IBL Research Colorado Alpine One major component of IBL research involves the biogeochemistry of alpine tundra and subalpine forest ecosystems in the Colorado Front Range area. Much of this work is done as part of the Niwot Ridge Long Term Ecological Research Program based at INSTAAR, and focuses on the effects of increasing nitrogen (N) deposition to alpine ecosystems. In collaboration with Jason Neff, Scott Lehman and Bill Bowman, work in our lab showed that increasing N inputs to alpine tundra soils may have significant effects on the turnover and storage of soil carbon, but that the responses are likely to be complex and varied among soil carbon fractions. For example, fertilization with N led to a strong increase in the decomposition of soil C fractions with turnover times in the roughly 10-40 year range, but also appeared to cause increased stabilization of C into longer-term reservoirs. We are currently exploring the response of soils from other ecosystem types, including grasslands and temperate forests, to see if the Niwot results are common across multiple systems. Additional work focuses on the potential fate of N inputs to tundra ecosystems. In a 15N labeling experiment led by former graduate student Keri Holland, we showed that unlike patterns common to many temperate forests, a substantial fraction of added N rapidly appears in plant biomass, and this pattern holds across a range of N fertilization levels. Alpine tundra is also quite diverse with respect to plant species, and the partitioning of 15N among such species varies considerably in ways that are not simply a function of their cover, thus suggesting that added N will likely lead to shifts in plant species (as showed by Bill Bowman's group), and that such shifts will in turn affect the partitioning of N inputs within the ecosystem. As with the temperate forest experiments, a significant fraction of added N also is stored in soil organic matter (SOM), with the bulk appearing to reside in the more labile light density fraction. Other work in the Niwot region demonstrated landscape level differences in soil P availability that are a function of past geologic and geomorphic history, and that such patterns lead to differences in the cover and activity of the plants with N-fixing symbioses (Trifolium spp.).