Ironically, most methods I can think of to "take out" or "phase out" the n-fixers will end up leaving a lot of their nitrogen in the soil (which is usually what you want, but maybe not in this case?)
I guess this really would need to be a phasing out, where you gradually hack back the n-fixer as the tree it is near grows to the size you're wanting. I guess I just wanted to point out that most methods of removing or phasing out the tree are going to leave more available nitrogen in the soil than leaving it there.
