Environmental stability of soil carbon   Soils contain the largest near surface terrestrial reservoir carbon on Earth. However, the stability of the soil carbon reservoir to environmental perturbation, such as natural and anthropogenic climate change and altered nitrogen deposition remain poorly understood. Working with a number of INSTAAR ecologists (among others), we sought to evaluate changes in the processing and residence time of soil carbon in control and N-fertilized study plots at CU-LTER's Niwot Ridge site. NSRL-NOSAMS performed high precision 14C measurements on recent plant harvests (Figure 1) and density-fractionated soil organic matter (Figure 2). Our initial results, presented in Nature [Neff et al., 2002] indicate that N additions greatly accelerated decomposition of soil C that had been resident in the soil for years to decades, while possibly stabilizing other soil C components. If these findings can be generalized to other locations, they call for significant revision of models currently used to predict response of the carbon cycle to anthropogenic N loading. To evaluate this, we are currently extending this measurement campaign to other NSF-LTER study sites. Figure 1: D14C of Niwot Ridge plant harvest compared to atmospheric D14CO2 from Germany [from Levin and Kromer, 1997]. Plant measurements were used to verify activity of organic input to the soil carbon reservoir [after Neff et al., 2002]. Figure 2: Variation in SOM D14C in control and fertilized light and heavy SOM fractions from Niwot Ridge. Error bars represent standard errors for n=10 measurements in each category. The statistically significant reduction (indicated by contrasting letters) in D14C of the light fraction of SOM following a decade of N fertilization indicates a reduction in residence time and increased utilization of the light SOM fraction- a conclusion that is also supported by changes in measured abundance and carbon isotopic composition of individual plant biomarkers isolated from the soils [Neff et al., 2002]. References cited: Neff, J.C., Townsend, A.R., Gleixner, G., Lehman, S.J., Turnbull, J., & W.D. Bowman (2002). Variable effects of nitrogen additions on the stability and turnover of soil carbon. Nature 419: 915-917. Levin, I. & Kromer, B. (1997) 14CO2 records from Schauinsland. In Trends: A compendium of data on global change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, TN, USA.