Biogeochemical Cycles

Carbon and water: the two fast cycles

The carbon cycle moves carbon between four reservoirs: the atmosphere, the oceans, the biosphere, and rock plus fossil fuels. Photosynthesis is the main flux out of the atmosphere; respiration, decomposition, and combustion are the fluxes back in. The largest reservoir by far is sedimentary rock (mostly limestone), but it turns over on geological timescales; the ocean is the largest actively exchanging reservoir. Burning fossil fuels transfers carbon from a slow reservoir into a fast one, which is why the atmospheric pool is growing.

The water cycle

Driven by solar energy and gravity. Key fluxes: evaporation, transpiration from plants (together evapotranspiration), condensation, precipitation, runoff, and infiltration that recharges groundwater. Paving a watershed cuts infiltration and raises runoff — the same rainfall then arrives as a flood peak instead of soaking in.

Nitrogen and phosphorus: the two limiting nutrients

Atmospheric $\mathrm{N_2}$ has a triple bond no plant can break, so nitrogen must be fixed to $\mathrm{NH_3}$ by bacteria (largely Rhizobium in legume root nodules), by lightning, or industrially by the Haber–Bosch process. Nitrification then oxidises $\mathrm{NH_4^+}\to\mathrm{NO_2^-}\to\mathrm{NO_3^-}$ and needs oxygen; denitrification returns $\mathrm{N_2}$ to the air and runs only where oxygen is absent.

Phosphorus has no atmospheric phase. It is released by weathering of rock, cycles through organisms, and is buried in sediment. That slow release is why phosphorus is the usual limiting nutrient in fresh water, and why a sudden fertiliser input triggers eutrophication so violently.