Topic 1.5 Notes – The Nitrogen Cycle

What the Nitrogen Cycle Is

Nitrogen is an essential macronutrient. Living things need it to build:

• amino acids and proteins
• nucleic acids like DNA and RNA
• chlorophyll
• other important biological compounds

The nitrogen cycle is the movement and chemical transformation of nitrogen among these reservoirs:

• atmospheric $N_2$, the biggest reservoir, about 78% of air
• soil nitrogen compounds
• dissolved nitrogen in water and groundwater
• organic nitrogen in biomass, waste, and dead matter
• ocean water and sediments

Here’s the key idea that drives the whole topic. Nitrogen atoms are conserved, but their chemical form changes.

That matters because abundance is not the same as availability. Atmospheric nitrogen gas, $N_2$, has a very strong triple bond, so most organisms cannot use it directly. Producers usually take up ammonium $NH_4^+$ and nitrate $NO_3^-$ instead. Most non-atmospheric reservoirs hold nitrogen only for short periods before it gets moved or transformed again.

This overview diagram pulls those reservoirs and transformations together before you learn the individual steps.

The Main Nitrogen Forms and Processes

Nitrogen forms

You need to recognize these together:

• $N_2$ = atmospheric nitrogen gas, mostly unusable to organisms
• $NH_3$ = ammonia
• $NH_4^+$ = ammonium, available to producers
• $NO_2^-$ = nitrite, an intermediate
• $NO_3^-$ = nitrate, available to producers and very mobile in water
• organic nitrogen = nitrogen in biomass, waste, detritus
• $N_2O$ = nitrous oxide, a greenhouse gas and possible intermediate/product

Five core processes

• Nitrogen fixation
  $N_2 \rightarrow NH_3 \rightarrow NH_4^+$
  Done mainly by microorganisms. Examples you should know:
  • Rhizobium in legume root nodules
  • cyanobacteria
  • lightning
  • Haber-Bosch industrial fixation
• Nitrification
  $NH_3/NH_4^+ \rightarrow NO_2^- \rightarrow NO_3^-$
  This is a microbial process in oxygen-rich soil or water.
• Assimilation
  $NH_4^+$ or $NO_3^-$ \rightarrow organic nitrogen in producers
  Consumers then get nitrogen by eating.
• Ammonification
  organic nitrogen $\rightarrow NH_3/NH_4^+$
  Decomposers break down waste and dead organisms.
• Denitrification
  $NO_3^- \rightarrow N_2O \rightarrow N_2$
  This returns nitrogen to the atmosphere, usually in low-oxygen conditions.

How the Nitrogen Cycle Moves Through Ecosystems

It helps to picture the cycle as a path:

1. Atmospheric $N_2$ enters ecosystems through fixation.
2. In soil, ammonia becomes ammonium.
3. Nitrification can turn ammonium into nitrite and then nitrate.
4. Producers assimilate ammonium or nitrate.
5. Consumers get organic nitrogen through feeding.
6. Waste and dead matter enter the detritus pool.
7. Ammonification returns organic nitrogen to ammonium.
8. Denitrification can return nitrate to the atmosphere as gases.

Nitrogen also moves sideways between reservoirs through runoff, leaching, groundwater flow, and deposition.

A common diagram question tests whether you know the forms at each end of an arrow. Don’t identify a process by arrow direction alone. Identify it by starting form and ending form.

Microorganisms and Environmental Conditions

Most of the chemical conversions are done by microorganisms:

• nitrogen-fixing microbes do fixation
• nitrifying microbes do nitrification
• bacteria and fungi do ammonification
• denitrifying bacteria do denitrification

Plants are central to assimilation, but they usually do not perform the major chemical conversions themselves.

Environmental conditions shape which process happens:

• oxygen-rich conditions favor nitrification
• low-oxygen or waterlogged conditions favor denitrification

Mobility matters too:

• ammonium is less mobile because it can bind to soil particles
• nitrate is more mobile because it is soluble and easily leaches or runs off

Why the Nitrogen Cycle Matters

Nitrogen is often a limiting nutrient for primary production in many terrestrial and marine ecosystems. So even though the atmosphere is full of nitrogen, growth can still be limited if there isn’t enough usable nitrogen.

The rate of nitrogen fixation helps control how much usable nitrogen enters ecosystems. If available nitrogen is added, producer growth often increases until some other factor becomes limiting.

Humans increase reactive nitrogen through:

• synthetic fertilizer from Haber-Bosch fixation
• planting nitrogen-fixing crops
• fossil-fuel combustion that releases nitrogen oxides

Major effects of excess reactive nitrogen:

• increased crop production
• nitrate leaching into groundwater
• runoff into aquatic systems
• algal growth and eutrophication
• shifts in species composition and lower biodiversity
• increased $N_2O$ emissions