Environmental controls on observed spatial variability of soil pore water geochemistry in small headwater catchments underlain with permafrost

Nathan Alec Conroy, Jeffrey M. Heikoop, Emma Lathrop, Dea Musa, Brent D. Newman, Chonggang Xu, Rachael E. McCaully, Carli A. Arendt, Verity G. Salmon, Amy Breen, Vladimir Romanovsky, Katrina E. Bennett, Cathy J. Wilson, Stan D. Wullschleger

Research output: Contribution to journalArticlepeer-review

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 modeling where accurate representation of sub-grid variability may be necessary to predict watershed-scale behaviors. Our research goal is 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 identify the key factors controlling concentrations of important pore water solutes in these systems. The SPW geochemistry of 18 locations spanning two small Arctic catchments was examined for spatial variability and its dominant environmental controls. The primary environmental controls considered were vegetation, soil moisture and/or 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.

Original languageEnglish
Pages (from-to)3987-4006
Number of pages20
JournalCryosphere
Volume17
Issue number9
DOIs
StatePublished - Sep 14 2023

Funding

We would like to thank the Sitnasuak Native Corporation and the Mary's Igloo Native Corporation for their guidance and for allowing us to conduct this research on the traditional homelands of the Inupiat people. Funding was provided by the Next-Generation Ecosystem Experiments (NGEE Arctic) project, supported by the Office of Biological and Environmental Research in the US DOE Office of Science. We wish to thank Lauren Charsley-Groffman and Nathan Wales for their assistance with fieldwork as well as George Perkins, Oana Marina, Rose Harris, and Emily Kluk for their assistance with laboratory analyses. This study takes advantage of a scientifically diverse array of observations and datasets made available by the Next-Generation Ecosystem Experiments (NGEE) Arctic project, sponsored by the US Department of Energy Office of Science. Most of the locations studied herein were selected by the NGEE Arctic project to provide co-located measurements in a wide range of vegetation types, nested within representative hillslopes and catchments. Although selected largely to represent a range of vegetation structure, such as shrub abundance and canopy height, these locations also have considerable variability in other environmental parameters including but not limited to soil moisture and temperature, presence or absence of near-surface permafrost, and maximum observed thaw depth (Tables 1 and 2). The vegetation-delineated sampling approach provides an opportunity to not only quantify the biogeochemical variability of SPW in Arctic environments but also to investigate the root causes of that observed variability. Data from additional sampling locations, available from a co-located study, were also utilized when possible.

FundersFunder number
Office of Biological and Environmental Research in the US DOE
Office of Science

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