I once worried over the whole 'N' loss thing too but have been told by a LSU Ag professor that as long as you are not mixing this into the soil but only using saw dust as a top dressing (mulch) you will have no big problems.
And on changing soil pH again from the same gentleman ... soil pH is impossible to change. Anything you do to your soil to change pH is a passing thing at best.
So I have been told by one who should know and so I pass along ;o)
Mike