I am tilling under leaves in my new, never gardened plot this Fall.
Should I be adding some nitrogen to assist the leaf decomposition? If so, how much per 100 or 1000 sq. feet?
Yes, you should, but the amount is going to vary somewhat depending upon the type of leaves, type of soil, what was previously grown there, pre-existing soil N level.
You can approach this one of two ways -- you could have a soil test done, and go from there.
OR, you could just get a high N fertilizer and apply at the rate suggested on the bag. Personally, I would do the latter, much simpler.
I actually had a class in Crop and Soil Science, and for some reason the figure of 1 lb of actual elemental N per 1,000 square feet sticks in my head as a general supplemental amount for vegetable gardens, but it was almost 25 years ago, so I could be way off, or just plain confused in my old age.
I have tons of worms in my soil. I turn the leaves under give the worms some time then put down manure and legumes. I also use legumes as a green mulch or living mulch. Legumes most always will return the nitrogen back in for you. Over doing it with dead leaves without taking some action leaves the soil weak.