Biology 12

nitrogen-fixing

bacteria in root

nodules of legumes

nitrogen-fixing soil bacteria

ammonification

nitrification

assimilation

nitrifying bacteria

nitrifying

bacteria

denitrifying

bacteria

plants

decomposers

(aerobic and anaerobic

bacteria and fungi)

nitrogen in atmosphere ( )N 2

nitrates ( )NO 3 −

ammonium ( )NH 4 + nitrites ( )NO 2 −

454 MHR • Unit 5 Population Dynamics

converted to these materials in the atmosphere if

it becomes dissolved in rain or attached to dust

particles. It can then enter the soil and be taken

up by plants (see Figure 13.28).

More commonly, the conversion of nitrogen to a

form useful to plants occurs as a result of the

activity of bacteria. Certain bacterial species are

capable of a process known as nitrogen fixation,

which converts nitrogen gas to ammonium. Other

types of bacteria can convert ammonium to nitrite

(NO 2 −), in processes referred to as nitrification.

Bacteria and cyanobacteria are the only organisms

that carry out these processes. Without them, very

little nitrogen would be available for living things

in ecosystems.

Nitrogen is a vital component of amino acids,

which are the building blocks of which proteins are

made. Since proteins make up the majority of the

structural material of living organisms, act as

enzymes, and perform many other functions, it is

clear that without the bacteria involved in the

nitrogen cycle the biosphere as we know it today

would be devoid of life.

The Phosphorus Cycle

Water, carbon, and nitrogen exist as gases in the

atmosphere and therefore cycle over large areas.

In contrast, phosphorus is an example of a nutrient

that cycles on a more localized scale (see Figure

13.29). Most phosphorus is cycled through food

webs in both aquatic and terrestrial ecosystems.

Some phosphorus escapes from the land and enters

Earth’s supply of ground water (the reserve of

underground water), drains into rivers, and

eventually flows into the sea. However, an almost

equal amount re-enters the soil from the

weathering of rocks.

Over long periods of time, phosphates that reach

the ocean accumulate in sediments and eventually

become incorporated into rock. These rocks may

re-enter terrestrial environments as a result of

geological processes that either raise the sea floor

or lower seawater levels at a particular location.

Most phosphorus cycles locally through soil,

plants, and consumers, but some will cycle over a

longer time and larger scale. This is true for most

elements that do not exist in the atmosphere as a gas.

Figure 13.28The width of the arrows in this diagram

represents the relative amount of nitrogen being moved by

each process. Why do you think some gardeners inoculate

some types of plants with a specially prepared mix of

bacteria before planting them?