13 May 2022
 | 13 May 2022

Environmental Controls on Observed Spatial Variability of Soil Pore Water Geochemistry in Small Headwater Catchments Underlain with Permafrost

Nathan Alec Conroy, Jeffrey Heikoop, Emma Lathrop, Dea Musa, Brent Newman, Chonggang Xu, Rachael McCaully, Carli Arendt, Verity Salmon, Amy Breen, Vladimir Romanovsky, Katrina Bennett, Cathy Wilson, and Stan Wullschleger

Abstract. Soil pore water (SPW) chemistry can vary substantially across multiple scales in Arctic permafrost landscapes. The magnitude of these variations and their relationship to scale are critical considerations for understanding current controls on geochemical cycling and for predicting future changes. These aspects are especially important for Arctic change modelling where accurate representation of sub-grid variability may be necessary to predict watershed scale behaviours. Our research goal was to characterize intra- and inter-watershed soil water geochemical variations at two contrasting locations in the Seward Peninsula of Alaska, USA. We then attempt to establish which environmental factors were important for controlling concentrations of important pore water solutes in these systems. The SPW geochemistry of 18 locations spanning two small Arctic catchments were examined for spatial variability and its dominant environmental controls. The primary environmental controls considered were vegetation, soil moisture/redox condition, water/soil interactions and hydrologic transport, and mineral solubility. The sampling locations varied in terms of vegetation type and canopy height, presence or absence of near-surface permafrost, soil moisture, and hillslope position. Vegetation was found to have a significant impact on SPW NO3 concentrations, associated with the localized presence of nitrogen-fixing alders and mineralization and nitrification of leaf litter from tall willow shrubs. The elevated NO3 concentrations were however, frequently equipoised by increased microbial denitrification in regions with sufficient moisture to support it. Vegetation also had an observable impact on soil moisture sensitive constituents, but the effect was less significant. The redox conditions in both catchments were generally limited by Fe reduction, seemingly well-buffered by a cache of amorphous Fe hydroxides, with the most reducing conditions found at sampling locations with the highest soil moisture content. Non-redox-sensitive cations were affected by a wide variety of water-soil interactions that affect mineral solubility and transport. Identification of the dominant controls on current SPW hydrogeochemistry allows for qualitative prediction of future geochemical trends in small Arctic catchments that are likely to experience warming and permafrost thaw. As source areas for geochemical fluxes to the broader Arctic hydrologic system, geochemical processes occurring in these environments are particularly important to understand and predict with regards to such environmental changes.

Nathan Alec Conroy et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on egusphere-2022-137', Christopher Campe, 14 Sep 2022
    • AC3: 'Reply on CC1', Nathan Conroy, 23 Nov 2022
  • RC1: 'Comment on egusphere-2022-137', Anonymous Referee #1, 28 Sep 2022
    • AC1: 'Reply on RC1', Nathan Conroy, 23 Nov 2022
  • RC2: 'Comment on egusphere-2022-137', Anonymous Referee #2, 13 Oct 2022
    • AC2: 'Reply on RC2', Nathan Conroy, 23 Nov 2022

Nathan Alec Conroy et al.

Nathan Alec Conroy et al.


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Short summary
This study combines field observations, non-parametric statistical analyses, and thermodynamic modeling to characterize the environmental causes of the spatial variability in soil pore water solute concentrations across two Arctic catchments with varying extents of permafrost. Vegetation type, soil moisture and redox conditions, weathering and hydrologic transport, and mineral solubility were all found to be the primary drivers of the existing spatial variability of some soil pore water solutes.