Preprints
https://doi.org/10.5194/tc-2021-166
https://doi.org/10.5194/tc-2021-166

  11 Jun 2021

11 Jun 2021

Review status: this preprint is currently under review for the journal TC.

High Temporal and Spatial Nitrate Variability on an Alaskan Hillslope Dominated by Alder Shrubs

Rachael E. McCaully1,2, Carli A. Arendt1,2, Brent D. Newman1, Verity G. Salmon3, Jeffrey M. Heikoop1, Cathy J. Wilson1, Sanna Sevanto1, Nathan A. Wales1, George B. Perkins1, Oana C. Marina1, and Stan D. Wullschleger3 Rachael E. McCaully et al.
  • 1Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, 87545, United States
  • 2Department of Marine Earth and Atmospheric Sciences, North Carolina State University, Raleigh, 27695, United States
  • 3Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, 37830, United States

Abstract. In Arctic ecosystems, increasing temperatures are driving the expansion of nitrogen (N) fixing shrubs across tundra landscapes. The implications of this expansion to the biogeochemistry of Arctic ecosystems is of critical importance, yet many details about the form, location, and availability of N from these shrubs remain unknown. To address this knowledge gap, the spatiotemporal variability of nitrate (NO3) and its environmental and edaphic controls were investigated at an alder (Alnus viridis spp. fruticosa) dominated permafrost tundra landscape in the Seward Peninsula, Alaska, USA. Soil pore water was collected from locations within alder shrubland growing along a well-drained hillslope and compared to soil pore water collected from locations outside (upslope, downslope, and between) the alder shrubland. δ15N and δ18O of soil pore water were consistent with the predicted range of NO3 produced through microbial degradation of N-rich alder shrub organic matter. Soil pore water collected within alder shrubland had an average NO3 concentration of (4.27 ± 8.02 mg L−1) and differed significantly from locations outside alder shrubland (0.23 ± 0.83 mg L−1; p < 0.05). Temporal variation in NO3 within and downslope of alder shrubland corresponded to precipitation events, where NO3 accumulated in the soil was flushed downslope during rainfall. Enrichment of both δ15N and δ18O isotopes at wetter downslope locations indicate that denitrification buffered the mobility and spatial extent of NO3. These findings have important implications for nutrient production and mobility in N-limited permafrost systems that are experiencing shrub expansion in response to a warming Arctic.

Rachael E. McCaully et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2021-166', Anonymous Referee #1, 17 Jul 2021
  • RC2: 'Comment on tc-2021-166', Anonymous Referee #2, 15 Aug 2021

Rachael E. McCaully et al.

Rachael E. McCaully et al.

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Short summary
Degrading permafrost and shrub expansion are critically important to tundra biogeochemistry. We characterized sources and spatiotemporal trends of soil pore water NO3 in an alder dominated permafrost hillslope in Alaska (USA) and observed significant variability. Proximity to alder shrubs and downslope denitrification strongly influence NO3 availability. The highly dynamic nature of labile N on small spatiotemporal scales has key implications for nutrient mobility in a warming Arctic.