Nitrogen is fundamental to all life on Earth, forming the backbone of crucial biological molecules like DNA and proteins. When plants and animals die, the nitrogen they contain is temporarily locked away in complex organic matter. This bound nitrogen must be freed and converted back into a usable form so that new life can be sustained. Recycling this biological nitrogen from dead organisms back into the environment is a continuous natural process.
Conversion to Ammonium
The first step in liberating nitrogen from dead biomass begins with decomposition, driven by bacteria and fungi in the soil. These decomposer organisms release enzymes that break down the large, complex organic molecules from the dead tissues. This process, known as mineralization, transforms the nitrogen from its organic form into simple inorganic compounds.
This initial breakdown leads directly to the release of ammonia (\(NH_3\)), which quickly reacts with water in the soil to form ammonium ions (\(NH_4^+\)). The conversion of organic nitrogen into inorganic ammonium is termed ammonification. This is the first stage where the nitrogen becomes available in the soil environment. The resulting ammonium is a simple, positively charged ion that can sometimes be directly taken up by plant roots. If the ammonium is not immediately used, it enters the next transformative stage in the soil.
Further Transformation in the Soil
Once ammonium is present in the soil, it is subject to a two-step biological process called nitrification, which requires the presence of oxygen. This transformation is carried out by specialized groups of autotrophic bacteria that use the oxidation of nitrogen compounds as their energy source. The first stage involves bacteria like Nitrosomonas, which oxidize ammonium (\(NH_4^+\)) into a compound called nitrite (\(NO_2^-\)).
Nitrite is toxic to plants and thus cannot accumulate in the soil without negative effects. A second group of bacteria, including species like Nitrobacter, rapidly converts the nitrite into nitrate (\(NO_3^-\)). This second oxidation step completes the process and yields the form of nitrogen most readily absorbed by the majority of plants. Nitrate is a highly mobile, negatively charged ion that is easily dissolved in soil water, making it the preferred nitrogen source for plant uptake.
Re-entering the Atmosphere or Biomass
The nitrate and ammonium ions newly available in the soil face two primary fates: re-entry into the living biomass or a return to the atmosphere. Plants rapidly absorb these inorganic nitrogen forms through their roots in a process called assimilation, incorporating them into new proteins, DNA, and other organic molecules. This allows the nitrogen to continue cycling through the ecosystem as the plants are consumed by animals, which in turn use the nitrogen to build their own tissues.
The other major outcome, which removes nitrogen from the soil system entirely, is denitrification. This process occurs under anaerobic conditions, typically in waterlogged soils where oxygen is scarce, forcing certain bacteria to use nitrate as an alternative electron acceptor for their metabolism. Denitrifying bacteria, such as those in the Pseudomonas genus, convert the nitrate (\(NO_3^-\)) back into gaseous forms, primarily nitrogen gas (\(N_2\)) and sometimes nitrous oxide (\(N_2O\)). These gases then escape from the soil and are released back into the atmosphere, completing the long-term cycle of nitrogen between the living world and the air.