Abstract: Nutrient cycles in Arctic tundra ecosystems are tight, with relatively small inputs balanced by relatively small losses. Biological nitrogen fixation (BNF), the conversion of atmospheric N2 gas to bioavailable forms of nitrogen, is the main input of nitrogen in Arctic tundra. As climate change warms and ultimately thaws permafrost, decomposition of previously frozen organic matter releases nutrients, which could tip the tundra nitrogen cycle out of balance unless BNF declines in concert. This occurs in many ecosystems as BNF is energetically expensive. However, nitrogen-fixing organisms found in tundra (prokaryotes living in bryophytes, lichens, litter, and soils) are often disconnected from the soil nitrogen supply. To test the impact of increased nutrient supply on BNF and community composition, we sampled from long term plots at Toolik Field Station, Alaska, where a gradient of nutrient fertilization has been applied annually since 2006 in an effort to simulate elevated decomposition due to climate change. These treatments are 0, 0.5, 1, 2.5, 5, and 10 g N m−2 y−1, with phosphorus also applied at half the rate. We extracted ten 5 cm x 5 cm substrate samples per treatment for each type of nitrogen-fixer (bryophytes, lichens, litter, and soil) in July 2022. The samples were incubated with 15N2, dried, and analyzed for isotopic content and nitrogen concentration.
We found that BNF rates per unit nitrogen-fixing substrate were similar between soil and litter. Bryophytes fixed at a rate 1 order of magnitude higher than soil and litter, and lichen fixed at an additional order of magnitude higher than bryophytes. However, there was no statistically significant difference in BNF rates between fertilization treatments. BNF rates for bryophytes, lichens, litter, and soil averaged 4.080x10-5, 3.544x10-4, 7.80x10-6, and 1.0x10-6 g N g dry mass−1 day−1, respectively. Analyzing % cover data collected annually in the plots, we found that fertilization led to higher % cover of litter, but lower % cover of bryophytes and lichens. Determining how BNF responds to nutrient inputs is crucial to predicting climate change and biodiversity in the Arctic. While studies have examined the direct effects of warming on BNF in the Arctic, there are few data on how BNF responds to increased levels of nutrient supply. If BNF activity persists as we determined, it could further disrupt the Arctic nutrient cycle. Potential consequences include increased nitrogen export to aquatic systems, increased emissions of the greenhouse gas nitrous oxide, and decreasing biodiversity.
