## 2.18.4. Nitrogen Retranslocation[¶](#nitrogen-retranslocation "Permalink to this headline")
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The retranslocation uses an iterative algorithm to remove Nitrogen from each piece of falling litter. There are two pathways for this, ‘free’ uptake which removes the labile N pool, and ‘paid-for’ uptake which uses C to extract N from increasingly more recalcitrant pools.

At each timestep, the pool of carbon in falling leaves (\\(C\_{fallingleaf}\\), g m\-2) is generated from the quantity of litterfall on that day (see Phenology chapter for details). The amount of N in the litter pool (\\(N\_{fallingleaf}\\), g m\-2) is calculated as the total leaf N multiplied by the fraction of the leaf pool passed to litter that timestep.

> \\\[N\_{fallingleaf} = N\_{leaf}.C\_{fallingleaf}/C\_{leaf}\\\]

The carbon available at the beginning of the iterative retranslocation calculation is equal to the \\(C\_{avail}\\) input into FUN.

> \\\[C\_{avail,retrans,0} = C\_{avail}\\\]

