## 2.18.3. Resolving N cost across simultaneous uptake streams[¶](#resolving-n-cost-across-simultaneous-uptake-streams "Permalink to this headline")
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The total cost of N uptake is calculated based on the assumption that carbon is partitioned to each stream in proportion to the inverse of the cost of uptake. So, more expensive pathways receive less carbon. Earlier versions of FUN [(Fisher et al., 2010)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#fisheretal2010)) utilized a scheme whereby plants only took up N from the cheapest pathway. [Brzostek et al. (2014)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#brzosteketal2014) introduced a scheme for the simultaneous uptake from different pathways. Here we calcualate a ‘conductance’ to N uptake (analagous to the inverse of the cost function conceptualized as a resistance term) \\(N\_{conductance}\\) ( gN/gC) as:

> \\\[N\_{conductance,f}= \\sum{(1/N\_{cost,x})}\\\]

From this, we then calculate the fraction of the carbon allocated to each pathway as

> \\\[C\_{frac,x} = \\frac{1/N\_{cost,x}}{N\_{conductance}}\\\]

These fractions are used later, to calculate the carbon expended on different uptake pathways. Next, the N acquired from each uptake stream per unit C spent (\\(N\_{exch,x}\\), gN/gC) is determined as

> \\\[N\_{exch,x} = \\frac{C\_{frac,x}}{N\_{cost,x}}\\\]

We then determine the total amount of N uptake per unit C spent (\\(N\_{exch,tot}\\), gN/gC) as the sum of all the uptake streams.

> \\\[N\_{exch,tot} = \\sum{N\_{exch,x}}\\\]

and thus the subsequent overall N cost is

> \\\[N\_{cost,tot} = 1/{N\_{exch,tot}}\\\]
> 
> Retranslocation is determined via a different set of mechanisms, once the \\(N\_{cost,tot}\\) is known.

