The word is:
My first letter is related to root-nodules
Root nodules are to do with plant roots and Nitrogen, the chemical symbol for which is N.
My second letter is a start
The start of the alphabet is A.
My third letter is related to Loki
Loki likes to turn into a snake (to trick Thor, among other things). Snakes are often associated with the letter S due to their shape and they start with S.
My fourth is the same as the second
In whole I am known to hide many secrets
NASA are thought to be hiding secrets of aliens and the like.