Nitrogen Cycle
Soil 206 – Soil Ecosystem Lab
Objectives:
After completing this laboratory, the student should be able to:
1.
Explain the difference between heterotrophic and autotrophic soil organisms.
2.
Define the terms mineralization, immobilization, and nitrification and discuss the significance of each
of these processes in relation to the availability of soil nitrogen.
3.
Discuss the organic or inorganic oxidations and terminal electron acceptors in various N cycle
processes.
4.
Indicate whether a given N cycle process is accomplished to a) obtain energy, b) use N for a
terminal electron acceptor, or c) use N for cellular components
5.
Explain the results obtained from various treatments in a soil incubation experiment.
Introduction
Soils are complex biogeochemical materials on which plants grow and humans live. Soil has physical,
chemical, and biological properties that distinguish it from the material of its origin. Soils are also
dynamic ecosystems inhabited by diverse populations of plants, animals and many kinds of
microorganisms. While each of these groups occupies its own niche, they are also interrelated and highly
dependent on one another for nutrient and energy cycling.
In the following exercise, you will observe how different soil conditions influence the process of
mineralization, immobilization, nitrification and denitrification. Although you will study only a small
part of the soil ecosystem, the principles you learn will be applicable to many different forms of terrestrial
life.
Ecology of Soil Microorganisms
Many conditions affect the growth of soil microorganisms. Among the most important are the supplies of
oxygen and water, temperature, pH and the amount and type of nutrients present. Carbon (C) and
nitrogen (N) are the most common nutritional constraints faced by soil bacteria. This is because the
majority of the C and N in soils are in a form not readily available to most microorganisms. Large
quantities of C and N are present in the plant and animal materials continually added to soil, however
much of the C and N in these materials is tied up in large macromolecules such as cellulose, lignin, and
proteins. Thus many soil microorganisms must wait while a hierarchy of decay organisms decomposes
the macromolecules. As the large molecules are broken into smaller ones, C and N are released in a
bioavailable form. Soil microbes derive their life-sustaining energy from oxidation reactions so they are
classified as chemotrophs. Autotrophic organisms obtain their carbon from carbon dioxide while
heterotrophic organisms derive their carbon form organic material. The nitrifying bacteria are classified
as chemoautotrophs since they oxidize inorganic ammonium or nitrite as their energy source and use
carbon dioxide as their carbon source. The microbes that are responsible for denitrification are mostly
chemoheterotrophic, anaerobic bacteria. They are classified as heterotrophs as they obtain their energy
and their carbon from the oxidation of organic compounds.
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The Nitrogen Cycle
Few, if any, plant nutrients are as complex to study as soil N. The availability of N to plants in intricately
linked to microbial decomposition of soil organic matter. Nitrogen may be added and lost, transformed
and translocated in soil by a large and interrelated number of processes. Nitrogen comprises 78% of the
earth's atmosphere yet this element is quite often the most limiting of plant nutrients. The amount of soil
N present in plant available forms is small while the quantity lost through leaching and consumption by
crops is large. To attain maximum vegetative growth on intensively used soils, it has become
standard practice to apply nitrogen fertilizers on an annual basis.
There are three major forms of N in soils:
1. organic-N associated with soil humus,
2. ammonium-N fixed by clay minerals and
3. soluble inorganic-N compounds.
These three forms of N are related through a complex series of biochemical and physical processes
known as the N cycle.
The majority of soil N is associated with organic matter. The amount of N in the form of soluble
ammonium and nitrate compounds is seldom more than 1 - 2% of the total present, except where
applications of N fertilizers have been made. Organic N must undergo microbial decomposition to
inorganic forms such as NH4+ or NO3- before becoming available to plants. This slow conversion of
organic N to inorganic N is known as mineralization. Mineralization is accomplished by a large variety of
chemoheterotrophic soil microorganisms including actinomycetes, fungi, and bacteria.
Nitrification, the enzymatic oxidation of ammonium to nitrate, proceeds in two steps but is accomplished
by only a few species of chemoautotrophic bacteria (Figure 1). Note that the conversion of ammonium
to nitrite (NO2-) results in the release of hydrogen ions. Nitrification has been associated with a gradual
acidification of soils in certain areas, including Northern Idaho. Nitrifying organisms are unusually
sensitive to the soil environment, particularly soil acidity and aeration. Nitrification is inhibited when the
soil pH is low. Thus in highly acid soils, the primary form of available N are the ammonium
ion. Nitrification is an oxidative process; that is, an adequate supply of oxygen is required for the
conversion of ammonium to nitrate. Factors associated with good soil aeration are therefore important
factors in nitrate production. An adequate source of NH4+ is also necessary as this ion serves as the
substrate for the first step in nitrification.
The type and amount of organic matter present can indirectly influence the nitrification. When organic
matter is added to soil, the heterotrophic decay organisms become active and multiply rapidly. Their
increased activity creates such a demand for inorganic N (NH4+ and NO3-) that, for a period of time, very
little N is available to plants and other organisms. Under these conditions, nitrification is inhibited, the
level of nitrate in solution drops significantly, and the majority of the soil N is tied up as microbial
biomass. This conversion of N from an inorganic to a microbial (organic) form is called immobilization.
The nitrogen content of the incorporated organic matter influences the potential for N immobilization.
Residues with a low N content such as wheat straw favor immobilization. Legume residues (i.e. alfalfa,
beans, and peas) have a relatively high N content which is usually sufficient to satisfy the N demand for
both plants and decay microorganisms. It should be stressed that immobilization is a temporary
phenomenon as eventually the activity of the decay organisms will subside due to a lack of oxidizable
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carbon and the biomass-N converted to an inorganic form via mineralization. The level of inorganic N
increases when this occurs. Refer to Figure 1.
Nitrogen may be lost to the atmosphere when nitrate ions are converted to gaseous forms of nitrogen by
a series of widely occurring biochemical reduction reactions termed denitrification. The organisms that
carry out this process are commonly present in large numbers and are mostly facultative anaerobic
bacteria. Denitrifying bacteria have been identified as heterotrophs, which obtain their energy and carbon
from the oxidation of organic compounds and autotrophs which obtain their energy from the oxidation of
sulfide and their carbon from carbon dioxide. The conditions that favor denitrification include the
presence of nitrate, readily decomposable organic compounds, soil air with less than 10% oxygen and a
soil temperature between 25 and 35 degrees Celsius.
For further discussion of immobilization, mineralization, and denitrification read pp. 546-564 in Brady and
Weil. A summary of the nitrogen reactions for immobilization and mineralization is found in Figure 2 and
a diagram of the Nitrogen cycle is found in Figure 3.
Figure 1: Cyclical Relationship Between Time, Microbial Activity and Nitrate Level
High C:N Residues Added
Low C:N Residues Remain
Microbial Population,
Evolution of CO2
Nitrate Level in Soil
Increasing
Concentration
Increasing Time
Source: N.C. Brady, The Nature and Properties of Soils, 10ed., p. 293. Macmillian, 1990 (used with permission)
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Figure 2: Simplified Illustration of Mineralization and Nitrification
Mineralization
Organic-N
NH4+ + Energy
Nitrification
NO2- + 4H+ + Energy
NO3- + Energy
.
Figure 3: Nitrogen Cycle
NH3(g)
N2(g)
denitrification
mineralization
NH4+
Organic-N
immobilization
ammonium
fixation
nitrification
NO3-
leaching
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