THE NITROGEN CYCLE
Nitrogen comprises about 80 percent of the earth's atmosphere and is incorporated in all
plant and animal proteins. It is a required element for all forms of life. The chemical form of
nitrogen is important in relation to its availability to plants and animals. Nitrogen in its elemental
form is a gas with molecules composed of two nitrogen atoms, N2. Elemental N is relatively
inert, but it undergoes oxidation and reduction reactions to form several chemically active species
(Fig. 2). The N cycle shows the transformations of N from elemental through nitrite (NO2"),
nitrate (NO3), ammonium (NH4j), nitrous (N20) and nitric (NO) oxides, and organic nitrogen.
The transformations are performed biochemically by a number of species of soil bacteria.
Nitrogen enters the soil/plant system, under natural conditions, through rainfall where is
has been converted to nitrate through lightning, or as ammonia, or by being fixed through the
root systems of leguminous plants. Legumes have Rhizobium bacteria associated with the roots
in nodules. It is the bacteria that actually fix N from the air. As plants die or shed leaves, plant
residues enter the soil and are decomposed releasing nitrogen to complete the cycle.
Row crops and fruit and nut trees require more N for adequate yields than can be supplied
by natural means. Thus, agricultural production is dependent on addition of N in the form of
fertilizer. A crop of corn is capable of removing 150 to 200 pounds of N per acre. When an
adequate supply of N is not available, yields of crops are reduced. Nitrogen is frequently the
limiting nutrient in agricultural production.
Fertilizer N is generally added as ammonium or nitrate, although there are areas where
there is widespread use of anhydrous ammonia. Nitrifying bacteria transform ammonia by
oxidizing it to nitrate. Ammonium ion interacts with soil and may be fixed in certain types of
clay. More often, ammonium is simply sorbed on the soil particles and slowed in its migration
through soil. As it becomes oxidized to nitrate, the nitrate is either absorbed by plants or soil
organisms, or is leached beyond the root zone and finally to the water table. Over fertilization
can result in groundwater nitrate concentrations that exceed the drinking water concentration limit
of 10 mg/L (milligrams/Liter or parts per million) as nitrate-N. Runoff from agricultural fields
into lakes and streams is also of concern as the added nutrient may cause eutrophication of the
Nitrogen is the most transient nutrient in the soil system. It has numerous pathways in
which it is lost from soil as dissolved ions or gases. A large effort in studying the means of
controlling N reactions in soil has been expended to increase the efficiency of N utilization by
plants and to reduce N losses to ground and surface waters. Nitrate in excess in drinking water
can cause a condition known as methemoglobinemia in infant and geriatric populations. This is
a reduction in the capacity of blood to carry oxygen because of absorption of nitrite ion formed
from nitrate reduction in the stomach.
I eder associates consulting-gineers p.c.
THE NITROGEN CYCLE
(after Stevenson, 1982)