Nitrogen is the primary limiting nutrient for Arctic plant productivity so inputs of available N via symbiotic fixation has the potential to alter plant, soil and microbial interactions in these rapidly warming ecosystems. Alnus viridis (alder) is a deciduous shrub that forms a symbiotic relationship with Frankia bacteria. These bacteria fix atmospheric nitrogen (N) into biologically available forms within nodules formed on the shrub’s roots. To assess the impact of alder on N cycling within nearby plant communities and evaluate alder’s inclusion as novel plant functional type (PFT) in earth system models, in 2016 we established vegetation plots across the six most common plant communities at the Kougarok hillslope near Nome, AK on the Seward Peninsula. Aboveground biomass, inorganic N and P availability in surface soils, and foliar %N and natural abundance δ15N were measured across all six plant communities. We quantified N inputs associated with alder by measuring N fixation rates using 15N-N2 and nodule biomass associated with tall stature alder individuals growing in alder shrublands communities. Nodule biomass associated with short stature alder growing in water tracks across tussock tundra was also assessed and the presence of alder across the site was determined using point-center-quarter transects.
Results/Conclusions
Although there was a high degree of variation in observed characteristics across the six plant communities, soil inorganic N availability and foliar %N were highest within plots located near alder, suggesting that symbiotic N fixation inputs from alder shrubland communities impacts N availability of neighboring microsites. Variation in N inputs via symbiotic N fixation were driven primarily by variation in nodule biomass rather than by the observed rates of N fixation within root nodules. Nodule biomass for alder growing in shrublands was significantly higher than that of alder growing in water tracks (18.54 g/m2 vs 3.64 g/m2, p =0.04). Basal area of alder in shrublands was also higher (6.49 cm2/ m2 vs 1.14 cm2/ m2, p=0.02) as was height (264.5 cm vs 125 cm, p<0.001). The dichotomous grouping of aboveground and belowground alder traits at this site suggests that an alder PFT could be parameterized differently for the two plant communities found at the Kougarok site. Overall, our results show that the inclusion of a symbiotic N-fixing shrub in dynamic vegetation models will be crucial for capturing N cycling in these nutrient poor high latitude ecosystems.