Salix arctica changes root distribution and nutrient uptake in response to subsurface nutrients in High Arctic deserts

Abstract

Moisture is critical for plant success in polar deserts but not by the obvious pathway of reduced water stress. We hypothesized that an indirect, nutrient-linked, pathway resulting from unique water/frozen soil interactions in polar deserts creates nutrient-rich patches critical for plant growth. These nutrient-rich patches (diapirs) form deep in High Arctic polar deserts soils from water accumulating at the permafrost freezing front and ultimately rising into the upper soil horizons through cryoturbated convective landforms (frost boils). To determine if diapirs provide an enhanced source of plant-available N for Salix arctica (Arctic willow), we characterized soil, root, stem, and leaf N natural abundance across 24 diapir and non-diapir frost boils in a High Arctic granitic semi-desert. When diapir horizons were available, S. arctica increased its subsurface (i.e., diapir) N uptake and plant root biomass doubled within diapir. Plant uptake of enriched N injected into organic rich soil patches was 2.5-fold greater in diapir than in non-diapir frost boils. S. arctica percent cover was often higher (7.3 ± 1.0 [mean ± SE]) on diapiric frost boils, compared to frost boils without diapirs (4.4 ± 0.7), potentially reflecting the additional 20% nitrogen available in the subsurface of diapiric frost boils. Selective N acquisition from diapirs is a mechanism by which soil moisture indirectly enhances plant growth. Our work suggests that diapirs may be one mechanism contributing to Arctic greening by shrub expansion.

Could Cryoturbic Diapirs Be Key for Understanding Ecological Feedbacks to Climate Change in High Arctic Polar Deserts?

“…To assess if these were diapiric frost boils (i.e., Bhy horizon present), we quantified the SOC within each soil profile using a field‐portable visible and near‐infrared (vis‐NIR) reflectance spectrophotometer with a soil probe attachment (Guy et al, ). We classified frost boils as diapiric if there was an abrupt increase of at least 0.2 log % SOC in the subsurface—indicative of a diapiric Bhy horizon and nondiapiric in the absence of increasing SOC concentrations with depth (Muller et al, ). Pits were excavated into diapiric and nondiapiric frost boils to expose the soil profile.…”

“…We hypothesize that (1) frost boil diapirism increases key soil properties such as soil organic C and N, which leads to altered microbial nutrient (i.e., C and N) cycling, and (2) frost boil diapirism decreases SOC quality for soil microbes, which links to GHG emissions. Confirming these hypotheses would help explain how desert frost boils influence GHG emissions and why S. arctica aggressively forages for nutrients in frost boils (Muller et al, ).…”

“…Frost heave sorts soil materials by size, producing repeated circular patches or frost boils (<3 m diameter) at the ground surface, with finer materials in the centers and larger materials in the margins. Frost boils are common in polar deserts (Muc et al, ), with surfaces that are either barren or sparsely vegetated (Muller et al, ; van Everdingen, ). Frost boil cryoturbation may control biological production of GHGs from nutrient cycling, as organic matter leaches downward from the surface and then moves upward with frost heave (Walker et al, ).…”

“…Diapirism intrudes soil materials from the top of the permafrost table (ice‐rich horizon) into the overlying horizon. Within frost boils, surface meltwater leaching (Muller et al, ; Schaeffer et al, ; Tye et al, ) can accumulate in ice‐rich horizons above concavities in the permafrost table (Boike et al, ; Cannone et al, ; Walker et al, ); therefore, diapirism can carry nutrients upward creating nutrient‐rich soil horizons (Bhy). Diapiric nutrient intrusions occur when this nutrient‐enriched low bulk density (higher buoyancy) Bhy horizon “floats” through the higher bulk density overlying layer into the upper soil (Swanson et al, ).…”

“…How plant and soil biology interacts with diapirism and how this interaction may contribute to GHG production from frost boil systems are not clear. Muller et al () found that Salix arctica (arctic willow) roots actively targeted Bhy horizons for N, though microbial C respiration was lower in diapiric relative to nondiapiric frost boils (Brummell et al, ). In other arctic regions, C and N mineralization was lower in cryoturbated relative to noncryoturbated horizons (Gillespie et al, ; Kaiser et al, ; Wild et al, ).…”