Dear DSSATers
 
I have some queries regarding the nitrification part in the CERES nitrogen
routines, where the `capacity' index for nitrification is calculated. Both
relate to where the effect of NH4 supply is estimated with the two
variables SANC and SARNC. I would be very grateful if there is anybody
that can help.
 
1. I am not sure why it is the SNH4 variable (in kg/ha) that is used here
rather than the NH4 (in mg/kg) to calculate the supply effect - I would
have thought that the concentration term (i.e. NH4) was the relevant
variable, as SNH4 is dependent on the thickness of the soil layer
concerned which in turn is dependent on the specific dataset being used.
 
2. Where SANC is calculated, the equation used is
 
       SANC = 1-EXP(-0.01363*SNH4)
 
   whereas SARNC is calculated as
 
       SARNC = 1-EXP(-0.1363*SNH4).
 
i.e. there is a factor of 10 difference in the constant. As far as I can see,
these variables are similar, the latter being used to calculate the lag effect
of nitrification. Is this correct (if so, why are different constants
used?), or is it an error (if so, which constant is the correct one?). I
notice that the different constants are also in the listing in the relevant
chapter in Hanks & Ritchie (1991), so it doesn't seem to be just a typo in
the version of the routines that I have.
 
3. Does anyone know of any references where the derivations of the above
equations from actual data are described?
 
Thanks in advance.
 
Robin Matthews
 
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   Robin Matthews                           Tel: +44 1525 863008
   Department of Water Management           Fax: +44 1525 863001
   Silsoe College, Cranfield University     Email: [log in to unmask]
   Bedfordshire MK45 4DT                    Tlx: 826838 SILCAM G
   UNITED KINGDOM.
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