This is a very complicated "what if" question. I doubt engineering
plants to fix nitrogen would have a major impact on global warming for
the following reasons:

1. Plant nitrogen fixation requires a lot of photosynthetic energy.
Plants that naturally fix nitrogen often produce less dry matter if
they have to fix all their own nitrogen than when they are fertilized
with nitrogen.

2. The main reason to engineer plants to fix nitrogen is to save on
the cost of fertilizers. Fertilizer production uses a lot of fossil
fuel, and contributes to global warming, so there might be some
savings in fossil fuel use in fertilizer production. However, that
fossil fuel will simply be burned for something else given the
increasing demand for fossil fuels.

3. Plants that will be engineered to fix nitrogen will be high value
food crops such as corn, wheat and rice. The biomass those food plants
produce is simply plowed under or burned each season so no carbon
dioxide is permanently removed from the air.

4. Plant growth is often limited by factors other than nitrogen,
especially water, temperature, other mineral nutrients, insects, plant
diseases, etc. Plants engineered to fix nitrogen will not
automatically have greater growth rates than nonengineered plants.

5. Fighting global warming by sequestering recently produced plant dry
matter cannot have a significant impact because we are burning fossil
fuels and lowering the world's photosynthetic capacity via forest
destruction, building, paving, etc. at such a rapid rate. One estimate
is that one gallon of gasoline required 98 tons of buried plant
matter.

"Bad Mileage: 98 tons of plants per gallon"
http://www.innovations-report.de/htm...cht-22773.html

David R. Hershey
http://www.angelfire.com/ab6/hershey/bio.htm