What are the ecological effects of moderately high and prolonged concentrations of nitrogen deposition on an ecosystem?
Too much nitrogen can run off during rain and collect in ponds, leading to eutriphication, find more here: https://en.wikipedia.org/wiki/Eutrophication

It's probably also responsible for dead zones: https://en.wikipedia.org/wiki/Dead_zone_%28ecology%29

This is an important question because we've more than doubled the amount of bioavailable nitrogen on Earth in the last 100 years simply because of the haber process and man made fertilizers. This amount of nitrogen has never been available before.