Summary:

## Carbon Isotope Discrimination During Photosynthesis

Photosynthesis in the Community Land Model (CLM) is modeled as a two-step process: the diffusion of CO2 into the stomatal cavity, followed by enzymatic fixation. Each step has an associated kinetic isotope effect.

The kinetic isotope effect during CO2 diffusion through the stomatal opening is 4.4‰. The kinetic isotope effect during CO2 fixation with Rubisco is around 30‰, but since 5-10% of carbon in C3 plants reacts with phosphoenolpyruvate carboxylase (PEPC), the net kinetic isotope effect during fixation is around 27‰ for C3 plants.

For C4 plants, only the diffusion effect is important. The fractionation factor equations for C3 and C4 plants are provided:

- For C4 plants: α_psn = 1 + 4.4/1000
- For C3 plants: α_psn = 1 + (4.4 + 22.6*c_i*/pCO2)/1000

The kinetic isotope effect during CO2 fixation is dependent on the intracellular CO2 concentration (c_i*), which is calculated based on the net carbon assimilation, leaf boundary layer conductance, and leaf stomatal conductance.

The isotopic fractionation code is compatible with a multi-layered canopy parameterization, but the number of canopy layers is currently set to one by default.