There are many sources of nitrogen which eventually get deposited out of the atmosphere and onto the land/surface. Typically higher levels of nitrogen compounds are emitted near urban and roadway locations, raising the levels of nitrogen deposited on the nearby landscape. What are the ecological effects of moderately high (at least double background) and prolonged concentrations (10+ years) of nitrogen deposition on an ecosystem? Please be specific about which type of ecosystem your comments are intended.
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.
Up to a certain point, adding nitrogen to soil will increase plant growth. However, too much nitrogen will "burn" plants- stunting root growth and making the plant more susceptible to diseases. Unfortunately, I don't know at exactly what threshold nitrogen becomes "too much"- I suppose it would vary depending on the plants.