Understanding Nutrient Cycles: The Earth’s Recycling System 

 

What Are Nutrient Cycles?

Nutrient cycles are nature's way of recycling essential elements, such as carbon and nitrogen, so they can be used again by living organisms. These cycles ensure that elements flow between the environment and living organisms.

 

The Carbon Cycle

The carbon cycle describes how carbon moves through the Earth’s systems. It starts as carbon dioxide $(CO_2)$ in the air and ends up in living organisms or back in the atmosphere.

Steps in the Carbon Cycle:

1. Photosynthesis
   Plants take in $(CO_2)$ from the atmosphere to make glucose: $$6CO_2 + 6H_2O \xrightarrow[Light] C_6H_12O_6 + 6O_2$$

   • Key Players: Plants, algae, and phytoplankton.
2. Respiration
   Plants, animals, and decomposers release $CO_2$ back into the atmosphere by breaking down glucose for energy: $$C_6H_12O_6 + 6O_2 \xrightarrow  6CO_2 + 6H_2O + \text{Energy}$$

3. Feeding
   Animals eat plants (or other animals), passing carbon up the food chain.

4. Decomposition
   Dead organisms and waste are broken down by decomposers, releasing $CO_2$

5. Fossil Fuels
   Dead plants and animals that don't decompose fully become coal, oil, or gas over millions of years.

6. Combustion
   Burning fossil fuels releases stored $CO_2$ back into the air

 

 

The Nitrogen Cycle

Nitrogen is needed by living organisms to make proteins and DNA. However, nitrogen gas $(N_2)$ 

in the air must be converted into usable forms.

Steps in the Nitrogen Cycle:

1. Nitrogen Fixation

   • Lightning or nitrogen-fixing bacteria convert $N_2$ into ammonium $(NH_4^-)$
   • Key Players: Bacteria in soil and plant root nodules.

2. Nitrification

   • Nitrifying bacteria convert ammonium into nitrites $(NO_2^-)$, and then into nitrates $(NO_3^-)$ that plants can absorb.
3. Absorption by Plants
   Plants take up $(NO_3^-)$ and use it to make proteins.

4. Feeding and Digestion
   Animals eat plants, gaining nitrogen to build their own proteins.

5. Decomposition
   Dead plants and animals are broken down by decomposers, releasing ammonium.

6. Denitrification
   Denitrifying bacteria convert nitrates back into nitrogen gas $(N_2)$

 

Microorganisms in the Nitrogen Cycle

Role	Microorganism Action
Nitrogen Fixation	Convert $\text{N}_2$ into ammonium ($\text{NH}_4^+$).
Nitrification	Convert ammonium to nitrates ($\text{NO}_3^-$​).
Decomposition	Break down dead organisms to release ammonium.
Denitrification	Convert nitrates back into nitrogen gas.

 

Key Differences Between the Cycles

Feature	Carbon Cycle	Nitrogen Cycle
Element	Carbon ($\text{C}$)	Nitrogen ($\text{N}$)
Importance	Forms glucose for energy.	Builds proteins and DNA.
Main Processes	Photosynthesis, respiration, decomposition, combustion.	Nitrogen fixation, nitrification, absorption, decay.