Given the task definition and input, reply with output. In this task, you are given a paragraph, and your task is to construct a question from the given passage. Construct question in such a way that (i) it is well explained in the paragraph, (ii) the words may/may not be present in the paragraph, (iii) it should start with what/how/describe. Avoid creating a question that (i) is just relevant to one or two sentences, and (ii) not relevant to passage. 

Bacteria changes nitrogen into ammonium through fixation. The nitrification process is where ammonium gets changed into nitrates by bacteria. Plants absorb the nitrates from soil into their roots through assimilation. When plants die, fungi and bacteria turn nitrogen back into ammonium through ammonification. The denitrification process pushes extra nitrogen from soil into the air. The cycle repeats from step 1. 

Describe the nitrogen cycle