## 2.18.1. Introduction[¶](#introduction "Permalink to this headline")
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The Fixation and Uptake of Nitrogen model is based on work by [Fisher et al. (2010)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#fisheretal2010), [Brzostek et al. (2014)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#brzosteketal2014), and [Shi et al. (2016)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#shietal2016). The concept of FUN is that in most cases, Nitrogen uptake requires the expenditure of energy in the form of carbon, and further, that there are numerous potential sources of Nitrogen in the environment which a plant may exchange for carbon. The ratio of carbon expended to Nitrogen acquired is referred to here as the cost, or exchange rate, of N acquisition (\\(E\_{nacq}\\), gC/gN)). There are eight pathways for N uptake:

1.  Fixation by symbiotic bacteria in root nodules (for N fixing plants) (\\(\_{fix}\\))
    
2.  Retranslocation of N from senescing tissues (\\(\_{ret}\\))
    
3.  Active uptake of NH4 by arbuscular mycorrhizal plants (\\(\_{active,nh4}\\))
    
4.  Active uptake of NH4 by ectomycorrhizal plants (\\(\_{active,nh4}\\))
    
5.  Active uptake of NO3 by arbuscular mycorrhizal plants (\\(\_{active,no3}\\))
    
6.  Active uptake of NO3 by ectomycorrhizal plants (\\(\_{active,no3}\\))
    
7.  Nonmycorrhizal uptake of NH4 (\\(\_{nonmyc,no3}\\))
    
8.  Nonmycorrhizal uptake of NO3 (\\(\_{nonmyc,nh4}\\))
    

The notation suffix for each pathway is given in parentheses here. At each timestep, each of these pathways is associated with a cost term (\\(N\_{cost,x}\\)), a payment in carbon (\\(C\_{nuptake,x}\\)), and an influx of Nitrogen (\\(N\_{uptake,x}\\)) where \\(x\\) is one of the eight uptake streams listed above.

For each PFT, we define a fraction of the total C acquisition that can be used for N fixation (\\(f\_{fixers}\\)), which is broadly equivalent to the fraction of a given PFT that is capable of fixing Nitrogen, and thus represents an upper limit on the amount to which fixation can be increased in low n conditions. For each PFT, the cost calculation is conducted twice. Once where fixation is possible and once where it is not. (\\(f\_{fixers}\\))

For all of the active uptake pathways, whose cost depends on varying concentrations of N through the soil profile, the costs and fluxes are also determined by soil layer \\(j\\).

