Nitrogen Cycle (words)

Fixation: N₂ → usable nitrogen compounds (e.g. ammonia / ammonium). Nitrification: ammonium → nitrite → nitrate (microbial oxidation). Assimilation: plants take up nitrates / ammonium into organic nitrogen. Ammonification: organic N → ammonium (decay / decomposers). Denitrification: nitrates → N₂ (returned to air). Cycle = closed loop of these named words.

Atmospheric N₂ is plentiful but not directly usable by most plants. Fixation (biological, industrial Haber link, or lightning in some syllabi) converts N₂ into combined forms such as ammonia/ammonium. Nitrifying bacteria oxidise ammonium through nitrite to nitrate — nitrification. Plants then assimilate nitrate or ammonium into proteins and other organic nitrogen — assimilation. When organisms die or excrete, decomposers release ammonium again — ammonification. Denitrifying bacteria convert nitrates back to N₂ gas — denitrification — closing the cycle. Exam answers win by defining each word precisely and showing the sequence of chemical forms: N₂ → NH₄⁺ → NO₂⁻ → NO₃⁻ → organic N → NH₄⁺ → N₂.

Nitrogen moves in a cycle between air, soil, and living things. Five words to learn: fixation, nitrification, assimilation, ammonification, denitrification. Fixation starts the usable-nitrogen story; denitrification sends nitrogen back to the air as N₂.

Map each word to a chemical change. Fixation: N₂ → combined N. Nitrification: NH₄⁺ → NO₂⁻ → NO₃⁻. Assimilation: inorganic N into plant/animal organic N. Ammonification: organic N → NH₄⁺. Denitrification: NO₃⁻ → N₂. Link Haber as an industrial fixation route when asked, but keep the biological cycle vocabulary separate and clear.

School cycle answers are process-name accurate rather than full microbial taxonomy. Emphasise redox ideas lightly: nitrification is oxidation of nitrogen in ammonium toward nitrate; denitrification is reduction of nitrate back toward N₂. Human fertiliser use (from Haber ammonia) increases available soil nitrogen and can intensify runoff / denitrification discussion in extended answers.

Fixation → Nitrification → Assimilation → Ammonification → Denitrification (loop back to N₂).