I thought it was the opposite

If you look at the roots of broad beans for instance you can see the nitrogen nodules. If you cut them off at ground level once they are finished this nitrogen should release into the soil as the root dies? Its a minimal amount and hardly worth the bother tbh? 
Put the roots on the compost heap and the nitro will help activate it.

Most flowers are usually treat 'rough' with no fertiliser I always thought?