Nitrogen Cycle

Nitrogen CycleThe nitrogen cycle is the movement of nitrogen through different environmental segments.

Nitrogen

Nitrogen is essential for life. It is found in amino acids, proteins, and genetic material.

Nitrogen in our environment

Nitrogen is the most abundant element in the atmosphere (~78%). However, gaseous nitrogen must be 'fixed' into another form so that it can be used by living organisms.Forms of Nitrogen useable for plantsUrea CO(NH2)2Ammonia NH3 (gaseous)Ammonium NH4Nitrate NO3Nitrite NO2Nitrogen FixationNitrogen fixation means making atmospheric nitrogen (N2) available for plant consumption through ammonia and nitrates. Nitrogen fixation occurs naturally in the following ways.LighteningBiological fixationAmmonificaiton Nitrification

Fixation by LightningThe energy from lightning causes nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground, where they can be assimilated by plants.

Biological FixationAbout 90% of nitrogen fixation is done by bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium.N2+ 3H2 2NH3Ammonia can be used by plants directly. Ammonia and ammonium may be further reacted in the nitrification process.

Ammonification When plants and animals die, bacteria convert nitrogen nutrients back into ammonium salts and ammonia. This conversion process is called ammonification.

Amino acids + 11/2O2 CO2 + H2O + NH3+ 736kJ This process liberates a lot of energy which can be used by the Saprotrophic microbes8NitrificationNitrification occurs by the following reactions:2 NH3+ 3 O2 2 NO2+ 2 H++ 2 H2O2 NO2-+ O2 2 NO3-Aerobic bacteria use oxygen to convert ammonia and ammonium. Nitrosomonas bacteria convert nitrogen into nitrite (NO2-) and then nitrobacter convert nitrite to nitrate (NO3-).

The nitrogen fixersCyanobacteria are nitrogen fixers that also fix carbon (these are photosynthetic)Rhizobium bacteria are mutualistic with certain plant species e.g. LegumesThey grow in root nodulesAzotobacter are bacteria associated with the rooting zone (the rhizosphere) of plants in grasslands 10Nitrogen fixation by bacteriaAtmospheric nitrogen is chemically fixed by bacteria to ammonium (NH4+1) , which is used by plants as a fertilizer.Some bacteria exist in a symbiotic relationship with plants (legumes and some root-nodule species). Plants utilize the nitrate as a nutrient. Animals obtain nitrogen by eating plants or plant-eating animals.

11Only prokaryotes show nitrogen fixationThese organisms possess the nif gene complex which make the proteins, such as nitrogenase enzyme, used in nitrogen fixationNitrogenase is a metalloprotein, protein subunits being combined with an iron, sulphur and molybdenum complexThe reaction involves splitting nitrogen gas molecules and adding hydrogen to make ammonia

N2 2N- 669 kJ2N + 8H+ NH3 + H2 + 54 kJ

This is extremely energy expensive requiring 16 ATP molecules for each nitrogen molecule fixedThe microbes that can fix nitrogen need a good supply of energy 12NitrificationThis involves two oxidation processesThe ammonia produced by ammonification is an energy rich substrate for Nitrosomas bacteriaThey oxidise it to nitrite:NH3 + 11/2O2 NO2- + H2O + 276kJ

This in turn provides a substrate for Nitrobacter bacteria oxidise the nitrite to nitrate:NO3- + 1/2O2 NO3- + 73 kJ

This energy is the only source of energy for these prokaryotes so they are called chemoautotrophs.

13Return to the atmosphere: DenitrificationNitrates and nitrites can be used a source of oxygen for Pseudomonas bacteria Favourable conditions: Cold waterlogged (anaerobic) soils2NO3- 3O2 + N22NO2- 2O2 + N2 providing up to 2385kJThe liberated oxygen is used as an electron acceptor in the processes that oxidise organic molecules, such as glucoseThese microbes are, therefore, heterotrophs14

Industrial N-FixationThe Haber-Bosch Process N2 + 3H2 2NH3- 92kJThe Haber process uses an iron catalyst High temperatures (500C) High pressures (250 atmospheres)The energy required comes from burning fossil fuels (coal, gas or oil)Hydrogen is produced from natural gas (methane) or other hydrocarbon 16

Nitrogen Cycle

Atmospheric PollutionThe exhaust emissions of cars contribute a lot to atmospheric pollution in the form of NOxThese compounds form photochemical smogs They are green house gasesThey dissolve in rain to contribute to acid rain in the form of nitric acidThe rain falling on soil and running into riversThey contribute to the Eutrophication of water bodies19How to make things better?The need for synthetic fertilisers can be reduced by cultural practicesAvoiding the use of soluble fertilisers in sandy (free draining soil) prevents leachingRotating crops permits the soil to recover from nitrogen hungry crops (e.g. wheat)Adding a nitrogen fixing crop into the rotation cyclePloughing aerates the soil and reduces denitrificationDraining water logged soil also helps reduce denitrification20