good responses so far.

To address some of your other questions,

1. Beans don't fix N

2. Rhizobia bacteria fix N2  - thats why we innoculate beans. http://en.wikipedia.org/wiki/Rhizobia

3. N2 is 78% of the air you breathe http://en.wikipedia.org/wiki/Air  It is also where the fertilizer plant that sells you nitrogen get their N. (they pull it out of the air)

4. N2 is inert and contains no usable energy and requires energy to Fix.  http://en.wikipedia.org/wiki/Nitrogen_fixation 

5. Beans store sunlight energy in the form of sugar, absorb N2 from the air, then send sugar and N2 down to their roots and trade with the bacteria in exchange for the bacteria fixing N to give back to the plant.

6. Bean residue has more protein than corn stalks do.  that means it has a higher ratio of Nitrogen to Carbon.

7. The higher ratio of Nitrogen to Carbon in the residue means that microbes that eat it will not need as much of your fertilizer N when consuming the residue.

http://en.wikipedia.org/wiki/Root_nodule