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Carbon and Nitrogen Cycles

OpenStaxCollege

Learning Objectives

By the end of this section, you will be able to:

  • Discuss the biogeochemical cycles of carbon and nitrogen
  • Explain how human activities have impacted these cycles and the potential consequences for Earth

Energy flows directionally through ecosystems, entering as sunlight (or inorganic molecules for chemoautotrophs) and leaving as heat during the many transfers between trophic levels. However, the matter that makes up living organisms is conserved and recycled. The six most common elements associated with organic molecules—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—take a variety of chemical forms and may exist for long periods in the atmosphere, on land, in water, or beneath the Earth’s surface. Geologic processes, such as weathering, erosion, water drainage, and the subduction of the continental plates, all play a role in this recycling of materials. Because geology and chemistry have major roles in the study of this process, the recycling of inorganic matter between living organisms and their environment is called a biogeochemical cycle.

The cycling of these elements is interconnected. For example, the movement of water is critical for the leaching of nitrogen and phosphate into rivers, lakes, and oceans. Furthermore, the ocean itself is a major reservoir for carbon. Thus, mineral nutrients are cycled, either rapidly or slowly, through the entire biosphere, from one living organism to another, and between the biotic and abiotic world. In this section we will focus on carbon and nitrogen. Carbon is found in all organic macromolecules and is an important constituent of fossil fuels. Nitrogen is a major component of our nucleic acids and proteins and is critical to human agriculture.

The Nitrogen Cycle

Diagram of the nitrogen cycle including plants, bacteria, and nitrogen compounds.Figure 1. Nitrogen Cycle. Nitrogen enters the living world from the atmosphere via nitrogen-fixing bacteria. This nitrogen and nitrogenous waste from animals is then processed back into gaseous nitrogen by soil bacteria, which also supply terrestrial food webs with the organic nitrogen they need. (modification of work by John M. Evans and Howard Perlman, USGS)

We are surrounded by a lot of nitrogen, N2 comprises approximately 78 percent of the atmosphere. However, nitrogen in this form is not available to most living organisms. Getting nitrogen into the living world is difficult. Plants and phytoplankton are not equipped to incorporate nitrogen from the atmosphere (which exists as tightly bonded, triple covalent N2). Nitrogen enters the living world via free-living and symbiotic bacteria, which incorporate nitrogen into their macromolecules through nitrogen fixation. Cyanobacteria live in most aquatic ecosystems where sunlight is present; they play a key role in nitrogen fixation. Cyanobacteria are able to use inorganic sources of nitrogen to “fix” nitrogen. Rhizobium bacteria live symbiotically in the root nodules of legumes (such as peas, beans, and peanuts) and provide them with the organic nitrogen they need. Free-living bacteria, such as Azotobacter, are also important nitrogen fixers.

Organic nitrogen is especially important to the study of ecosystem dynamics since many ecosystem processes, such as plant growth and decomposition, are limited by the available supply of nitrogen. As shown in Figure 1, the nitrogen that enters living systems by nitrogen fixation is successively converted from organic nitrogen back into nitrogen gas by bacteria. This process occurs in three steps in terrestrial systems: ammonification, nitrification, and denitrification. First, the ammonification process converts nitrogenous waste from living animals or from the remains of dead animals into ammonium (NH4+) by certain bacteria and fungi. Second, the ammonium is converted to nitrites (NO2−) by nitrifying bacteria, such as Nitrosomonas, through nitrification. Subsequently, nitrites are converted to nitrates (NO3−) by similar organisms. Third, the process of denitrification occurs, whereby bacteria, such as Pseudomonas and Clostridium, convert the nitrates into nitrogen gas, allowing it to re-enter the atmosphere.

NH4+, NO2– and NO3– can all be absorbed by the root systems of plants through a process called assimilation. Once in the plant, these forms of nitrogen will be used to produce amino acids and nucleic acids, which in turn contribute to the production of proteins and nucleotides respectively. Limitations of nitrogen in the soil are common and can lead to growth deficits in plants. To complete the cycle, decomposers digesting the proteins and nucleotides present in plants release nitrogen in the form of NH4+, NO2– and NO3– back into the soil. This is why compost piles often contain lots of nitrogen and can function as a fertilizer that can be used to supplement the soil and promote enhanced plant growth.

Human activity can release nitrogen into the environment by two primary means: the combustion of fossil fuels, which releases different nitrogen oxides, and by the use of artificial fertilizers in agriculture, which are then washed into lakes, streams, and rivers by surface runoff. Atmospheric nitrogen is associated with several effects on Earth’s ecosystems including the production of acid rain (as nitric acid, HNO3) and greenhouse gas (as nitrous oxide, N2O) potentially causing climate change. A major effect from fertilizer runoff is saltwater and freshwater eutrophication, a process whereby nutrient runoff causes the excess growth of microorganisms, depleting dissolved oxygen levels and killing ecosystem fauna.

A similar process occurs in the marine nitrogen cycle, where the ammonification, nitrification, and denitrification processes are performed by marine bacteria. Some of this nitrogen falls to the ocean floor as sediment, which can then be moved to land in geologic time by uplift of the Earth’s surface and thereby incorporated into terrestrial rock. Although the movement of nitrogen from rock directly into living systems has been traditionally seen as insignificant compared with nitrogen fixed from the atmosphere, a recent study showed that this process may indeed be significant and should be included in any study of the global nitrogen cycle.

The Carbon Cycle

Carbon is the second most abundant element in living organisms. Carbon is present in all organic molecules, and its role in the structure of macromolecules is of primary importance to living organisms. Carbon compounds contain especially high energy, particularly those derived from fossilized organisms, mainly plants, which humans use as fuel. Since the 1800s, the number of countries using massive amounts of fossil fuels has increased. Since the beginning of the Industrial Revolution, global demand for the Earth’s limited fossil fuel supplies has risen; therefore, the amount of carbon dioxide in our atmosphere has increased. This increase in carbon dioxide has been associated with climate change and other disturbances of the Earth’s ecosystems and is a major environmental concern worldwide. Thus, the “carbon footprint” is based on how much carbon dioxide is produced and how much fossil fuel countries consume.

Diagram of the carbon cycle showing exchanges and storage of carbon in the atmosphere, land, and ocean.

Figure 2. The Carbon Cycle. Carbon enters the atmosphere as carbon dioxide gas that is released from human emissions, respiration by all organisms, decomposition, and volcanic emissions. Carbon dioxide is removed from the atmosphere by marine and terrestrial photosynthesis. Carbon from the weathering of rocks becomes soil carbon, which over time can become fossil carbon. Carbon enters the ocean from land via leaching and runoff. Uplifting of ocean sediments can return carbon to land. (modification of work by John M. Evans and Howard Perlman, USGS)

The carbon cycle is most easily studied as two interconnected sub-cycles: one dealing with rapid carbon exchange among living organisms and the other dealing with the long-term cycling of carbon through geologic processes.

The Biological Carbon Cycle

Living organisms are connected in many ways, even between ecosystems. A good example of this connection is the exchange of carbon between autotrophs and heterotrophs within and between ecosystems by way of atmospheric carbon dioxide. Carbon dioxide is the basic building block that most autotrophs use to build multi-carbon, high energy compounds, such as glucose. The energy harnessed from the sun is used by these organisms to form the covalent bonds that link carbon atoms together. These chemical bonds thereby store this energy for later use in the process of respiration. Most terrestrial autotrophs obtain their carbon dioxide directly from the atmosphere, while marine autotrophs acquire it in the dissolved form (carbonic acid, H2CO3−). However carbon dioxide is acquired, a by-product of the process is oxygen. The photosynthetic organisms are responsible for depositing approximately 21 percent oxygen content of the atmosphere that we observe today.

Heterotrophs and autotrophs are partners in biological carbon exchange (especially the primary consumers, largely herbivores). Heterotrophs acquire the high-energy carbon compounds from the autotrophs by consuming them, and breaking them down by respiration to obtain cellular energy, such as ATP. The most efficient type of respiration, aerobic respiration, requires oxygen obtained from the atmosphere or dissolved in water. Thus, there is a constant exchange of oxygen and carbon dioxide between the autotrophs (which need the carbon) and the heterotrophs (which need the oxygen). Gas exchange through the atmosphere and water is one way that the carbon cycle connects all living organisms on Earth.

The Biogeochemical Carbon Cycle

The movement of carbon through the land, water, and air is complex, and in many cases, it occurs much more slowly geologically than as seen between living organisms. Carbon is stored for long periods in what are known as carbon reservoirs, which include the atmosphere, bodies of liquid water (mostly oceans), ocean sediment, soil, land sediments (including fossil fuels), and the Earth’s interior.

As stated, the atmosphere is a major reservoir of carbon in the form of carbon dioxide and is essential to the process of photosynthesis. The level of carbon dioxide in the atmosphere is greatly influenced by the reservoir of carbon in the oceans. The exchange of carbon between the atmosphere and water reservoirs influences how much carbon is found in each location, and each one affects the other reciprocally. Carbon dioxide (CO2) from the atmosphere dissolves in water and combines with water molecules to form carbonic acid, and then it ionizes to carbonate and bicarbonate ions

On land, carbon is stored in soil as a result of the decomposition of living organisms (by decomposers) or from weathering of terrestrial rock and minerals. This carbon can be leached into the water reservoirs by surface runoff. Deeper underground, on land and at sea, are fossil fuels: the anaerobically decomposed remains of plants that take millions of years to form. Fossil fuels are considered a non-renewable resource because their use far exceeds their rate of formation. A non-renewable resource, such as fossil fuel, is either regenerated very slowly or not at all. Another way for carbon to enter the atmosphere is from land (including land beneath the surface of the ocean) by the eruption of volcanoes and other geothermal systems. Carbon sediments from the ocean floor are taken deep within the Earth by the process of subduction: the movement of one tectonic plate beneath another. Carbon is released as carbon dioxide when a volcano erupts or from volcanic hydrothermal vents.

Carbon dioxide is also added to the atmosphere by the animal husbandry practices of humans. The large numbers of land animals raised to feed the Earth’s growing population results in increased carbon dioxide levels in the atmosphere due to farming practices and the respiration and methane production. This is another example of how human activity indirectly affects biogeochemical cycles in a significant way. Although much of the debate about the future effects of increasing atmospheric carbon on climate change focuses on fossils fuels, scientists take natural processes, such as volcanoes and respiration, into account as they model and predict the future impact of this increase.

Which of the following statements about the nitrogen cycle is false?

  1. Ammonification converts organic nitrogenous matter from living organisms into ammonium (NH4+).
  2. Denitrification by bacteria converts nitrates (NO3−) to nitrogen gas (N2).
  3. Nitrification by bacteria converts nitrates (NO3−) to nitrites (NO2−).
  4. Nitrogen fixing bacteria convert nitrogen gas (N2) into organic compounds.

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Section Summary

Mineral nutrients are cycled through ecosystems and their environment. Of particular importance are water, carbon, nitrogen, phosphorus, and sulfur. All of these cycles have major impacts on ecosystem structure and function. As human activities have caused major disturbances to these cycles, their study and modeling is especially important. A variety of human activities, such as pollution, oil spills, and events) have damaged ecosystems, potentially causing global climate change. The health of Earth depends on understanding these cycles and how to protect the environment from irreversible damage.

Art Connections

[link] Which of the following statements about the nitrogen cycle is false?

  1. Ammonification converts organic nitrogenous matter from living organisms into ammonium (NH4+).
  2. Denitrification by bacteria converts nitrates (NO3−) to nitrogen gas (N2).
  3. Nitrification by bacteria converts nitrates (NO3−) to nitrites (NO2−).
  4. Nitrogen fixing bacteria convert nitrogen gas (N2) into organic compounds.

[link] C: Nitrification by bacteria converts nitrates (NO3−) to nitrites (NO2−).

Review Questions

The movement of mineral nutrients through organisms and their environment is called a ________ cycle.

  1. biological
  2. bioaccumulation
  3. biogeochemical
  4. biochemical

C

Carbon is present in the atmosphere as ________.

  1. carbon dioxide
  2. carbonate ion
  3. carbon dust
  4. carbon monoxide

A

The majority of water found on Earth is:

  1. ice
  2. water vapor
  3. fresh water
  4. salt water

D

The average time a molecule spends in its reservoir is known as ________.

  1. residence time
  2. restriction time
  3. resilience time
  4. storage time

A

The process whereby oxygen is depleted by the growth of microorganisms due to excess nutrients in aquatic systems is called ________.

  1. dead zoning
  2. eutrophication
  3. retrofication
  4. depletion

B

The process whereby nitrogen is brought into organic molecules is called ________.

  1. nitrification
  2. denitrification
  3. nitrogen fixation
  4. nitrogen cycling

C

Free Response

Describe nitrogen fixation and why it is important to agriculture.

Nitrogen fixation is the process of bringing nitrogen gas from the atmosphere and incorporating it into organic molecules. Most plants do not have this capability and must rely on free-living or symbiotic bacteria to do this. As nitrogen is often the limiting nutrient in the growth of crops, farmers make use of artificial fertilizers to provide a nitrogen source to the plants as they grow.

What are the factors that cause dead zones? Describe eutrophication, in particular, as a cause.

Many factors can kill life in a lake or ocean, such as eutrophication by nutrient-rich surface runoff, oil spills, toxic waste spills, changes in climate, and the dumping of garbage into the ocean. Eutrophication is a result of nutrient-rich runoff from land using artificial fertilizers high in nitrogen and phosphorus. These nutrients cause the rapid and excessive growth of microorganisms, which deplete local dissolved oxygen and kill many fish and other aquatic organisms.

Why are drinking water supplies still a major concern for many countries?

Most of the water on Earth is salt water, which humans cannot drink unless the salt is removed. Some fresh water is locked in glaciers and polar ice caps, or is present in the atmosphere. The Earth’s water supplies are threatened by pollution and exhaustion. The effort to supply fresh drinking water to the planet’s ever-expanding human population is seen as a major challenge in this century.

Figure Descriptions

Figure 1. The image illustrates the nitrogen cycle, showcasing the transition of nitrogen through different forms and processes in the ecosystem. At the top, “Atmospheric nitrogen (N₂)” is depicted inside a box with an arrow leading to plants in the center. A rabbit is nearby, suggesting the consumption of plants. Below are arrows indicating processes such as “Assimilation” by plants and “Decomposers (aerobic and anaerobic bacteria and fungi)” converting organic matter. The lower section represents bacteria like nitrogen-fixing bacteria living in legume root nodules and soil bacteria, with processes labeled as “Ammonification,” “Nitrification,” and “Denitrification.” Molecules like “Ammonium (NH₄⁺),” “Nitrites (NO₂⁻),” and “Nitrates (NO₃⁻)” are shown with corresponding transformation processes, involving nitrifying and denitrifying bacteria. A lightning bolt near a cloud symbolizes nitrogen fixation by lightning.

Figure 2. The image illustrates the carbon cycle, showing the movement and storage of carbon across different components of the Earth’s system. In the center is a cross-sectional view of land and ocean. On land, there are trees, vegetation, animals, and a factory. Arrows indicate carbon transfer between these components, with numbers showing the magnitude of fluxes and stocks. A large sun is depicted in the top left, and atmospheric carbon is shown with arrows pointing towards and away from the atmosphere. The ocean is divided into surface and deep ocean with arrows illustrating carbon exchange. Text boxes indicate carbon storage in gigatons and fluxes in gigatons per year.

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