I'm getting some mixed information here. A source on google books states that "the signatories were still bound to the Convention even when one of the parties to a conflict was not among the signatories."(294)
However, when I'm reading a different source, a General Reinecke defends himself in front of a court, stating the since Germany and the Soviet Union were not binded by the Geneva Convention, "War captivity was solely protective custody, and to prevent prisoners from further participating in war," which was only the normal international law.
The main problem is that the author of the second source states that General Reinecke's argument was legally true, but that contrasts with what the first author said about the Geneva Convention, which was that even though the Soviet Union didn't sign, Germany still had to comply.
Thank you for all your help!
Legally, Germany was obliged to uphold the Geneva Convention; however, Wilhelm Keitel of the OKH (Army High Command) declared that because the Soviet Union was not a signatory, German soldiers would not be obliged to uphold it. This lead to horrific atrocities against Red Army POWs almost from the very beginning of the Barbarossa campaign.