Passive N uptake in CLM5-FUN?

[whh_xkp]

What version of the code are you using?
CESM2.1.3


Have you made any changes to files in the source tree?
Yes. Modified src/biogeochem/CNFUNMod.F90 to implement transpiration-driven passive N uptake. Re-calculated soil solution N concentration using mineral N pools (smin_vr_no3 and smin_vr_nh4) instead of plant-available flux variables, and added a capacity limit (n_passive_cap) based on plant N demand and C availability.


Describe every step you took leading up to the problem:
N/A (Methodology / Design Inquiry)


If this is a port to a new machine: Please attach any files you added or changed for the machine port (e.g., config_compilers.xml, config_machines.xml, and config_batch.xml) and tell us the compiler version you are using on this machine.
Please attach any log files showing error messages or other useful information.
N/A


Describe your problem or question:
I have a question regarding the representation of Passive Nitrogen Uptake (transpiration-driven mass flow) within the CLM5-FUN module:

1. In standard CLM5, passive N uptake outputs as zero or is omitted. Is this intentionally due to FUN's carbon-cost optimization framework (where passive uptake has zero C cost), or was it omitted due to numerical issues encountered during development (e.g., concentration scaling or C-pool depletion)?
2. What is the recommended way to cap/constrain passive N uptake to prevent breaking plant C-N stoichiometry or over-depleting C pools when enabling it in CLM5?

 

[wwieder]

Hello Haohao, I hadn't noticed that passive N uptake was zero in more recent tags, but it looks like this is also true on CLM5 results. I'm not sure why this is happening, but I'm assuming it's likely because inorganic N concentrations are so low (or there's a bug in the FUN module). As implemented, FUN really works as an additional tax on autotrophic respiration. As such, I'm not sure how to respond to your question on "over-depleting C pools". Are you seeing evidence of "breaking plant C-N stoichiometry" in your results too?

 

[whh_xkp]

Hello Haohao, I hadn't noticed that passive N uptake was zero in more recent tags, but it looks like this is also true on CLM5 results. I'm not sure why this is happening, but I'm assuming it's likely because inorganic N concentrations are so low (or there's a bug in the FUN module). As implemented, FUN really works as an additional tax on autotrophic respiration. As such, I'm not sure how to respond to your question on "over-depleting C pools". Are you seeing evidence of "breaking plant C-N stoichiometry" in your results too?

Hello Will, Thanks for the reply! I double-checked CNFUNMod.F90 in standard CLM5, and you are completely right—passive N uptake is indeed initialized to zero and never updated in current tags.

I am currently testing a modification to implement transpiration-driven passive N uptake using soil mineral N pools, constrained by plant N demand and available C to avoid stoichiometry imbalances.

Are you aware of any ongoing official CESM developments or community efforts working on reinstating passive N uptake in CLM5-FUN? Please let me know if there's any existing work to align with, and I'll keep you posted as I make progress!

 

[wwieder]

I'm not aware of other work planned with FUN. Please circle back when you have results to share. It would be interesting to hear what you find.