1.5 Nitrogen Cycle

Nitrogen Fixation

The conversion of atmospheric nitrogen gas (N2) into biologically usable forms like ammonia (NH3) or ammonium (NH4+), performed by nitrogen-fixing bacteria or lightning.

Assimilation

The process by which producers absorb inorganic nitrogen compounds (NH4+, NO3-) through their roots and incorporate them into biological tissues (proteins and nucleic acids).

Ammonification

The conversion of organic nitrogen from dead organisms and biological waste into ammonia (NH3) or ammonium (NH4+) by decomposers.

Nitrification

A two-step process carried out by soil bacteria that converts ammonia/ammonium (NH3/NH4+) into nitrite (NO2-) and then into nitrate (NO3-).

Denitrification

The conversion of soil nitrates (NO3-) back into atmospheric nitrogen gas (N2) by specialized anaerobic bacteria, completing the cycle.

Atmospheric Nitrogen (N2)

The largest reservoir of nitrogen on Earth, making up approximately 78% of the atmosphere, though it is inert and unusable by most organisms without fixation.

Legumes

Plants (such as beans and peas) that have root nodules housing symbiotic nitrogen-fixing bacteria.

Haber-Bosch Process

An industrial process that converts atmospheric nitrogen (N2) gas into synthetic ammonia (NH3) for use in commercial agricultural fertilizers.

Eutrophication

An environmental process caused by excess nitrogen/phosphorus runoff leading to algal blooms, high decomposer oxygen demand, hypoxiadead zones, and fish kills.

Acid Precipitation (Nitric Acid)

Acid rain (HNO3) formed when atmospheric nitrogen oxides (NOx) react with water vapor (H2O), lowering soil and lake pH.

Nitrate (NO3-)

The primary form of inorganic nitrogen preferred and absorbed by plants during assimilation.

Ammonium (NH4+)

An inorganic form of nitrogen produced by ammonification and nitrogen fixation that can be assimilated by plants or converted during nitrification.

Nitrogen Sink / Reservoir

A location where nitrogen is stored; the atmosphere is the primary reservoir, while biomass, soils, and sediments serve as secondary sinks.

Nitrogen Oxides (NOx)

Gaseous nitrogen compounds released by fossil fuel combustion that contribute to photochemical smog and acid precipitation (HNO3).

The starting molecule for nitrogen fixation

atmospheric nitrogen (N2)

The product of nitrogen fixation

ammonia (NH3), which can be utilized by plants.

The starting molecule for nitrification

ammonia (NH3)

The product of nitrification

nitrate (NO3-), which plants can absorb and utilize.

The starting molecule for denitrification

nitrate (NO3-)

The product of denitrification

nitrogen gas (N2), which is released into the atmosphere.

The starting molecule for assimilation

ammonium (NH4+)

Where nitrogen ends up after assimilation

in plant tissues

The starting point for ammonification

organic matter

The product of ammonification

is ammonium (NH4+)