Some of the plant decomposes to the atmosphere, but some stays in the soil too. It is pretty easy to demonstrate it to yourself, with fast-growing plants, after a few years you can have quite a bit of extra carbon-based matter in the soil.
There does appear to be a limit to carbon content in soil, though. It isn’t an endless sink.
Some interesting research which I can’t find great sources for indicates that microbial content of the soil can increase the carbon capacity, but I’d need to see better sources to verify that. An example of research around that would be BEAM/Johnson-Su bio reactors and the compost they use to improve agricultural yield. Part of their work indicates that the more they feed the soil with microbe and fungus-rich compost, the better the carbon-retention becomes. There are others like it, but there’s not a large enough body to work to show how well this scales and how well it works across different media.
If I had another life to live I think I’d love to do this kind of research.
A point made in the article, more than halfway down, is that modern intensive agriculture is less of a carbon sink than previous approaches.
There are people seeking to alter modern wheat | canola | et al to leave more carbon behind in larger root systems that remain within the soil and build deeper "peatier" soil bases.
This has the potential to knock a significant percentage from the global annual atmosperic carbon increase .. and every bit helps.
