Visualizing Environmental Science

PROCESS DIAGRAM

nitrogen is so stable that it does not readily combine with

other elements. Atmospheric nitrogen must first be bro-

ken apart before the nitrogen atoms combine with other

elements to form proteins and nucleic acids.

There are five steps in the nitrogen cycle, in which

nitrogen cycles between the abiotic environment and

organisms: nitrogen fixation, nitrification, assimila-

tion, ammonification, and denitrification (Figure 5.11).

by increasing glacial and polar ice-cap melting and by in-

creasing evaporation in some areas.

The Nitrogen Cycle

Nitrogen is critical for all organisms because it is an

essential part of biological molecules such as proteins

and nucleic acids (for example, DNA). The atmosphere

is 78 percent nitrogen gas (N 2 ). However, atmospheric

The Cycling of Matter in Ecosystems 109

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Atmospheric

nitrogen (N 2 )

Plant and animal proteins

Assimilation

(nitrates, ammonia, or

ammonium absorbed by roots and

used to make organic compounds)

Nitrification

(nitrifying bacteria)

Internal cycling

(nitrification, assimilation,

ammonification on land)

1200

Decomposition

(ammonification by

ammonifying bacteria)

Nitrate (NO 3 )

Ammonia (NH 3 )

and ammonium (NH 4 )

+

Nitrogen fixation

from

human activity

100

Denitrification

(denitrifying bacteria)

reverses action of

nitrogen fixation and

nitrification

200

Biological nitrogen fixation

(nitrogen-fixing bacteria

in root nodules and soil)

140

The movement of nitrogen between the abiotic environment (primarily the atmosphere)

and living organisms is known as the nitrogen cycle. The five steps of the nitrogen cycle

are nitrogen fixation, nitrification, assimilation, ammonification, and denitrification. The

values shown for some of the active pools in the global nitrogen budget are expressed as

1012 g of nitrogen per year. For example, each year humans fix an estimated 100 × 10^12 g

of nitrogen.

Values are from Schlesinger, W. H.

Biogeochemistry: An Analysis of Global Change

, 2nd edition. Academic

Press, San Diego (1997) and based on several sources.

✓✓THE PLANNER

How might a large increase in

livestock production alter the nitrogen cycle?

Think Critically