Plants pull carbon from both the ground and the air. Rich topsoil is full of carbon by definition.
Thus Human->ground (assuming you mean whole, preserved corpses going into the ground) doesn't really mean much, and means even less when you consider that sequestering bodies in caskets isn't done in most places (preferring non-preserving, or burning), the meat that is buried is insignificant. Even with 55 million humans dying per year, consider that we kill over 50 billion chickens every year, 40 million cows, and 100 million pigs. All of those are also eating plants, so they're consuming carbon. If what you said about them being carbon sinks was correct, we wouldn't have the issues with atmospheric carbon that we do.
In other words, that is not the cycle.
Some of our waste is just thrown in the ground, but that is not a good way to treat it. In order to prevent water contamination and diseases, our waste is filtered, blended, treated with bacteria, methane reclaimed, dried, chopped up, treated to remove pathogens, and used as fertilizer or otherwise churned back into the soil. Basically, it's fed to plants, and it goes back to us one way or another, in an altered form. Some of it goes right back into the atmosphere.