Dear DSSAT colleagues,
 
This is about a possible bug with DSSAT models.  The default initial soil
NO3 and NH4 in DSSAT is 0.1 ppm per layer (IPSLIN).  The user may override
this value in the experiment file (FILEX).  If the user enters a value of <
0.1 ppm for the top few layers, then something unusual happens:  The top
layers have 0.01 ppm NO3 or NH4, deeper layers have 0.1 ppm (the default).
 
This is only a minor problem.  A bigger problem is that NO3 and NH4 needs
to be at least 0.5 ppm for the N uptake and transformation routines
(NUPTAK, NTRANS) to work.  In those routines the minimum allowed soil
concentration is 0.25-0.50 ppm.  If the user initialized the model with
less than 0.5 ppm, then these equations give negative values and bad things
happen:
 
Subroutine NUPTAK:
RNH4U(L) = RFAC*SMDFR*FNH4*(NH4(L)-0.5)*DLAYR(L)
 
Subroutine NTRANS:
B2      = (NH4(L)-0.5)/FAC(L)
 
I came across this problem when I simulated a sequence of several years of
unfertilized corn.  My temporary fix was to add these lines to the input
program, at the end of IPSLIN:
 
do i = 1, nlayr                                                      !AG
   iNO3(i) = max (0.5, iNO3(i))                                      !AG
   iNH4(i) = max (0.5, iNH4(i))                                      !AG
enddo                                                                !AG
 
Aris Gerakis
 
 
 
 
 
 
 
 
Dept. of Crop and Soil Sciences                 Voice: (517) 353 8537
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http://nowlin.css.msu.edu/aris/