The nitrogen, water and phosphorus cycles

Nitrogen — the bacterial cycle

• Plants can only absorb nitrogen as NO₃⁻ or NH₄⁺, never as N₂ gas, so fixation is indispensable.
• Sequence: N₂ —fixation→ NH₃/NH₄⁺ —nitrification→ NO₂⁻ → NO₃⁻ —assimilation→ plant and animal protein —death and excretion— NH₄⁺ —ammonification→ —denitrification→ N₂.
• Legumes host Rhizobium in root nodules in a mutualism: the bacterium gets sugars, the plant gets amino acids — the basis of crop rotation with clover, beans and alfalfa.
• Human interference: the Haber–Bosch process fixes more nitrogen industrially than all natural fixation, and excess fertiliser runs off into water (eutrophication) or is denitrified to N₂O, a powerful greenhouse gas.

Water

Evaporation from oceans, transpiration from plants, condensation into clouds, precipitation, then run-off, infiltration into aquifers and return to the sea — driven by solar energy and gravity, so it is a cycle that needs no chemical change of the substance.

Phosphorus — no gaseous phase

Phosphate is released by the weathering of rocks, taken up by plants, passed through consumers, returned by decomposers, and finally precipitates in sediments on the sea floor; it returns to land only by uplift or by guano and fishing. Because there is no air reservoir, phosphorus is the limiting nutrient in many freshwaters, and its addition from detergents triggers algal blooms.