Document Type

Thesis - Open Access

Award Date


Degree Name

Master of Science (MS)

Department / School

Biology and Microbiology

First Advisor

Robert L. Todd


Availability of nitrogen to the plant is a paramount concern in agricultural production today. Crop yields are most often dependent on the level of available nitrogen than any other element. Nitrogen can be supplied to the soil environment by either fertilizers or by the natural processes of decomposition of nitrogen containing compounds and biological fixation of nitrogen. The most abundant source of nitrogen on the earth is the atmosphere. Although the atmosphere is approximately 79% nitrogen, this form is generally unavailable for plant use. Atmospheric nitrogen contains two nitrogen atoms held together by a triple bond which must be reduced before the resulting NH3 can be assimilated by the plant. The conversion of atmospheric N to a usable form ammonium via ammonia by microbial populations is termed nitrogen fixation. This process is not only an energy requiring reaction but occurs under a limited set of environmental conditions provided by only a few biological systems. Lightning and combustion reduce small quantities of atmospheric nitrogen, but the majority of nitrogen transferred on a global scale from the atmosphere to the soil is by microbial organisms. Biological nitrogen fixation can be performed by either symbiotic, associative or free-living microorganisms. Plant roots produce nodules which are the site of fixation when infected by these symbiotic fixers. Associative fixers do not invade the plant root but reside in the root zone in close contact with the plant. Free-living fixers are not directly associated with any plant but are distributed throughout the soil profile. However, the greatest numbers are found in the rhizosphere or the zone of soil inhabited by plant roots. Other microbes are responsible for further cycling of nitrogen. Ammonia produced from fixation is oxidized to nitrite then nitrate by a process termed nitrification. Nitrate can undergo reduction to nitrite and ammonia or reduction through denitrification, resulting in the release of some form of nitrogen gas - N2o, N02, NO and N2 into the atmosphere. Nitrogen fixation represents a key link in the cycling of nitrogen both on a global and on an individual plant scale. An understanding of nitrogen fixation and the factors that affect this process are essential to maintaining adequate plant production to feed an ever-growing world population.

Library of Congress Subject Headings

Nitrogen -- Fixation
Soils -- Nitrogen content



Number of Pages



South Dakota State University


No Copyright - United State