Will my booby create nitrogen depletion in my soil?

Over the last few years i've been using leaves to mulch my flower beds. Then the following year i dig it into the soil and reapply more leaf mulch. The beds are a mix of shrubs and flowers. Some of the shrubs have grown and some have remained the same size as the day i planted them, we are talking a couple of years.
I recall some time ago reading that mulch should never be covered over with soil because in order for the decomposition process to take place it requires nitrogen, and the only place to get it from, once it is covered, is the soil. Which means that the necessary compound to help my plants thrive is being used to break down the organic matter i mulch with! Has anyone ever heard this before or am i over reacting?