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Soil biogeochemical testbed
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litterfall stoichiometric differences #52

Open wwieder opened 2 years ago

wwieder commented 2 years ago

I've been digging some into the stoichiometry differences in CASA and MIMICS and was surprised to see how different the litterfall C:N fluxes are.

These are calculated for MIMICS by summing the following fluxes MIM_cnLIT = (ds_1.cLitInput_metb + ds_1.cLitInput_struc) / (ds_1.nLitInput_metb + ds_1.nLitInput_struc)

Similarly, in CASA, these are the sum of CAS_cnLIT = (ds.cLitInptMet + ds.cLitInptStruc) / (ds.nLitInptMet + ds.nLitInptStruc)

Why is the litterfall C:N so much higher for MIMICS? It seems like CWD fluxes are being included in both fluxes It's not obvious that the vegetation looks very different (below).

image

Here a maps of the same data image

wwieder commented 2 years ago

In conversations with @melanniehartman it looks like this CWD litterflux is CASA is purely diagnostic, with calculations starting here. https://github.com/wwieder/biogeochem_testbed_1.1/blob/630389decceaac88d7ab8e09339daa75f960ba32/SOURCE_CODE/casa_cnp.f90#L1149

It seems like the N associated with this flux should be handled as it is in the main code for gross N mineralization (e.g. without using the associated CO2 flux in casaflux%fromLtoS?

casaflux%Nlittermin(nland) = casaflux%Nlittermin(nland) &
             + casaflux%klitter(nland,j) * casapool%Nlitter(nland,j)

https://github.com/wwieder/biogeochem_testbed_1.1/blob/630389decceaac88d7ab8e09339daa75f960ba32/SOURCE_CODE/casa_cnp.f90#L1171

The main fluxes that are actually used by CASA must be calculating an associated N immobilization flux that does account for the C lost to CO2 and the C:N of the donor pool 

casaflux%Nsimm(nland) = casaflux%Nsimm(nland) &
            - casaflux%fromLtoS(nland,kk,jj) &
            * casaflux%klitter(nland,jj)     &
            * casapool%Clitter(nland,jj)     &
            * casapool%ratioNCsoilnew(nland,kk)

https://github.com/wwieder/biogeochem_testbed_1.1/blob/630389decceaac88d7ab8e09339daa75f960ba32/SOURCE_CODE/casa_cnp.f90#L1180

Subsequently, net N mineralization fluxes are calculated.

I think that for MIMICS we don't want to mess with the potential immobilization flux associated with the CWD2LIT fluxes. But instead, we follow the same logic for calculating

  1. CWD2CO2 fluxes as is done in CASA. This is already being done: https://github.com/wwieder/biogeochem_testbed_1.1/blob/630389decceaac88d7ab8e09339daa75f960ba32/SOURCE_CODE/mimics_cycle_CN.f90#L592
  2. Do we do the same thing for N, as in the gross N min flux? nwd2str(npt) = casaflux%klitter(npt,cwd) * casapool%nlitter(npt,cwd)

If so, it seems the CASA diagnostic should be handled similarly?

wwieder commented 2 years ago

@melanniehartman also suggested we "could just put ALL the decaying CWD into structural litter and let MIMICS microbes compute the associated respiration and mineralization/immobilization."

cwd2co2(npt) = 0.0
cwd2str(npt) = casaflux%klitter(npt,cwd) * casapool%clitter(npt,cwd)

This idea is nice, since it avoids trying to replicate the CASA approach to CWD, but I'm concerned that it also:

  1. introduces inconsistencies in the CO2 fluxes from CWD decomposition and
  2. would exacerbate the differences in litterfall C:N ratios that the two models see.
wwieder commented 2 years ago

This also made me realize that CASA sends CWD to soil_mic and soil_slow, with different respiration fractions associated with each flux ( that are a product of wood lignin concentrations)

!   casaflux%fromLtoS(:,mic,cwd)   = 0.40*(1.0-casabiome%fracLigninplant(veg%iveg(:),wood))                                          
! CWD -> fmic
!   casaflux%fromLtoS(:,slow,cwd)  = 0.7 * casabiome%fracLigninplant(veg%iveg(:),wood)        
! CWD -> slow

It looks like wood lignin fraction is 0.4 for all PFTs, meaning that fractions of C passed from CWD to soil pools fromLtoS are 0.24 and 0.28, respectively. The remainder, 48%, goes to CO2?

This raises two more questions.

  1. In CASA, are there actually two C fluxes leaving CWD (one each to mic and slow pools)? If so, does this mean the decomposition rate k is effectively doubled?

  2. In MIMICS, what's being done?

https://github.com/wwieder/biogeochem_testbed_1.1/blob/630389decceaac88d7ab8e09339daa75f960ba32/SOURCE_CODE/mimics_cycle_CN.f90#L259

This is identical to how CASA is handling the CO2 fluxes, but MIMICS doesn't have to split the CWD flux to two different pools.

The CWD decay rates look to be calculated similarly as in CASA, modified by CASA temperature and moisture scalars.

https://github.com/wwieder/biogeochem_testbed_1.1/blob/630389decceaac88d7ab8e09339daa75f960ba32/SOURCE_CODE/mimics_cycle_CN.f90#L244

OK, now I'm more convinced that CASA and MIMCS are handling C consistently, making me think it's just the differences in N we need to focus on?