Here is a concise summary of the article:

## Isotope Symbols, Units, and Reference Standards

The article discusses the isotopic properties and notations used for carbon in the Community Land Model (CLM).

Key points:
- Carbon has two primary stable isotopes: 12C and 13C, with 12C being the most abundant.
- The isotope ratio (R) is the mass ratio of the rare isotope (13C) to the abundant isotope (12C).
- Carbon isotope ratios are often expressed using delta (δ) notation, which represents the difference between the isotope ratio of a compound and the Pee Dee Belemnite (PDB) standard.
- Isotopic fractionation can be expressed using alpha (α) or epsilon (ε) notation, which describe the ratio of isotope ratios between two reservoirs.
- The unstable isotope 14C is also included in CLM and can be described using delta (δ14C) or delta-delta (Δ14C) notation, with the latter correcting for mass-dependent fractionation.
- For 14C calculations in CLM, the fractionation is set to zero, such that δ14C = Δ14C.