Upendra Singh provide the follwing response to Hiroshi's question:
 
(1)  The effect of soil texture is reflected through soil moisture and
oxygen supply (soil moisture/porosity used as a surrogate) effects on
mineralization, nitrification and denitrification
(2) In soils with amorphous (allophane) mineralogy, e.g., Andisols, the
DMOD value (default of 1.0) is reduced to reflect higher surface
area/adsorption of organic matter.
 
None of the N transformation processes use clay content direclty in the
model.
 
Regards,
 
Upendra Singh