A biogeochemical cycle, or a cycle of matter, is the movement and change of chemical elements and compounds between living things, the atmosphere, and Earth's crust. Major cycles include the carbon cycle, the nitrogen cycle, and the water cycle. In each cycle, a chemical element or molecule is changed and moved by living organisms and through different parts of Earth, such as the atmosphere, soil, and oceans. These cycles show how a chemical substance moves through the living parts of Earth, like plants and animals, and the non-living parts, such as air, rocks, and water.

For example, in the carbon cycle, plants take in carbon dioxide from the air through photosynthesis, changing it into organic compounds that help plants grow and provide energy. Carbon is later released back into the air through respiration and when plants and animals decay. Carbon is also stored in fossil fuels, and human activities like burning fossil fuels release it into the air. In the nitrogen cycle, plants change nitrogen gas from the air into forms like ammonia and nitrates through a process called nitrogen fixation. These forms are used by other living things, and nitrogen returns to the air through processes like denitrification. In the water cycle, water evaporates from Earth's surface and forms clouds. It then falls back as rain or snow, seeping into the ground to become groundwater or flowing into rivers and lakes. Water underground can eventually flow into the ocean, carrying nutrients with it.

Other elements, such as oxygen, hydrogen, phosphorus, calcium, iron, sulfur, mercury, and selenium, also have biogeochemical cycles. Molecules like water and silica are also part of these cycles. There are also larger cycles, like the rock cycle, and human-made cycles for substances like polychlorinated biphenyls (PCBs). Some cycles have storage areas where substances can stay for long periods.

Biogeochemical cycles involve the interaction of biological, geological, and chemical processes. Microorganisms, such as bacteria and fungi, play a key role in these cycles by helping to move nutrients and chemicals through ecosystems. These tiny organisms carry out processes that are essential for life on Earth. Without them, many natural processes would not happen, which could harm ecosystems and the planet. Changes in these cycles can affect human health. These cycles are connected and help regulate Earth's climate, support the growth of plants, phytoplankton, and other living things, and keep ecosystems healthy. Human activities, such as burning fossil fuels and using large amounts of fertilizer, can disrupt these cycles, leading to problems like climate change, pollution, and harm to the environment.

Overview

Energy moves through ecosystems in one direction, starting as sunlight or chemical compounds for certain organisms and ending as heat after passing through different levels of the food chain. However, the materials that make up living things are not lost. The six most common elements in living things—carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur—can change forms and remain in the air, soil, water, or underground for long periods. Natural processes like weathering, erosion, water movement, and the movement of Earth's plates help recycle these materials. Because both geology and chemistry are important in studying this process, the recycling of non-living materials between living things and their environment is called a biogeochemical cycle.

These six elements are used by living things in many ways. Hydrogen and oxygen are part of water and organic molecules, which are essential for life. Carbon is found in all organic molecules, while nitrogen is important for DNA and proteins. Phosphorus is used to create DNA and the building blocks of cell membranes. Sulfur helps shape the structure of proteins. The movement of these elements is connected. For example, water movement helps carry sulfur and phosphorus into rivers, which then flow into oceans. Minerals move between living and non-living parts of ecosystems and between different organisms.

Ecosystems have many biogeochemical cycles, such as the water cycle, carbon cycle, and nitrogen cycle. All chemical elements in living things are part of these cycles. These elements also move through non-living parts of ecosystems, like water, land, and air. The living parts of Earth, including all plants, animals, and microorganisms, are called the biosphere. Nutrients like carbon, nitrogen, oxygen, phosphorus, and sulfur are part of a closed system, meaning they are reused rather than lost or constantly added like in an open system.

The biosphere is connected through the movement of chemical elements and compounds in biogeochemical cycles. In many of these cycles, living organisms play an important role. Volcanoes release materials from Earth's interior. The atmosphere exchanges some chemicals quickly with living things and oceans. Exchanges between rocks, soil, and oceans happen more slowly.

Energy movement in ecosystems is an open system because the Sun constantly provides energy as light, which is eventually lost as heat through the food chain. Carbon is used to make carbohydrates, fats, and proteins, which are major sources of food energy. These compounds are broken down to release carbon dioxide, which plants use to create organic compounds. This process uses sunlight as energy.

Sunlight is needed to combine carbon with hydrogen and oxygen to create energy, but ecosystems in the deep sea, where no sunlight reaches, use sulfur instead. For example, giant tube worms near hydrothermal vents use hydrogen sulfide for energy. In the sulfur cycle, sulfur is reused as an energy source. Energy is released when sulfur compounds are changed through chemical reactions, like turning sulfur into sulfite and then sulfate.

• Examples of major biogeochemical processes
• The oceanic whale pump, which shows how whales move nutrients through ocean water
• Changes in the global carbon cycle caused by human activities are a concern for scientists.

Although Earth receives energy from the Sun, its chemical makeup stays mostly the same, with new materials only added rarely by meteorites. Since chemicals are not constantly added like energy, all processes that use them must reuse them. These cycles include both the living biosphere and the non-living parts of Earth, such as the land, air, and water.

Biogeochemical cycles differ from geochemical cycles. Geochemical cycles only involve Earth's crust and layers below it, even though some processes overlap with biogeochemical cycles.

Compartments

Biogeochemical cycles move substances through Earth's living and non-living parts. The living part includes all living things, called the biosphere. The non-living parts are the atmosphere (air), lithosphere (land), and hydrosphere (water). Microorganisms play a major role in these cycles.

The global ocean covers more than 70% of Earth's surface and has many different areas. Productive marine regions and coastal ecosystems make up a small part of the ocean's surface, but they greatly influence Earth's biogeochemical cycles. Microbial communities, which make up 90% of the ocean's living material, drive these cycles. Recent studies have focused on the movement of carbon and key nutrients like nitrogen, phosphorus, and silicate. Other elements, such as sulfur and trace elements, have been studied less due to challenges in research.

Human activities are causing major changes in these marine areas. For example, cultural eutrophication happens when runoff from farms adds too much nitrogen and phosphorus to coastal ecosystems. This increases productivity, leading to algal blooms, low oxygen levels in water, and more greenhouse gas emissions. These changes affect both local and global carbon and nitrogen cycles. Runoff of organic matter from land to coasts is one threat, but climate change also impacts marine systems. Melting glaciers and permafrost alter ocean layers, while changes in oxygen levels in different environments are rapidly changing microbial communities.

Global changes are affecting important processes, such as the growth of plants and algae, the fixing of carbon dioxide and nitrogen gas, the breakdown of organic matter, and the sinking and burial of carbon. Oceans are also becoming more acidic, with a pH drop of about 0.1 since pre-industrial times. This affects chemical balances in the ocean and harms plankton, especially those that build shells.

Changes in ocean conditions have also altered the production of gases like nitrous oxide and methane, which contribute to global warming. These gases are released in areas with very low oxygen, called oxygen minimum zones. Other harmful substances, such as hydrogen sulfide, threaten marine life like fish and shellfish.

As global changes continue, scientists are learning more about the complexity of marine ecosystems and the vital role of microbes in keeping these systems healthy.

Reservoirs

Chemicals can sometimes stay in one place for a long time. This place is called a reservoir. An example of a reservoir is coal deposits, which hold carbon for a long period. When chemicals stay in one place for only a short time, they are in an exchange pool. Examples of exchange pools include plants and animals.

Plants and animals use carbon to make carbohydrates, fats, and proteins. These materials help build their bodies or provide energy. After using carbon, plants and animals release it back into the air or surrounding environment. Reservoirs are usually non-living factors, while exchange pools are living factors. Carbon stays in plants and animals for a short time compared to how long it stays in coal deposits. The time a chemical stays in one place is called its residence time, turnover time, renewal time, or exit age.

Box models

Box models are commonly used to study systems that involve Earth's processes and living things. These models simplify complex systems by dividing them into boxes, which represent storage areas for chemical materials. These boxes are connected by material fluxes, or movements of materials. Simple box models have few boxes, and the properties of these boxes, such as volume, remain the same over time. The boxes are assumed to mix evenly. These models help scientists create formulas that describe how chemical materials change over time and reach a steady state, where their amounts remain constant.

The diagram on the right shows a basic one-box model. The box represents a storage area for a specific material, defined by chemical, physical, or biological properties. The source is the movement of material into the box, and the sink is the movement of material out of the box. The budget refers to the balance between the material entering and leaving the box. A box is in a steady state when the amount of material entering equals the amount leaving, meaning there is no change over time.

The residence or turnover time is the average time a material stays in a box. In a steady state, this time is the same as the time needed to fill or empty the box. If τ represents the turnover time, then τ equals the total material in the box divided by the amount leaving the box each unit of time. The equation used to describe how the material in a box changes over time is also applied in these models.

When two or more boxes are connected, materials can move between them in predictable patterns. More complex models with many boxes are usually solved using numerical methods.

Global biogeochemical box models often measure the movement of materials between different parts of Earth's systems. The diagram on the left shows a simplified model of ocean carbon flows. It includes three interconnected boxes: the euphotic zone, the ocean interior (dark ocean), and ocean sediments. In the euphotic zone, phytoplankton produce about 50 petagrams of carbon each year. Of this, 10 petagrams move to the ocean interior, while 40 petagrams are released back into the water. Organic carbon breaks down as particles fall through the ocean interior. Only 2 petagrams reach the seafloor, while 8 petagrams are released in the dark ocean. In sediments, the time available for carbon to break down increases greatly, so 90% of the carbon is broken down, leaving only 0.2 petagrams of carbon buried in sediments each year.

The diagram on the right shows a more complex model with many interacting boxes. The boxes represent carbon stocks, measured in petagrams of carbon. Carbon moves between the atmosphere and its main sinks, the land and the ocean. The black numbers and arrows show the estimated amounts of carbon stored in each box and the movement of carbon between them in 1750, before the Industrial Revolution. The red arrows and numbers show changes in carbon movement caused by human activities between 2000 and 2009. The red numbers in the boxes show the total changes in carbon stored in each area since the start of the Industrial Period in 1750.

Fast and slow cycles

There are two types of biogeochemical cycles: fast and slow. Fast cycles occur in the biosphere, which includes living things, while slow cycles happen in the lithosphere, which includes rocks and Earth's crust. Fast cycles move substances between the atmosphere and biosphere quickly, often within years. Slow cycles take much longer, sometimes millions of years, to move substances through Earth's crust, soil, oceans, and atmosphere.

The fast carbon cycle is shown in the diagram on the right. This cycle involves short-term processes that move carbon between the environment and living organisms. It includes the movement of carbon between the atmosphere, land and ocean ecosystems, soil, and ocean floor sediments. The fast cycle includes yearly processes like photosynthesis and processes that happen over decades, such as plant growth and decay. How the fast carbon cycle responds to human activities will affect short-term changes in Earth's climate.

The slow carbon cycle is shown in the other diagram. It involves long-term processes connected to the rock cycle. The exchange of carbon between the ocean and atmosphere can take centuries, and the breakdown of rocks can take millions of years. Carbon from the ocean settles on the ocean floor, where it can form sedimentary rock and be pushed deep into Earth's mantle. Mountain-building processes bring this carbon back to Earth's surface. Once there, rocks break down, and carbon returns to the atmosphere through volcanic activity or to the ocean through rivers. Some carbon also returns to the ocean through the release of calcium ions from underwater vents. Each year, between 10 and 100 million tonnes of carbon move through this slow cycle. Volcanoes release carbon directly into the atmosphere as carbon dioxide. However, this is less than 1% of the carbon dioxide added to the atmosphere by burning fossil fuels.

Deep cycles

The terrestrial subsurface is the largest storage of carbon on Earth, holding 14 to 135 petagrams of carbon and 2 to 19% of all living matter. Microorganisms in this environment drive changes in both organic and inorganic compounds, which helps control biogeochemical cycles. Most of what we know about the microbial life in the subsurface comes from studying 16S ribosomal RNA (rRNA) gene sequences. Recent estimates show that less than 8% of 16S rRNA sequences in public databases are from subsurface organisms, and only a small number of these have complete genetic information or have been grown in laboratories. This means there is very little reliable information about how microorganisms in the subsurface use energy or interact with one another. Scientists know little about how organisms in subsurface ecosystems are connected through their metabolic processes. Some studies using laboratory-grown communities and small-scale genetic analyses of natural groups suggest that organisms may share chemical products from their reactions. However, no complex subsurface environments have been fully studied to understand the complete networks of these interactions. This limits the ability of models to accurately describe key parts of carbon and other nutrient cycles. New methods, such as genome-resolved metagenomics, which can create detailed genetic maps of organisms without needing to grow them in labs, may help scientists better understand these processes.

Some examples

Some of the more well-known biogeochemical cycles are shown below:

• Carbon cycle
• Oxygen cycle
• Nitrogen cycle
• Nutrient cycle
• Phosphorus cycle
• Sulfur cycle
• Rock cycle
• Water cycle

Many biogeochemical cycles are being studied for the first time. Climate change and human activities are changing the speed, intensity, and balance of these less understood cycles, which include:

• the mercury cycle, and
• the human-caused cycle of PCBs.

• Chloroplasts help plant cells and other eukaryotic organisms perform photosynthesis.
• Kerogen cycle
• Coal stores carbon.

Biogeochemical cycles always involve active balance: a balance in the movement of elements between different parts of Earth. However, overall balance may involve parts of Earth that are spread across the entire globe.

Because biogeochemical cycles describe the movement of substances around the entire planet, studying them requires knowledge from many different fields. For example, the carbon cycle may involve research in ecology and atmospheric sciences. Biochemical processes may also relate to the fields of geology and pedology.