The largest nitrogen reservoir on Earth is the atmosphere, which holds about 78% nitrogen gas (N₂). Nitrogen enters this reservoir primarily through denitrification , where soil bacteria convert nitrates (NO₃⁻) back into atmospheric N₂ gas.

Key Nitrogen Reservoirs

The atmosphere dominates as the biggest pool of nitrogen, vastly outpacing others:

  • Atmosphere : ~3.9 × 10¹⁵ tons of N₂, inert and stable.
  • Lithosphere (rocks/sediments) : Stores fixed nitrogen but much smaller scale.
  • Oceans/Biosphere : Minor compared to air; hold dissolved or organic forms.

Deep Earth mantle may rival it geologically, but surficial (surface) cycles focus on the atmosphere.

Pathways Adding Nitrogen to Atmosphere

Since "which of the following" implies multiple-choice options (common in quizzes), here's how pathways work in the nitrogen cycle:

  • Denitrification (e.g., bacteria like Pseudomonas → NO₃⁻ to N₂): Primary pathway adding N₂ to atmosphere. Releases fixed nitrogen back to air.
  • Nitrogen fixation (lightning/bacteria → N₂ to NH₃): Pulls N₂ out of atmosphere into usable forms.
  • Nitrification (NH₄⁺ to NO₃⁻): Soil process; doesn't add to air.
  • Decomposition : Recycles organic N in soil; indirect at best.

If options were listed (e.g., from quiz sources), select the one showing denitrifying bacteria or NO₃⁻ → N₂.

Why Denitrification Matters

This anaerobic bacterial process in wet, low-oxygen soils prevents nitrogen buildup in ecosystems. Human fertilizers boost it, contributing to ~100 Tg N/year globally returned to air. Without it, soils would overload with nitrates.

Pathway| Adds to Atmosphere?| Direction| Example
---|---|---|---
Denitrification 5| Yes| NO₃⁻ → N₂| Bacteria in wetlands
Fixation 9| No| N₂ → NH₃| Lightning, legumes
Nitrification 5| No| NH₄⁺ → NO₃⁻| Nitrosomonas bacteria
Decomposition 10| Indirect/No| Organic → NH₄⁺| Fungi, bacteria

TL;DR: Denitrification pathway adds nitrogen to the atmosphere (largest reservoir).

Information gathered from public forums or data available on the internet and portrayed here.