## 2.18.6. Modifications to allow variation in C:N ratios[¶](#modifications-to-allow-variation-in-c-n-ratios "Permalink to this headline")
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The original FUN model as developed by [Fisher et al. (2010)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#fisheretal2010) and [Brzostek et al. (2014)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#brzosteketal2014) assumes a fixed plant tissue C:N ratio. This means that in the case where N is especially limiting, all excess carbon will be utilized in an attempt to take up more Nitrogen. It has been repeatedly observed, however, that in these circumstances in real life, plants have some flexibility in the C:N stoichiometry of their tissues, and therefore, this assumption may not be realistic. However, the degree to which the C:N ratio varies with N availability is poorly documented, and existing global nitrogen models use a variety of heuristic methods by which to incorporate changing C:N ratios (Zaehle and Friend 2010; Ghimire et al. 2016). This algorithm exists as a placeholder to allow variable C:N ratios to occur, and to allow exploration of how much the parameters controlling their flexibility has on model outcomes. Incorporation of emerging understanding of the controls on tissue stoichiometry should ultimately replace this scheme.

Thus, in CLM5, we introduce the capacity for tissue C:N ratios to be prognostic, rather than static. Overall N and C availability (\\(N\_{uptake}\\) and \\(C\_{growth}\\)) and hence tissue C:N ratios, are both determined by FUN. Allocation to individual tissues is discussed in the allocation chapter

Here we introduce an algorithm which adjusts the C expenditure on uptake to allow varying tissue C:N ratios. Increasing C spent on uptake will directly reduce the C:N ratio, and reducing C spent on uptake (retaining more for tissue growth) will increase it. C spent on uptake is impacted by both the N cost in the environment, and the existing tissue C:N ratio of the plant. The output of this algorithm is \\(\\gamma\_{FUN}\\), the fraction of the ideal \\(C\_{nuptake}\\) calculated from the FUN equation above

> \\\[C\_{nuptake} = C\_{nuptake}.\\gamma\_{FUN}\\\]

