Assessment of Atmospheric Nitrogen Deposition: Possible Effects on Alpine Ecosystems Above 9000 Feet In Grand Teton National Park

Atmospheric N deposition is becoming a stressor on ecosystems in the western U.S. There are few National Atmospheric Deposition Program (NADP) monitoring sites and little is known about N deposition impacts on terrestrial ecosystems in the Intermountain West. Alpine ecosystems may be particularly sensitive to changes in N inputs because of the shallow soils, short growing seasons, and sparse plant cover. This study focused on N deposition effects on an alpine ecosystem in Grand Teton National Park located along a modeled N deposition gradient (Moose Basin high, Paint Brush Medium, Rendezvous Mtn. low) and across contrasting edaphic conditions using a two-factorial design. At each location, we estimated N deposition and measured soil moisture and temperature across edaphic conditions, soil parameters (total and extractable N, available N, net mineralization, and nitrification potential), and plant community characteristics (species richness, species composition, percent cover, plant and root biomass, N content, and above and belowground plant components). These response variables were used to test whether there is a north to south N deposition gradient, if N deposition and N status are affected by soil moisture content, and whether soil and/or plant properties were affected by N deposition and edaphic conditions and if the response variables can serve as indicators as early warning signs of N saturation. The Tetons receive 1.42 kg N ha-1 yr-1 with more in winter (0.85-1.17 kg N ha-1 yr-1) than during the summer (0.25 kg N ha-1 yr-1). Soil moisture content was related to snowpack accumulations and melt but did not affect N status. Moose Basin (i.e., high N deposition) showed characteristics of an N-rich site shown by higher soil N content and extractable soil NH4+, higher nitrification potential, low C:N ratios, more aboveground biomass, and higher foliar N content compared to the RDV location. Rendezvous Mountain (i.e., low N deposition) showed characteristics of an N-poor site having lower soil extractable N, high C:N ratios in soil and roots, and low N mineralization potential. Paint Brush was highly variable in soil and plant characteristics and most clearly showed differences between wet and dry sites. In terms of N status, it was intermediate and shared similarities with both N-poor and N-rich sites. This study shows that it is important to consider both soil and plant indicators (i.e., total and extractable N in soils, soil nitrification potential, above and belowground biomass, and N content) together to assess N status. The C:N ratio of plants and soils was less informative than anticipated. Species richness and composition was a less sensitive indicator of N-induced change and should be considered for long-term assessments only