Verity G. Salmon
October 16, 2019
Symbiotic nitrogen fixation by alder shrubs influences the availability of a key limiting nutrient in tundra ecosystems.
Inputs of nitrogen by alder, a deciduous shrub that associates with nitrogen-fixing bacteria, were quantified in two tundra plant communities, and the ecosystem-scale effects on nitrogen cycling were assessed. The results from this study demonstrate that tall alder shrubland communities had high nitrogen inputs that were associated with high levels of available nitrogen in adjacent soils and plant communities. These tall alder shrublands can be identified in satellite and aerial imagery and have expanded their range during the last half century.
Aerial imagery collected from 1956 to 2014 indicated that alder shrublands at this study site expanded 40%, and researchers from Oak Ridge National Laboratory (ORNL) and the Next-Generation Ecosystem Experiments (NGEE)–Arctic team calculated that this expansion may have increased nitrogen inputs by 22%. These findings suggest quantifying nitrogen fixation at the landscape scale is feasible and important for predicting future nutrient availability of warming tundra ecosystems.
Primary productivity of tundra plants is strongly limited by nitrogen availability, so plants capable of symbiotic nitrogen fixation have the potential to alter plant, soil, and microbial interactions in rapidly warming Arctic ecosystems. The ORNL research team, therefore, investigated the impact that alder, a nitrogen-fixing deciduous shrub, has on tundra nitrogen cycling at a hillslope located on Alaska’s Seward Peninsula. The team quantified nitrogen fixation in two distinct alder communities at this site: tall-statured alder shrublands located on well-drained, rocky outcroppings in the uplands and relatively short statured alder savannas located in water tracks along the moist toe slope of the hill. Annual nitrogen fixation rates in alder shrublands were 1.95 ± 0.68 grams of nitrogen (g N) per m2 per year, leading to elevated nitrogen levels in adjacent soils and plants. Alder savannas had lower nitrogen fixation rates (0.53 ± 0.19 g N per m2 per year), perhaps due to low phosphorus availability and poor drainage in highly organic soil profiles underlain by permafrost. In addition to supporting higher rates of nitrogen fixation, alder shrublands had different foliar traits than alder in savannas, providing an opportunity to link estimates of nitrogen fixation to remotely sensed data products. Analysis of historic aerial and satellite imagery showed that the area covered by alder shrublands at this hillslope site has increased by 40% from 1956 to 2014. The team estimates this increase was associated with a 22% increase in nitrogen inputs from fixation. Study results suggest that expansion of alder shrublands has the potential to substantially alter nitrogen cycling in upland tundra regions. An improved understanding of the consequences of nitrogen fixation within nitrogen-limited tundra plant communities will, therefore, be crucial for predicting the biogeochemistry of warming Arctic ecosystems.
BER Program Manager
U.S. Department of Energy Office of Science, Office of Biological and Environmental Research
Earth and Environmental Systems Sciences Division (SC-33.1)
Environmental System Science
Verity G. Salmon, R&D Associate
Oak Ridge National Laboratory
Environmental Science Division; Climate Change Science Institute
Oak Ridge, TN 37831
This work is supported (in part) by UT-Battelle, LLC, under Contract No. DE-AC05-00OR22725, with the U.S. Department of Energy (DOE) Office of Science and by the Next-Generation Ecosystems Experiments (NGEE)–Arctic project in the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research (BER), within the DOE Office of Science.
Salmon, V. G., et al. “Alder distribution and expansion across a tundra hillslope: Implications for local N cycling.” Frontiers in Plant Science 10, 1099 (2019). [DOI:10.3389/fpls.2019.01099].