The Nitrogen, Phosphorus, and Water Cycles

Nitrogen gas makes up 78 percent of the atmosphere, but its two atoms are joined by a triple bond so stable that plants cannot break it. Plants need nitrogen to build amino acids and nucleotides, but they can only absorb it in fixed forms like ammonium (NH₄+) or nitrate (NO₃-). Nitrogen-fixing bacteria, living freely in soil or in nodules on legume roots, have the enzymes needed to break that triple bond and combine nitrogen with hydrogen. Other bacteria then convert ammonium to nitrate through nitrification, making it available for uptake. Denitrifying bacteria eventually send nitrogen back to the atmosphere as N2, completing the loop.

Phosphorus never takes this atmospheric detour. It has no stable gas phase, so it cycles entirely through solid and dissolved forms: phosphate locked in rock is released by weathering, dissolves into soil and water, gets taken up by plants, passed through food chains, and eventually returns to sediment or rock over long timescales. That makes phosphorus the slower, more geological of the two cycles, and often the nutrient that limits growth in freshwater ecosystems.

The water cycle links both of them. Evaporation from oceans and lakes, and transpiration from plant leaves, send water vapor into the atmosphere; it condenses into clouds and falls as precipitation, which drives weathering and carries dissolved nitrogen and phosphorus into soil and waterways. All three cycles show the same big idea in Unit 8: matter is finite and reused, unlike energy, which flows through an ecosystem once and is lost as heat.