The rate of yearly, natural nitrogen fixation, thought to be about 110 million metric tons on land, plays a crucial role in the organization and functioning of the biological world. It is now estimated that human activity has at least doubled this amount. The major sources of this nitrogen include industrial fertilizers, fossil fuel combustion, and large-scale cultivation of soybeans, peas, and other crops that host symbiotic nitrogen fixers. Additionally, human activity is speeding up the release of biologically usable nitrogen from long-term storage in soils and organic matter. Nitrous oxide is long-lived in the atmosphere and contributes to enhancing the greenhouse effect that is likely to warm Earth's climate.
Nitric oxide plays an important role in acid rain and smog.
Besides having the potential to become a pollutant, excess nitrogen can decrease the biodiversity and disrupt functioning of an ecosystem. For instance, native plants that have evolved to prosper with limited nitrogen may be pushed out by other plants that perform well in ecosystems with abundant nitrogen. The report also documents several areas where more studies need to be done, including whether elevated nitrogen levels are affecting biological processes in the open ocean. The report closes by looking at what will need to be done in the future to deal with and minimize human changes to the nitrogen cycle. And while economically more developed countries have become more efficient, applications in developing countries are rising dramatically.