The figure shows the nitrogen cycle.  Which microbial activity is most directly responsible for returning nitrogen from soil compounds back into atmospheric N₂?

The cycle has two key microbial steps. (i) Nitrogen-fixers (Rhizobium, cyanobacteria) take N₂ from the air and lock it into compounds plants can use. (ii) Denitrifying bacteria do the reverse — they convert nitrogen compounds in the soil back into N₂ gas, which is released into the atmosphere. The chapter says: 'Certain other bacteria convert some part of [the nitrogenous compounds] to nitrogen gas which goes back into the atmosphere.' That balances the cycle and keeps atmospheric N₂ at ~78%.