A Land Plant that has lived dies, and decomposes. Then, it becomes a part of the soil, being decomposed and eaten my worms, etc. Sometimes, when the ocean meets the soil, it can seep into the soil and dry out, leaving the carbon in the soil.
How Does Carbon Leave the Soil?
When plants use their roots to get the water from the ground, they suck up the carbon from the soil too. Also, when the ocesn meets the soil, the soil's carbon dissolves into the ocean.