Most decomposers are bacteria or fungi, which live on decaying organic matter. They absorb soluble molecules such as amino acids, organic acids and mineral salts. Insoluble materials such as starch, cellulose, fats and resins are broken down first. Decomposers digest complex molecules by secreting digestive enzymes onto their food surface, digesting it externally and then absorbing the broken down products.
The process results in the formation of ammonia in the soil. This is the point were nitrogen enters the biological cycle. These ammonium salts, plus any newly fixed molecules not yet absorbed by plants may be used as an energy substance by soil bacteria of the genus Nitrosomonas. These nitrifying bacteria oxidise ammonia to nitrite ions, which in turn can be used as an energy substrate by species of a second genus of soil bacteria, nitrobacteria. These bacteria further oxidise nitrite ions to nitrates, a form of nitrogen, which is once again available for the uptake by plant roots, providing another route within the nitrogen cycle.
This process is summarised by the equations
2NH3 + 3O2->2HNO2 +2H2O + Energy (nitrifying bacteria e.g. Nitrosomonas)
2HNO2 -> 2HNO3 +Energy (nitrifying bacteria e.g. Nitrobacter)
Nitrogen pays an important role in biology because of its incorporation in the structures of purines and pyramidines, which leads to the formation of DNA. The DNA molecule is the bases of reproduction, which constitutes partly of nitrogen. Nitrogen is also required in the process of respiration because of its formation in ATP molecules. Without the presence of nitrogen fixing bacteria, it will be inevitable to see the death and extinction of plants, which would severely damage the ecosystem. Rhizobium, which is present in root nodules of leguminous plants, is an example of mutual relationship. While the bacteria Rhizobium lives within the root nodules, the rhizobium converts atmospheric nitrogen into nitrates, which can be utilised by the plant. Alongside this, the root of the plant can fertilise the soil. This is of great benefit of the sustainability of the ecosystem, as chemical fertilisers will not have to be used. This reduces the effects of eutrophication, which can damage the ecosystem in aquatic ecosystems. Also, a lack in primary producers will without doubt decrease the energy available to all other trophic levels, thus affecting the biodiversity of the ecosystem. The most effected species will be the secondary and tertiary consumers as very little energy will be available to these trophic levels.
Another essential nutrient, which is vital to the fertility of the ecosystem, is carbon. Carbon exists as the key component of organic matter. This is due to its chemical structure in which the its four bonds can encompass other atoms to form complex molecules. It is also a vital feature in the sustainability of the soil in any ecosystem. Soil organic matter (SOM) constitutes of carbon and with the loss of carbon, the process of erosion will significantly enhance. This will lead to the conversion of fertile land into barren landscape. However, the sustainability of carbon can only be achieved through soil organisms like earthworms. Earthworms are an important detritivore in the formation and sustainability of soil. Not only is dead organic matter utilised, but also the structure within the soil caused by the earthworm is of benefit to other functions within the soil. The tunnelling of the worm allows the presence of oxygen, which can then be used in the process of nitrification of ammonia into nitrites or nitrites into nitrates. This then provides plants the sources for growth and reproduction.
It is quite obvious that soil organism is vital to the sustainability of any ecosystem. Without the recycling of vital nutrients such as carbon and nitrogen, the key components of life cannot be available to any organisms. This will lead to the reduction of biodiversity. It can even lead to the extinction of organisms in the higher trophic levels as energy and resources are not sufficient in the lower levels. Therefore, humans owe these soil organisms the diversity and substantial fertility of species within ecosystems.
