The correct answer is D. By denitrification.
Nitrogen fixation is the process by which atmospheric nitrogen (N2) is converted into ammonia (NH3). This process is essential for life on Earth, as ammonia is a key component of proteins and nucleic acids.
Nitrogen fixation can occur through a number of natural processes, including:
- Biological nitrogen fixation: This is the process by which bacteria convert atmospheric nitrogen into ammonia. This process is carried out by a variety of bacteria, including cyanobacteria, rhizobia, and Azotobacter.
- Industrial nitrogen fixation: This is the process by which ammonia is produced from atmospheric nitrogen using a process called the Haber-Bosch process. This process is used to produce ammonia for fertilizer and other industrial purposes.
- Lightning: Lightning can also fix nitrogen by converting atmospheric nitrogen into ammonia. This process is not very efficient, but it is an important source of nitrogen in some ecosystems.
Denitrification is the process by which bacteria convert ammonia back into atmospheric nitrogen. This process is important in the nitrogen cycle, as it helps to return nitrogen to the atmosphere. However, denitrification can also be a problem in agricultural settings, as it can lead to the loss of nitrogen from the soil.
In conclusion, the correct answer is D. By denitrification.