Radioactive tracers (radiotracers) are very helpful in a number of cases. Isotopes are nuclides which have the same number of protons (but not neutrons) and and electrons, therefore having the same chemical behaviour. Radioactive isotopes chemically behave like their stable counterparts, but can be very easily detected as they decay and emit ionising radiation.

We use them:

1. When process rates are very low, e.g. turnover of biochar 
2. When you want to chase different C sources in the same system of the same element (e.g., [13C]malic acid and [14C]cellulose)
3. When you have no choice. ³¹P only has radioactive isotopes.
4. For imaging purposes