three-month column experiments. Different depths are indicated with different colors and symbols (cooler colors near the top of the soil column and warmer colors at the base). Data include pH (a and b), the ferrous:total iron ratio (c and d), dissolved organic carbon (DOC) (e and f), and specific UV absorbance (SUVA) (g and h) for the upland and thermokarst columns. Grey areas indicate the period of soil water saturation.

Fig. 4

Extracted iron from the upland (UC) and thermokarst (TC) columns comparing soil Fe concentrations before the experiment (pre) and after the experiment (post) for the three different homogenized soil sections (org = organic, min = mineral/transition, and deep = deep/permafrost). Details for extracting exchangeable, organic bound, poorly-crystalline, and crystalline iron are included in the Methods and Materials. Exchangeable, organic bound, and poorly crystalline extractions were conducted in triplicate, and mean values are shown with standard deviations. Only one sample was measured for the crystalline extractions.

Fig. 5

Relative microbial composition for methanogens and iron-active bacteria (part a), and log-scale plot for magnification of low relative abundances methanogens (part b). TC = thermokarst soil column; UC = upland tundra soil column. Org, Min, and Perm/Deep) = homogenized initial organic, mineral, and permafrost (deep) soil horizons, respectively. * Indicates initial soil microbial community pre-experiment. Depth increments (0–10, 10–20, 20–30, 30–40, and 40–50 cm) represent post-experiment community characterization along the soil column.

Fig. 6

PFLOTRAN simulations compared to column experiment data for the upland column (UC) in part a and thermokarst column (TC) in part b. Headspace CO₂ fluxes from the experiment (blue-colored markers) are plotted with PFLOTRAN simulation results (black dashed lines).

Fig. 7

PFLOTRAN simulations for different saturated hydraulic conductivities (K_sat) compared to the base case (black line). Increasing K_sat by an order of magnitude (10 × K_sat, red dashed line) and decreasing by an order of magnitude (0.1 × K_sat, blue dotted line) resulted in variations in response variables (liquid saturation, pH, Total aqueous Fe, Free DOM, Total aqueous acetate, and Total CH₄) for both upland simulations (top, green outline) and thermokarst simulations (bottom, blue outline). Bold vertical axis titles indicate different vertical axis scales for upland and thermokarst simulations. Additional simulation results and a sensitivity analysis for the PFLOTRAN simulations are presented in Supporting Information.

Fig. 8

Research site near Council Road mile marker 71. Aerial view of site showing thermokarst channels (a), and a ground-view of the thermokarst channels showing variation in vegetation from the upland with low shrubs and tussock to the thermokarst channels with relatively more sedges (b). The channels are formed from thawing permafrost and land subsidence.

Fig. 9

Soil column experiment with sampling ports for Rhizon samplers and sensors for water content and oxygen. Air was continuously pumped through the headspace at the top of the column. During draining phases, the bottom outflow sample port was open.