The process called nitrogen cycle is when nitrogen is converted to other forms of the element. There are two ways that this process is carried out and it’s either a biological process or a non-biological process. The important processes for the nitrogen cycle include nitrification, fixation, denitrification and mineralization. 78% of the Earth’s atmosphere is composed of nitrogen which is why it’s the largest sources of nitrogen but this atmospheric nitrogen is not available for biological use which leads to the scarcity of the nitrogen that is usable for the ecosystem. Ecologists are interested with the nitrogen cycle because the availability of nitrogen can affect the ecosystem process rate that includes the primary production up to the decomposition. The global nitrogen cycle had been significantly altered by human activities like the fossil fuel combustion and the use of artificial fertilizers containing nitrogen and also by releasing waste water with nitrogen.
Ecological functions of Nitrogen
Nitrogen is important part of different processes and it’s crucial for any living thing here on Earth. It’s in amino acids, protein, nucleic acids like our DNA and our RNA. Plants need nitrogen too for their chlorophyll that are essential molecules for photosynthesis and for the growth of plants. Most of the nitrogen from the atmosphere are useless to plants. This nitrogen needs to undergo a chemical process called natural fixation wherein it’s converted by bacteria to make it useful for live organisms. This is why nitrogen is an important element for the production of food. Our food supply is dictated by the amount of reactive nitrogen available on the land where it’s grown.
Processes of the nitrogen cycle
Nitrogen is in the environment in many different chemical forms such as the organic nitrogen, the NH4 ammonium ion, nitrate or no3- and the nitrogen gas or N2. The nitrogen cycle process transforms one form of nitrogen to another and most often it’s the microbes that carry out this process in order to produce energy or to accumulate the nitrogen in the needed form for their growth.
The atmospheric nitrogen needs to be processed through nitrogen fixation for it to be useful to plants. Nitrogen fixation can occur with lightning strikes but most often it’s caused by symbiotic bacteria or the free living bacteria. Nitrogenase enzymes from these bacteria combine with the N2 gas with Hydrogen in order to produce the ammonia which is then being converted to the organic compounds. Rhizobium is one example of these bacteria and they live in the roots of legumes like beans or peas. They are in a mutual relationship with the legumes where they produce the needed ammonia while they are receiving carbohydrates from the plants. Soils that are deficient in nutrients can regain the nutrients by planting legumes on them to increase the nitrogen. Few plants can do this too. Now, 30% of the fixed nitrogen is being manufactured by chemical plants for ammonia.
Conversion of N2
The conversion of the gas nitrogen from the atmosphere to a form that is readily available to plans is important for animals and humans as it’s an important step for the nitrogen cycle that distributes the essential nutrient. There are 4 ways to convert the atmospheric nitrogen gas to more reactive chemical forms. First is the biological fixation. Symbiotic bacteria most often found on legumes and some other free-living bacteria cn fix the nitrogen to be organic nitrogen. Rhizobium bacteria is one example of this type of bacteria and they live in the root of legume nodules. They are diazotrophs. Free-living bacteria example is the Azotobacter.
At 600 degrees centigrade and with iron as a catalyst, the atmospheric nitrogen combined with hydrogen from petroleum or natural gas, at really high pressure would be turned to ammonia or NH3. Explosives and fertilizers use the Haber-Bosch process of converting the gaseous nitrogen and hydrogen to NH3. Fossil fuel combustion release NOx. There are other processes that could form NO from the Nitrogen and Oxygen from the atmosphere such as the photons caused by lightning.
There are plants that absorb nitrogen from the soil in the form of ammonium or nitrate ions. Nitrogen obtained by animals and humans can be traced to the foods they eat. Plants have root hairs that absorb the ammonium or the nitrate ions from the soil. Nitrate is firstly reduced to nitrite ions then to ammonium ions to be incorporated into amino acids, chlorophyll and nucleic acids. Plants that have a mutual relationship with the rhizobia bacteria, some of the nitrogen is absorbed as ammonium ions from the nodules directly. Animals, heterotrophic organisms and fungi are able to absorb nitrogen in the form of amino acids, small organic molecules or as nucleotides.
Ammonification is the process wherein the organic form of nitrogen is turned to ammonium ion again. This happens when an animal or plant dies or from their expelled waste. Enzymes that are involved in this process are GS or Gln Synthetase, GOGAT, and GDH.
Ammonium conversion to nitrate is primarily performed by the soil-living bacteria as well as other nitrifying bacteria. The first stage is the oxidation of the ammonium ion that is performed by these bacteria like the Nitrosomonas that converts the ammonium to NO2 ions. Nitrobacter and other species of bacteria then convert nitrites to nitrates. This is important because too much of the nitrite ions are already toxic to plants.
The denitrification process reduces the nitrates to the inert nitrogen gas again and this is what completes the nitrogen cycle. Pseudomonas and the Clostridium bacteria are in charge of this process in an anaerobic condition. They use the nitrate in place of the oxygen for respiration as an electron acceptor.