September 14, 2023

Improved Understanding of Controls on Arctic Soil Pore Water Variability

Characterizing Arctic watershed soil water geochemical variations and determining primary environmental controls helps predict future change.

Median concentrations (gray diamonds with dashed black lines) of calcite, strontium, and magnesium with distance downslope at the Teller-27 catchment (blue) and the Kougarok-64 hillslope (yellow) along topographic transects in Alaska. The elevation profiles of the hillslopes are plotted in green (right axes).

[Reprinted under a Creative Commons Attribution 4.0 International License (CC BY 4.0) from Conroy, N. A., et al. "Environmental Controls on Observed Spatial Variability of Soil Pore Water Geochemistry in Small Headwater Catchments Underlain with Permafrost." The Cryosphere 17 (9), 3987–4006 (2023). DOI:10.5194/tc-17-3987-2023.]

The Science

Permafrost thaw in the Arctic is causing significant changes to landscape structure, hydrology, vegetation, and biogeochemistry. These changes produce carbon fluxes and increased nutrients in Arctic rivers, leading to enhanced nutrient loadings with strong implications for the global carbon cycle. Many recent studies focus on environmental change observed and expected as a result of Arctic warming, but only a limited understanding exists of the key environmental controls on the spatial distribution of soil pore water (SPW) solute concentrations. This study analyzes the primary drivers of these changes.

The Impact

This study quantitatively evaluates the spatial variability of SPW geochemistry within and between 2 distinct catchments underlain with permafrost and seeks to identify the observed spatial variability’s source. Identifying the dominant controls on solute concentration variability within and across catchments will facilitate better projections of soil pore hydrogeochemistry in permafrost landscapes and improve understanding of how these signatures are related to changing soil moisture and increasing tundra shrub abundance in the Arctic. Changes in hydrogeochemistry in small Arctic catchments not only have larger-scale impacts but also impact the future hydrogeochemistry of larger Arctic rivers.

Summary

To address knowledge gaps in understanding biogeochemical cycles in a changing Arctic, this study analyzed data from 2 contrasting hillslope sites on the Seward Peninsula in Alaska. A team of researchers sampled SPW from the upper 30 cm of soil with fiberglass wicks and MacroRhizons across the study sites.

This data was paired with additional observations of vegetation characteristics, soil moisture, and permafrost extent to analyze the dominant environmental controls of solute concentrations within SPWs at the sites. Researchers then conducted thermodynamic modeling with PHREEQC to understand what could control SPW solute concentrations. The approach identified mineral phases that may control solute generation processes through solubility limitations.

Vegetation significantly impacted SPW concentrations and was associated with the localized presence of nitrogen-fixing alders and mineralization and nitrification of leaf litter from tall willow shrubs. Vegetation also had a less significant impact on soil moisture–sensitive constituents.

The redox conditions in both catchments were generally limited by iron reduction, with the most reducing conditions found at sampling locations with the highest soil moisture content. Nonredox-sensitive cations were affected by various water-soil interactions that affect mineral solubility and transport. Topographic differences and lack of well-defined drainage channels were the likely environmental controls causing systematically higher SPW solute concentrations at one study site.

Overall, the study provides directions for future work and suggests that evaporative concentration could be a significant control on SPW solute concentrations in permafrost catchments, particularly in those with limited drainage and therefore a perched near-surface water table. Future efforts to predict SPW solute and nutrient dynamics should directly address evaporative concentration’s impacts on permafrost catchments, especially with future permafrost thaw.

Principal Investigator

Nathan Conroy
Los Alamos National Laboratory
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This research was supported by the Biological and Environmental Research program in the U.S. Department of Energy’s Office of Science as a contribution to the Next-Generation Ecosystem Experiments Arctic project.

References

Conroy, N. A., et al. "Environmental Controls on Observed Spatial Variability of Soil Pore Water Geochemistry in Small Headwater Catchments Underlain with Permafrost." The Cryosphere 17 (9), 3987–4006 (2023). https://doi.org/10.5194/tc-17-3987-2023.