The major pathway for nitrogen to enter an ecosystem is through nitrogen fixation. Certain bacteria, just like lightning and volcanic activity, can fix nitrogen naturally.
Which statement best describes the role of soil bacteria in nitrogen cycling?
A. They convert inorganic nitrogen to organic nitrogen.
B. They convert rock to ammonium, which is converted into nitrogen gas released to the atmosphere.
C. They convert nitrogen gas to amino acids, which are taken up by plants and recycled when they die.
D. They convert unavailable nitrogen into nitrogen that is available to plants, and back again.