Re: D 90 low iso settings
Lo ISO does not add noise, in fact the opposite, but it loses contrast.
High ISO shifts the data up, and the noise comes up with it, more visible.
Low ISO (lower than the base ISO) shifts the data down, and the noise goes lower, but so do the high values. The histogram shown to us is adjusted, but LO 1 loses half the dynamic range (contrast). Probably the opposite of what you want at sunrise and sunset.
Manuals for models later than D90 say:
D300:
LO 0.3–LO 1
The settings [LO 0.3] through [LO 1] correspond to ISO sensitivities 0.3–
1 EV below ISO 200 (ISO 160–100 equivalent). Use for larger apertures
when lighting is bright. Contrast is slightly lower than normal; in most
cases, ISO sensitivities of ISO 200 or above are recommended. (this one is a ISO 200 camera model)