Controls on Fine-Scale Spatial and Temporal Variability of Plant Available Inorganic Nitrogen in a Polygonal Tundra Landscape

NGEE Arctic researchers documented strong relationship between soil moisture, inorganic N availability, and plant N content across the polygonal ground of the Barrow Environmental Observatory

Nitrogen availability in the Arctic strongly influences plant productivity and distribution, and in permafrost systems with patterned ground, ecosystem carbon and nutrient cycling can vary substantially over short distances. Improved understanding of fine-scale spatial and temporal variation in soil N availability is needed to better predict tundra responses to a warming climate. We quantified plant-available inorganic N at multiple soil depths in 12 micro-habitats associated with a gradient from low-center ice-wedge polygons to high-center polygons in coastal tundra at Barrow, Alaska. We measured vegetation composition, biomass, N content, and rooting depth distribution, as well as soil temperature, moisture, pH, and thaw depth to determine relationships between the spatial and temporal variability in N availability and environmental and vegetation drivers. Micro-habitat strongly influenced the form of available inorganic N, with ammonium predominating in polygon troughs and low-centers, and nitrate predominating in drier high-centers, flat-centers and rims. Soil moisture was the most important variable linked to distribution of both ammonium and nitrate. Total inorganic N availability increased as the active layer thawed, but the newly available N near the permafrost boundary late in the season was apparently not available to roots and did not contribute to plant N content. Nitrate in the drier sites also was not associated with plant N content, raising the possibility of N losses from this N-limited ecosystem. These results imply that understanding hydrological changes with permafrost degradation is key to determining nutrient cycling responses in complex polygonal tundra landscapes. Reference: Norby RJ, VL Sloan, CM Iversen, and J Childs. 2018. Controls on fine-scale spatial and temporal variability of plant available inorganic nitrogen in a polygonal tundra landscape. Ecosystems (in review).

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Richard Norby