The Nitrogen Cycle
~6 min read · Tier toggle changes depth (T1 plain · T2 standard · T3 rigour)
You should understand the basic concept of biogeochemical cycles from the previous lecture on the Carbon Cycle and be familiar with the roles of primary producers, consumers, and decomposers in an ecosystem.
Why this matters
Nitrogen is a limiting nutrient for most life on Earth, and its availability dictates the productivity of ecosystems.
Atmospheric nitrogen is far more abundant than atmospheric carbon dioxide, but unlike CO2, which plants fix directly, N2 is inaccessible to plants in its atmospheric form. The nitrogen cycle describes the complex set of biological and chemical processes that convert inert atmospheric nitrogen into forms usable by living organisms, and then return it to the atmosphere or soil. Understanding this cycle is crucial because it explains why fertilisers are necessary in agriculture, why eutrophication occurs in water bodies, and how microbial communities drive global nutrient availability. This lecture focuses on the key transformations: fixation, nitrification, assimilation, ammonification, and denitrification.
The Nitrogen Cycle in Plain Language
Imagine the atmosphere as a vast reservoir of nitrogen gas, \(\mathrm{N_2}\), which makes up about 78% of the air we breathe. However, this gas is chemically inert; the triple bond between the two nitrogen atoms is extremely strong, making it difficult for most organisms to break. Plants cannot use this gas directly to build proteins and nucleic acids. They need nitrogen in a "fixed" form, such as ammonia (\(\mathrm{NH_3}\)) or nitrate (\(\mathrm{NO_3^-}\)). This is where the magic happens. Specialised bacteria, either living in the soil or in symbiosis with plant roots (like legumes), perform nitrogen fixation. They break the \(\mathrm{N_2}\) bond and convert it into ammonia, a process that requires significant energy. Once plants absorb this ammonia or nitrate from the soil, they incorporate it into amino acids and DNA. When animals eat these plants, they gain this fixed nitrogen. Eventually, when organisms die or excrete waste, decomposers break down the organic nitrogen back into ammonia. This process is called ammonification. Other bacteria then oxidise this ammonia into nitrite and then nitrate, a process known as nitrification. Finally, under anaerobic conditions, some bacteria convert nitrate back into nitrogen gas, returning it to the atmosphere. This completes the cycle, ensuring that nitrogen is continuously recycled between the atmosphere, soil, and living biomass.
Biological Nitrogen Fixation
Nitrogen fixation is the reduction of atmospheric nitrogen to ammonia. This reaction is energetically demanding because it involves breaking the strong \(\mathrm{N \equiv N}\) triple bond. The enzyme responsible for this is nitrogenase, which is sensitive to oxygen. Therefore, free-living nitrogen fixers either avoid oxygen (anaerobes such as Clostridium) or protect the enzyme; Azotobacter, a strict aerobe, does so by respiring so fast that the oxygen inside the cell stays low. In symbiotic relationships, such as those between rhizobia and legumes, the bacteria live inside root nodules. The plant provides carbohydrates for energy, while the bacteria provide fixed nitrogen. The nodule contains a protein called leghaemoglobin, which binds oxygen to keep concentrations low enough to protect nitrogenase but high enough to support aerobic respiration. The stoichiometry of the fixation reaction can be represented as: \[ \mathrm{N_2} + 8\mathrm{H^+} + 8\mathrm{e^-} + 16\mathrm{ATP} \rightarrow 2\mathrm{NH_3} + \mathrm{H_2} + 16\mathrm{ADP} + 16\mathrm{P_i} \] This shows that a significant amount of cellular energy is required to drive this process, highlighting why nitrogen fixation is a critical bottleneck in ecosystem productivity.
Nitrification and Assimilation
Once nitrogen is fixed into ammonia, it is often converted into nitrate through nitrification. This is a two-step aerobic process carried out by different groups of bacteria. First, ammonia-oxidising bacteria (AOB) convert ammonia to nitrite: \[ \mathrm{NH_4^+} + 1.5\mathrm{O_2} \rightarrow \mathrm{NO_2^-} + 2\mathrm{H^+} + \mathrm{H_2O} \] Then, nitrite-oxidising bacteria (NOB) convert nitrite to nitrate: \[ \mathrm{NO_2^-} + 0.5\mathrm{O_2} \rightarrow \mathrm{NO_3^-} \] Nitrate is highly soluble and mobile in soil, making it readily available for plant uptake. Plants assimilate both ammonia and nitrate into organic molecules. Ammonia is incorporated into glutamate via the glutamine synthetase/glutamate synthase (GS/GOGAT) pathway. Nitrate must first be reduced to nitrite by nitrate reductase, and then to ammonia by nitrite reductase, before it can enter the GS/GOGAT pathway. This assimilation process is crucial because it locks nitrogen into the biomass, transferring it up the food chain. The availability of nitrate often limits plant growth in terrestrial ecosystems, a phenomenon known as nitrogen limitation.
Ammonification and Denitrification
When organisms die or excrete waste, the organic nitrogen in their tissues is broken down by decomposers. This process, ammonification, converts organic nitrogen back into ammonia (\(\mathrm{NH_3}\)) or ammonium (\(\mathrm{NH_4^+}\)). This returns nitrogen to the inorganic pool, making it available for plant uptake or nitrification again. In waterlogged soils or sediments where oxygen is scarce, denitrification occurs. Denitrifying bacteria, such as Pseudomonas and Paracoccus, use nitrate as an alternative electron acceptor for respiration. They reduce nitrate stepwise to nitrite, nitric oxide, nitrous oxide, and finally nitrogen gas: \[ \mathrm{NO_3^-} \rightarrow \mathrm{NO_2^-} \rightarrow \mathrm{NO} \rightarrow \mathrm{N_2O} \rightarrow \mathrm{N_2} \] This process removes fixed nitrogen from the ecosystem and returns it to the atmosphere, completing the cycle. Denitrification is a significant source of nitrous oxide (\(\mathrm{N_2O}\)), a potent greenhouse gas, linking the nitrogen cycle to climate change. The balance between nitrification and denitrification depends heavily on soil moisture and oxygen availability, illustrating the dynamic nature of biogeochemical cycles.
The concept
Biogeochemical cycles
How carbon and nitrogen move through ecosystems.
A common misconception is that nitrogen fixation is the only source of nitrogen for plants. In reality, plants can also absorb nitrate directly, and in some ecosystems, atmospheric deposition of nitrogen compounds (like rain containing nitrates) contributes significantly to soil nitrogen pools. Additionally, the rate of denitrification can vary drastically with small changes in soil moisture, meaning that nitrogen losses can be highly unpredictable and spatially variable. C
Worked example
Problem. A soil sample has a total nitrogen content of 2.0% by mass. If 10% of this nitrogen is in the form of nitrate (\(\mathrm{NO_3^-}\)) and the rest is organic or ammonium, calculate the mass of nitrate nitrogen in a 1000 g sample of soil. Assume the molar mass of nitrogen is 14.0 \(\mathrm{g\,mol^{-1}}\).
- Determine the total mass of nitrogen in the soil sample. Total mass = 1000 g. Nitrogen percentage = 2.0%. Mass of N = \(1000 \times 0.02 = 20\) g.
- Calculate the mass of nitrogen present as nitrate. Nitrate fraction = 10%. Mass of N in nitrate = \(20 \times 0.10 = 2.0\) g.
- Verify the unit consistency. The question asks for the mass of nitrate nitrogen, which is a mass in grams. The calculation yields grams.
- Consider if the question asked for the mass of the nitrate ion (\(\mathrm{NO_3^-}\)) instead. Molar mass of \(\mathrm{NO_3^-}\) = \(14.0 + 3 \times 16.0 = 62.0\) \(\mathrm{g\,mol^{-1}}\). Ratio of N to \(\mathrm{NO_3^-}\) is \(14.0/62.0\). Mass of \(\mathrm{NO_3^-}\) = \(2.0 \times (62.0/14.0) \approx 8.86\) g.
- Since the question specifically asks for "nitrate nitrogen", the answer is the mass of the nitrogen atom within the nitrate ion, which is 2.0 g.
Units check. The input mass is in grams, and the percentage is dimensionless, so the output is correctly in grams.
Three quick tests
- What is the primary energy requirement for biological nitrogen fixation?
Answer
ATP (Adenosine Triphosphate) is required to power the nitrogenase enzyme. - Which process converts nitrate back into nitrogen gas?
Answer
Denitrification. - Why is leghaemoglobin important in root nodules?
Answer
It binds oxygen to maintain low concentrations, protecting the oxygen-sensitive nitrogenase enzyme while allowing aerobic respiration.
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Lecture notes checked line by line by the editor; every number in the worked example was recomputed.