Nitrogen Cycle Processes Overview

Nitrogen Fixation

• Certain bacteria and organisms, including those in legume roots, convert atmospheric nitrogen (N₂) into ammonia (NH₃) or ammonium ions (NH₄⁺). This is called nitrogen fixation.
• Lightning can also fix nitrogen, creating nitrogen oxides (that become usable forms like nitrate (NO₃⁻)).

Nitrification

• Nitrifying bacteria (like Nitrosomonas) convert ammonia into nitrites (NO₂⁻).
• Other nitrifying bacteria (like Nitrobacter) convert nitrites into nitrates (NO₃⁻).
• Plants primarily absorb nitrates, making them essential.

Assimilation

• Plants absorb nitrates or ammonium from the soil to build essential compounds like proteins.
• Animals get nitrogen by consuming plants or other animals, and incorporate it into their own biological molecules (proteins and DNA).

Ammonification (Decomposition)

• When organisms die, decomposers (bacteria and fungi) break down organic matter, releasing nitrogen as ammonia (NH₃) or ammonium (NH₄⁺).
• This released nitrogen can be utilized by plants or further processed.

Denitrification

• In oxygen-poor environments (like waterlogged soil), denitrifying bacteria convert nitrates (NO₃⁻) back into nitrogen gas (N₂).
• This released nitrogen gas returns to the atmosphere, completing the cycle.

Human Impact

• Excessive use of nitrogen fertilizers can lead to nitrate runoff, harming aquatic ecosystems (eutrophication) and contaminating water sources.
• Burning fossil fuels and industrial processes release nitrogen oxides (NOₓ), causing air pollution and acid rain.
• Changes in land use (e.g., deforestation) disrupt this natural cycle.