The Nitrogen and Phosphorus Cycles

Nitrogen (N₂) makes up 78% of the atmosphere but is unusable by most organisms in its gaseous form. Nitrogen fixation—performed by specialized bacteria in soil and root nodules of legumes—converts N₂ into ammonia (NH₃), which plants can absorb. Nitrifying bacteria convert ammonia to nitrite (NO₂⁻) and then nitrate (NO₃⁻), the form most readily used by plants. Animals obtain nitrogen by eating plants. When organisms die, decomposers release nitrogen as ammonia (ammonification). Denitrifying bacteria convert nitrates back to N₂ gas, completing the cycle. Lightning also fixes a small amount of nitrogen naturally.