Which nutrient cycle needs a nutrient to be fixed prior to use by producers and consumers answer
Answers
The nitrogen cycle.
Free nitrogen, N2, is in the atmosphere, but it is unusable by both producers and consumers. Some bacteria in the soil are able to "fix" the nitrogen by breaking it apart and attaching hydrogen atoms. This makes ammonia NH3, which is still unusable by producers or consumers. Another type of bacteria picks up the ammonia, removes the hydrogens and adds 2 oxygen atoms making nitrites NO2. Another type of bacteria then takes the nitrites and adds another oxygen atom, NO3, to make nitrates. These nitrates are then able to be absorbed by producers, and then eaten by consumers. To complete the cycle, waste from consumers, dead consumers and producers, are then broken down by decomposers that make ammonia.
To add a little more complexity, there are some other bacteria that can take nitrates out of the soil and free up the nitrogen as N2 and release it back into the atmosphere. There are some varieties of bacteria that live in nodules on the roots of some legumes that are able to convert N2 from the air and convert it into a usable form that the plants roots can absorb. Some nitrogen fixation can also occur in the atmosphere from lightning, where atmospheric N2 and O2 are converted into nitrates that dissolve in the rain and are brought down to the soil.
Nitrogen cycle because they are fixed by certain rizobium, azotobacter and blue green algae. These fix nitrogen in the soil but not in the form plants want. So they are modified by another bacteria and then used by plants. As for rizobium they are present in leguminous plants which enriches the soil by themselves