Smell
After a long period of time fighting, bodies lay strewn across no-man's land and in the trenches. These bodies start to decompose and smell, which is a huge demoralizer for the soldiers.
Bodies
No one is able to retrieve the bodies in fear of being shot, so the bodies stay in no-man's land for long periods of time. Rats and insects decompose the bodies, which are mostly devoured by the time they are recovered.
Records were kept so that when the bodies were retrieved, they could be identified.