Soil respires CO2 (from microbial digestion, primarily), actually probably more actively than your compost pile does because it increases the surface area / mass ratio.

Exactly how long carbon is sequestered in plant litter is an actively researched topic, and the general consensus seems to be that it depends a lot on temperature, soil composition, etc. But the overview is that worldwide, the feeling is that about 3x as much carbon is contained in soil as in living plants, almost all of that carbon being the result of plant litter. On a macroscopic level across all time-frames, clearly the carbon cycle is basically closed, so the amount respired out by the litter is the same amount as in. On a more local level, we may now have a lower-than-usual amount of carbon in the soil, as modern agricultural techniques and deforestation may have released more carbon from the soil than usual.

Empirical results seem to confirm the intuition that woods and such sequester carbon longer than quicker-decaying substances, and indeed how could they not?

So as a very, very rough rule of thumb, I'd say that you'd expect a fast-growing plant to sequester carbon for roughly 2-3x its lifespan (and of course some of that is carbon release happens quickly and some more slowly).