## 2.22.4. Biological Nitrogen Fixation[¶](#biological-nitrogen-fixation "Permalink to this headline")
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The fixation of new reactive nitrogen from atmospheric N2 by soil microorganisms is an important component of both preindustrial and modern-day nitrogen budgets, but a mechanistic understanding of global-scale controls on biological nitrogen fixation (BNF) is still only poorly developed ([Cleveland et al. 1999](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#clevelandetal1999); [Galloway et al. 2004](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#gallowayetal2004)). CLM5.0 uses the FUN model (chapter [2.18](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/FUN/CLM50_Tech_Note_FUN.html#rst-fun)) to calculate the carbon cost and nitrogen acquired through symbotic nitrogen fixation. This nitrogen is immediately available to plants.

[Cleveland et al. (1999)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#clevelandetal1999) suggested an empirical relationships that predicts BNF as a function of either evapotranspiration rate or net primary productivity for natural vegetation. CLM5.0 adopts the evapotranspiration approach to calculate asymbiotic, or free-living, N fixation. This function has been modified from the [Cleveland et al. (1999)](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#clevelandetal1999) estimates to provide lower estimate of free-living nitrogen fixation in CLM5.0 (\\({CF}\_{ann\\\_ET}\\), mm yr\-1). This moves away from the NPP approach used in CLM4.0 and 4.5 and avoids unrealistically increasing freeliving rates of N fixation under global change scenarios ([Wieder et al. 2015](https://escomp.github.io/ctsm-docs/versions/master/html/tech_note/References/CLM50_Tech_Note_References.html#wiederetal2015) The expression used is:

(2.22.1)[¶](#equation-22-1 "Permalink to this equation")\\\[NF\_{nfix,sminn} ={0.0006\\left(0.0117+CF\_{ann\\\_ ET}\\right)\\mathord{\\left/ {\\vphantom {0.0006\\left(0.0117+ CF\_{ann\\\_ ET}\\right) \\left(86400\\cdot 365\\right)}} \\right.} \\left(86400\\cdot 365\\right)}\\\]

Where \\({NF}\_{nfix,sminn}\\) (gN m\-2 s\-1) is the rate of free-living nitrogen fixation in [Figure 2.22.1](#figure-biological-nitrogen-fixation).

![Image 1: ../../_images/image11.png](https://escomp.github.io/ctsm-docs/versions/master/html/_images/image11.png)

Figure 2.22.1 Free-living nitrogen fixation as a function of annual evapotranspiration. Results here show annual N inputs from free-living N fixations, but the model actually calculates inputs on a per second basis.[¶](#id2 "Permalink to this image")

As with Atmospheric N deposition, free-living N inputs are added directly to the NH4+ pool.

