What converts nitrogen into a usable substance during the nitrogen cycle?

A. Consumers

B. Decomposers

C. leaches

D. Nitrogen fixing bacteria

D.) Nitrogen fixing bacteria

"Nitrogen is converted from atmospheric nitrogen (N2) into usable forms, such as NO2-, in a process known as fixation. The majority of nitrogen is fixed by bacteria, most of which are symbiotic with plants. Recently fixed ammonia is then converted to biologically useful forms by specialized bacteria."