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

Norby, R. J., Sloan, V. L., Iversen, C. M., & Childs, J. . 2018.

Abstract

Nitrogen availability in the Arctic strongly influ- ences 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 nitrogen at multiple soil depths in 12 microhabitats associated with a gradient from low-center ice-wedge polygons to high-center polygons in coastal tundra at Utqia_gvik (formerly Barrow), Alaska. We measured vegetation composition, biomass, N content, and rooting depth distribution, as well as soil tempera- ture, moisture, pH, and thaw depth to determine relationships between the spatial and temporal variability in N availability and environmental and vegetation drivers. Soil moisture varied across the microhabitats of the polygonal terrain and was the most important variable linked to distribution of both ammonium and nitrate, with ammonium predominating in wetter areas and nitrate pre- dominating in drier areas. Total inorganic N avail- ability increased as the soil in the active layer thawed, but the newly available N near the per- mafrost 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 ecosys- tem. The strong relationship between soil moisture, inorganic N availability, and plant N content im- plies that understanding hydrological changes that may occur in a warming climate is key to deter- mining nutrient cycling responses in complex polygonal tundra landscapes.

Key Words

Arctic; ice-wedge polygons; tundra; microhabitat; thaw; active layer; nitrate; ammo- nium; plant-available nitrogen; root distribution.