Summary:

## Modifications to Allow Variation in C:N Ratios

The original FUN model assumed a fixed plant tissue C:N ratio, where all excess carbon would be utilized to take up more nitrogen when nitrogen is limited. However, observations indicate that plants have flexibility in their C:N stoichiometry, which is not well-documented.

To address this, CLM5 introduces the capacity for tissue C:N ratios to be prognostic, rather than static. The C expenditure on uptake is adjusted to allow varying tissue C:N ratios. Increasing C spent on uptake will directly reduce the C:N ratio, while reducing C spent on uptake (retaining more for tissue growth) will increase it.

The algorithm determines the fraction of the ideal C_nuptake calculated from the FUN equation, denoted as γ_FUN. The final C_nuptake is then calculated as:

C_nuptake = C_nuptake * γ_FUN

This algorithm serves as a placeholder to allow exploration of how the flexibility in tissue stoichiometry impacts model outcomes, until a more robust understanding of the controls on tissue stoichiometry can be incorporated.