Summary of the Article:

**Title: Carbon Isotopes in the Community Land Model (CLM)**

The article discusses the implementation of carbon isotopes (13C and 14C) in the Community Land Model (CLM). Key points:

**Overview**
- CLM includes a fully prognostic representation of carbon isotope fluxes, storage, and discrimination.
- The implementation takes advantage of CLM's hierarchical data structures, tracking total carbon and both isotopes separately at the column and Plant Functional Type (PFT) level.

**Approach for 13C**
- For most fluxes and carbon state variable updates, 13C is calculated directly from the corresponding total C fluxes.
- Special calculations are required in a few cases, such as where isotopic discrimination occurs or where necessary isotopic ratios are undefined.

**Special Cases**
- The article briefly mentions that the general approach for 13C flux and state variable calculation is described, followed by a description of all the places where special calculations are required.

In summary, the article outlines how CLM incorporates a detailed representation of carbon isotopes, utilizing the model's hierarchical structure to track total carbon and the two isotopes separately, with some specific calculations needed in certain situations.