Summary:

Nitrogen Uptake Cost and Flexible C:N Ratios

The environmental cost of nitrogen (N_cost,tot) is used to determine the variable γ_FUN, which represents the response of carbon (C) expenditure to nitrogen uptake cost. This relationship is expressed as:

γ_FUN = max(0.0, 1.0 - (N_cost,tot - a_cnflex) / b_cnflex)

where:
- a_cnflex and b_cnflex are parameters fitted to allow flexible C:N ratios within the model's operating range of nitrogen costs.
- a_cnflex represents the N_cost,tot threshold above which there is a modification in C expenditure, allowing for higher C:N ratios.
- b_cnflex is the scalar that determines the degree of C expenditure modification for a given discrepancy between a_cnflex and the actual cost of uptake.

The calibration of these parameters is intended as a placeholder to enable some flexible stoichiometry, in the absence of adequate understanding of this process. Future testing in idealized experimental settings is recommended to refine the understanding and parameterization of this flexible C:N relationship.