Yes they will. You should also research your species of leguminous tree in order to make sure that the leaves don't contain large amounts of allellopathic chemicals.
Any green material with a decent protein content will provide nitrogen. In the world of plant nutrient analysis we assume that all plant protein is 16% nitrogen. Of course, nitrogen fixing plants like alfalfa or clover tend to have high protein levels in their vegetation, because they have more nitrogen to work with than available to most other plants.
Lush grass or cereal grains in the vegetative stages when cut and mulched onto or incorporated into gardens will still contribute nitrogen, as will green leaves from most trees. Legumes simply fix that nitrogen from the air, where other plants will just be mining it from what ever source is being made available to them.
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