Summary:

## Carbon Expenditure on Fixation and Active Uptake

The article discusses the calculation of the overall cost of nitrogen (N) uptake in terms of carbon to nitrogen (C:N) ratios within the model. At each time step, the available carbon (C_avail) is allocated to two outcomes: payment for N uptake or conservation for growth.

Key points:

1. The carbon available (C_avail) is equal to the carbon spent on N uptake (C_nuptake) plus the carbon saved for growth (C_growth).

2. The nitrogen acquired from the expenditure of N must equal the N required to match the growth carbon.

3. The ideal C_nuptake can be calculated using the equation:
   C_nuptake = C_tospend / ((1.0 + f_gr * (CN_target / N_cost)) + 1)

4. Other C and N fluxes can be determined based on the logic outlined in the article.

The summary provides a concise overview of the main points regarding the carbon expenditure on nitrogen fixation and active uptake within the model, as described in the provided text.