The Nitrogen Cycle: A brief introduction.

I thought I'd start with a basic overview of the functioning of the nitrogen cycle.  Hopefully this will make my blog a little more readable when things get more in-depth later on.  Anybody who is well acquainted with this topic should probably give this a miss and move on to the more interesting stuff to follow.

Nitrogen has a vital role in the biosphere, it is an essential nutrient and major component of nucleic acids and proteins. Almost 80% of the atmosphere is made up of N2 gas, however before nitrogen is available to be used to support growth it must first be used to form ammonium NH4 or nitrate NO3 ions.  The availability of nitrogen in this 'fixed' form is the key limiting factor for plant growth and controls the productivity of most ecosystems.  Nitrogen fixation is a process performed by nitrogen-fixing organisms and involves the conversion of atmospheric nitrogen to ammonia by the nitrogenase enzyme.  Leguminous plants and some nitrogen fixing bacteria have a mutualistic relationship, where the bacteria live in the root nodules of the legume and produce ammonia in exchange for carbohydrate.  

The quality of soils can be improved by planting legumes to increase the soil nitrogen content, this is a fairly simplistic way for humans to influence the nitrogen cycle.  Nitrogen fixation also occurs artificially, the Haber-Bosch process is used in industry to create ammonia by combining N2 and Hydrogen at high temperature and pressure.  The ammonia formed industrially accounts for 30% of all fixed nitrogen, this is a major anthropogenic influence on the natural cycle. The combustion of fossil fuels is another anthropogenic process where atmospheric nitrogen is converted to a reactive form as a result of human activity.

The next process in the cycle is nitrification, this is where nitrifying bacteria oxidise ammonium in order to gain energy and create nitrate in the process, they also use carbon dioxide to synthesise organic compounds.  This causes problems as nitrate is easily leached from the soil, resulting in a loss of soil fertility and a potential build up of nitrate in groundwater, which can later contaminate drinking water supplies. The nitrate present can then be reduced to nitrite, a highly reactive compound. The health risks associated with this include 'blue baby syndrome' (respiratory problems in infants) and  the formation of carcinogenic compounds in the gut.

Other processes in this cycle replace nitrogen to the atmosphere. This is denitrification, which involves the conversion of nitrate to gaseous nitrogen compounds (e.g. nitric oxide, nitrous oxide and N₂ ).  In the natural nitrogen cycle the nitrogen released to the atmosphere by denitrification is approximately equal to the amount of nitrogen fixation occurring.  

For some further information and an outstanding summary of the processes of the global nitrogen cycle, as well as disturbances and their impacts that will be the subject of subsequent posts....see the Bernhard (2010) summary in Nature Education Knowledge.