The article introduces the Fixation and Uptake of Nitrogen (FUN) model, which is based on the work of several researchers. The key points are:

1. Introduction
   - The FUN model is based on the concept that nitrogen (N) uptake requires the expenditure of carbon (C) by plants.
   - There are eight pathways for N uptake, including fixation by symbiotic bacteria, retranslocation from senescing tissues, and active/non-mycorrhizal uptake of NH4 and NO3.
   - Each pathway has associated cost (C expended per N acquired), payment (C uptake), and influx (N uptake) terms.
   - The fraction of total C acquisition that can be used for N fixation (f_fixers) is defined for each plant functional type (PFT).
   - The cost calculations are done twice: once where fixation is possible and once where it is not.
   - For active uptake pathways, the costs and fluxes are determined by soil layer.

The summary captures the main components of the FUN model, including the different N uptake pathways, the associated cost and flux terms, and the role of the f_fixers parameter. The key details are conveyed in a clear and concise manner, following the guidelines provided.