Nitrogen is one of the most abundant elements on Earth – it makes up 78% of the atmosphere, 16% of all protein diets, and is present at a concentration of 7.5 kilograms for every 250 kilograms of human feces.
The different forms of nitrogen undergo various chemical and physical transformations that are all equally critical to the global nitrogen cycle. Over the last few decades, excessive fertilizer and fossil fuel usage have lead to serious environmental problems, which have increased disease and pollution. Some of these issues include nitrate-contaminated groundwater, eutrophication, and increased production of carbon dioxide, methane, and other harmful greenhouse gases in the carbon cycle.
Accordingly, researchers have focused on balancing the cost to the benefits of human activities. As part of the global nitrogen cycle, soil is heavily studied not only because a significant amount of nitrogen is stored in the soil, but also because the increasing human population demands more nitrogen for food production.
In soil, most of the chemical transformations of nitrogen are due to microbial activities, such as nitrogen fixation, nitrification, immobilization, and denitrification. Learning how organisms behave in soil is critical for humans to understand the complex nitrogen cycle.
Function of Nitrogen Cycle
Nitrogen is an essential ingredient for life as we know it. Its unique chemical bonding properties allow it to create structures such as DNA and RNA nucleotides, and the amino acids from which proteins are built. Without nitrogen, these molecules would not be able to exist.
It’s thought that the first nucleotides and amino acids formed naturally under the volatile conditions of early Earth, where energy sources like lightning strikes could cause nitrogen and other atoms to react and form complex structures
This process might have naturally produced self-replicating organic chemicals – but in order to reproduce and evolve, life needed to figure out how to make these nitrogen compounds on demand.
Today, “nitrogen fixers” are organisms that can turn nitrogen gas from the atmosphere into nitrogen compounds that other organisms can use to produce nucleic acids, amino acids, and more. These nitrogen fixers are such a vital part of the ecosystem that agriculture cannot occur without them.
Ancient peoples learned that if they did not alternate growing nitrogen-consuming crops with nitrogen-fixing crops, their farms would become fallow and unable to support growth. Today, most artificial fertilizers contain life-giving nitrogen compounds as their main ingredient to make soil more fertile.
Nitrogen Cycle Steps
The basic steps of the nitrogen cycle are illustrated here:
Nitrogen Fixation in Nitrogen Cycle
In nitrogen fixation, bacteria turn nitrogen gas from the atmosphere into ammonia.
These nitrogen-fixing bacteria, often called “diazotrophs,” have an enzyme called “nitrogenase” which combines nitrogen atoms with hydrogen atoms. Ammonia is a nitrogen compound that can dissolve in water, and is easier for other organisms’ enzymes to interact with.
Interestingly, the enzyme nitrogenase can only function when oxygen isn’t present. As a result, organisms that use it have had to develop oxygen-free compartments in which to perform their nitrogen fixation!
Common examples of such nitrogen-free compartment sare the Rhizobium nodules found in the roots of nitrogen-fixing legume plants.
The hard casing of these nodules keeps oxygen out of the pockets where Rhizobium bacteria do their valuable work of converting nitrogen gas into ammonia.
You can see the oxygen-free Rhizobium nodules, visible as big round lumps, on the roots of this cowpea plant:
Nitrification In Nitrogen Cycle
In nitrification, a host of soil bacteria participate in turning ammonia into nitrate – the form of nitrogen that can be used by plants and animals. This requires two steps, performed by two different types of bacteria.
First, a soil bacteria such as Nitrosomonas or Nitrococcus convert ammonia into nitrogen dioxide. Then another type of soil bacterium, called Nitrobacter, adds a third oxygen atom to create nitrate.
These bacteria don’t convert ammonia for plants and animals out of the goodness of their hearts. Rather, they are “chemotrophs” who obtain their energy from volatile chemicals. By metabolizing nitrogen along with oxygen, they obtain energy to power their own life processes.
The process can be thought of as a rough (and much less efficient) analog to the cellular respiration performed by animals, which extract energy from carbon-hydrogen bonds and use oxygen as the electron acceptor, yielding carbon dioxide at the end of the process.
Nitrates – the end product of this vital string of bacterial reactions – can be made artificially, and are the main ingredient in many soil fertilizers. You may actually hear such fertilizer referred to as “nitrate fertilizer.” By pumping the soil full of nitrates, such fertilizers allow plants to grow large quickly, without being dependent on the rate at which nitrogen-fixing bacteria do their jobs!
Interestingly, high-energy environments such as lightning strikes and volcanic eruptions, can convert nitrogen gas directly into nitrates – but this doesn’t happen nearly enough to keep modern ecosystems healthy on its own!
Assimilation in Nitrogen Cycle
In assimilation, plants finally consume the nitrates made by soil bacteria and use them to make nucleotides, amino acids, and other vital chemicals for life.
Plants take up nitrates through their roots and use them to make amino acids and nucleic acids from scratch.
Animals that eat the plants are then able to use these amino acids and nucleic acids in their own cells.
Ammonification in Nitrogen Cycle
Now we have moved nitrogen from the atmosphere into the cells of plants and animals.
Because there is so much nitrogen in the atmosphere, it may seem that the process could stop there – but the atmosphere’s supply is not infinite, and keeping nitrogen inside plant and animal cells would eventually result in big changes to our soil, our atmosphere, and our ecosystems!
Fortunately, that’s not what happens. In a robust ecosystem like ours, anywhere that energy has been put into creating an organic chemical, there is another form of life that is waiting to extract that energy by breaking those chemical bonds.
In this case, a process called “ammonification” is performed by soil bacteria which decompose dead plants and animals. In the process, these decomposers break down amino acids and nucleic acids into nitrates and ammonia, and release those compounds back into the soil.
There, the ammonia may be taken up again by plants and nitrifying bacteria. Or…
Denitrification in Nitrogen Cycle
In the final step of the nitrogen cycle, anaerobic bacteria can turn nitrates back into nitrogen gas.
This process, like the process of turning nitrogen gas into ammonia, must happen in the absence of oxygen. As such it often occurs deep in the soil, or in wet environments where mud and muck keep oxygen at bay.
In some ecosystems, this denitrification is a valuable process to prevent nitrogen compounds in the soil from building up to dangerous levels.