FUN active uptake terminology

[ecaas]

I'm currently looking into variables related to the FUN code, and I'm a bit confused about the terminology between mycorrhizal and nomycorrhizal N uptake, and cost related to it.

In the code, it seems like the nonmycorrhizal pathway counts as "active uptake":

CTSM/src/biogeochem/CNFUNMod.F90

Line 1512 in f47ce5c

npp_Nactive(p) = npp_Nactive_no3(p) + npp_Nactive_nh4(p) + npp_Nnonmyc_no3(p) + npp_Nnonmyc_nh4(p)

CTSM/src/biogeochem/CNFUNMod.F90

Line 1496 in f47ce5c

Nactive(p) = Nactive_no3(p) + Nactive_nh4(p) + Nnonmyc_no3(p) + Nnonmyc_nh4(p)

while in the declarations, and in the history field longnames, these variables are described as mycorrhizal only:

CTSM/src/biogeochem/CNFUNMod.F90

Lines 586 to 587 in f47ce5c

	npp_Nactive => cnveg_carbonflux_inst%npp_Nactive_patch , & ! Output: [real(r8)
	! (:) ] Mycorrhizal N uptake used C (gC/m2/s)

CTSM/src/biogeochem/CNFUNMod.F90

Lines 632 to 633 in f47ce5c

	Nactive => cnveg_nitrogenflux_inst%Nactive_patch , & ! Output: [real(r8) (:)
	! ] Mycorrhizal N uptake (gN/m2/s)

In the documentation the nonmycorrhizal pathway is listed under the header "Cost of Active uptake" (https://escomp.github.io/ctsm-docs/versions/release-clm5.0/html/tech_note/FUN/CLM50_Tech_Note_FUN.html#cost-of-active-uptake), however only the mycorrhizal cost is labeled as "active".

Is the nonmycorrhiza pathway in fact active, and the longnames and descriptions should be updated, or am I missing something?

The background for the issue is that in my single-site simulations of Norwegian forests, I find that the active N uptake is completely dominated by the nonmycorrhizal pathway, as shown in the below figure. Based on Shi et. al 2018, I would expect a dominance of the ECM pathway, and therefore wonder if I am interpreting the variables correctly.

[wwieder]

These are good questions.

It looks like the variable n_active_*_acc is used to sum up different pathways for active N uptake, with the ecm diagnostic saved here.

CTSM/src/biogeochem/CNFUNMod.F90

Line 1418 in f47ce5c

n_ecm_no3_acc(p) = n_active_no3_acc(p,istp)

To circle back to your question about large non-mycorrhizal fluxes, I've seen similar patterns in global-scale runs.
If you look at this table from CLM5 results:

• Total Nuptake ~ 1000 Tg N y-1
• AM N uptake ~ 100 Tg N y-1
• ECM N uptake ~ 10 Tg N y-1

We haven't really worried about the different active N uptake pathways being used in FUN, because it doesn't meaningfully feedback on the soil biogeochemistry. With your changes with MIMICS, however, these feedbacks matter. I'd suggest modifying the FUN costs associated with different uptake pathways so that you generate fluxes that seem more appropriate for the system you're working in (e.g. higher active N uptake costs, or lower ECM N uptake costs should favor the ECM pathway). Just check that vegetation doesn't die out when you run the model from a cold start with the new parameter values.

Hopefully this helps.