Download Biogeochemical Cycles of Life

Biogeochemical Cycles of Life
At a meeting not long ago someone asked how much nitrogen was released into the soil by lightning. I
thought the answer would be simple but, after browsing a bit, discovered the subject was far more
complex and interesting. The release of nitrogen by lightning is small part of the Nitrogen Cycle, one of
the essential biogeochemical cycles of life. Plants and animals that live and then die are the bio part; the
earth that they decompose into comprises the geo part; and the process by which organic matter returns
to the chemical elements in the earth is explained by the chemical part. There are four biogeochemical
cycles, and each of them returns to the earth important elements that are required in living organisms.
They are the Water Cycle, Carbon Cycle, Phosphorus Cycle and Nitrogen Cycle. - Robyn Wilkerson,
Master Rosarian
The Water Cycle
Water is always recycled through the water cycle, as shown in the diagram. The water undergoes evaporation,
condensation, and precipitation; falling back to Earth clean and fresh. Elements, chemical compounds, and other
forms of matter are passed from one organism to another and from one part of the biosphere to another through
biogeochemical cycles.
Illustration source University of Colorado.edu.
The Carbon Cycle
Plants take in carbon dioxide for photosynthesis. Animals consume plants or other animals and all living things
contain carbon. Carbon is what makes organic molecules ‘organic’ (living). Carbon is necessary for the creation of
molecules such as carbohydrates, proteins, and fats. Plants release carbon dioxide when they decompose. Animals
release carbon dioxide when they decompose or respire. Animals take in oxygen and release carbon dioxide when
they breathe. Carbon dioxide also is released when organic matter such as wood, leaves, coal, or oil are burned.
The carbon dioxide returns to the atmosphere, where it can be taken in by more plants that are then consumed by
animals. Decomposing animals and plants leach carbon into the ground, forming fossil fuels such as coal or oil.
Peat also forms from the decomposition of organic matter. Some carbon is stored in the form of cellulose in the
wood of trees and bushes.
Illustration source Wikipedia.org.
The Phosphorus Cycle
Phosphates move quickly through plants and animals. However, the processes that move them through the soil or
ocean are very slow, making the phosphorus cycle overall one of the slowest biogeochemical cycles.
Initially, phosphate weathers from rocks and minerals. Overall small losses occur in terrestrial environments by
leaching and erosion, through the action of rain. In soil, phosphate is absorbed in iron oxides, aluminum
hydroxides, clay surfaces, and organic matter particles becoming incorporated (immobilized or fixed). Plants and
fungi can also be active in making P soluble.
Illustration source Environmental Engineering Blog: envi-scie.blogspot.com
The Nitrogen Cycle
Nitrogen is essential to all living systems, which makes the nitrogen cycle one of Earth's most important nutrient
cycles. Eighty percent of Earth's atmosphere is made up of nitrogen in its gas phase.
Atmospheric nitrogen becomes part of living organisms in two ways. The first is through bacteria in the soil that
form nitrates out of nitrogen in the air. The second is through lightning. During electrical storms, large amounts of
nitrogen are oxidized and united with water to produce an acid that falls to Earth in rainfall and deposits nitrates in
the soil.
Plants take up the nitrates and convert them to proteins that then travel up the food chain through herbivores and
carnivores. When organisms excrete waste, the nitrogen is released back into the environment. When they die and
decompose, the nitrogen is broken down and converted to ammonia. Plants absorb some of this ammonia; the
remainder stays in the soil, where bacteria convert it back to nitrates. The nitrates may be stored in humus or
leached from the soil and carried into lakes and streams. Nitrates may also be converted to gaseous nitrogen
through a process called denitrification and returned to the atmosphere, continuing the cycle.
1.
Nitrogen Fixation
Nitrogen in the air becomes a part of biological matter mostly through the actions of bacteria and algae in a
process known as nitrogen fixation. Legume plants (e.g. peas, beans, soybeans, clover, lentils, peanuts, etc.) form
nodules on the roots where nitrogen fixing bacteria take nitrogen from the air and convert it into ammonia (NH 3).
The ammonia is further converted by other bacteria first into nitrite ions, NO 2 , and then into nitrate ions, NO3 .
Another method of nitrogen fixation takes place in atmosphere. Lightning breaks nitrogen molecules into atoms
which combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, which are
carried to the earth.
2.
Decay
The proteins made by plants enter and pass through food webs. At each level, their metabolism produces organic
nitrogen compounds that return to the environment, chiefly in excretions. The final beneficiaries of these
materials are microorganisms of decay which break down the molecules in excretions and dead organisms into
ammonia.
3.
Nitrification
Soil-living and nitrifying bacteria convert ammonia to nitrate. Bacteria of the genus Nitrosomonas oxidize NH 3 to
−
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nitrites (NO2 ) then bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO 3 ). In this way, nitrogen is
made available to the roots of plants. Archael microbes present in soil and ocean convert ammonia to nitrites.
Many legumes, in addition to fixing atmospheric nitrogen, also perform nitrification (conversion of organic
nitrogen to nitrites and nitrates). These reach the soil when they shed their leaves.
4.
Denitrification
Denitrification is the reduction of nitrates back into nitrogen gas (N 2). Bacteria live deep in soil and in aquatic
sediments where conditions are anaerobic. They use nitrates as an alternative to oxygen for the final electron
acceptor in their respiration.
Illustration source National Center for Atmospheric Research and University Corporation for Atmospheric Research
Sources: National Center for Atmospheric Research and University Corporation for Atmospheric Research; Encyclopaedia
Brittanica; For Dummies®Making Everything Easier; University of Colorado.edu; Wikipedia.org.; Environmental Engineering
Blog: envi-scie.blogspot.com