Biogeochemical Cycles: Videos & Practice Problems

Biogeochemical cycles are essential processes that recycle chemical nutrients between biotic (living) and abiotic (non-living) components of ecosystems. Key cycles include the water cycle, carbon cycle, nitrogen cycle, and phosphorus cycle. Each cycle involves various reservoirs, such as the atmosphere, oceans, and sedimentary rocks, and processes like evaporation, photosynthesis, and decomposition. Understanding these cycles is crucial for grasping how elements like carbon and nitrogen are utilized by organisms, impacting ecosystems and global climate dynamics.

Introduction to Biogeochemical Cycles

Biogeochemical cycles are essential processes that describe the movement of chemical nutrients and elements between living organisms (biotic components) and the Earth (abiotic components). The term "biogeochemical" combines "bio," meaning life, and "geo," referring to Earth, highlighting the interconnectedness of life and the environment in nutrient cycling.

These cycles play a crucial role in recycling vital elements, ensuring that they are available for various biological processes. Key biogeochemical cycles include the water cycle, carbon cycle, nitrogen cycle, and phosphorus cycle. Each of these cycles involves both biological processes, such as respiration and photosynthesis, and geological processes, like erosion and sedimentation, which facilitate the transfer of nutrients between different reservoirs.

Reservoirs are storage spaces for these chemical elements and nutrients, and they can be found in various forms, such as the atmosphere, oceans, soil, and living organisms. The movement of nutrients between these reservoirs is represented by processes indicated by arrows in diagrams, illustrating how elements are cycled through the ecosystem.

Understanding these cycles is vital for grasping how ecosystems function and maintain balance, as they ensure the continuous availability of essential nutrients necessary for life. As we delve deeper into each specific cycle, we will explore the unique reservoirs and processes involved, enhancing our comprehension of the intricate relationships within ecosystems.

Biogeochemical Cycles Example 1

In understanding geological processes, it's essential to differentiate between biological and geological activities. Geological processes are those that shape or change the Earth, while biological processes are associated with living organisms. For instance, biogeochemical cycles encompass both types of processes, but when identifying a geological process, one must focus on activities that alter the Earth's structure or composition.

In a recent example, several options were presented to identify a geological process. The options included:

• Photosynthesis - This is a biological process performed by plants to convert sunlight into energy, thus it is not geological.
• Cellular Respiration - Another biological process where organisms convert glucose and oxygen into energy, releasing carbon dioxide and water.
• Excretion - This biological process involves the removal of waste products from an organism's body.
• Decomposition - Carried out by decomposers, this process breaks down dead organic material, again a biological function.

After eliminating these biological processes, the only remaining option was Weathering. Weathering is indeed a geological process that involves the breakdown of rocks and minerals at the Earth's surface, influenced by factors such as climate and environmental conditions. This process plays a crucial role in the movement of nutrients between different reservoirs, contributing to the overall dynamics of Earth's systems.

In summary, recognizing weathering as a geological process highlights its significance in shaping the Earth and facilitating nutrient cycling, distinguishing it from the biological processes that are vital for life.

Water Cycle

The water cycle, a fundamental biogeochemical cycle, is crucial for sustaining life on Earth. It ensures the availability of water across various ecosystems, including both aquatic and terrestrial environments. Without this cycle, life as we know it would not be possible. The cycle begins with the oceans, which contain approximately 97% of Earth's water, making them the largest reservoir.

In the water cycle, net evaporation occurs over the oceans, where water vapor rises into the atmosphere. As it cools, this vapor condenses to form rain clouds. It's important to note that evaporation rates over the ocean exceed precipitation rates, which is why the evaporation arrow is larger than the precipitation arrow in diagrams. The atmosphere also acts as a reservoir, with rain clouds being transported by winds over land.

Evapotranspiration plays a significant role in this process. This term encompasses both evaporation from bodies of water and transpiration, which is the release of water vapor from plants into the atmosphere. Over land, net precipitation occurs, meaning that more water falls as rain than is lost through evapotranspiration. This precipitation provides essential water for terrestrial organisms.

Once liquid water reaches the land, it can either run off into oceans through streams and rivers or percolate down through the soil, replenishing groundwater sources like aquifers. This underground water can eventually flow back into the oceans, completing the water cycle. Understanding these processes is vital for grasping how water moves through our environment and supports life.

Biogeochemical Cycles Example 2

In understanding the distribution of Earth's water, it's essential to recognize that approximately 97% of it is salt water found in oceans, while only about 3% is fresh water. This fresh water is primarily stored in glaciers and snow (about 70%), with the remaining 30% comprising groundwater, including aquifers and soil moisture. Notably, less than 1% of fresh water exists in lakes and rivers.

When considering processes that move water, evaporation from oceans is significant due to the vast volume of salt water they contain, making it an unlikely candidate for the smallest volume of water moved. Similarly, precipitation over oceans also involves large quantities of water, leading to its elimination as a potential answer.

This leaves us with two options: evaporation from freshwater rivers and lakes, and evapotranspiration over land. Evapotranspiration is a comprehensive term that includes both evaporation from water bodies and transpiration from plants, thus moving a larger volume of water than evaporation from freshwater sources alone. Given that evaporation from freshwater rivers and lakes represents a very small fraction of the total water volume, it becomes clear that this process moves the smallest volume of water among the options provided.

Therefore, the correct answer is that evaporation from freshwater rivers and lakes moves the smallest volume of water compared to the other processes discussed.

Imagine you’re tracking a water molecule frozen in snow on land. Which of the following is a possible path it could take to eventually end up as part of the ocean?

• A. Melting into a stream, ingested by an animal, percolate into the ocean.
• B. Melting into a stream, evaporating into the atmosphere, then runoff into the ocean.
• C. Melting into soil, taken up by a plant, evapotranspiration into the ocean.
• D. Melting into soil, percolating into an aquifer, underground runoff into the ocean.

Answer: (D)

Carbon Cycle

The carbon cycle is a fundamental biogeochemical cycle that plays a crucial role in sustaining life on Earth. Carbon is an essential element, forming the structural backbone of vital biological molecules such as proteins, nucleic acids, lipids, and carbohydrates. This cycle ensures that carbon is recycled and available in various forms to all living organisms across different ecosystems, including both aquatic and terrestrial environments.

One of the primary reservoirs of carbon is the atmosphere, where it exists as carbon dioxide (CO₂). This atmospheric carbon is vital for primary producers, such as phytoplankton in aquatic ecosystems and trees in terrestrial ecosystems, which utilize CO₂ during photosynthesis to create organic matter. As organisms consume these primary producers, carbon moves up the food chain, allowing higher trophic levels to obtain carbon in their biomass.

Cellular respiration by these organisms releases CO₂ back into the atmosphere, contributing to the cyclical nature of the carbon cycle. When organisms die, decomposition—primarily carried out by microbes—further releases CO₂ into the atmosphere. Under specific conditions, organic matter can transform into fossil fuels like oil and coal, which humans exploit for energy. The combustion of these fossil fuels releases stored carbon back into the atmosphere, significantly increasing CO₂ levels and contributing to global warming, as CO₂ is a greenhouse gas.

Additionally, volcanic activity can also release substantial amounts of CO₂ into the atmosphere over time. Beyond the atmosphere, the oceans serve as another significant carbon reservoir. When CO₂ dissolves in ocean water, it forms carbonic acid, which can alter the water's pH and potentially harm aquatic life. Given that approximately 97% of Earth's water is found in oceans, they play a critical role in the carbon cycle, but they cannot absorb all atmospheric CO₂ without consequences.

Understanding the carbon cycle is essential for recognizing the interconnectedness of life and the impact of human activities on global climate patterns. This knowledge will be foundational as we explore further concepts related to ecosystems and environmental science.

Biogeochemical Cycles Example 3

In ecosystems, the availability of essential nutrients like nitrogen, phosphorus, and water can be significantly affected by local conditions such as heavy rainfall or drought. However, carbon behaves differently due to its unique source. Plants do not produce their own carbon; instead, they utilize carbon dioxide (CO₂) from the atmosphere through the process of photosynthesis. This process allows them to fix atmospheric carbon into their biological systems, making carbon a constant and readily available resource.

When considering the role of decomposers, it's important to note that they do not generate their own carbon either. Instead, they release carbon dioxide as a byproduct when breaking down organic materials. This further emphasizes that carbon is not produced but rather recycled within the ecosystem.

While some might think that plants can absorb carbon from the soil, this is not accurate. Plants primarily acquire carbon directly from the air, which distinguishes carbon from other nutrients like nitrogen and phosphorus. Nitrogen exists in the atmosphere as nitrogen gas (N₂), an unusable form for plants, while phosphorus lacks an atmospheric component altogether. Water, although present in the atmosphere, must be in a liquid state for plants to absorb it from the soil, rather than directly from the air.

Thus, the unique ability of plants to utilize carbon dioxide gas from the atmosphere directly is what sets carbon apart from other essential nutrients, making it a constant source for plant growth and development.

Which of the following statements is false?

• A. Biomass is one of the largest carbon reservoirs in the biosphere.
• B. Burning plants increases the amount of atmospheric CO₂.
• C. The carbon cycle has maintained a constant atmospheric CO₂ concentration over the last million years.
• D. Carbon can be stored in soil, rocks, and ocean water.

Answer: (C)

Nitrogen Cycle

The nitrogen cycle is a crucial biogeochemical cycle that ensures the availability of nitrogen in various forms for all living organisms. Nitrogen is a fundamental element found in amino acids, which compose proteins, and in the nitrogenous bases of nucleotides that form DNA and RNA. The cycle is vital for life, as it recycles nitrogen across different ecosystems, including terrestrial and aquatic environments.

In the atmosphere, nitrogen gas (N₂) constitutes about 78% of the air, but this form is biologically unusable for most organisms. The cycle begins with nitrogen fixation, where nitrogen-fixing bacteria, found in soil or in the root nodules of certain plants, convert N₂ into usable forms such as ammonium (NH₄⁺) and nitrates (NO₃^-). These forms can be readily absorbed by plants, allowing nitrogen to enter the food chain.

Following nitrogen fixation, the process of nitrification occurs, where nitrifying bacteria convert ammonium (NH₄⁺) into nitrates (NO₃^-), which are also usable by plants. As organisms consume these plants, nitrogen moves up the food chain. Eventually, when organisms die, decomposition occurs through ammonification, where organic nitrogen is broken down into ammonium (NH₄⁺) again, allowing the cycle to continue.

The cycle can conclude with denitrification, where denitrifying bacteria convert nitrates back into nitrogen gas (N₂), releasing it into the atmosphere and completing the cycle. This process is mirrored in aquatic environments, where cyanobacteria primarily carry out nitrogen fixation.

Human activities also impact the nitrogen cycle significantly. Industrial nitrogen fixation produces fertilizers that add nitrates to the soil, influencing both terrestrial and aquatic nitrogen cycling. However, the burning of fossil fuels releases reactive nitrogen gases, contributing to greenhouse gas effects, acid rain, and ozone layer depletion. Additionally, abiotic processes such as lightning and volcanic activity can also fix nitrogen, further contributing to the cycle.

Understanding the nitrogen cycle is essential for recognizing the interconnectedness of ecosystems and the impact of human activities on environmental health. The cycle's five key steps—nitrogen fixation, nitrification, assimilation, ammonification, and denitrification—highlight the dynamic processes that sustain life by recycling nitrogen in various forms.

You are tasked with restoring a degraded ecosystem that has experienced a shortage of nitrogen. Which of the following steps could you take?

• A. Introduce nitrogen-fixing bacteria, which convert nitrogen gas to nitrogen-containing compounds.
• B. Introduce denitrifying bacteria, which convert nitrate to nitrogen gas.
• C. Introduce nitrifying bacteria, which convert nitrate to ammonium.
• D. All of the above.

Answer: (A)

Which of the following answers correctly completes the diagram below?

• A. 1 – nitrification, 2 – ammonification, 3 – nitrogen fixation. 
• B. 1 – denitrification, 2 – nitrification, 3 – nitrogen fixation.
• C. 1 – nitrogen fixation, 2 – nitrification, 3 – denitrification.
• D. 1 – nitrogen fixation, 2 – ammonification, 3 – denitrification.

Answer: (D)

Phosphorus Cycle

The phosphorus cycle is a crucial biogeochemical cycle that ensures the availability of phosphorus, an essential element for all life forms. Phosphorus is a key component of nucleic acids like DNA and RNA, as well as ATP, the energy currency of cells. Unlike other cycles such as the water, carbon, and nitrogen cycles, the phosphorus cycle does not involve the atmosphere as a reservoir. Instead, it primarily occurs in local areas, making it distinct in its cycling patterns.

The most significant reservoir of phosphorus is found in phosphate-containing sedimentary rocks. Through the process of weathering, these rocks release phosphates into the soil, primarily in the form of ${\mathrm{PO}}^4-$. Primary producers, such as plants, assimilate these phosphates, which then move up the food chain through consumption. When organisms die, decomposition by fungi and bacteria returns phosphorus back to the soil, completing the terrestrial cycling of phosphorus.

In aquatic ecosystems, phosphates can enter through weathering of rocks or leaching from the soil, resulting in dissolved phosphates (${\mathrm{PO}}^4-$) in water bodies. Similar to terrestrial ecosystems, these dissolved phosphates are assimilated by primary producers like phytoplankton, consumed by higher trophic levels, and eventually returned to the water through decomposition. The cycle is further completed by sedimentation, where phosphates can reform into sedimentary rocks, which may later be re-exposed to the atmosphere through geological uplift.

Understanding the phosphorus cycle is vital for recognizing how phosphorus is recycled in ecosystems, ensuring its availability across different trophic levels and environments. This cycle highlights the interconnectedness of terrestrial and aquatic systems and the importance of phosphorus in sustaining life.

Biogeochemical Cycles Example 4

The phosphorus cycle is a crucial biogeochemical cycle that illustrates the movement of phosphorus through various environmental compartments. The largest reservoir of phosphorus is found in phosphorus-containing rocks, which serve as the starting point for the cycle. This is represented as the first step in the cycle.

When these rocks undergo weathering, they release phosphates into the environment. This process is essential as it allows phosphorus to enter the soil and freshwater systems, making it available for biological uptake. The second step in the cycle is weathering, which facilitates the release of phosphates.

Once in the soil and freshwater, phosphates can be assimilated by organisms, contributing to biomass. This incorporation into living organisms is a vital aspect of the cycle, as it supports the growth and development of various life forms. The third step involves decomposition, where the breakdown of organic matter returns phosphates back to the soil and freshwater, ensuring a continuous supply of this essential nutrient.

From freshwater systems, phosphates can eventually run off into oceans, marking the fourth step in the cycle. In the ocean, phosphates can settle and contribute to sediment formation, which may eventually lead to the reformation of phosphorus-containing rocks, thus completing the cycle. This cyclical process highlights the interconnectedness of geological and biological systems in maintaining phosphorus availability in ecosystems.

Which of the following statements is true?

• A. Unlike nitrogen, carbon, and water, the atmosphere is not a significant reservoir of phosphorus.
• B. Sedimentary rock is one of the largest reservoirs of phosphorus.
• C. Phosphorus is essential to ecosystems as it’s a major component of nucleic acids & ATP.
• D. All of the above are true.

Answer: (D)

Here’s what students ask on this topic:

What are biogeochemical cycles and why are they important?

Biogeochemical cycles are processes that recycle chemical nutrients and elements between the biotic (living) and abiotic (non-living) components of ecosystems. These cycles include the water cycle, carbon cycle, nitrogen cycle, and phosphorus cycle. Each cycle involves various reservoirs, such as the atmosphere, oceans, and sedimentary rocks, and processes like evaporation, photosynthesis, and decomposition. Understanding these cycles is crucial because they ensure the availability of essential elements like carbon, nitrogen, and phosphorus, which are vital for life. These cycles influence ecosystem health and stability by regulating nutrient flow and maintaining balance within ecosystems. Without these cycles, life as we know it would not be sustainable, as organisms rely on these processes to obtain the nutrients necessary for growth, reproduction, and survival.

How does the water cycle function and why is it essential for life on Earth?

The water cycle is a biogeochemical cycle that ensures the continuous movement of water within the Earth and its atmosphere. It involves processes such as evaporation, condensation, precipitation, and runoff. Water evaporates from oceans, lakes, and rivers, rises into the atmosphere, and condenses to form clouds. It then precipitates as rain or snow, replenishing terrestrial and aquatic ecosystems. The cycle is essential for life because it distributes water, a critical resource for all living organisms, across different ecosystems. It supports plant growth, regulates climate, and maintains habitats. Without the water cycle, ecosystems would lack the necessary water supply, leading to the collapse of life-supporting systems on Earth.

What role does the carbon cycle play in ecosystems?

The carbon cycle is a crucial biogeochemical cycle that regulates the flow of carbon among the Earth's atmosphere, oceans, and living organisms. Carbon is an essential element for life, forming the backbone of organic molecules like proteins, nucleic acids, and carbohydrates. The cycle involves processes such as photosynthesis, where plants and phytoplankton absorb atmospheric carbon dioxide (CO₂) to produce organic matter. This carbon is transferred through the food chain via consumption and is eventually released back into the atmosphere through respiration and decomposition. The carbon cycle ensures the availability of carbon for all trophic levels, supporting life in both terrestrial and aquatic ecosystems. It also plays a significant role in regulating Earth's climate by controlling CO₂ levels, a greenhouse gas.

How does the nitrogen cycle contribute to ecosystem health?

The nitrogen cycle is a vital biogeochemical cycle that ensures the availability of nitrogen, an essential element for life, in ecosystems. Nitrogen is a key component of amino acids, proteins, and nucleic acids. The cycle involves processes like nitrogen fixation, where atmospheric nitrogen (N₂) is converted into biologically usable forms such as ammonium (NH₄⁺) and nitrate (NO₃^-) by bacteria. These forms are assimilated by plants and transferred through the food chain. Decomposition and ammonification return nitrogen to the soil, while denitrification releases it back into the atmosphere. The nitrogen cycle maintains ecosystem health by ensuring a continuous supply of nitrogen for plant growth and supporting biodiversity. It also influences soil fertility and productivity, impacting agricultural practices and food security.

Why is the phosphorus cycle different from other biogeochemical cycles?

The phosphorus cycle is unique among biogeochemical cycles because it does not involve a gaseous phase or atmospheric component. Instead, phosphorus primarily cycles through rocks, soil, water, and living organisms. The main reservoir of phosphorus is phosphate-containing sedimentary rocks. Weathering releases phosphates into the soil, where they are absorbed by plants and move through the food chain. Decomposition returns phosphorus to the soil, and it can also enter aquatic systems through runoff. Unlike other cycles, phosphorus cycling is more localized, as it lacks atmospheric transport. This local cycling is crucial for providing phosphorus, a key component of DNA, RNA, and ATP, to organisms, supporting energy transfer and genetic functions essential for life.