What is the significance of the nitrogen cycle in ecosystems? Using suitable examples, discuss the role of microorganisms in the Nitrogen Cycle. How can Man influence the Nitrogen Cycle?

Some bacteria belonging to the genus Rhizobium live in close association with the roots of leguminous seed plants such as peas, soybeans and alfalfa. These bacteria are normally free-living in the soil but when they are close to the root of a legume they infect it. They invade the root hairs and cause the production of an infection thread in the root hair cell (Heriot-Watt University, 2002). This enables the bacteria to penetrate deeper into the root tissue and infect the cells there. The infected root cells become enlarged and undergo rapid division to form a mass of cells called root nodules. The plant then supplies the bacteria with sugars which they use to provide energy for nitrogen fixation. Also the plant makes a compound called leghaemoglobin which traps any oxygen present. This is important because nitrogen fixation is an anaerobic process and the presence of oxygen deactivates the nitrogenase.

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

Nitrification

Nitrification is the conversion of ammonia to nitrate by soil bacteria. It is a two stage process. In the first stage Nitrosomonas utilise ammonia (NH ) as an energy source to oxidise it to nitrites

( NO ). In the second stage Nitrobacter oxidise the nitrites to nitrates (NO ). Both of these stages require oxygen and so must be performed in the presence of free oxygen. These two groups of autotrophic bacteria are called nitrifying bacteria. Plants take up the NO released by the bacteria and convert it to organic molecules such as amino acids.

Assimilation

This is the uptake of nitrite or ammonia by primary producers and its incorporation into proteins or nucleic acids. These materials are then passed along the food chain through digestion and assimilation at each trophic level.

Denitrification

This is the conversion of nitrates to unusable gaseous nitrogen to the atmosphere. The process is carried out by denitrifying bacteria living deep in the soil in aquatic sediments where the conditions are anaerobic. An example is the bacteria Pseudomonas who use nitrites or nitrates as an alternative to oxygen for the final acceptor in their respiration.

Leaching

Another loss of nitrogen form the ecosystem is through leaching. This is when some of the highly soluble nitrate is taken from the soil by percolating water abd ends up running into rivers and streams and eventually to the ocean.

Importance of the Nitrogen Cycle

Nitrogen is an essential element in the structure of DNA, RNA and ATP. It is needed for protein synthesis as this process involves the use of RNA. The three types of protein are fibrous protein, globular protein and conjugated protein. Fibrous protein is needed for muscles, bones and connective tissue. Globular protein is needed for the production of enzymes, hormones and antibodies. Conjugated protein is needed for haemoglobin and mucus. Thus the cycling of nitrogen is essential for life. As nitrogen is constantly lost form the system, it needs to be fixed into the substances such as ammonia and nitrates to be assimilated by plants then to animals and through the trophic levels eventually being returned to the atmosphere by denitrification.

Man's Influence on the Nitrogen Cycle

Man has caused major influences on the nitrogen cycle mainly through agriculture. When a farmer plants a non-leguminous plant continuously on the same soil the nitrogen availability decreases. A way this is solved is by planting leguminous plants which enrich the soil by the root nodule bacteria releasing nitrogen. This can also benefit plants nearby as it increases the amount of nitrogen available in the area. Crop rotation involving periods of growing leguminous plants can be used to help improve the soil. This means farmers don't always need to use expensive and environmentally damaging synthetic nitrogen fertilisers. When used, often the nutrients from the fertilisers can be leached from the soil especially when they have been applied at the wrong time of year, just before heavy rain or in excessive quantities. This means the plants are unable to use the nutrients before they are leached from the soil. The excess fertiliser is carried away to adjacent water bodies where toxic cyanobacteria can increase due to the increase in nutrient levels. This can also lead to deoygenation of the water column due to the increased respiration or decomposition. This nutrient enrichment of natural water bodies as a result of pollution is called eutrophication.