Nitrogen is an essential nutritional element for all plants and animals. There is a large reservoir of nitrogen in the air, but it is in a chemical form which is unavailable to plants and must therefore be “fixed” into usable form as nitrates or nitrites by soil microorganisms. As much as 130 metric tons of nitrogen is fixed annually by terrestrial systems, and is then available for use by plants. Nitrogen is released into the soil and water when the plants die, or when the herbivores which have consumed the plants die, or excrete nitrogen compounds. Humans have greatly altered this cycle by their own fixation of additional nitrogen for fertilizers (more than 80 million metric tons in 1990) and by the release of nitrogen into the atmosphere by fossil fuel combustion and land conversion. Human cultivation of legumes also increases nitrogen entry into the soil. All in all, human activities add as much fixed nitrogen to the land as comes from natural sources (Vitousek, et al., 1997).
Because of the great extent of rainforests, they are a significant ingredient in the global nitrogen cycle. They fix a great deal of nitrogen by means of their huge microbial populations; nevertheless, many tropical rainforests are limited in their growth by low nitrogen levels because they lose tremendous quantities of nitrogen into the soil and, to almost as great an extent, into water as dissolved nitrogen (Perakis and Hedin, 2002).