Isotopic tracers are used to examine processes in natural systems, especially terrestrial and aquatic environments. In soil science N tracers are used extensively to study nitrogen cycling, whereas C and C, stable and radioisotopes of carbon respectively, are used for studying turnover of organic compounds and fixation of CO 2 by autotrophs. For example, Marsh et al. (2005) used dual labeled (N- and C) urea to demonstrate utilization of the compound by ammonia oxidizers as both an energy source (ammonia oxidation) and carbon source (chemoautotrophic carbon fixation). Deuterated water is also used for tracing the fate and ages of water in a tree or in an ecosystem.