How can I change notification volume on Android 6.0? I have an option to change alarm or ringtone volume in Settings, but not notifications as I had in 4.3.

How can I change notification volume on Android 6.0 without changing ringtone volume? 

I would prefer a solution that doesn't require root, but would accept one that requires root if it's impossible otherwise.

3rd-party app is acceptable if there's no way to do so in the OS.

I already considered a hack of setting ringtone to a very loud sound and notifications to artificially quiet sound file, please don't offer that as a "solution" since it doesn't allow adjustments.
I'm not aware of a native solution, but my app listing for Volume control has an app called Ringer &amp; Notification Unlink. It requires root and the XPosed framework, but should do its job. From the app description:


  Since ICS, Google has decided to link the volume of ringer and notification volume together. This Xposed module can separate the two streams as they were before ICS.


I have not tried it myself and especially cannot tell whether it works fine with Marshmallow – but the changelog for the latest version explicitly mentions it should: "support for Android 6".