8th Grade Science Study Notes: The Nitrogen Cycle

Big Picture and Core Facts

• The Nitrogen Cycle: The constant movement of nitrogen between living (biotic) and non-living (abiotic) parts of an ecosystem.

• Atmospheric Nitrogen ($N_2$): Makes up $78\%$ of the air surrounding Earth.

• Organism Requirements: All living things need nitrogen to survive, including plants, bacteria, and animals.

• Direct Usage: Plants and animals cannot absorb or use atmospheric nitrogen gas ($N_2$) directly from the air.

• Biological Purpose of Nitrates: Plants and animals use nitrates to create the protein necessary to live, grow, and build biological tissue.

Order, Path, and Human Nitrogen Acquisition

• Sequential Path through the Cycle:

  • $\text{Air} \rightarrow \text{Bacteria} \rightarrow \text{Plants} \rightarrow \text{Animals}$

• Human Acquisition of Nitrogen: Humans cannot obtain nitrogen by breathing it in. Humans acquire nitrogen by consuming food, specifically meat and vegetables.

• Full Process from Atmosphere to Humans:

  1. Atmospheric nitrogen gas ($N_2$) in the atmosphere is pulled into the soil.

  2. Soil bacteria fix $N_2$ into usable nitrogen compounds.

  3. Plants take in the nitrogen compounds from the soil through their roots.

  4. Animals and humans eat the plants (or consume animals that ate plants), passing the nitrogen up the food web.

Chemical Compounds and Their Sources

• Ammonia ($NH_3$):

  • Produced by nitrogen-fixing bacteria located in soil and plant roots.

  • Produced through lightning fixation.

  • Produced when decomposers break down animal waste and dead biological matter.

• Nitrite ($NO_2$):

  • Formed in the soil during nitrification when specialized bacteria convert ammonia ($NH_3$) into nitrites ($NO_2$).

• Nitrate ($NO_3$):

  • Formed in the soil during nitrification when bacteria convert nitrites ($NO_2$) into nitrates ($NO_3$).

  • Can originate directly from lightning fixation carried down to the soil by precipitation.

• Plant Assimilation Preference: Plants specifically assimilate Nitrate ($NO_3$).

Key Vocabulary and Biological Processes

• Nitrogen Fixation:

  • Definition: The process of combining atmospheric nitrogen gas ($N_2$) with other elements to convert it into usable compounds.

  • Performers: Carried out by bacteria in the soil, bacteria residing in the root nodules of legumes (such as beans and peas), and atmospheric lightning.

• Nitrification:

  • Definition: The process of converting ammonia ($NH_3$) into nitrites ($NO_2$) and subsequently into nitrates ($NO_3$).

  • Location: Occurs in the soil.

• Assimilation:

  • Definition: The process by which plants take in and absorb nitrogen compounds (nitrates) from the soil through their root systems.

• Decomposition (Ammonification):

  • Definition: A component of the decay process where dead organisms and animal wastes are broken down by bacteria and fungi, converting the bound nitrogen back into ammonia ($NH_3$).

• Denitrification:

  • Definition: The biological process where excess nitrogen compounds in the soil are converted back into atmospheric nitrogen gas ($N_2$) and released back into the air; carried out by soil bacteria.