Plant and soil nitrogen relationships across polygonal ground at Barrow, Alaska

Background/Question/Methods

A reorganization of Arctic plant communities may be a significant result of climate change that drives important feedbacks to the atmosphere and to permafrost stability. Identification and quantification of the key processes linking plant community structure and function to soil moisture and nutrient availability are essential for refining mechanistic-based models of Arctic ecosystems and for linking biogeochemical cycling models to vegetation dynamics models in an integrated, coupled land-climate model framework for both regional and global scales. Representation of plant function at the climate grid-cell scale should be based on relationships between plant community composition and geomorphic units that are observable at the sub-grid-cell scale. A relevant geomorphic framework on Alaska’s north slope is the polygonal ground formed by permafrost processes. We measured nitrogen (N) in plants and soil across the moisture gradients of centers, ridges, and troughs of the polygonal ground of the Barrow Environmental Observatory, including high-centered polygons indicative of permafrost degradation. Plant community composition, aboveground biomass and leaf area, and foliar N content were measured. Plant available NO₃^-, NH₄⁺, and other nutrients were measured using Plant Root Simulators™.

Results/Conclusions

The site was dominated by Carex aquatilis, which comprised 54% of aboveground plant biomass and 59% of aboveground plant N. Foliar N concentration in Carex was uniform across the site, averaging 2.6%. The biomass-weighted average foliar N of the plant community across the land surface was 2.4%, but it was distinctly less (1.5%) in the dry centers of high-centered polygons, reflecting the presence of the evergreen shrub Vaccinium vitis-idaea, which had lower N concentration than other species (1.0 %). Vegetation N on a land area basis was well predicted by leaf area index (LAI), regardless of plant community composition, but this analysis does not include mosses and lichens. Time-integrated total N availability in soil was dominated by NH₄⁺ except in drier locations (e.g., polygon rims and centers of high-centered polygons) where NO₃^- was predominant. Aboveground N content increased with soil moisture and with NH₄⁺ availability. These survey results support the use of LAI as a scalar for plant productivity across this landscape, but increased resolution for predictions related to climate change can be expected with more detailed relationships between plant community composition, soil N availability, and plant N content across contrasting geomorphic units.