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Rhizobium Collection
Plant Sciences Insitute
Beltsville Agricultural Research Center
Logo, Soybean Genomics and Improvement Laboratory

Nitrogen is an essential element for life
because it is a fundamental component of
protein and nucleic acids. Unfortunately
the supply of nitrogen for plants in soil is
limited and for economic returns in
agriculture management of the crop's
nitrogen supply is critical. Application of
fertilizer nitrogen can be costly increasing
food prices, its manufacture uses the
earth's dwindling energy reserves, and its
use significantly impacts our environment.
Gaseous dinitrogen constitutes 78% of
Earth's atmosphere, but most life forms
are unable to use it. The exception is some
microorganisms that use gaseous dinitrogen directly
from air. In nature, nitrogen cycles between the atmosphere, the soil, and life.
In agriculture, the most important nitrogen-fixing bacteria are called rhizobia because they form a very close symbiotic relationship with leguminous plants. Many leguminous plants are important crops. The rhizobia invade the roots and cause swellings in the root cortex. These swellings are called root nodules. The bacteria are located inside the nodules where they reduce gaseous dinitrogen to ammonium. The ammonium combines with sugars and other organic compounds supplied by the plant to produce proteins and nucleic acids.

Chart showing Nitrogen cycle
Biological Nitrogen Fixation