In August, 2011, the US Environmental Protection Agency's Science Advisory Board released their report, "Reactive Nitrogen in the United States: An Analysis of Inputs, Flows, Consequences and Management Options". Note that Reactive nitrogen (Nr) is not regulated, at all, and according to Robert Socolow of Princeton University, "Globally, until nitrogen fixation is balanced by denitrification, the amount of excess fixed nitrogen in the world will grow relentlessly, with increasing consequences for ecosystems and the public health."
This is how the Advisory Board describes the "nitrogen cascade" (p. i):
In the United States, human activities across multiple sources currently introduce more than five times the Nr into the environment than natural processes. The largest U.S. sources of new Nr entering the U.S. environment include the creation and use of synthetic fertilizers, Nr created by legumes, and the combustion of fossil fuels.
Much of the Nr used to ensure a plentiful supply of food, fiber and biofuel is released to the environment, as is the Nr formed during fossil fuel combustion.
The introduction of human created Nr into the environment degrades air and water quality, which can cause harmful algae blooms, hypoxia, fish kills, loss of drinking water potability, loss of biodiversity, forest declines, and human health problems resulting in losses of billions of dollars per year.
The most important task for EPA and allied agencies and departments will be to effectively inform the public of the costs and dangers of excess Nr. Without strong public support, the widespread efforts necessary to control Nr will not be possible.
[note that last sentence, emphasis added - it's there because widespread efforts monumental sacrifices will be required in order to stop the cascade, which is why "the National Academy of Engineering has identified management of the nitrogen cascade one of the 'grand challenges' facing this country". It's a grand challenge near impossibility because it would require rationing fuel and fertilizer use, and, as Socolow suggests, a "focus on the food choices of the prosperous." meaning, a reduction in the consumption of meat. Ha!]
The effects are often magnified because the same atom of nitrogen can cause multiple effects in the atmosphere, in terrestrial ecosystems, in freshwater and marine systems, and on human health. We call this sequence of effects the nitrogen cascade.