Questions about the N fixation

[jinmuluo]

Hi,

I have some questions about the FUN module and N fixation in CLM.
(1) Does anyone know about the relationship between fluxes <ffix_to_sminn > and <nfix_to_sminn> in CLM, it looks like if we turn the FUN on, "nfix_to_sminn" should be zero (CNNDynamicsMod, line 300), but I can still see the values from the <nfix_to_sminn> even I turn FUN on.

(2) If I turn on FUN, are all N fixation fluxes included in <ffix_to_sminn >, even in soy fixation?

Best,

Jinmu

 

[wwieder]

Hi thanks for your question. This is a confusing topic that I've looked into before and that I can answer with pretty high confidence quickly. That said, you may want to dive into the code to check some of my statements for accuracy.

N fix in CLM5 with FUN on has two components symbiotic N fixation, which does directly to plants and free living (or asymbiotic) N-fixation, which is a function of ET and goes to the mineral nitrogen pool. These fluxes are NFIX, and FFIX_TO_SMINN, respectively. The history variable nfix_to_sminn is ambiguous, and not identical to NFIX (e.g., in global runs the two fluxes are similar, but not identical), and I honestly can't remember why.

In crops, only soy fixes N (NFIX > 0), but all crops and native vegetation should be receiving N inputs to from free-living N-fix

 

[jinmuluo]

Hi thanks for your question. This is a confusing topic that I've looked into before and that I can answer with pretty high confidence quickly. That said, you may want to dive into the code to check some of my statements for accuracy.

N fix in CLM5 with FUN on has two components symbiotic N fixation, which does directly to plants and free living (or asymbiotic) N-fixation, which is a function of ET and goes to the mineral nitrogen pool. These fluxes are NFIX, and FFIX_TO_SMINN, respectively. The history variable nfix_to_sminn is ambiguous, and not identical to NFIX (e.g., in global runs the two fluxes are similar, but not identical), and I honestly can't remember why.

In crops, only soy fixes N (NFIX > 0), but all crops and native vegetation should be receiving N inputs to from free-living N-fix

Hi Will,

Thank you for your explanations. If I understand correctly, with the FUN on, part of fixed N going to the plant N pool (NFIX) and the other part going to the soil mineral pool (FFIX_TO_SMINN)? If I want to calculate the total soil nitrogen input from fixation, I should only sum the FFIX_TO_SMINN and SOYFIXN_TO_SMINN together?

Another weird thing I have found is in the "SoilBiogeochemNStateUpdate1Mod". FFIX_TO_SMINN is added to the soil mineral pool if use_fun = .true., otherwise NFIX_TO_SMINN will be added to the soil mineral pool. But in "CNFUNMod", it looks like <nfix_to_sminn_col> was converted from the <Nfix_patch>... I'm kind of confused about the flows of different fluxes, could you please tell me more about that?

Best,

Jinmu

 

[wwieder]

I would sum NFIX + FFIX_TO_SMINN if you want total N fixation rates. I would also ignore the diagnostic NFIX_TO_SMINN in CLM5 runs, which have FUN on by default.

Is that what you're asking?