The figure shows the nitrogen cycle.  Which microbial activity is most directly responsible for returning nitrogen from soil compounds back into atmospheric N₂?
ADecomposer fungi breaking dead leaves into humus
BRhizobium fixing nitrogen from air into root nodules
CYeast respiring and releasing carbon dioxide
DCertain soil bacteria converting nitrogenous compounds in the soil into nitrogen gas
Answer & Solution
Correct answer: D. Certain soil bacteria converting nitrogenous compounds in the soil into nitrogen gas
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%.
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