The dynamics of organic matter in soil size and density fractions traced by stable carbon isotopes

On the basis of different photosynthetic pathways, there is an obvious difference in δ13C values between C3 plants and C4 plants. In terms of this characteristic, we analyzed the δ13C values in different size and density fractions from two profile-soil samples either in farmland and forestlands near the Maolan Karst virgin forest, Southwest China, where there were developed C3 plants previously and now are C4 plants. Results showed that the δ13C values of different size fractions in forestland soil are δl3Ccoarse sand < δ13Cfine sand < δ13Ccoarse silt < δ13Cclay < δ13 Cfine silt, and the δ13 C values of different size fractions in farmland soil are δ13 Ccoarse sand > δ13 Cfine sand > δ13 Ccoarse silt > δ13 Cclay > δ13 Cfine silt, indicating that soil organic matter is fresh in coarse sand and oldest in fine silt. The δ13 C values of different density fractions in forestland soil are δ13 Clight < δ13 Cheavy, and the δ13 C values of different density fractions in farmland soil are δ13Clight > δ13Cheavy, indicating that the soil organic matter is fresh in light fractions and old in heavy fractions.