On the basis of different photosynthetic pathway,there's obvious difference in
δ 13 C values between C\-3 plants
and C\-4 plants.Use this characteristic,the
δ 13 C values in different size and density fractions of two profile soil samples either in farm land and forest lands near Maolan Karst virgin forest was analyzed,there plant C\-3 plants previously and plant C\-4 plants now.Results showed that the δ 13 C values of different size fractions in forest soil aere δ 13 C coarse sand < δ 13 C fine sand < δ 13 C coarse silt < δ 13 C clay < δ 13 C fine silt ,and the δ 13 C values of different size fractions in farmland soil were δ 13 C coarse sand > δ 13 C fine sand > δ 13 C coarse silt > δ 13 C clay > δ 13 C fine silt ,it indicated that the
soil organic matter was fresh in coarse sand and oldest in fine silt.The δ 13 C values of different density fractions in forest soil ware δ 13 C light < δ 13 C heavy ,and the δ 13 C values of different density fractions in farmland soil were δ 13 C light > δ 13 C heavy ,it also indicated that the soil organic matter was fresh in light fractions and old in heavy fractions.