The study looked at soil in karst areas in Southwest China to understand how organic carbon is distributed vertically. They focused on limestone and yellow soil types, measuring pH, organic carbon content, and carbon isotopes in plants and soil. Limestone soil had more organic carbon than yellow soil. Organic carbon in yellow soil was concentrated in the top 20 cm, while in limestone soil, it decreased with depth. Carbon isotopes showed differences in how organic carbon changed with depth in the two soil types. This information helps us understand how plant residue breaks down and accumulates in soil, shedding light on the soil carbon cycle and formation process.