The Nitrogen Cycle
Nitrogen is also one of the most abundant elements in living things. It’s important for
constructing both proteins and nucleic acids like DNA. The great irony of the nitrogen
cycle is that nitrogen gas (N 2 ) comprises the majority of the air we breathe, and yet is not
accessible to us or plants in the gaseous form (Figure24.11). In fact, plants often suffer
from nitrogen deficiency even through they are surrounded by plenty of nitrogen gas!
In order for plants to make use of nitrogen, it must be converted into compounds with
other elements. This can be accomplished several different ways. First, Nitrogen gas can
be converted to nitrate (NO 3 - ) through lightning strikes. Alternatively, special nitrogen-
fixing bacteria can also convert nitrogen gas into useful forms, a process callednitrogen
fixation.These bacteria live in nodules on the roots of plants in the pea family. In aquatic
environments, bacteria in the water can fix nitrogen gas into ammonium (NH 4 +), which can
be used by aquatic plants as a source of nitrogen.
Nitrogen also is released to the environment through decaying organisms or decaying wastes.
These wastes often take on the form of ammonium. Ammonium in the soil can be converted
to nitrate by a two-step process completed by two different types of bacteria. In the form of
nitrate, it can be used by plants through a process calledassimilation.
The conversion of nitrate back into nitrogen gas happens through the work of denitrifying
bacteria. These bacteria often live in swamps and lakes. The release of nitrogen gas would
equal the amount of nitrogen gas taken into living things if human activities did not influence
the nitrogen cycle. These human activities include the burning of fossil fuels, which releases
nitrogen oxide gasses into the atmosphere, leading to problems like acid rain.
Lesson Summary
- During the water cycle, water enters the atmosphere through evaporation, and water
returns to land through precipitation. - During the carbon cycle, animals add carbon dioxide to the atmosphere through res-
piration and plants remove carbon dioxide through photosynthesis. - During the nitrogen cycle, gaseous nitrogen is converted into water-soluble forms that
can be used by plants, while denitrifying bacteria convert nitrate back to gaseous
nitrogen.
Review Questions
- What human activities have thrown the carbon cycle off balance?
- What biological process “fixes” carbon, removing it from the atmosphere?
- What is the significance of nitrogen-fixing bacteria?
- What is the term for the remains of organisms that are burned for energy?