Changes between Version 26 and Version 27 of DevelopmentActivities/MergeOCN/Vuichard

Timestamp:

2014-02-06T14:30:17+01:00 (10 years ago)

Author:

nvuilsce

Comment:

--

DevelopmentActivities/MergeOCN/Vuichard

@@ -68 +68 @@
 * '''Coefficient K_N_min'''. It corresponds to the [NH4+] (resp. [NO3-]) for which the Nuptake equals vmax/2. Kronzucker, 1995 reports values that range between 20 and 40 umol for NH4+ uptake. OCN seems to use 30 umol for both NH4+ and NO3-, because the value used in the code of OCN expressed in (gN m-2) is 0.84 (see [source:branches/orchidee-N/ORCHIDEE/src_stomate/stomate_soilcarbon.f90@19#L897 here]). In addition, in Kronzucker, 1995 and 1996, the concentrations are always expressed in umol. There is no clear reference about the standard volume it is related to. I assumed it is litter.
 
-* Conversion factor from (umol per litter) to (gN m-2) for K_N_min'''. I assumed conv_fac_concent equals to 14 * 1e3 * 1e-6 * 2 with
+* '''Conversion factor from (umol per litter) to (gN m-2) for K_N_min'''. I assumed conv_fac_concent equals to 14 * 1e3 * 1e-6 * 2 with
   - 14   : molar mass for N (gN mol-1)
   - 10^3 : conversion factor (dm3 to m3)
@@ -75 +75 @@
  I wonder why it assumes 2 m3 per m2 of soil. The depth of the soil is 2 meter but it doesn't mean that all the column contains only water, there is also soil, no ? The question is : what is the volume that is considered for the concentration of NH4+ and NO3-. Is it a volume of soil or of solution ? If it is a volume of solution, I would suggest to multiply by a factor corresponding to the relative volume of water within the soil (like soilhum). Not done yet. 
 
+* '''Coefficient low_K_N_min'''. See eq. 8 of SM of Zaehle et al. (2010) and Table S1. In table S1, it is defined as the "Rate of N uptake not associated with Michaelis- Menten Kinetics" and the given value is 0.05 (-). It is mentioned also (unitless) but to my opinion it should have the same unit that 1/K_N_min or 1/N_min, so ((gN m-2)-1). So far, I cannot relate the value of low_K_N_min (0.05) to any reference, especially Kronzucker (1996). If I refer to Figure 4 of Kronzucker showing the NH4+ influx as a function of NH4+ concentration, the slope of the relationship could be used to define Vmax*low_K_N_min. For a concentration of NH4+ of 50 mmol, the influx equals 35 umol g-1 h-1. For a concentration of NH4+ of 20 mmol, the influx equals 17 umol g-1 h-1. So the slope can be written as 
+ slope = 10-3 * (35 - 17) / (50 - 20) = 10-3 * 18 / 30 = 0.0006 g-1 h-1 
+ and consequently low_K_N_min = slope / Vmax = 0.0006 / 3 = 0.0002 (umol)-1 
+ Using the Conversion factor from (umol per litter) to (gN m-2) conv_fac_concent = 14 * 1e3 * 1e-6 * 2, one should get 
+ low_K_N_min = 0.0002 / ( 14 * 1e3 * 1e-6 * 2 ) = 0.007 ((gN m-2)-1)
+ The value of 0.007 does not match with the one in OCN (0.05). This needs to be clarified
 
 
+