Biogeochemical Cycles of Life: The Water Cycle, Carbon Cycle, Phosphorus Cycle and Nitrogen Cycle

by Robyn Wilkerson, Master Rosarian, Rose Society of Greater St. Louis

This is a 2014 AOM article

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. ABOVE: Illustration source University of Colorado.edu

Illustration source Wikipedia.org.

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 Environmental Engineering Blog: envi-scie.blogspot.com

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.

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 (NH3). The ammonia is further converted by other bacteria first into nitrite ions, NO2-, 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 NH3 to nitrites (NO2−) then bacteria of the genus Nitrobacter oxidize the nitrites to nitrates (NO3−). 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 (N2). 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