Re: The nitrogen problem? also this
- nitrogen cycle, the continuous flow of nitrogen through the biosphere by the processes of
nitrogen fixation, ammonification (decay), nitrification, and denitrification. Nitrogen is vital
to all living matter, both plant and animal; it is an essential constituent of amino acids,
which form proteins of nucleic acids, and of many other organic materials.
Although the earth's atmosphere is 78% nitrogen, free gaseous nitrogen cannot be utilized
by animals or by higher plants. They depend instead on nitrogen that is present in the soil.
To enter living systems, nitrogen must be "fixed" (combined with oxygen or hydrogen)
into compounds that plants can utilize, such as nitrates or ammonia. A certain amount of
atmospheric nitrogen is fixed by lightning and by some cyanobacteria (blue-green algae).
But the great bulk of nitrogen fixation is performed by soil bacteria of two kinds: those
that live free in the soil and those that live enclosed in nodules in the roots of certain
leguminous plants (e.g., alfalfa, peas, beans, clover, soybeans, and peanuts). Among the
free-living forms are species of Clostridium, discovered c.1893 by Sergei Winogradsky,
and Azotobacter, discovered c.1901 by M. W. Beijerinck. Both Clostridium and Azotobacter
are generally present in agricultural soils, and both are saprophytes, i.e., they use the
energy from decaying organic matter in the soil to fuel soil processes, including nitrogen
Bacteria that live in the roots of legumes are of the genus Rhizobium, first isolated c.1888
by Beijerinck. These rod-shaped bacteria enter the roots chiefly through the root hairs and
then work their way to the inner root tissues. There they stimulate the growth of tumorlike
nodules. Within the nodules the bacteria develop into forms called bacteroids, which live in
a symbiotic (mutually beneficial) relationship with the green plant. The bacteroids take
carbohydrates from the plant for energy to fix nitrogen and synthesize amino acids; the
plants take the amino acids elaborated in the nodule to build plant tissue. Animals in turn
consume the plants and convert plant protein into animal protein. Rhizobia can be found
free-living in the soil, but they cannot fix nitrogen in the free state, nor can the legume
root fix nitrogen without Rhizobia.
The exact biochemistry of nitrogen fixation within the nodule is not yet understood. It is
estimated that more than 300 lbs of nitrogen per acre (340 kg per hectare) can be fixed by
fields of alfalfa and other legumes. After a harvest legume roots left in the soil decay,
returning organic nitrogen compounds to the soil for uptake by the next generation of
plants. For this reason crop rotation in which a leguminous crop is rotated with a
nonleguminous one is a common practice for maintaining soil fertility.
Other Aspects of the Nitrogen Cycle
Decomposing animal remains and animal wastes also return organic nitrogen to the soil as
ammonia. Many different kinds of decay microorganisms participate in ammonification.
The nitrifying bacteria of the genus Nitrosomonas oxidize the ammonia to nitrites, and
Nitrobacter oxidize the nitrites to nitrates. The nitrates can then be taken up again by the
green plant. The cycle of fixation-decay-nitrification-fixation can proceed indefinitely
without any nitrogen being returned to a gaseous state. But still another group of
microorganisms, the denitrifying bacteria, can reduce nitrates all the way to molecular
nitrogen. Denitrification occurs only in the absence of oxygen and is not common in well-
Effects of Artificial Fixation
Nitrogen fixation can also be accomplished artificially by various methods (see nitrogen).
Humans annually fix vast amounts of nitrogen for industrial purposes and for use as
fertilizer. Unfortunately, large-scale legume cultivation and artificial fixation may be
upsetting the natural nitrogen cycle in the biosphere. There is some question whether
natural denitrification can keep pace with fixation. For one thing, run-off of nitrate
fertilizer can cause eutrophication of lakes and streams (see water pollution) and can foul
drinking supplies. Another environmental problem is that inorganic fertilizers tend to
depress legume fixation. As a consequence, root tissue remaining after harvest is poorer,
and thus more fertilizer must be applied the following year.