Summary
Highlights
Atmospheric nitrogen, at 78%, exists as inert diatomic molecules, making it unusable by most organisms. It must be converted into usable forms, a process central to the nitrogen cycle. Animals and plants cannot directly utilize atmospheric nitrogen even through respiration, necessitating its conversion in the soil.
The nitrogen cycle involves five main processes: nitrogen fixation, assimilation, ammonification (decomposition), nitrification, and denitrification. These processes are interconnected, transforming inert atmospheric nitrogen into forms usable by living organisms and returning it to the atmosphere.
Nitrogen fixation converts inert atmospheric nitrogen into usable forms in the soil. This is primarily done by nitrogen-fixing bacteria like Rhizobium (in legume root nodules) and Azotobacter (free-living in soil), which convert nitrogen gas into ammonium. Lightning also contributes by converting atmospheric nitrogen into nitrogen dioxide, which dissolves in rain to form nitric acid, eventually becoming nitrates in the soil.
Assimilation is the process where organisms acquire and use nitrogen to build essential molecules. Plants absorb nitrates and ammonium from the soil through their roots. Animals obtain nitrogen by consuming plants or other animals. This nitrogen is crucial for synthesizing proteins, DNA, and other vital components for healthy growth.
When plants and animals die, decomposer organisms (like bacteria, fungi, and worms) in the soil break down their organic matter. This process, called ammonification, converts the nitrogen compounds in dead organisms into ammonia, which then dissolves in soil water to form ammonium. This returns nitrogen from dead biomass back to the soil.
In aerobic conditions (with oxygen), ammonium compounds are oxidized in a two-step process called nitrification. Nitrosomonas bacteria convert ammonium to nitrites, and then Nitrobacter bacteria convert nitrites to nitrates. Nitrates are a readily usable form of nitrogen for plants and are crucial for their growth.
Denitrification is the process where denitrifying bacteria (e.g., Clostridium, Pseudomonas) convert nitrates in the soil back into nitrogen gas and oxygen. This process occurs under anaerobic conditions (lack of oxygen) and returns nitrogen to the atmosphere, completing the cycle. It reduces the amount of nitrates available in the soil.
The nitrogen cycle involves the continuous circulation of nitrogen through the atmosphere, soil, water, and living organisms via these five interconnected processes. These processes can be categorized as those that add usable nitrogen to the soil (nitrogen fixation, nitrification, assimilation, lightning, fertilizers) and those that remove it (denitrification, erosion, leaching). The balance of these processes is essential for maintaining stable nitrogen levels in the environment.