Why care about Nitrogen? Well it's not a greenhouse gas like carbon is, but in certain forms it's it's own sort of pollutant. In the Chesapeake Bay, it fuels a chain reaction of algae explosions and bacterial decomposition that leaves the water starved of oxygen and life.
It comes from fertilizer on lawns and farms, and from autmobile and power plant exhaust, so it's something that everyone is responsible for creating in some way. The Web site has recommendations on how to reduce your nitrogen foot print by changing eating and travel habits.