August 14, 2018

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Controls on Nitrogen Availability in the Arctic Tundra

Hydrological changes are key to determining nutrient cycling responses in complex polygonal tundra landscapes.

The Science

The unique aspects of the permafrost environment create new challenges for representing plant-nitrogen interactions in the Arctic tundra. Next-Generation Ecosystem Experiments (NGEE)–Arctic scientists from Oak Ridge National Laboratory measured how nitrogen availability to plants varies spatially and temporally in the Arctic tundra in relation to microhabitats and permafrost thaw.

The Impact

Arctic models should not assume that increasing thaw depth with warming of the Arctic will release additional nitrogen to the benefit of plants. Increased production of inorganic nitrogen that is not coupled to plant uptake could lead to nitrogen losses from the system and degradation of the ecosystem.

Summary

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 nitrogen availability is needed to better predict tundra responses to a warming climate. NGEE-Arctic scientists from ORNL quantified plant-available inorganic nitrogen 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 Utqiagvik (formerly Barrow), Alaska.  They measured vegetation composition, biomass, nitrogen 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 nitrogen availability and environmental and vegetation drivers. Soil moisture variability across the complex polygonal terrain of the Barrow Environmental Observatory was the primary determinant of nitrogen availability. Drier habitats had a greater proportion of their nitrogen economy as nitrate rather than ammonium, but the plant species present could not exploit this resource. Nitrogen availability increased as the soil thawed during the summer, but the newly available nitrogen near the permafrost boundary late in the growing season was not available to roots. The strong relationship between soil moisture, inorganic nitrogen availability, and plant nitrogen content implies that understanding hydrological changes that may occur in a warming climate is key to determining nutrient cycling responses in complex polygonal tundra landscapes.

Principal Investigator

Richard Norby
Oak Ridge National Laboratory
norbyrj@ornl.gov

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This work was funded by the Next-Generation Ecosystem Experiments (NGEE)–Arctic) project. The NGEE-Arctic project is supported by the Office of Biological and Environmental Research within the U.S. Department of Energy (DOE) Office of Science.

Related Links

References

Norby, R. J., V. L. Sloan, C. M. Iversen, and J. Childs. "Controls on fine-scale spatial and temporal variability of plant-available inorganic nitrogen in a polygonal tundra landscape". Ecosystems 22 528–543  (2019). http://dx.doi.org/10.1007/s10021-018-0285-6.