Elevated ambient levels of nitrogen (N) deposition, initiated during the Industrial Revolution, have been implicated in large-scale ecosystem modifications throughout the 20th and 21st centuries. There are many environmental consequences associated with increased N availability, that include increased N concentrations in plant tissues, changes in plant population and community metrics, as well as more extreme changes in soil chemistry following a hierarchy of ecological indicators. What is less understood, however, are potential stabilizing feedbacks preventing change in ecological indicators with increases in N deposition. Community scale processes, including increased genetic and trait based diversity, have been shown to offer stabilizing feedbacks to environmental changes in many ecosystems, although how important these processes are in driving ecosystem response to low dosages of N deposition is relatively unknown. This study compares plant and soil ecological indicators of N deposition among three alpine communities (dry, moist, and wet meadows) varying in evenness and diversity of native plants using experimental N additions of 0, 3, 12 and 30 kgN ha-1 yr-1. Alpine ecosystems are particularly susceptible to increased inputs of N, due to a short growing season, and therefore it is critical to understand potential stabilizing feedbacks to N deposition in these areas.
The three alpine communities included in our study spanned a species richness gradient moist>wet>dry, and included variation in dominant plant species N-use strategies. Both the dry and moist meadows experienced changes in concentrations of N in dominant plant species biomass or litter (dry: 12 kgN ha-1 yr-1; moist: 30 kgN ha-1 yr-1). Changes in cover associated with dominant species occurred in all three communities (moist: 12 kgN ha-1 yr-1; wet: 12 kgN ha-1 yr-1; dry: 30 kgN ha-1 yr-1). Diversity was also affected for both the dry (3 kgN ha-1 yr-1) and moist meadow (30 kgN ha-1 yr-1). Soil indicators of ecological response to N deposition included resin extractable N, and we only observed significant increases in the dry and wet meadows (30 kgN ha-1 yr-1). For some indicators (cover) we found that the more diverse communities (dry, wet) required higher levels of N additions before change observed, however this was not true across the board. Many indicators showed changes in the dry meadow at levels lower than either the moist or wet meadows (diversity), suggesting that there are potential functional or resource-use traits that are equally important in predicting ecosystem responses to N deposition.