Source identification and chemical compositions of particulate and mineral-associated organic matter in the deposited sediments of a dam-controlled watershed

Abstract

Understanding the soil fraction controls on the source and chemical composition of soil organic matter (SOM) in buried sediments is essential for revealing the underlying mechanisms of deposited carbon (C) fate and stabilization on the earth. In this study, we fractionated the bulk SOM into particulate organic matter (POM) and mineral-associated organic matter (MAOM) in deposit profiles trapped by check dam on the Chinese Loess Plateau and then attempted to explore the sources and chemical compositions of the POM and MAOM fractions by combining physical fractionation with elemental, isotopic, and spectroscopic analyses. The results showed that the main C source of deposited SOM from upland soils varied with soil fractions. The primary contributor of the POM fraction in deposited sediments originated from grassland and the gully’s contribution was minimal, whereas the cropland was the main contributor to the MAOM fraction and the lowest contribution was from forest. The relative enrichments of δ13C and δ15N and the lower C/N ratio in the MAOM fraction indicated that it had undergone more microbial processing than the POM had. Moreover, the specific peak data of Fourier Transform Infrared spectra (FTIR) demonstrated that there were higher proportions of the labile functional groups (i.e., alcohol COH/CNH, polysaccharide CO, and aliphatic CH) than the recalcitrant functional group (i.e., aromatic CC/CN) in the POM fraction in entire deposit profiles. The poor relationships between 13CSOC and log-transformed SOC in different soil fractions further confirmed a lower decomposition rate of MAOM compared to the POM. The results indicate that the POM fraction might have a faster SOM biogeochemical cycling than the MAOM fraction in buried sediments. Our results provide important implications for accurately estimating the nature and persistence of the deposited OM retained by check dams.