In nitrogen is too unreactive to be used by

Inthe light independent reaction CO2 is also converted to O2 bythe plant.

It is taken up by the leaf through the stomata and diffuses throughto the chloroplast, where it combines with RuBP to form an unstable six-carbonmolecule. This breaks down into two GP molecules which are converted to TPusing the NADPH and ATP from the light dependent reaction. The two TP moleculescombine to form a hexose sugar that can be used to form other compounds. Afterdisassociating with other compounds, O2 is released into the air.  Inrespiration, more CO2 is released into the atmosphere and chemicalenergy is produced from internal processes in a plant in several stages. Duringglycolysis, two ATP molecules are used to phosphorylate glucose and break itdown into two 3-carbon molecules. These are then converted to pyruvate,reducing two molecules of NAD and using the energy released to convert fourmolecules of ADP to ATP. Pyruvate diffuses from the cytoplasm to the mitochondrialmatrix in the link reaction, where it gets converted into acetyl coenzyme A andreleases a CO2 molecule as well as reducing another NAD molecule.

Inthe Krebs cycle, acetyl coenzyme A is combined into a six-carbon compound.Carbon bonds are broken down step-by-step, releasing CO2 and energyin the form of ATP and NADH with each broken bond.  Nitrogenis used in the formation of amino acids, which are used to construct proteins.Although the air is around 80% nitrogen, in this form nitrogen is too unreactiveto be used by plants. Nitrates, however, can be used. This is achieved by thenitrogen cycle, wherein gaseous nitrogen is converted to nitrates.

Nitrogenfixing bacteria in the soil and nodules of plant roots convert nitrogen in theair to inorganic nitrogen compounds, as does lightning. The Haber processconverts gaseous nitrogen to ammonia, which are then converted to nitrates bynitrifying bacteria found in the soil. These compounds are all taken up by theplants, which use them to create proteins from amino acids. When these plantsare eaten by animals, their biomass is transferred to the animal.

Wastematerial is broken down by decomposers, returning nitrogen to the soil in theform of ammonia where it is converted to nitrates by nitrifying bacteria again.Decomposers also break down the bodies of dead animals, returning the nitrogenin their biomass to the soil as ammonia as well.  Eutrophicationis a process that kills all organisms living in a body of water.

Excessnutrients from fertilisers travel through underground water channels intorivers or lakes, or are flushed into them by rainwater. These cause an algalbloom, a large growth of plants that require these nutrients to grow. Thisblocks out the sunlight and uses up the oxygen in the lake in a positivefeedback loop, killing plants that required sunlight. These plants aredecomposed, using up even more oxygen. The ecosystem ultimately dies when somuch oxygen is used up that none of the lifeforms can survive.