Biogeochemical Cycles Explained

We explain the routes and importance of the main biogeochemical pathways (e.g. the carbon cycle, the nitrogen cycle) through the Earth's atmosphere, biosphere, hydrosphere, pedosphere and lithosphere.
Greenhouse gases are invisible but they remain a serious threat to our climate
Photo (CC-by-3.0)

The term “biogeochemical cycle” is a massively oversimplified description of a complex, repetitive process in which a chemical is shifted through different physical areas of Planet Earth, changing its state en route from gas to liquid or solid and back again, as required. 1

More technically, a biogeochemical cycle is a pathway along which a particular chemical substance passes continually through living (biotic) and non-living (abiotic) compartments or spheres of Earth. The living compartment is known as the biosphere, while the non-living compartments include: the atmosphere (air), pedosphere (soil & surface rock), lithosphere (deep rock) and hydrosphere (rivers, lakes, oceans). The speed with which materials move through the different spheres, varies from a few days to millions of years. The cycles are called ‘biogeochemical’, because they involve a variety of biological, geological, and chemical processes. They are also entwined with climate change and several climate feedbacks.

The 6 best known biogeochemical cycles include: The Carbon Cycle, the Water Cycle, the Nitrogen Cycle, the Phosphorus Cycle, the Sulfur Cycle, and the oxygen cycle, while others circulate calcium, hydrogen, mercury, selenium, and silica.

Water Cycle: 3 Stages Diagram
The three stages of the water cycle (also called the hydrologic cycle), is the journey water makes from one state to another. As the word ‘cycle’ suggests there is no starting point. Image: © NASA

Why Are Biogeochemical Cycles Important?

These cycles are important because the chemicals being circulated are critical for the biological processes of life. The cycling process is vital for all living organisms, as the chemicals involved represent the essential building blocks of life. They are necessary for essential processes, including metabolism, the formation of amino acids, cell respiration and the formation of tissues. For example, oxygen and carbon account for 80 percent of the weight of a human being.

What’s more, the Earth is a closed system, so critical materials must be reused. After the death of a living thing, for example, the elements fixed in its bodily remains are recycled back into the environment, through the actions of decomposers such as insects, bacteria and fungi, to be reused by other living organisms. Even waste products excreted during the lifetime of an organism are typically broken down by bacteria and recycled.

Because Earth is a closed system as far as matter is concerned, the substances that move along biogeochemical pathways today, derive from elements that were present in the Earth’s crust when it came into existence about 4.6 billion years ago. Carbon, for example, is comparatively rare in the Earth’s crust, while nitrogen, which makes up 78 percent of the atmosphere is not available in a form that can be used by living creatures.

As well as supporting the biodiversity of the planet, some of these chemical cycles – the carbon cycle in particular – have a major influence on Earth’s climate system, through their regulation of the greenhouse effect that keeps Earth’s temperature at a cosy 59 degrees Celsius. 2

The water cycle also has a significant impact on climate and weather patterns as it interacts with its surroundings (as vapor, liquid or ice). It changes the pressure and temperature of the air in the troposphere, creates storms, wind, rain and ocean currents, and its precipitation alters the structure of earth and rock through erosion and weathering.

The Oxygen Cycle
The Oxygen Cycle. Image: © Fred the Oyster (CC BY-SA)

What’s the Route of a Typical Biogeochemical Cycle?

Each cycle has numerous possible pathways and many reservoirs (sinks), where chemicals may reside for short or long periods of time. Cycles can be fast or slow, depending on the reactivity of the chemical involved and whether or not it exists as a gas, since gaseous molecules can be transported much more quickly.

A fast cycle one typically goes from the atmosphere (air) to plants or animals (biosphere) and from there back to the atmosphere via the pedosphere (soil) or hydrosphere (water body). The process is estimated to take no longer than a human lifetime, although it can be much shorter. An average drop of water, for instance, remains in the atmosphere for about ten days before it falls to the ground as rain.

By contrast, a slow biogeochemical circuit usually enters the lithosphere (underground rock formations) through a process of sedimentation. The chemical in question sinks into the soil or the ocean floor, and is gradually compacted and heated over millions of years until lithified in sedimentary rock. The rock is then moved around by tectonic plates until it (and its chemical) is finally outgassed into the atmosphere by volcanic activity.

There’s also a medium-paced pathway in the carbon cycle, for instance, during which carbon is diverted into the deep ocean (below about 3,000 m (10,000 feet). Here it is moved around the globe by a serpentine network of deep-water thermohaline currents, a journey that oceanographers estimate takes between 1,000 and 1,600 years to complete.

Nitrogen Cycle: Simple Diagram
The Nitrogen Cycle. © Hattiel (CC BY-SA 3.0)

The Two Basic Types of Biogeochemical Cycle

Biogeochemical cycles divide into two main types: gaseous and sedimentary.

In a gaseous cycle, elements move through the atmosphere and the main storage areas or reservoirs are the atmosphere and the ocean. Gaseous cycles typically move faster than sedimentary ones and react more easily to changes in the biosphere. Gaseous cycles include those of nitrogen, oxygen, water vapor and carbon, as well as the trace gases ammonia, methane, several oxides of nitrogen and sulfur. 3

In a sedimentary cycle, earth-based chemicals pass from earth to water to sediment. Main storage sites are the soil and a variety of sedimentary rocks. See also: Why is Soil so Important to the Planet?

A sedimentary cycle typically has a solution phase and a rock phase. Chemical weathering liberates minerals from the Earth’s crust in the form of salts, which become dissolved in water, are used by a series of organisms in the biosphere, and eventually reach the ocean.

Here, they form sediments that are lithified and recycled back to the surface only after geological time periods, through the upwelling of ocean waters or weathering of rocks.

Because these minerals take so long to re-emerge from the lithification process, soils can become more and more depleted of them over time. Sedimentary cycles include those of iron, phosphorus, sulfur, calcium, among others.

Human Interference with Biogeochemical Cycles

Biogeochemical cycles are subject to interference (good and bad) by human activities.

On the one hand, the introduction of more nitrogen and phosphorus into the soil has proven to be highly beneficial for soil health and crop yields – albeit with reservations concerning non-trivial but essentially manageable problems like run-off and eutrophication (hyper-nutritional state in ponds, lakes and the ocean).

On the other hand, some man-made industrial processes disrupt and pollute the water cycle, corrupt the sulfur cycle through the phenomenon of acid rain, and – more seriously – destabilize the carbon cycle.

Human interference with the carbon cycle has very serious impacts on the climate system and the health of the planet. The problem stems from the excavation and extraction of large deposits of carbon-rich fossil fuels which, when burned to produce energy, emit huge quantities of so-called greenhouse gases such as carbon dioxide (CO2) and methane (CH4). These greenhouse gas emissions are the dominant climate forcing and the direct cause of global warming throughout our planet. 4

In a nutshell, over the past two and a half centuries, we humans have extracted and burned billions of tons of hydrocarbon sediment – better known as coal, oil and natural gas – that were buried deep in the Earth’s crust for millions of years. Think of it for a moment. A huge underground reservoir of carbon, set aside by nature to maintain a balance between the various spheres of Earth, suddenly emptied within the space of a few cosmic seconds by mankind.

Add to this the deforestation of the Congo rainforest, and the Amazon rainforest, and inappropriate land use – such as the replacement of ecologically important mangrove swamps with commercial shrimp farms – and that’s another 10 percent of carbon emissions. Is it any wonder we have a climate crisis when we allow this sort of thing to happen?

Further Reading

• “12. Biogeochemical cycles”. Ecological processes handbook. Palmeri, Luca; Barausse, Alberto; Jorgensen, Sven Erik (2013). Boca Raton: Taylor & Francis. ISBN 9781466558489

• “Earth system science from biogeochemical cycles to global change.” (2nd ed.) Jacobson, Michael C.; Charlson, Robert J.; Rodhe, Henning; Orians, Gordon H. (2000). San Diego, Calif.: Academic Press. ISBN 9780080530642.

• “Global biogeochemical cycles.” Butcher, Samuel S., ed. (1993). London: Academic Press. ISBN 9780080954707.

• “Earth Materials: Introduction to Mineralogy and Petrology.” (2nd Edition). by Klein, Cornelis; Philpotts, Anthony. Cambridge University Press; 2 edition (December 24, 2016) ISBN-10: 1316608859 – ISBN-13: 978-1316608852

To understand more about the timeline of biogeochemical pathways, see: History of Earth in One Year (Cosmic Calendar).

References

  1. Environmental Biology.[]
  2. Biology Libre Texts.” []
  3. BiologyDictionary.” []
  4. 2014 National Climate Assessment.[]
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