Summary:

## Biological Nitrogen Fixation

The article discusses the role of biological nitrogen fixation (BNF) in the global nitrogen budget. Key points:

1. BNF by soil microorganisms is an important component of both preindustrial and modern nitrogen budgets, but a comprehensive understanding of global-scale controls on BNF is still lacking.

2. CLM5.0 uses the FUN model to calculate the carbon cost and nitrogen acquired through symbiotic nitrogen fixation, which is immediately available to plants.

3. For asymbiotic (free-living) nitrogen fixation, CLM5.0 adopts an empirical relationship based on evapotranspiration rate, rather than the previously used net primary productivity approach.

4. The expression used to calculate the rate of free-living nitrogen fixation (NF_nfix,sminn) is provided, which incorporates annual evapotranspiration (CF_ann_ET).

5. The free-living nitrogen inputs are added directly to the ammonium (NH4+) pool in the model.

The article highlights the importance of understanding and modeling biological nitrogen fixation processes in the context of global nitrogen budgets and the development of the CLM5.0 model.