Nitrogen critical loads for alpine vegetation and soils in Rocky Mountain National Park

We evaluated the ecological thresholds associated with vegetation and soil responses to nitrogen (N) deposition, by adding NH₄NO₃ in solution at rates of 5, 10 and 30 kg N ha^‑1 yr^‑1 to plots in a species rich dry meadow alpine community in Rocky Mountain National Park receiving ambient N deposition of 4 kg N ha^‑1 yr^‑1. To determine the levels of N input that elicited changes, we measured plant species composition annually, and performed one-time measurements of aboveground biomass and N concentrations, soil solution and resin bag inorganic N, soil pH, and soil extractable cations after 3 years of N additions. Our goal was to use these dose–response relationships to provide N critical loads for vegetation and soils for the alpine in Rocky Mountain National Park. Species richness and diversity did not change in response to the treatments, but one indicator species, Carex rupestris increased in cover from 34 to 125% in response to the treatments. Using the rate of change in cover for C. rupestris in the treatment and the ambient plots, and assuming the change in cover was due solely to N deposition, we estimated a N critical load for vegetation at 3 kg N ha^‑1 yr^‑1. Inorganic N concentrations in soil solution increased above ambient levels at input rates between 9 kg N ha^‑1 yr^‑1 (resin bags) and 14 kg N ha^‑1 yr^‑1 (lysimeters), indicating biotic and abiotic sinks for N deposition are exhausted at these levels. No changes in soil pH or extractable cations occurred in the treatment plots, indicating acidification had not occurred after 3 years. We conclude that N critical loads under 10 kg ha^‑1 yr^‑1 are needed to prevent future acidification of soils and surface waters, and recommend N critical loads for vegetation at 3 kg N ha^‑1 yr^‑1 as important for protecting natural plant communities and ecosystem services in Rocky Mountain National Park.