Nitrogen cycle

Most of our air is made of nitrogen.

But plants cannot use it from the air. Tiny bugs in the soil help them. These bugs change the nitrogen into food. Plants drink this food through their roots. Then, animals eat the plants to grow. When plants and animals die, bugs break them down. This puts the nitrogen back into the dirt. It is a big circle that never ends.

Most of our air is made of nitrogen.

But plants cannot use nitrogen gas from the air. It must change into other forms first. This set of steps is called the nitrogen cycle.

One way nitrogen changes is through nitrogen fixation. This is when gas turns into forms like nitrates. Lightning can do this. Also, tiny bugs in the soil help. These bugs are called diazotrophs. Some bugs live in the roots of plants like peas. They give the plant nitrogen. In return, the plant gives them food.

Plants take up this nitrogen through their roots. This is called assimilation. When plants and animals die, the nitrogen is in their bodies. Tiny bugs then break it down. This part is called ammonification. It turns the nitrogen into ammonia.

Other bugs turn ammonia into nitrates. This is called nitrification. This step is important because ammonia can hurt plants. Finally, some bugs turn nitrates back into gas. This is called denitrification. This completes the big circle.

Nitrogen is a very important part of our world. About 78% of the air around us is made of nitrogen gas. Even though there is so much, most living things cannot use it directly from the air. It must change into different forms to help life grow. This constant movement of nitrogen is called the nitrogen cycle. This cycle helps control how much life can grow in different places.

The cycle works through several important steps. First, nitrogen fixation turns gas into usable forms like nitrates. This can happen through lightning strikes or tiny bacteria called diazotrophs. Some bacteria, like Rhizobium, live in the roots of plants like peas. They give the plant nitrogen and get food in return. Next, plants use assimilation to take up these nutrients through their roots. When living things die, a process called ammonification turns their remains into ammonia.

Microbes play a huge role in how these changes happen. Soil bacteria like Nitrosomonas turn ammonia into nitrites. Then, other bacteria like Nitrobacter turn those nitrites into nitrates. This step is called nitrification. It is very important because ammonia can be toxic to plants. Finally, a process called denitrification turns nitrates back into nitrogen gas. This happens in places without much oxygen, like waterlogged soil.

Scientists study these changes to understand our planet better. In the ocean, nitrogen enters through rain or runoff. Tiny organisms called cyanobacteria perform fixation in the water. Without new nitrogen, the ocean's supply might run out in 2,000 years. Humans have also changed the cycle in big ways. We use the Haber-Bosch process to make industrial fertilizer. This process uses high heat and pressure to make ammonia.

You can see the nitrogen cycle in your own backyard. When you see a pea plant growing, you are seeing nitrogen fixation. When leaves turn green, they are using nitrogen for chlorophyll. Even the rain helps move nitrogen from the sky to the ground. The cycle connects the air, the soil, and the deep blue sea. It is a giant, invisible web that keeps all living things going.

The nitrogen cycle is a complex biogeochemical cycle. It describes how nitrogen changes into many different chemical forms. These forms circulate through the atmosphere, the land, and the oceans. Nitrogen is essential for life because it helps build amino acids, nucleic acids, and chlorophyll. Most of Earth's atmosphere consists of 78% nitrogen gas. However, this gas is mostly unavailable to living things in its current state. This scarcity of usable nitrogen can limit how much life grows in an ecosystem.

The cycle begins with nitrogen fixation. This is the process of converting nitrogen gas into usable nitrates and nitrites. Lightning strikes can fix between 5 and 10 billion kg of nitrogen every year. Most fixation is actually done by specialized bacteria called diazotrophs. These bacteria use an enzyme called nitrogenase to combine nitrogen gas with hydrogen. This creates ammonia, which is then turned into organic compounds. Some bacteria, like Azotobacter, live freely in the soil. Others, like Rhizobium, live in the root nodules of legumes such as peas or alfalfa. These bacteria form a mutualistic relationship with the plant. The bacteria provide ammonia, and the plant provides carbohydrates in return.

Humans have added a new, massive step to this cycle through industrial processes. About 30% of all fixed nitrogen is now produced using the Haber-Bosch process. This industrial method uses very high temperatures and high pressures. It combines nitrogen gas with a hydrogen source, such as petroleum or natural gas, to create ammonia. This ammonia is used to make artificial fertilizers. While this helps grow more food, it also changes the natural balance of the planet. Human activities like fossil fuel combustion and wastewater release have dramatically altered the global cycle.

Once nitrogen is fixed, plants use a process called assimilation. Plant roots absorb nitrate or ammonium from the soil. If a plant absorbs nitrate, it must first reduce it to nitrite and then to ammonium. These ions are then used to build essential molecules like DNA. Animals and fungi obtain their nitrogen by eating these organic molecules. When organisms die or produce waste, the nitrogen enters the ammonification stage. Bacteria and fungi use enzymes like proteases to break down organic matter. This process, also called mineralization, converts organic nitrogen back into ammonia or ammonium.

Nitrification is the next critical step in the sequence. This process converts ammonium into nitrate through two distinct stages. First, bacteria like Nitrosomonas oxidize ammonium into nitrites. Next, different bacteria, such as Nitrobacter, oxidize those nitrites into nitrates. This step is vital because ammonia gas can be toxic to plants. However, nitrates are highly soluble and do not stick to soil well. This means they can easily leak into groundwater. High nitrate levels in water can cause blue-baby syndrome in infants. It can also lead to eutrophication, which causes massive algal blooms in water bodies.

The cycle completes through denitrification. This is the reduction of nitrates back into nitrogen gas. This process is carried out by bacteria like Pseudomonas under anaerobic conditions. Anaerobic means the environment lacks oxygen, such as in waterlogged soils. These bacteria use nitrate instead of oxygen during respiration. Another process called ANAMMOX, or anaerobic ammonium oxidation, also plays a role. In this reaction, ammonia and nitrite are converted into diatomic nitrogen gas and water. This specific process is a major part of how nitrogen moves in the oceans.

The marine nitrogen cycle follows similar rules but has different players. Nitrogen enters the ocean through runoff, rain, or the atmosphere. Cyanobacteria perform nitrogen fixation in the water to help phytoplankton grow. Without new nitrogen entering the marine cycle, the supply would run out in about 2,000 years. Phytoplankton release ammonia and urea into the water when they excrete waste. This organic matter eventually sinks to deeper layers of the ocean. This movement of nitrogen connects the surface waters to the deep sea, supporting life throughout the entire ocean system.