Mechanism deciphering of variation of soluble organic carbon storage during the sedimentary period in lacustrine sediments.

Stable organic carbon (OC) burial into lacustrine sediments was the important C fixation path for atmospheric C reduction. However, OC retention effect and stability mechanisms in sediments was still unclear at the molecular scale during the burial period. Chronology and OC fractions were measured for the sediments from two freshwater lakes. Three-dimensional fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) were used to detect the change in the forms and molecular structure of soluble OC (SOC). Remarkably increasing TOC and total nitrogen (TN) concentrations along with the decreasing TOC/TN (C/N) values upwards occurred in the sediment columns indicated the weakened terrigenous OC and N inputs from about the 1980s. The OC mineralization contributed to the decrease of C burial in deeper sedimentary layers. However, higher SOC concentrations was found in deeper depths, suggesting that SOC facilitated the OC accumulation and fixation in the sediments and triggered the higher OCBR values over time. Additionally, obvious fluorescence intensity existence throughout the the whole sediment cores suggested the synchronous accumulation of both humic acid-like and fulvic acid-like materials over time as dominant SOC fractions. Higher fluorescence intensity at upper sediment layers indicated the SOC burial as humic acid-like materials with stronger sequestration potential. The persistence of fluorescence signal suggested that fulvic acid-like materials dominated the SOC immobilization and resistance to mineralization in the sediments. Finally, FT-ICR-MS analysis showed that remarkable accumulation of lipids with abundances from 31 to 44 % of organics was attributed to the decomposition of lignins and dominated CHO and CHON formulas and OC compounds. Lipids and lignins with higher abundances dominated the SOC burial and fixation over time even though mineralization occurred. Our work testified that lipids and lignins contributed to the majority of humic acid-like materials and subsequent OC retention in the lacustrine sediments at the geological time scale.