Alpine Microbial Community Responses to Climate Change and Atmospheric Nitrogen Deposition in Rocky Mountain National Park

Remote alpine ecosystems of the western US exhibit vulnerability to anthropogenic drivers of change. Atmospheric nitrogen (N) deposition and a changing climate introduce nutrients, alter hydrological processes, and expose soils to modified temperature regimes. We cannot yet predict the interacting effects and far-reaching biogeochemical consequences of this influence. Importantly, long-term data reveal headwater nitrate (NO3-) concentration trends increasing >50% from the 1990s to 2006 along the Colorado Front Range in conjunction with warm summer temperatures. Such a change in nutrient cycling raises concern for eutrophication in nutrient-poor alpine lakes. Increasing stream NO3- suggests terrestrial microbes may be responding to changes in important controls of community development and activity: temperature and ammonium (NH4+) availability. Nitrifying bacteria and archaea strongly influence alpine soil NO3- concentrations. Little is understood about alpine microbes. Our research characterizes nitrifier abundance and activity in alpine substrates by exposing them to experimental NH4+ and temperature treatments. Soil substrates fall along a gradient of succession commonly represented in alpine catchments due to deglaciation. These include well-developed meadow soils, unvegetated talus substrate, and newly-exposed glacial sediments. All three substrate types were collected from the Loch Vale watershed in Rocky Mountain National Park, a long-term research site in the Colorado Front Range known to receive elevated levels of atmospheric N deposition. All soils have been evaluated for initial %C, %N, microbial biomass, NO3-, NH4+, and DOC concentrations, and nitrifier abundance. After temperature and NH4+ treatments, samples will be evaluated for changes in biomass and nitrifier abundance as well as net and gross nitrification. Linking the influence of relative soil temperature and NH4+ concentrations on alpine substrates, at a range of successional stages, will provide insight into control thresholds, optima, and synergistic interactions. Characterizing microbial NO3- production in the alpine will help us evaluate the importance of biological, as opposed to physical, sources of stream NO3-. It will also inform our ability to forecast and mitigate consequences of anthropogenic drivers of change on these systems.