Plus and Minus Barrels make an appearance in only two levels of Donkey Kong Country 2: Diddy's Kong Quest, "Target Terror" and "Haunted Hall".
A Plus Barrel has a green plus sign on the barrel and is used to open gates and to increase the time on the timer in the level. A Minus Barrel has a red minus sign on the barrel and is used to close gates and to decrease the time on the timer in the level.
Additionally, a single Plus Barrel can be found at the beginning of "Rickety Race". This Plus Barrel works differently; it speeds up the Kongs' Skull Cart.