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Course: High school biology > Unit 9

Lesson 6: Biogeochemical cycles
Science
High school biology
Ecology
Biogeochemical cycles
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Biogeochemical cycles review

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Key terms

TermMeaning
Biogeochemical cycleThe ways in which an element or compound moves between living and nonliving forms and locations
NutrientElements that an organism needs to sustain life
HydrosphereThe set of places where water can be found as it cycles on Earth
GeosphereAll of the rock at and below Earth's surface
Limiting nutrientThe nutrient that's in shortest supply and limits growth

Biogeochemical cycles

Matter flows through trophic levels and elements are recycled among ecosystems using biogeochemical cycles.
As nutrients move through ecosystems, the compounds they form are usually transformed.

The water cycle

The water cycle is complex and involves state changes in water, as well as the physical movement of water through and between ecosystems.
The hydrosphere is large and diverse. Water is present as a liquid on the Earth's surface and underneath the ground, as ice in the polar ice caps and glaciers, and as water vapor in the atmosphere.
Water evaporates from the ocean surface and forms clouds by condensation. Water in clouds may fall as precipitation, over either the land or the sea. Clouds formed over the sea may move over the land. When rain falls over the land, it may flow along the surface, infiltrate the soil (move into it from above ground), and percolate through the soil, moving downward to become groundwater. Groundwater in upper levels may flow into rivers, lakes, or oceans. Water near the soil surface may be taken up by plants and move out of their bodies through transpiration from the leaves. Snowmelt runoff and sublimation of snow and ice are other processes that contribute to the water cycle.
The water cycle. Image from NOAA National Weather Service, CC BY 2.0.
Water molecules typically enter the atmosphere as vapor as they leave from bodies of water during evaporation or from the leaves of plants during transpiration.
The vapor may be transported through the atmosphere. As the vapor cools, the water condenses into droplets that form clouds and result in precipitation like rain or snow.

The carbon cycle

Carbon is an essential element in the bodies of living organisms. It is also economically important to modern humans, in the form of fossil fuels.
  • Carbon dioxide (CO2) from the atmosphere is taken up by photosynthetic organisms and used to make organic molecules, which travel through food chains. In the end, the carbon atoms are released as CO2 in respiration.
  • Slow geological processes, including the formation of sedimentary rock and fossil fuels, contribute to the carbon cycle over long timescales.
  • Some human activities, such as burning of fossil fuels and deforestation, increase atmospheric CO2 and affect Earth's climate and oceans.
The carbon cycle. Image from OpenStax, CC BY 4.0.
Autotrophs (producers) remove carbon from the atmosphere through photosynthesis and uses it to make organic compounds such as glucose. Heterotrophs (consumers) such as humans consume the organic molecules, and the organic carbon is passed through food chains and webs.
Carbon is released back into the atmosphere when autotrophs or heterotrophs undergo cellular respiration, or when decomposers may break down the bodies of dead organisms.
Human activity, such as burning fossil fuels, may also release carbon into the atmosphere.
Carbon enters the geosphere when dead organic matter gets buried and transformed. This process, which ultimately creates fossil fuels, can take millions of years.

The nitrogen cycle

Nitrogen is an important nutrients for all living organisms, as it is found in proteins, DNA, and RNA.
The illustration shows the nitrogen cycle. Nitrogen gas from the atmosphere is fixed into organic nitrogen by nitrogen fixing bacteria. This organic nitrogen enters terrestrial food webs. It leaves the food webs as nitrogenous wastes in the soil. Ammonification of this nitrogenous waste by bacteria and fungi in the soil converts the organic nitrogen to ammonium ion (NH4 plus). Ammonium is converted to nitrite (NO2 minus), then to nitrate (NO3 minus) by nitrifying bacteria. Denitrifying bacteria convert the nitrate back into nitrogen gas, which reenters the atmosphere. Nitrogen from runoff and fertilizers enters the ocean, where it enters marine food webs. Some organic nitrogen falls to the ocean floor as sediment. Other organic nitrogen in the ocean is converted to nitrite and nitrate ions, which is then converted to nitrogen gas in a process analogous to the one that occurs on land.
The nitrogen cycle. Image from OpenStax, CC BY 4.0.
Nitrogen exists in the atmosphere as a gas. In nitrogen fixation, bacteria convert nitrogen gas into ammonia, a form of nitrogen usable by plants. When animals eat the plants, they acquire usable nitrogen compounds. Some bacteria also obtain energy through denitrification, in which nitrates are converted into nitrogen gas and then released into the atmosphere.
Nitrogen is a common limiting nutrient in nature (and agriculture). When the available supply of nitrogen is low, many processes, such as primary production and decomposition, are stunted.

Common mistakes and misconceptions

  • Not every nutrient will go through these cycles in the same path. People often think that biogeochemical cycles are linear, and that each atom or molecule goes through the cycle step-by-step. However, this is not true. The same atom or molecule may be stored for a long time in one stage of a cycle, cycled between the same two stages, or enter every stage.
  • Fossil fuels are considered a non-renewable resource. Although the process that creates fossil fuels happens naturally and continuously, fossil fuels are considered a non-renewable resource because they are being used up much faster than they can be produced by geological processes.

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