Carbon exists in elemental form as diamond and graphite.

Earth’s relatively thin atmosphere primarily consists of a mixture of nitrogen (78%) and oxygen (21%) gases. The remaining 1% contains several inactive gases (i.e. argon, neon, helium, hydrogen, and xenon) and several other gases that vary in concentration (i.e. water vapour, carbon dioxide, methane, nitrous oxide, ozone, and chlorofluorocarbons).

Aerobic respiration  is the major process that utilises oxygen gas to produce energy in the form of ATP.

The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air, forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth.

Rhizobium is a soil bacterium that fixes nitrogen (diazotrophs) after becoming established inside the root nodules of legumes (Fabaceae).

 Oxygen constitutes 21% of the atmosphere.

Nitrogen is a vital ingredient to plant life. Most industrial fertilisers contain high concentrations of nitrogen compounds, such as ammonium nitrate. Animals eat plants and turn the nitrogen into proteins. When the animals die, bacteria break down the proteins, releasing N₂ back into the atmosphere to begin the cycle again.

Nitrate reduction in plants is carried out in two steps. Nitrate is first reduced to nitrite (NO²⁻) in the cytosol by nitrate reductase using NADH or NADPH. Nitrite is then reduced to ammonia in the chloroplasts (plastids in roots) by a ferredoxin-dependent nitrite reductase. In photosynthesising tissues, it uses an isoform of ferredoxin (Fd1) that is reduced by PSI, while in the root, it uses a form of ferredoxin (Fd3) that has a less negative midpoint potential and can be reduced easily by NADPH. In non-photosynthesising tissues, NADPH is generated by glycolysis and the pentose phosphate pathway. In the chloroplasts, glutamine synthetase incorporates this ammonia as the amide group of glutamine using glutamate as a substrate. Glutamate synthase (Fd-GOGAT and NADH-GOGAT) transfer the amide group onto an 2-oxoglutarate molecule producing two glutamates. Further transaminations are carried out to make other amino acids (most commonly aspargine) from glutamine. 

 Green house effect raises the temperature on Earth leading to global warming.

Denitrification is the reduction of nitrates back into the largely inert nitrogen gas (N₂), completing the nitrogen cycle.

Amino acids play a central role, both as building blocks of proteins and as intermediates in metabolism.

Condensation is the change of the physical state of matter from gaseous phase into liquid phase, and is the reverse of evaporation.

Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as kudzu, clovers, soybeans, alfalfa, lupines, peanuts, and rooibos. They contain symbiotic bacteria called rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants.

Alkaloids are a group of naturally occurring chemical compounds that contain mostly basic nitrogen atoms. Urea serves an important role in the metabolism of nitrogen-containing compounds by animals, and is the main nitrogen-containing substance in the urine of mammals.

 Among all the processes given, only combustion adds carbon dioxide to the atmosphere.

Underground water finds its way to the surface through springs.

Evaporation is a type of vaporisation of a liquid that occurs from the surface of a liquid into a gaseous phase that is not saturated with the evaporating substance.

Water seeping down from the land surface adds to the ground water and is called recharge water. Ground water is recharged from rain water and snowmelt or from water that leaks through the bottom of some lakes and rivers. Ground water also can be recharged when water-supply systems (pipelines and canals) leak and when crops are irrigated with more water than the plants can use.

Condensation is the change of water from its gaseous form (water vapour) into liquid water. Perspiration, also known as sweating or diaphoresis, is the production of fluids secreted by the sweat glands in the skin of mammals. Respiration is defined as the movement of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction. Transpiration is the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapour and is released in the atmosphere.