1. Department of Ecology & Evolutionary Biology, University of Colorado Boulder, Boulder, Colorado 80309-0216, United States
2. Cooperative Institute for Research in Environmental Sciences, University of Colorado Boulder, Boulder, Colorado 80309, United States
3. Department of Soil & Crop Sciences, Colorado State University, Fort Collins, Colorado 80523-1101, United States
4. Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, United States
5. AIRIE, Applied Isotope Research for Industry and the Environment, Fort Collins, Colorado 80524-2313, United States
6. Department of Geosciences, University of Oslo, Oslo NO-0316, Norway