Sign up to access the complete lesson and track your progress!
Unlock This CourseNitrogen is everywhere around us - it makes up about 78% of the air we breathe! But here's the catch: most living things can't use nitrogen gas directly from the atmosphere. It's like having a massive treasure chest that's locked tight. The nitrogen cycle is nature's way of unlocking this treasure and making it available to all living organisms.
Think of the nitrogen cycle as a recycling system that never stops working. It transforms nitrogen from the air into forms that plants can absorb, then passes it through food chains and eventually returns it back to the atmosphere. This process is essential for life on Earth because nitrogen is a key ingredient in proteins and DNA.
Key Definitions:
Nitrogen is like the building blocks of life. It's found in amino acids (which make proteins), nucleic acids (DNA and RNA) and chlorophyll (the green stuff in plants that captures sunlight). Without nitrogen, there would be no proteins, no genetic material and no photosynthesis!
Nitrogen fixation is like having a special key to unlock atmospheric nitrogen. Most organisms can't break the strong triple bond between nitrogen atoms in N₂ gas, but some bacteria have evolved this superpower!
There are two main types of nitrogen-fixing bacteria that do the heavy lifting in this process:
These bacteria live independently in soil and water. Examples include Azotobacter and Clostridium. They fix nitrogen for their own use, but some leaks into the soil for plants to absorb.
These bacteria form partnerships with plants, especially legumes like peas, beans and clover. They live in special root nodules and fix nitrogen in exchange for sugars from the plant.
Lightning provides enough energy to break nitrogen bonds and combine nitrogen with oxygen, forming nitrates that dissolve in rainwater and reach the soil.
Farmers have known for centuries that growing legumes like beans and peas actually improves soil fertility. This happens because Rhizobium bacteria in root nodules fix atmospheric nitrogen, converting it into ammonia. When the crop is harvested, nitrogen-rich nodules remain in the soil, naturally fertilising it for the next crop. This is why crop rotation often includes legumes - it's nature's own fertiliser factory!
Once nitrogen has been fixed into ammonia, it needs another transformation before most plants can use it effectively. This is where nitrification comes in - a two-step process carried out by specialised bacteria.
Nitrification happens in two distinct stages, each carried out by different types of bacteria:
Bacteria called Nitrosomonas oxidise ammonia (NH₃) into nitrites (NO₂⁻). These bacteria get energy from this chemical reaction, which is how they survive. Nitrites are still quite toxic to plants in large amounts.
Another group of bacteria called Nitrobacter convert nitrites into nitrates (NO₃⁻). Nitrates are the preferred form of nitrogen for most plants because they're easily absorbed by roots and aren't toxic.
This process is crucial because it creates the nitrogen compounds that plants can readily absorb through their root systems. Without nitrification, the ammonia produced during nitrogen fixation would largely remain unavailable to plants.
Assimilation is when plants finally get to use all that processed nitrogen! Plants absorb nitrates and ammonium ions through their roots and incorporate the nitrogen into their own proteins, nucleic acids and other essential molecules.
Once plants have absorbed nitrogen compounds, the nitrogen begins its journey through food chains:
Plants use nitrogen to make proteins, chlorophyll and DNA. The nitrogen becomes part of the plant's structure and is stored in leaves, stems and roots.
Herbivores eat plants and break down plant proteins to make their own proteins. The nitrogen is recycled and becomes part of animal tissues.
Carnivores eat herbivores and the nitrogen continues up the food chain. Each level recycles nitrogen into new proteins and other molecules.
In African savannas, the nitrogen cycle supports vast herds of grazing animals. Grasses absorb nitrates from soil, zebras and wildebeest eat the grasses and lions eat the herbivores. When animals produce waste or die, decomposers break down the nitrogen compounds and return them to the soil. This creates a continuous cycle that supports one of Earth's most productive ecosystems.
What goes up must come down and what gets taken from the atmosphere must eventually return. The final stages of the nitrogen cycle involve breaking down organic matter and returning nitrogen gas to the atmosphere.
When plants and animals die, or when they produce waste, decomposer bacteria and fungi get to work. They break down proteins and nucleic acids, releasing ammonia back into the soil. This process is called mineralisation or ammonification.
Bacteria like Bacillus and Clostridium break down dead organic matter in soil. They release ammonia, which can then be nitrified again or used directly by plants.
Fungi are particularly good at breaking down tough organic materials like wood and leaves. They release nitrogen compounds slowly over time as they digest organic matter.
Denitrification is the final step that completes the nitrogen cycle. In waterlogged soils or sediments where oxygen is scarce, special bacteria convert nitrates back into nitrogen gas, which escapes to the atmosphere.
This process is carried out by bacteria such as Pseudomonas and Paracoccus. They use nitrates instead of oxygen for respiration when oxygen levels are low. While this might seem wasteful, denitrification is essential for preventing the build-up of excess nitrates in ecosystems.
Humans have dramatically altered the nitrogen cycle, mainly through agriculture and industry. We've essentially turbocharged nitrogen fixation, but this has created both benefits and problems.
This industrial process fixes atmospheric nitrogen to make fertilisers. It has revolutionised agriculture and helped feed billions of people, but it also uses enormous amounts of energy and has environmental consequences.
Excess nitrogen from fertilisers can cause eutrophication in water bodies, leading to algal blooms and dead zones. It can also contribute to greenhouse gas emissions and soil acidification.
Every summer, a massive dead zone forms in the Gulf of Mexico due to nitrogen pollution from agricultural runoff. Excess nitrates from fertilisers flow down the Mississippi River, causing explosive algal growth. When the algae die and decompose, they use up oxygen in the water, creating areas where fish and other marine life cannot survive. This dead zone can be as large as the state of Connecticut, demonstrating how human interference with the nitrogen cycle can have far-reaching consequences.
The nitrogen cycle is a perfect example of how interconnected life on Earth really is. From tiny bacteria in soil to massive trees in forests, from microscopic algae to huge whales in the ocean - every living thing depends on this cycle.
Understanding the nitrogen cycle helps us appreciate why biodiversity is so important. Each type of bacteria, plant and animal plays a specific role in keeping nitrogen moving through ecosystems. When we disrupt these relationships, we can cause problems that ripple through entire food webs.
As we face challenges like climate change and feeding a growing human population, understanding and protecting the nitrogen cycle becomes increasingly important. By working with natural processes rather than against them, we can maintain healthy ecosystems while meeting human needs.