Opened 3 years ago

Last modified 3 months ago

#580 new enhancement

Calculation of resp_maint and its response to N-fertilization

Reported by: luyssaert Owned by: somebody
Priority: major Milestone: Not scheduled yet
Component: Anthropogenic processes Version: trunc
Keywords: Cc:

Description (last modified by luyssaert)

According to Vicca et al 2012 and Campiolli et al 2015 NPP/GPP varies between 0.33 and 0.63 depending on the nutrient availability at the site. Quick tests with ORCHIDEE-CN-CAN r6084 did not show any substantial response of the NPP/GPP ratio to N-availability (note that the absolute values of GPP and NPP changed but that their ratio was more or less inresponsive to N-fertilization).

The change in NPP/GPP was thought to be the outcome of C-subsidies to mycorrhizae. If the C/N ratio of the tissue is favorable, the plant will not have to give a lot of C to the myccorhizae. If the C/N ratio is unfavorable this subsidy will become substantial.

In ORCHIDEE the parameter COEFF_MAINT_RESP accounts for the maintenance respiration but also includes the C-losses due to leaching, BVOCs and C-subsidies to mycorrhizae. If we want to improve the model response to N-fertilization we will have add C-subsidies to mycorrhizae and take that carbon from the resp_maint flux. In reality this has nothing to do with maintenance respiration (senso stricto) but because of the way this component has been defined in ORCHIDEE (see above) this should be accounted by changing resp_maint in ORCHIDEE.

It is relatively easy to make such a correction (by for example making use of the sugar_load) but such a change would only affect the C-cycle whereas in reality the presence of mycorrhizae also affect plant_n_uptake.

!! Proposal to account for C-subsidy to mycorrhizae (NOT TESTED YET)
IF (tmp_bm(ipts,j,icarbres,icarbon) +tmp_bm(ipts,j,ilabile,icarbon) .GE. &
    reserve_pool + labile_pool ) THEN
   ! The plant is accumulating C in its labile and reserve pools. This hints
   ! at a N-shortage. Give some of excess C to the mycorrhizae so that they
   ! can take up N and give it to the plant.            
   ! A high sugar loading suggests too much C and too little N 
   ! so that seems to be the ideal condition to give some of 
   ! the nitrogen to the mycorrhize to make them happy and to
   ! let them take up more nitrogen. The code below will not 
   ! result in a better N-uptake but it accounts for the C given
   ! to the mycorrhizae and respired by the mycorrhizae. Here we
   ! don give the C to the mycorrhizae (because we don have such
   ! a pool in ORCHIDEE) but we respire the C right away. 
   ! Truncate the sugar loading to avoid too much and
   ! too sudden corrections. These boundaries are
   ! arbitrary. Move to local variable because sugar_load 
   ! is an inout and its original value is needed later in
   ! this routine and in stomate_vmax.
   ! NOTE: sugar_load from the previous time step is used here
   ! sugar loading is updated later in this code.
   IF (sugar_load(ipts,j).GT.un) THEN
       ! Actual reserves < target. The reserves cannot build
       ! up too high because there is enough N to allocate them.
       ! No need to give carbon to the mycorrhizae to take up
       ! even more nitrogen
       corr_sugar_load = zero
   ELSEIF (sugar_load(ipts,j).LT.0.5) THEN
       ! Actual > target. Stimulate the mycorrhizae. Truncate
       ! at an arbitrary value.
       corr_sugar_load  = 0.5
       ! Actual > target. Stimulate the mycorrhizae proportional
       ! to the excess sugar
       corr_sugar_load = (un - sugar_load(ipts,j))
   ! The plant will only use part of the excess carbon as a
   ! subsidy for the mycorrhizae.
   excess = tmp_bm(ipts,j,icarbres,icarbon) - reserve_pool 
   resp_excess(ipts,j) = resp_excess(ipts,j) + corr_sugar_load * excess 
   tmp_bm(ipts,j,icarbres,icarbon) = tmp_bm(ipts,j,icarbres,icarbon) - &
      corr_sugar_load * excess 
ENDIF !icarbres+ilabile .GT. labile_pool+reserve_pool

If we want to do this in a good way we would need to make plant_n_uptake a function of the mycorrhizal biomass. plant_n_uptake is calculate in stomate_soilcarbon.f90 in the subroutine nitrogen_dynamics.

This would also require improving the way Rm is calculated in the first place. In reality
GPP = Rm + Rg + biomass production + BVOCs + leaching + C-subsidy to the mycorrhizae.
GPP = Ra + NPP
NPP = biomass production + BVOCs + leaching + C-subsidy to the mycorrhizae
Ra = Rm + Rg
GPP = Rm’ + Rg + biomass production.
NPP = biomass production (= common approximation and a common mistake)
And thus
Ra = Rm + Rg + BVOCs + leaching + C-subsidy to the mycorrhizae
However we assume that
Ra = Rm + Rg (1)
We tune Rm such that biomass production/GPP matches the observations, we are confident with Rg and we don’t simulate BVOCs + leaching + C-subsidy to the mycorrhizae yet. Hence Rm’= Rm + BVOCs + leaching + C-subsidy to the mycorrhizae. If we want to be precise, we should say that Ra = Rm’+ Rg in ORCHIDEE. To keep this already long message short: Rm is a not Rm but a mixed salad of fluxes that we don’t understand very well or are extremely difficult to measure.

Change History (2)

comment:1 Changed 2 years ago by luyssaert

  • Milestone changed from ORCHIDEE 4.0 to Not scheduled yet
  • Version set to trunc

comment:2 Changed 3 months ago by luyssaert

  • Description modified (diff)
  • Summary changed from Response of resp_maint to N-fertilization to Calculation of resp_maint and its response to N-fertilization
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