If you put about a 6"+ of mulch on the a food forest floor, do you need to worry about nitrogen fixing?
You appear to have confused nitrogen fixing (the process where bacteria and legumes work together to fix nitrogen from the air into the soil in a plant-usable form) with nitrogen depletion, a temporary condition caused by the breakdown of carbonaceous materials tying up nitrogen reserves in the soil and keeping plants from using them.
MY observation and methodology is that mulch, on top of the soil, not mixed in, has a limited ability to pull nitrogen from the soil, since it is only contacting the surface of the soil. So, I don't get too concerned about it. When or if it gets mixed/tilled in, you'll need to wait 3 weeks or so for the stuff to decompose, or things planted there will be nitrogen-starved.
Depending what you are mulching with, 6" is probably way over the top, which seems to be a recurring theme with you. Straw, which is hollow, does not pack easily, and has lots of air-space might be OK - your coffee grounds that we keep telling you to put in the compost pile would be a moldy disaster 6" deep. You certainly need to maintain a clear area around the stems of plants and especially trees/shrubs or you will kill them with mulch that deep.