A recent research summary by Anne Simon Moffat (Science 279, 988-989) points out the critical nature of a global overload of nitrogen compounds. Fixed nitrogen (ammonia, nitrogen oxides) is beginning to overwhelm a wide range of forest, lake, river, and coastal ecosystems. One "ecological service" of natural ecosystems is to absorb or breakdown these compounds, but the increased volume of nitrogen compounds is increasingly overwhelming these natural systems.
Nitrogen is essential for plant growth, but excessive amounts lead to ecosystem takeover by certain species such as the notorious red and brown tides caused by excessive algal blooms in coastal areas. The recently reported hypoxic zone at the mouth of the Mississippi River is an example where excessive surface algal growth leads to oxygen depletion at lower levels when these organisms die, sink, and decay. Depleted oxygen supplies at low levels disrupts populations of deep-dwelling fish.
Sources of the excess nitrogen include increased use of nitrogen fertilizer and nitrates produced from fossil fuel combustion. It is estimated that humans produce 60% of the fixed nitrogen deposited on land each year, which is far more than can be absorbed by agricultural and natural ecosystems. These changes reduce biological diversity by preferential growth and possible ecosystem take over by species that thrive in high nitrogen levels.