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The nitrogen cycle is the bio and geochemical cycle by which nitrogen is converted into
multiple chemical forms as it circulates among atmosphere terrestrial and marine
ecosystems. The conversion of nitrogen can be carried out through both biological and
physical processes. Important processes in the nitrogen cycle include nitrogen fixation and
ammonification as well as nitrification and denitrification. The majority of atmosphere of
Earth is atmospheric nitrogen making it the largest source of nitrogen. However
atmospheric nitrogen has limited availability for biological use leading to a scarcity of
usable nitrogen in many types of ecosystems. The nitrogen cycle is of particular interest to
ecologists because nitrogen availability can affect the rate of key ecosystem processes
including primary production and decomposition. Human activities such as fossil fuel
combustion or use of artificial nitrogen fertilizers and release of nitrogen in wastewater
have dramatically altered the global nitrogen cycle. Human modification of the global
nitrogen cycle can negatively affect the natural environment systems and also human
health. Organic nitrogen can be present in the form of humus of a living organism or in the
intermediate products of organic matter decomposition. The goal of processes in the
nitrogen cycle is to transform nitrogen from one form to another. Many of these processes
are carried out by microbes either in their effort to harvest energy or to accumulate
nitrogen in a form needed for their growth. The nitrogenous wastes in animal urine are
broken down by nitrifying bacteria in the soil to be used by plants. The conversion of
nitrogen gas into nitrates and nitrites through atmospheric industrial and biological
processes is called nitrogen fixation. Atmospheric nitrogen must be processed or fixed into a
usable form to be taken up by plants. Most of the fixation is done by free living or symbiotic
bacteria known as diazotrophs. These bacteria have the an enzyme that combines gaseous
nitrogen with hydrogen to produce ammonia which is converted by the bacteria into other
organic compounds. Most biological nitrogen fixation occurs by the activity of a complex
enzyme found in a wide variety of bacteria. An example of free living bacteria is
Azotobacter. Symbiotic nitrogen fixing bacteria such as Rhizobium usually live in the root
nodules of legumes. Here they form a mutual relationship with the plant producing
ammonia in exchange for carbohydrates. Because of this relationship legumes will often
increase the nitrogen content of nitrogen poor soils. A few non legumes can also form such
symbioses. Today about thirty percent of the total fixed nitrogen is produced industrially
using the Haber Bosch processes which uses high temperatures and pressures to convert
nitrogen gas and a hydrogen source into ammonia. Plants can absorb nitrate or ammonium
from the soil by their root hairs. If nitrate is absorbed it is first reduced to nitrite ions and
then ammonium ions for incorporation into amino or nucleic acids and chlorophyll. In plants
that have a symbiotic relationship with rhizobia some nitrogen is assimilated in the form of
ammonium ions directly from the nodules. It is known that there is a more complex cycling
of amino acids between Rhizobia bacteria and plants. The plant provides amino acids to the
bacteria due to which ammonia assimilation is not required and the bacteria pass amino
acids back to the plant thus forming an interdependent relationship.