Nutrient Cycling: Carbon Cycle and Phosphorus Cycle, Practice Problems and FAQs

Agriculture forms an important part of civilisation. You know that farmers give fertilisers to the crop. But why do fertilisers need to be given to the plants?

Farmers provide fertilisers to the crop to fulfil their nutrient requirements. These fertilisers contain essential elements, such as nitrogen, phosphorus, carbon, sulphur, etc. Crops are given fertilisers when soil is not able to fulfil their nutrient requirements. Do you have any idea about the major elements required for the plants? Yes, you are correct. Plants require sodium (Na), potassium (K), nitrogen (N), carbon (C ) etc. 

                      Fig: Farmers provide fertiliser to the crop

Phosphorus (P) and carbon (C) are two essential elements that are required by every living organism. Therefore, they should always be available to the organisms. In order to maintain their concentration, nature regulates their amount in the atmosphere and lithosphere. This is known as the nutrient cycle. Let’s discuss more about the phosphorus cycle and carbon cycle in detail in this article.

Table of contents:

• Nutrients
• Nutrient cycling
• Carbon cycle
• Phosphorus cycle
• Carbon cycle vs Phosphorus cycle
• Practice Problems
• FAQs

Nutrients

Nutrients are described as the organic substances that are required in the optimum amount to regulate the body functions. To develop, reproduce, and control numerous physiological functions, organisms require a steady supply of nutrients. To maintain life, a variety of nutrients are required. The most essential nutrients are carbon, hydrogen, oxygen, phosphorus, nitrogen and sulphur. Apart from these, there are other nutrients needed like calcium, potassium, sodium, etc. 

                                             Fig: Nutrients

Biogenetic nutrients

These are essential elements required by the organisms for their growth, development and metabolism. These elements are provided by the Earth and return to Earth after the death and decay of organisms. Examples include carbon, nitrogen, phosphorus etc. 

Standing state

Standing state is calculated as the amount of nutrients present in the soil at any given point of time. The nutrients involved include carbon, nitrogen, calcium, phosphorus etc. The standing state of nutrients varies for different types of ecosystems. It also varies on a seasonal basis.

                             Fig: Standing state different in different types of ecosystems

Nutrient cycling

Nutrient cycle is also termed as biogeochemical cycle. It is the movement of nutrients through various components like living and nonliving parts of an ecosystem. The nutrient cycle is of two types based on their major reservoir as follows: 

• Gaseous cycle
• Sedimentary cycle 

Gaseous cycle

The reservoir of the gaseous cycle is present in the atmosphere. Examples include carbon, oxygen and nitrogen cycles. 

Sedimentary cycle

On the other hand, the reservoir of the sedimentary cycle is present in the Earth’s crust. Examples include phosphorus and sulphur cycles.

                                                           Fig: Types of nutrient cycles

Reservoir

The reservoir's purpose is to compensate for the deficit caused by an imbalance in the rate of nutrient influx and efflux.

                                       Fig: Reservoir

States of biogenetic nutrients

The biogenetic nutrients exist in two forms in the atmosphere as follows:

Reservoir pool

It is the pool of nutrients from which the nutrients are normally transferred into the cycling pool. 

Cycling pool

It is the pool of nutrients which is repeatedly exchanged between the biotic and abiotic components of the biosphere. 

Carbon cycle

The essential element found in all living beings is carbon. It constitutes around 49% of the dry weight of organisms. The carbon cycle occurs through the ocean, atmosphere, living and dead organisms. 

Importance of carbon

Carbon is very essential for us because it is the backbone of all biomolecules, such as amino acids, nucleotides, fatty acids and sugars. 

                                Fig: Carbon is a building block of biomolecules

Sources of carbon

Carbon is present in the abiotic environment in different forms as follows:

Carbon dioxide

In the atmosphere, carbon is found in the form of carbon dioxide. Out of the total global carbon, only 1% of it is found in the atmosphere. 

Carbonic acid and bicarbonates

71% of carbon is present in dissolved form in the ocean. It exists in the form of carbonic acid and bicarbonates. 

Fossil fuels

Fossil fuels like coal, petroleum etc., also possess carbon.

In rocks

Carbon is present in the form of carbonates and graphites in the rock. 

Cycling of carbon

Carbon dioxide is captured by the plants. Plants capture carbon dioxide by the process of photosynthesis and convert it into energy storage form and that is glucose. According to estimates 4 × 1013 kg of carbon is fixed annually in the biosphere through photosynthesis.

                              Fig: Carbon dioxide is fixed through photosynthesis

Animals feed on the plants and get the required carbon. For example, primary consumers, such as deer and goat feed on the plant and fulfil their carbon requirements. Therefore, primary consumers are also known as herbivores. Then, secondary consumers feed on primary consumers and tertiary consumers feed on secondary consumers to get the carbon. This will create a terrestrial food chain and food web, because the animals of different levels are interconnected to each other for their nutrient requirements.

When the animals and plants die, decomposers feed on them and release carbon. Decomposers primarily act on decomposing and decaying matter and release carbon dioxide to the atmosphere. This forms a detritus food chain. Carbon dioxide is also released in the atmosphere through respiratory activities of producers and consumers. 

                       Fig: Carbon is released in the form of carbon dioxide

Moreover, carbon dioxide is also released into the atmosphere through other activities, such as burning of wood, forest fire and combustion of organic matter, fossil fuels, and volcanic activity.

                                                    Fig: Release of carbon dioxide

Carbon cycle also occurs in the ocean. Here, in oceans, the underwater plants, seaweeds and phytoplanktons capture the carbon dioxide. Now, these producers are eaten by primary consumers, such as smaller fishes which are then eaten by secondary consumers, such as larger fishes. In this way, carbon is transferred from one organism to another when one feeds on the other. Carbon dioxide is also released into the atmosphere when these organisms respire.

                          Fig: Release of carbon dioxide in oceans

When aquatic organisms die, decomposers present in the ocean act on these and decompose them. This will release carbon in the form of carbon dioxide. Calcareous sediments are formed when the shells and skeletons of these animals are deposited in the deep ocean. As a result, some of the fixed carbon is lost to sediments and taken out of circulation. 

                    Fig: Decomposition releases carbon dioxide in ocean

Therefore, fossil fuel also represents a reservoir of carbon because they release carbon dioxide. Oil and natural gas (limestone and dolomite) are produced from sea sediments that have been accumulated for thousands of years. After thousands of years, dying trees on land are transformed to coal. Burning of these fossil fuels releases carbon dioxide back into the atmosphere.

                                                         Fig: Carbon cycle

The carbon cycle keeps the overall amount of carbon in the atmosphere stable and balanced. However, human activities have significant effects on the carbon cycle. There are some human activities that adversely affect the release of carbon dioxide into the atmosphere and as a result, global temperature increases. These activities involve rapid deforestation and massive burning of fossil fuels for energy and transport.

                    Fig: Increase in global temperature

Phosphorus cycle

Phosphorus is a biologically essential element. 

Importance of phosphorus

It serves as a major constituent of various biological membranes, ATP (energy currency) molecules and nucleic acids. They are also needed in significant amounts to create bones and teeth, as well as shells in some animals. 

                                                       Fig: Uses of phosphorus

Sources of phosphorus

Phosphorus cycle is a sedimentary cycle that means rocks are the major reservoir of phosphorus. Phosphorus is present mainly in the rocks in the form of phosphates. When rocks weather, minute amounts of phosphate are released into the soil solution.

 Fig: Release of phosphate ions during weathering of rocks

Cycling of phosphorus

The phosphate is released into the soil solution and then leached into the surface water. The plants absorb the dissolved phosphorus present in the soil.

                         Fig: Absorption of dissolved phosphorus by plants

The herbivores or primary consumers feed on plants and get phosphorus as per their requirements.

                   Fig: Primary consumers obtain phosphorus from plants

After the animals die, the phosphate-solubilizing bacteria in the soil breakdown phosphorus with other waste items. Bacteria that break down phosphorus release it back into the soil.

                        Fig: Release of phosphorus back into the soil

The phosphorus then accumulates in the rocks and soil. Some phosphorus washed into the sea along with rain water and floods. Sea weeds take this phosphorus. Now it enters into the food chain and food webs of aquatic environments. 

                         Fig: Phosphorus enters into aquatic environments

It becomes part of the fishes and seabirds. The seabirds deposit their faeces called guano which is rich in phosphorus on the land. 

                                              Fig: Phosphorus cycle

Carbon cycle vs Phosphorus cycle

The major differences between the carbon cycle and phosphorus cycle are as follows:

Practice Problems

Q 1. From the given cycles, determine the ones that represent the sedimentary cycle.
A - Phosphorus cycle, B - Sulphur cycle, C - Nitrogen cycle, D - Carbon cycle

a. A, B and C
b. B and C
c. C and D
d. A and B

Answer: Nutrient cycle is also termed as biogeochemical cycle. It is the movement of nutrients through various components like living and nonliving parts of an ecosystem. The nutrient cycle is of two types on the basis of their major reservoir as follows:

• The reservoir of the gaseous cycle is present in the atmosphere. Examples include carbon, oxygen and nitrogen cycles.
• The reservoir for the sedimentary cycle is present mainly on the lithosphere or Earth’s crust. Examples include phosphorus and sulphur cycles. Hence, the correct option is d.

Q 2. Identify the image and select the correct statement.

a. Phosphorus cycle, which is an example of gaseous cycle
b. Phosphorus cycle, which is considered as an example of sedimentary cycle
c. Carbon cycle, which is considered as an example of gaseous cycle
d. Carbon cycle, which is an example of sedimentary cycle

Answer: The given image represents the carbon cycle which is an example of a gaseous cycle. The reservoir of the carbon cycle is present in the atmosphere. The carbon cycle occurs through the ocean, atmosphere, living and dead organisms. Hence, the correct option is c.

Q 3. From the given cycles, how many cycles are sedimentary and how many are gaseous?
Nitrogen cycle, Carbon cycle, Sulphur cycle and Phosphorus cycle

a. 2, 2 
b. 1, 3
c. 3, 1 
d. 4, 0

Answer: From the given cycles, two cycles are sedimentary and two are gaseous. Nutrient cycle is the movement of nutrients through various components such as living and nonliving parts of an ecosystem. The nutrient cycle is of two types on the basis of their reservoir as follows:

• The reservoir of the gaseous cycle is present in the atmosphere. Examples include carbon, oxygen and nitrogen cycles.
• The reservoir of the sedimentary cycle is present on the Earth’s crust. Examples include phosphorus and sulphur cycles. Hence the correct option is a. 

Q 4. Identify the correct statement regarding standing state?

a. It is the amount of nutrients like nitrogen, carbon, and phosphorus present in the soil at a given point of time
b. It is the amount of phosphorus present in the atmosphere at a given time
c. It is the amount of water present in the pond at a given time
d. It is the amount of biotic components in the ocean at a given time

Answer: Standing state is calculated as the amount of nutrients present in the soil at any given point of time. The nutrients involved include carbon, nitrogen, calcium, phosphorus etc. The standing state of nutrients varies in different types of ecosystems. It also varies on a seasonal basis. Hence, the correct option is a.

Q 5. Define biogeochemical cycle?

Answer: Nutrient cycle is also termed as biogeochemical cycle. It is the movement of nutrients through various components of an ecosystem. The nutrient cycle is of two types based on the reservoir as follows:

• Gaseous cycle
• Sedimentary cycle

FAQs

Q 1. What is the main difference between the phosphorus cycle and the carbon cycle?
Answer: The phosphorus cycle and carbon cycle can be differentiated on the basis of reservoir. The reservoir of the carbon cycle is present in the atmosphere and hydrosphere, whereas the reservoir of the phosphorus cycle is present in the rocks (lithosphere).

Q 2. What are the main steps involved in the phosphorus cycle?
Answer: There are four main steps involved in the phosphorus cycle:

• Weathering rocks.
• Absorption of phosphorus by plants. 
• Absorption of phosphorus by animals.
• Release of phosphorus to the environment through decomposition. 

 Q 3. How will biogeochemical cycles affect humans?
Answer: Human activities have increased carbon dioxide levels, especially in the atmosphere and nitrogen levels mainly in the biosphere. Altered climate patterns combined with altered biogeochemical cycles increases the vulnerability of biodiversity, human health, food security, soil quality and water quality.

Q 4. What will happen to the matter in the biogeochemical cycle?
Answer: Matter always flows through the different trophic levels in the food chains. Elements are recycled between different ecosystems using the biogeochemical cycles. As these nutrients move through different ecosystems, the compounds formed are usually transformed.

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