1. Nitrogen cycle

3. The element Nitrogen also has the ability to cycle globally, because of its presence in the atmosphere. Nitrogen in the atmosphere is only found in the form N2. Only very few creatures can take in nitrogen in this form. Therefore, nitrogen-fixing bacteria in the soil take on the task. The bacteria come in two types: free bacteria and root nodule bacteria. Free bacteria survive on their own in the soil. These bacteria carry out the chemical reaction that changes N2 to ammonium (NH4+), called Nitrogen Fixation. Ammonium and nitrate (NO3-), another form of nitrogen, can be used by most plants and are found in Amino Acids. Nitrates come from nitrifying bacteria in the soil, which take in ammonium to create nitrates (called nitrification). These nitrates, in addition to supplying Amino Acids to plants, return to the atmospheric reservoir through a process called, incidentally, denitrification.

4. Finally we get to root nodule bacteria. These bacteria supply legumes, or pod-producing plants, with nitrogen that they cannot recieve on their own. This is a commensalistic relationship, because the bacteria do not benefit, but the legumes could not possibly survive without them. Another process in the Nitrogen Cycle is the decomposition of animals. As stated before, animals cannot absorb N2 on their own, so they must rely on the plants they eat (or lower level consumers) to get nitrogen. When these animals die, the nitrogen stays within them, as does Carbon, and detritovores transform it into ammonium, thus continuing the cycle. Abiotic reservoirs: atmosphere, soil Biotic reservoirs: animals, plants, detritivores, bacteria