I'm not certain about this, but I really don't think so.
I think the snails primarily eat detritus, bacteria, algae, fungus, biofilms, etc. Probably mostly stuff that would end up just decomposing anyways.
If anything, I think they are just temporarily diverting some of the nutrients out of the cycle, sorta like a terrace on a hill.
plus, the actual bio-mass of the snails is pretty small, and what they are eating/excreting is going to be an even smaller fraction of that.