I don't know about the laws from the 1st Session of the Confederate Congress that relate to this, but here are the laws from the 2nd Session of the Confederate Congress:

SATURDAY, February 18, 1865.
OPEN SESSION.

A bill (S. 197)declaring certain persons liable to duty in the reserve forces of the respective States;
which was read the first and second times and referred to the Committee on Military Affairs.

WEDNESDAY, March 1, 1865.
OPEN SESSION.

Mr. President: The House of Representatives have passed bills and a joint resolution of the Senate of the following titles:

S. 197. An act declaring certain persons liable to duty in the reserve forces of the respective States.

I *think* that prior to this national Confederate law, the reserves were formed under State law...in this case, under Alabama Law. And, this service was also mandatory. Hopefully I will have an opportunity to look this up tomorrow and report back.