Differences in ecological thresholds to nitrogen deposition among Rocky Mountain alpine meadow ecosystems

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The availability of biologically active nitrogen (N) drastically increased following human alteration of the N cycle with the production of synthetic sources of fertilizers. There are many environmental consequences associated with increased N availability, ranging from rates of nutrient cycling to changes in plant diversity, and some ecosystems are more susceptible to these consequences than others. Additionally, the amount of added N necessary to induce ecosystem changes varies among different types of potential responses, creating a hierarchy of expected changes with increasing N availability for a given ecosystem. This study compares ecological thresholds of atmospheric N deposition, using experimental additions in three different alpine meadow communities, as alpine ecosystems are particularly susceptible to increased inputs of N. Most studies examining the impact of N deposition on plant communities rely on levels of N addition far exceeding ambient conditions, and therefore may over or underestimate the threshold levels of N necessary for producing ecosystem responses. Our study focuses on adding N at 1x, 4x and 10x the ambient deposition rate of 3 kg N ha^-1 yr^-1, to better predict key thresholds for subtle changes in responses such as plant abundance that are overlooked in experiments with high rates of nutrient addition.

After two growing seasons of treatment, our results show that there are changes in the plant species composition, within a community type, associated with increased additions of N for both in the wet and dry meadows (F_3,99 = 3.18, p = 0.03). Specific plant compositional changes include a non-linear change in species diversity for dry meadows, where the greatest addition of N has increased diversity relative to control and treatments 2 and 3. In the moist meadow, the control plots have the least diversity, while treatment 2 and 3 have more, and treatment 4 is greatest. Changes in soil N processes in response to N additions generally take longer to become apparent, however we have seen increased soil N availability in plots receiving the most N as compared to other treatments for both the dry and moist meadows.  We have not seen an increase in N availability in the wet meadow community, potentially due to denitrification. Future work will examine potential drivers for differences in responses among alpine plant communities, including differences in soil structure and the effect of the plant community composition on other observed responses.