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 ============================================================================== 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. ==============================================================================