Biogeochemical cycles

Carbon: plants build organic matter from CO2 in photosynthesis; respiration and decay return it to the atmosphere; some is stored for long periods as limestone, coal, oil and peat. Burning fuels raises atmospheric CO2 and warms the planet. Nitrogen: the atmosphere is rich in N2 but most organisms cannot use it; nodule bacteria and free-living nitrogen fixers convert it to ammonia, decomposers split protein to ammonia (ammonification), nitrifying bacteria oxidise it to nitrite and nitrate, and denitrifying bacteria return it as N2. Excess nitrogen fertiliser pollutes water bodies. Oxygen is released in photosynthesis, used up in respiration and forms the ozone layer. Water evaporates with solar energy, condenses and returns as precipitation; the ocean supplies most of the evaporation. Phosphorus is almost absent from the atmosphere; it is taken from rocks, used as fertiliser, washes into water and slowly passes into sediments. Sulfur is taken up as sulfates; burning fuel releases SO2, which causes acid rain.