As far as adding nitrogen I planted a cover crop of clover witch is a nitrogen fixing plant, chop and drop with comfrey and other green plants also planted Mimosa tree and Autumn olive tree. They are both nitrogen fixing trees. I had the same question at first. The idea for me is to mimic the forest floor. Since we don't add nitrogen to the forest and there is woody material falling all the time I wondered how nature did it. After looking at my patch of woods and picking out the different species of trees and plants I noteced that the Mimosa tree, Autumn Olive and clover were the three main nitrogen fixers so those were the ones I went with.