Models just use previous results regarding CO2 effects on crops. There are several published results of FACE experiments conducted in USA and Europe that I suggest you to review. These assessments comprise C4 and C3 crops. Some of them can be found in volume 10 of the European Journal of Agronomy (1999).
I think that while other models might not take in account the fertilizer effects, DSSAT models do. I suggest you to review particularly the following paper:
Tubiello, F.N., Ewert, F. 2002. Simulating the effects of elevated CO2 on crops: approaches and applications for climate change. European J. Agron. 18:57-74.
However, there are some recent evidences that such positive effects might be overestimated. There was an interesting meeting in the Royal Society of London last year, "Food crops in a changing climate", where they conclude that:
"Experiments on crops under realistic field conditions have shown that the beneficial 'fertilisation' effect of increased levels of atmospheric carbon dioxide (CO2) could be less than that previously estimated".
The presentation in this regard was made by Professor Steve Long, University of Illinois at Urbana-Champaign, USA. He has some published papers concerning this.

At the end, I (and perhaps you and many more) do not know exactly what to do concerning the CO2 effect!. Perhaps the best choice is to perform two simulations with DSSAT. The first considering the CO2 effect and a second simulation later ignoring it. Confident results must be somewhere in between!.

Dr. Angel Utset
Research Division
ITACyL
Carretera de Burgos, Km.119
47071-VALLADOLID-SPAIN
Telf. +34 983 410 436 Fax.+34 983 414 780
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-----Mensaje original-----
De: DSSAT - Crop Models and Applications [mailto:[log in to unmask]] En nombre de Roberts, Michael
Enviado el: jueves, 02 de marzo de 2006 16:39
Para: [log in to unmask]
Asunto: The latest evidence on CO2 fertilization for corn, soybeans, and cotton

Dear listserv members,

I understand this forum mainly pertains to the DSSAT simulation software
and so this question may be a little off topic.  But I figured the
members would likely be very knowledgeable.

Can some one point me toward the latest evidence on the likely effects
of CO2 fertilization on corn, soybean, and cotton yields?  I have found
a few papers suggesting fairly large positive effects, and most of these
employ simulation models.  I have also seen papers suggesting that while
yields may increase, caloric content increases much less.  I'm
interested in the most compelling evidence and how this is folded into
the simulation models.  No need to spell out the details, but citations
would be greatly appreciated.

Many thanks,

Michael J. Roberts
USDA Economic Research Service
Washington, DC 20036
202-694-5557