Alpine moist meadow response to a regional gradient of nitrogen deposition in the Rocky Mountains
Human alteration of the nitrogen (N) cycle has resulted in a drastic increase in availability of biologically active N. Alpine ecosystems are particularly susceptible to increased inputs of N due to shallow soils, a short growing season, and more precipitation and therefore higher rates of N input than surrounding lower elevation areas. The objective of our study was to examine effects of ambient N deposition in alpine moist meadow communities, and determine whether changes in plant and ecosystem indicators are correlated with N deposition. We found that a number of ecosystem responses were correlated with ambient N deposition, including the tissue N concentration of dominant plant species at peak biomass, and abundance of a nitrophilic dominant grass species. These species are known to have an influence on N processing in soil, and are therefore important potential plant-soil feedback mechanisms facilitating future ecosystem responses to environmental changes. Additionally, we found that plant diversity was negatively correlated with increasing N deposition and there were significant differences in species composition among sites receiving different levels of N deposition. We found less support for a correlation between soil responses and N deposition, with only weak correlations between resin bag N or soil pore water concentrations of nitrate and N deposition during two years of sampling. These findings suggest that while the Rocky Mountain alpine is being affected by N deposition, impacts have been detected only for biotic changes while biogeochemical changes currently are minimal.