Describe and illustrate Earth's nitrogen cycle.
The nitrogen cycle is the process by which nitrogen is converted into different forms and cycles through the ecosystems on Earth. Nitrogen is an essential element for the growth and development of living organisms, including plants and animals. However, nitrogen gas (N2) in the atmosphere cannot be used directly by most organisms. Therefore, nitrogen must be converted into other forms before becoming available for use.
1. Nitrogen fixation: The nitrogen fixation process involves the conversion of atmospheric nitrogen gas (N2) into a usable form, such as ammonia (NH3) or nitrate (NO3-). This conversion is facilitated by certain bacteria, such as Rhizobium, which live in symbiosis with leguminous plants (e.g., peas, beans, clover). Lightning also contributes to nitrogen fixation by converting atmospheric nitrogen into nitrogen oxide compounds.
2. Assimilation: Assimilation is the process by which plants and animals take up the nitrogen compounds (e.g., ammonia or nitrates) from the soil and incorporate them into their own tissues. Plants primarily assimilate nitrogen in the form of nitrates through their roots, while animals obtain nitrogen by consuming plants or other nitrogen-rich organisms.
3. Ammonification: After the death and decomposition of plants and animals, nitrogen-rich organic matter, such as proteins, DNA, and urea, is converted back into ammonia (NH3) through a process called ammonification. This process is carried out by bacteria and fungi present in the soil.
4. Nitrification: Nitrification is a two-step process that converts ammonia (NH3) into nitrate (NO3-), which is the primary form of nitrogen used by plants for growth. The first step involves the oxidation of ammonia to nitrite (NO2-) by nitrifying bacteria, such as Nitrosomonas. The second step involves the oxidation of nitrite to nitrate by other nitrifying bacteria, such as Nitrobacter.
5. Denitrification: Denitrification is the opposite of nitrogen fixation, where nitrogen compounds are converted back into atmospheric nitrogen. This process is carried out by denitrifying bacteria under anaerobic conditions, such as in waterlogged soils or sediments. Denitrification helps to maintain the balance of nitrogen in the environment by preventing nitrogen accumulation.
The nitrogen cycle is a continuous process, driven by the interactions between plants, animals, bacteria, and other organisms. It plays a crucial role in maintaining the balance of nitrogen in ecosystems and is essential for sustaining life on Earth.