If the C:N ratio is wide and there is microbial activity, CO2 is respired which causes the ratio to narrow ...

In the process of decomposition of residue, the carbon breaks into CO2, and the part of CO2 oxidizes. same time, the part of N is used by the microbes as a food source. That's why the N level remains the same level in narrowing the C: N ratio.

@Praveen Kadam

Please give a reading to shared information (in the link below) for a detailed understanding. Nicely explained.

https://www.ecofarmingdaily.com/build-soil/soil-inputs/minerals-nutrients/carbon-nitrogen-ratio/

Good Luck

Thank you Mukesh Choudhary, sir.