The changes in soil organic matter in a forest-cultivation sequence traced by stable carbon isotopes

Abstract

There is an obvious difference in δ¹³C values between plants that assimilate carbon via the C₃ photosynthetic pathway and those that do so by the C₄ photosynthetic pathway. In terms of this characteristic, we analysed the organic carbon content and δ¹³C values of total soil and δ¹³C values in different size and density fractions of profile-soil samples either in farmland or in forestland near the Maolan Karst virgin forest, south-west China. This is an area where C₃ plants grew previously, now replaced by C₄ plants. Deforestation has accelerated the decomposition rate of soil organic matter and reduced the proportion of active components in soil organic matter and thus soil fertility. The δ¹³C values of different size fractions in forest soil are δ¹³C_{coarse sand} < δ¹³C_{fine sand} < δ¹³C_{coarse silt} < δ¹³C_clay < δ¹³C_{fine silt}, and the δ¹³C values of different size fractions in farmland soil are δ¹³C_{coarse sand} > δ¹³C_{fine sand} > δ¹³C_{coarse silt} > δ¹³C_clay > δ¹³C_{fine silt}, indicating that soil organic matter is fresh in coarse sand and oldest in fine silt. The δ¹³C values of different density fractions in forest soil are δ¹³C_light < δ¹³C_heavy, and the δ¹³C values of different density fractions in farmland soil are δ¹³C_light > δ¹³C_heavy, indicating that the soil organic matter is fresh in light fractions and old in heavy fractions.