Print PageRemote alpine ecosystems of the western US are especially vulnerable to anthropogenic drivers of change. Atmospheric nitrogen (N) deposition and changing climate introduce nutrients, alter hydrological processes, and expose soils to novel temperature regimes. We cannot yet predict the interacting effects and far-reaching biogeochemical consequences of these changes. Importantly, long-term data reveal that headwater nitrate (NO3-) concentrations increased >50% from the 1990s to 2006 along the Colorado Front Range, commensurate with warmer summer temperatures. This change in N cycling raises concern for nutrient enrichment in oligotrophic alpine lakes. Increasing stream NO3- suggests that terrestrial microbes may be responding to changes in important controls of community development and activity associated with a changing climate, namely temperature and moisture. Nitrifying bacteria and archaea play an important role in alpine N cycling and soil NO3- concentrations.
Results/Conclusions
Our research explores nitrifier abundance and activity in three alpine soils by exposing them to experimental temperature and moisture treatments. The soils fall along a gradient of succession commonly represented in alpine catchments due to deglaciation. These include well-developed meadow soils, poorly vegetated talus substrate, and newly-exposed glacial outwash. Soil samples were collected in August of 2011 from the Loch Vale watershed in Rocky Mountain National Park, a long-term research site in the Colorado Front Range. We found that soils were N-rich and contained NH4-N concentrations ~7 times higher than NO3-N with pH around 5. Bacterial nitrifier abundance ranged from 4E7 to 5E6 copies of amoA/gram of soil. Following a 45-day incubation at 4, 15, and 25oC and three moisture levels, samples will be evaluated for changes in microbial biomass, net nitrification, and nitrifier abundance using qPCR. Gross nitrification rates under these same treatments will be determined from a short-term incubation with the use of 15N. Linking the influence of temperature and moisture on alpine microbial communities 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.
