The amount of gaseous nitrogen being fixed at
any given time by natural processes represents only a
small addition to the pool of previously fixed nitrogen
that cycles among the living and nonliving components
of the Earths ecosystems. Most of that nitrogen, too,
is unavailable, locked up in soil organic matter partially rotted plant and animal remains that must be
decomposed by soil microbes. These microbes release
nitrogen as ammonium or nitrate, allowing it to be recycled
through the food web. The two major natural
sources of new nitrogen entering this cycle are nitrogenfixing
organisms and lightning.
Nitrogen-fixing organisms include a relatively
small number of algae and bacteria. Many of them live
free in the soil, but the most important ones are bacteria
that form close symbiotic relationships with higher plants.
Symbiotic nitrogen-fixing bacteria such as the Rhizobia,
for instance, live and work in nodules on the roots of
peas, beans, alfalfa and other legumes. These bacteria
manufacture an enzyme that enables them to convert
gaseous nitrogen directly into plant-usable forms.
Lightning may also indirectly transform atmospheric
nitrogen into nitrates, which rain onto soil.
Quantifying the rate of natural nitrogen fixation
prior to human alterations of the cycle is difficult but
necessary for evaluating the impacts of human-driven
changes to the global cycling of nitrogen. The standard
unit of measurement for analyzing the global nitrogen
cycle is the teragram (abbreviated Tg), which is equal to
a million metric tons of nitrogen. Worldwide, lightning,
for instance, fixes less than 10 Tg of nitrogen per year
maybe even less than 5 Tg. Microbes are the major
natural suppliers of new biologically available nitrogen.
Before the widespread planting of legume crops, terrestrial
organisms probably fixed between 90 and 140 Tg
of nitrogen per year. A reasonable upper bound for the
rate of natural nitrogen fixation on land is thus about
140 Tg of N per year.