Effects of land use change on soil organic carbon sources and molecular distributions: 6280 years of paddy rice cropping revealed by lipid biomarkers

PURPOSE: The sources of and changes in molecular composition of soil organic carbon (SOC) due to land use change in a paddy soil chronosequence ˃6000 years old were studied using a lipid biomarker approach. MATERIALS AND METHODS: Lipid biomarkers comprising fatty acids, aliphatic alcohols, and sterols were used to evaluate rice field reclamation and management over 6000 years on sources of and changes in molecular composition of SOC in two soil profiles comprising of the following three soil chronosequence layers (from bottom to top): 6280-year-old BP prehistoric paddy soil/pristine upland, 3320-year-old BP ancient paddy soil, and the present paddy soil. Furthermore, comparison with adjacent abandoned paddy soils was made to indicate the effect of abandoning cultivation due to urbanization on the sources and fluctuations in the molecular composition of SOC. RESULTS AND DISCUSSION: Fatty acids and aliphatic alcohols represented the most abundant fraction (∼80%) of autochthonous organic carbon sources from microbial activity in the prehistoric pristine upland. Reclamation of pristine upland to submerged rice fields led to 44.5 and 41.1% increases in the ratio of long-chain n-fatty acids and n-alcohols (≥C22), respectively, to the total saturated homologs indicating an increased input of allochthonous organic matter sources derived from rice cropping. The evolution of the paddy soil from 6280 to 3320 years BP and then to the present was accompanied by a slight and gradual increase in the proportion of the higher plant-derived organic components. Abandonment of rice cropping led to an apparent decrease in the concentration of the lipid biomarkers suggesting a decline in rice roots and stubble inputs and of microbial activity. CONCLUSIONS: Lipid biomarkers can be used to trace the sources and fluctuations in the molecular composition of SOC in paddies owing to land use change. Fatty acids were more suitable as the lipid proxy than aliphatic alcohols or sterols.