2.3.17 (HL)—Nitrogen cycle stores

Organic nitrogen is in proteins and other carbon-based matter; inorganic nitrogen includes N₂, ammonium, nitrite and nitrate.

Plants generally cannot use atmospheric N₂ directly, so microbial conversion to soluble forms controls biological availability. The atmosphere is the largest pool, not necessarily the most accessible.

Protein in a dead leaf is organic nitrogen; nitrate dissolved in soil water is inorganic and can be taken up by roots.

Nitrate, because roots can absorb it; N₂ requires fixation first.

Largest pool does not equal usable pool; classify by chemical form and access.