Nitrogen is an essential element found throughout living systems, playing a fundamental role in the structure of proteins and nucleic acids like DNA. The vast reservoir of nitrogen gas in the atmosphere is not directly usable by most organisms. When plants and animals conclude their life cycles, the nitrogen contained within their tissues does not simply disappear. Instead, a continuous recycling process ensures this valuable element is transformed and returned to the environment, becoming available for new life.
Decomposition and Initial Release
When plants and animals die, complex organic nitrogen compounds like proteins and nucleic acids break down. Decomposers, such as bacteria and fungi, carry out this initial stage. They consume organic matter, converting organic nitrogen into simpler, inorganic forms.
This conversion, called ammonification, transforms organic nitrogen into ammonium (NH4+). Microorganism enzymes degrade these substances, releasing ammonium into the soil or water. This makes nitrogen accessible for further ecosystem transformations.
Nitrification: Soil Transformations
The ammonium (NH4+) released during decomposition undergoes further transformations in the soil through nitrification, a two-step process. Specific nitrifying bacteria, which require oxygen, carry out this process. First, ammonia-oxidizing bacteria, such as Nitrosomonas species, convert ammonium into nitrite (NO2-).
Then, a different group of nitrifying bacteria, including Nitrobacter species, rapidly convert nitrite into nitrate (NO3-). This second step proceeds quickly, preventing nitrite accumulation. Nitrate is a highly mobile form of nitrogen in the soil and the primary form most plants readily absorb through their roots.
Denitrification: Return to the Air
Nitrogen also returns to the atmosphere through denitrification, primarily performed by certain bacteria. This process occurs in low-oxygen environments, such as waterlogged soils or deep sediments. Denitrifying bacteria convert nitrates (NO3-) back into gaseous forms of nitrogen.
These gaseous products include nitrogen gas (N2) and nitrous oxide (N2O). The bacteria use nitrate as an electron acceptor in the absence of oxygen, effectively “breathing” nitrate. This step completes a portion of the nitrogen cycle by returning nitrogen to its atmospheric reservoir.
Assimilation: Re-entering the Web of Life
Processed nitrogen, primarily nitrates and, to a lesser extent, ammonium, re-integrates into living systems through assimilation. Plants absorb these inorganic forms from the soil via their root systems. Inside the plant, this nitrogen incorporates into organic molecules like amino acids, proteins, and DNA, essential for plant growth and function.
This nitrogen then moves up the food chain when herbivores consume plants. Carnivores obtain nitrogen by consuming herbivores or other carnivores. This continuous uptake and transfer of nitrogen from the soil into living organisms and through the food web ensures its constant recycling and availability to support new life.