Both beans and corn require N for growth. The difference is that legumes end up putting about as much in the ground as they take (and so are sometimes known as N-neutral, when cropped - when the entire corpus is left, including seed, it can be a net gain). Of course most other crops will leave less N behind.

To specifically answer your question, while actively growing legumes have to have some N in place to start with (an ample amount to make an abundant crop), to say nothing of supplying it for other crops.