N is part of the Pitting Equivalence Number calc. And the fear is if it is lost it will reduce pitting resistance, generally part of what duplex is intended for. Being a gas it is argued it is lost in the welding process. It is normally only an issue with super duplexes with stringent G48 requirments and such.
The idea is to add it back in through the gas either shielding or purge. Which one depends upon testing regime, process, etc.
It will also effect phase balance so one should be careful when using it. It is an extreme austenite stabilizer. And can also form detrimental nitrides.